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To achieve fast, safe and economical drilling operations through formations containing natural fractures or vugs, all fractures and vugs must be perfectly sealed using proper fractures seal materials in a process called wellbore strengthening. Wellbore strengthening may elevate the overall tensile strength of the treated formation to a value approaches the intrinsic tensile strength of unfractured rock.Numerous studies have been conducted for testing fractures seal materials suitability for wellbore strengthening application in various formations. However, there is no solid and simple theoretical explanation on how wellbore strengthening process can recover the tensile strength and hence the fracturing pressure of the treated naturally fractured formation.In this study, test specimens cored from an artificial (building) sandstone bricks were used to measure the unconfined compressive strength (UCS), Brazilian indirect tensile strength (BTS), and establishing the multi-stage triaxial compression (MS-TCS) test. Core samples from the same sandstone were previously used for wellbore strengthening studies using crushed dates palm seeds. Crushed dates palm seeds succeeded to completely stop mud losses at a pressure equal to 6.9 MPa.Data obtained from unconfined compression, multi-stage triaxial compression, Brazilian indirect tensile, and wellbore strengthening tests were collectively used to develop a clear theoretical understanding of the mechanism of tensile strength recovery caused by wellbore strengthening effect.It was found that wellbore strengthening effect in sealing natural fractures and vugs is similar to the confining pressure effect in closing the induced shear fractures in the test specimen during a multi-stage triaxial compression test.For the tested sandstone, using crushed dates palm seeds as a fracture seal material has improved wellbore strengthening from zero tensile strength (complete loss of mud circulation) to 2.56 MPa tensile strength at which there was a complete fracture seal (zero mud loss). The recovered tensile strength (2.56 MPa) is approaching the intrinsic tensile strength (BTS = 2.7 MPa) of the same sandstone in its initial unfractured state.

Lost circulation is considered a time-consuming, costly problem during the construction of oil and gas wells. There are several preventive techniques to mitigate this problem. Stress cage technology is a mechanical lost circulation method, in which the formation at the wellbore wall is strengthened to stop the creation of induced fractures as one of the main causes of lost circulation. In this research, a two-dimensional numerical model, considering the elastic, poro-elastic, and thermo-poro-elastic behavior of the rock, is built to investigate the effectiveness of the stress cage method. Results show that better performance of the technology is achieved if the fractures are bridged close to their apertures. Additionally, it was found that the difference between the elastic, poro-elastic, and thermo-poro-elastic models is slightly visible. The conclusion states that the application of the stress cage methods leads to an increase in hoop stress and subsequent formation fracture gradient. [ABSTRACT FROM AUTHOR]

This research presents a comprehensive review of lost circulation and wellbore instability challenges in geothermal drilling, complemented by an original case study demonstrating the application of Expandable Profile Liner Technology (EPLT). The review synthesizes and analyzes current mitigation strategies, highlighting their limitations in complex geothermal environments. In response to the inadequacies of traditional methods and the imminent risk of well abandonment, EPLT was implemented as an innovative solution. The case study of the J-1 well demonstrates EPLT's novel application in severe lost circulation conditions, showcasing its ability to effectively seal large fractures and high-permeability zones without reducing wellbore diameter and breakthrough — a common limitation of conventional techniques. Quantitative analysis reveals significant improvements in drilling efficiency, wellbore stability, and cost-effectiveness. This research contributes to the field by introducing EPLT as a groundbreaking approach to lost circulation management and wellbore strengthening in geothermal drilling, potentially revolutionizing industry practices and enhancing drilling performance in high-temperature environments. • Comprehensively analyzing lost circulation and wellbore instability challenges in geothermal drilling. • Effectively addressing severe lost circulation with EPLT while preserving well diameter. • Demonstrating EPLT's effectiveness in handling severe lost circulation and collapse formations through J-1 well case study. • Reducing downtime and drilling costs with EPLT, enhancing geothermal project economics. [ABSTRACT FROM AUTHOR]

The problem of creating unwanted fractures while drilling ultradeep wells is mitigated with the application of wellbore strengthening techniques. Despite the numerous applications, an open question remains about the efficiency of the loss of circulation materials (LCM) and its implications on the closure stress distribution change during plugging. This work investigates the effects on the stress field before and after plugging along the fracture extension axis by introducing a hydrodynamic plug. With the hydrodynamic plug, fluid flow is constrained by pressure conditions at specific locations in the fracture simulating the LCM. Three different scenarios were considered. First, the efficiency of the bridge is simulated by varying the pressure drop. Second, the location of the bridge inside the fracture and finally a nearly packed fracture. The models are fully coupled and were solved with the finite element method in impermeable and permeable hard rocks. We find that for high‐efficiency bridges, narrower fracture profiles are predicted, which causes the induced closure stresses to increase significantly. On the other hand, when the bridge is close and near the wellbore area, the fracture profiles are maintained wide and narrow when it is nearest to the tip. The predicted fracture geometry induces higher closure stresses when the plug is near the well and slightly reduces when it is near the tip. Finally, the pressure profile resulting from the packed fracture significantly affects the fracture dimensions, resulting in narrower fracture, however resulting in a smooth variation of induced closure stresses with high magnitude comparable to the stresses at the state of propagation. The diffusion occurring in the permeable case creates back‐stresses that appear to have an additive contribution to the induced closure stresses. This underlines the significance of diffusion on the induced coupled closure stresses for large fractures while performing wellbore strengthening methodologies. [ABSTRACT FROM AUTHOR]

Wellbore strengthening by fracture plugging is an effective method for controlling fluid leakage in a fractured formation. However, few experiments have documented the effectiveness of fracture plugging on formation leakage. In this study, fracture plugging of fractured carbonate formation was conducted using a novel plugging dynamic core device. The samples were processed into standard cores before creating fractures. Fracture plugging was assessed using the fracture pressure bearing capacity, whose higher values indicate better plugging effectiveness. A generalized linear model (GLM) has been proposed to correlate the effects of confining pressure, fracture porosity, and plugging agent content (PAC) on the fracture pressure bearing capacity. According to this model, the fracture porosity has a greater effect on the fracture pressure than the confining pressure and plugging content. The model also revealed that as the fracture porosity increased by 10%, the fracture pressure bearing capacity decreased by 10.3%, while an increase in the confining pressure by the same magnitude resulted in a 5.3% increase in the fracture pressure. The experimental results were successfully verified in the field by applying a 30% plugging agent in two wells. An improvement in the fracture pressure bearing capacity in two vulnerable sections of the wells was observed. [ABSTRACT FROM AUTHOR]

Wellbore strengthening is a technique for increasing formation fracture pressure and widening the mud window. In this study, smart wellbore strengthening materials (WSMs) built on shape memory polyurethane (SMPU) are introduced. SMPU WSMs are programmed to release pre-determined recovery stress once unfold at predetermined temperature. The molecular structure of the SMPU WSM synthesized here is characterized and its shape memory, recovery stress and mechanical properties are determined. Compatibility of SMPU with xanthan gum-based drilling fluid is confirmed through mud rheology and filtration tests. SMPU WSMs mixed with SMPU lost circulation materials (LCMs) smoothly flow through the bit, pack against and seal fractures and release recovery stress once activated. Wellbore strengthening and fracture sealing of the SMPU WSM and LCM mixture show a recovery stress of 3.52 MPa in a constrained environment. The seal showed high plugging pressure of 110 bar with only 150 cm3 total fluid loss. [Display omitted] • Wellbore strengthening material based on shape memory polyurethane was synthesized. • The role of shape memory and recovery stress on wellbore strengthening is assessed. • Impact of hard segment content and programming method on recovery stress is detailed. • Fracture sealing efficacy of the smart wellbore strengthening materials is evaluated. [ABSTRACT FROM AUTHOR]

To achieve fast, safe and economical drilling operations through formations containing natural fractures or vugs, all fractures and vugs must be perfectly sealed using proper fractures seal materials in a process called wellbore strengthening. Wellbore strengthening may elevate the overall tensile strength of the treated formation to a value approaches the intrinsic tensile strength of unfractured rock. Numerous studies have been conducted for testing fractures seal materials suitability for wellbore strengthening application in various formations. However, there is no solid and simple theoretical explanation on how wellbore strengthening process can recover the tensile strength and hence the fracturing pressure of the treated naturally fractured formation. In this study, test specimens cored from an artificial (building) sandstone bricks were used to measure the unconfined compressive strength (UCS), Brazilian indirect tensile strength (BTS), and establishing the multi-stage triaxial compression (MS-TCS) test. Core samples from the same sandstone were previously used for wellbore strengthening studies using crushed dates palm seeds. Crushed dates palm seeds succeeded to completely stop mud losses at a pressure equal to 6.9 MPa. Data obtained from unconfined compression, multi-stage triaxial compression, Brazilian indirect tensile, and wellbore strengthening tests were collectively used to develop a clear theoretical understanding of the mechanism of tensile strength recovery caused by wellbore strengthening effect. It was found that wellbore strengthening effect in sealing natural fractures and vugs is similar to the confining pressure effect in closing the induced shear fractures in the test specimen during a multi-stage triaxial compression test. For the tested sandstone, using crushed dates palm seeds as a fracture seal material has improved wellbore strengthening from zero tensile strength (complete loss of mud circulation) to 2.56 MPa tensile strength at which there was a complete fracture seal (zero mud loss). The recovered tensile strength (2.56 MPa) is approaching the intrinsic tensile strength (BTS = 2.7 MPa) of the same sandstone in its initial unfractured state. [ABSTRACT FROM AUTHOR]

Abstract Determining the width of an induced hydraulic fracture is the first step for applying wellbore strengthening and hydraulic fracturing techniques. However, current 2-D analytical solutions obtained from the plane strain assumption may have large uncertainties when the fracture height is small. To solve this problem, a 3-D finite element method (FEM) is used to model wellbore strengthening and calculate the fracture width. Comparisons show that the 2-D plane strain solution is the asymptote of the 3-D FEM solution. Therefore, the 2-D solution may overestimate the fracture width. This indicates that the 2-D solution may not be applicable in 3-D conditions. Based on the FEM modeling, a new 3-D semi-analytical solution for determining the fracture width is proposed, which accounts for the effects of 3-D fracture dimensions, stress anisotropy and borehole inclination. Compared to the 2-D solution, this new 3-D semi-analytical solution predicts a smaller fracture width. This implies that the 2-D-based old design for wellbore strengthening may overestimate the fracture width, which can be reduced using the proposed 3-D solution. It also allows an easy way to calculate the fracture width in complex geometrical and geological conditions. This solution has been verified against 3-D finite element calculations for field applications.

Lost circulation is considered a time-consuming, costly problem during the construction of oil and gas wells. There are several preventive techniques to mitigate this problem. Stress cage technology is a mechanical lost circulation method, in which the formation at the wellbore wall is strengthened to stop the creation of induced fractures as one of the main causes of lost circulation. In this research, a two-dimensional numerical model, considering the elastic, poro-elastic, and thermo-poro-elastic behavior of the rock, is built to investigate the effectiveness of the stress cage method. Results show that better performance of the technology is achieved if the fractures are bridged close to their apertures. Additionally, it was found that the difference between the elastic, poro-elastic, and thermo-poro-elastic models is slightly visible. The conclusion states that the application of the stress cage methods leads to an increase in hoop stress and subsequent formation fracture gradient.

As one of the most widely used preventive treatment of lost circulation, wellbore strengthening techniques need to be further investigated to improve its efficiency. It is crucial to accurately predict the dynamic propagation of fractures for wellbore strengthening, while most of the numerical simulations neglect the distortion of fracture trajectory affected by fracture interference. Thus, a fully coupled poroelastic model is established by using the extended finite element method (XFEM) – based cohesive zone method (CZM) to investigate the effect of fracture interference. The rock deformation, seepage flow of pore fluid, fluid flow of fracture and leak-off of drilling fluid are coupled by the model. The sensitivity analysis is studied by capturing the change of fracture length, width and deviation angle. The results show that the distortion of the fracture trajectory and the closure extent of fracture surface after bridging can be controlled more easily with lower injection rate and larger spacing. A proper viscosity can balance between the distortion of the fracture trajectory and the tendency of fracture reopening after bridging. The anisotropic stress mainly affects the wellbore strengthening by influencing the propagation direction. Adjusting the inclination and azimuth of the wellbore to increase the effect of stress anisotropy can reduce the effect of stress shadow. [ABSTRACT FROM AUTHOR]

For years, drilling engineers have been faced with the challenge of drilling wells through naturally fractured reservoirs that are present around the world. During drilling, the pressure at the bottomhole of a well is frequently intentionally higher than formation pressure, which can result in the loss of mud in surrounding rocks. During well cementing, the bottomhole pressure is even higher than it is during drilling, because the cement slurry density is higher than the density of the mud. Therefore, if natural or induced fractures in the surrounding rocks are not plugged during drilling, the cement slurry can be lost to them, reducing their permeability which is undesirable in the case of a pay zone. To prevent the loss of circulation and the related consequences, it is necessary to apply good drilling and cementing practices and to use adequate methods and carefully selected materials for plugging the loss zones. The aim of this article is to give an overview of the preventive and corrective methods that can be applied in drilling and cementing through fractured zones as well as improvements in drilling and cementing technology to avoid lost circulation issues (e.g., aerated drilling fluid, casing while drilling, managed pressure drilling, expandable tubulars, lightweight cement slurries, etc.). [ABSTRACT FROM AUTHOR]

Abstract Although a large volume of mudcake filtration test data is available in the literature, effects of mudcake on wellbore strengthening cannot be quantified without incorporating the data into a stress-analysis model. Traditional models for determining fracture initiation pressure (FIP) either consider a wellbore with an impermeable mudcake or with no mudcake at all. An analytical model considering permeable mudcake is proposed in this paper. The model can predict pore pressure and stress profiles around the wellbore, and consequently the FIP, for different mudcake thickness, permeability, and strength. Numerical examples are provided to illustrate the effects of these mudcake parameters. The results show that a low-permeability mudcake enhances FIP, mainly through restricting fluid seepage and pore pressure increase in the near-wellbore region, rather than by mudcake strength. Fluid loss pressure (FLP) should be distinguished from FIP when a mudcake is present on the wellbore wall. Fracture may occur behind the mudcake at FIP without mudcake rupture. The small effect of mudcake strength on FIP does not mean its effect on FLP is small too. Mudcake strength may play an important role in maintaining integrity of the wellbore once a fracture has initiated behind the mudcake.

The selection of physical lost circulation materials (LCMs) type plays a crucial role in formulating effective plugging slurry. The structural stability of the plugging zone, formed by LCMs, is a decisive factor in determining their effectiveness in deep fractured reservoirs. In this study, combined with the Hertz contact model and calibration of contact forces of photoelastic particles, the color gradient mean square method was employed to quantitatively characterize the strong force chains evolution of the three typical types of plugging zones. Results showed that the incorporation of flexible particles into the LCMs enhanced the average shear strength, increased the proportion, and promoted the development of circular structures within the strong force chains, thereby facilitating stronger shear stability. This research provides new insight to understand the role of physical LCMs type on the mesoscale failure mechanism of plugging zone in deep fractured tight reservoir. [Display omitted] • Photoelastic images of rigid and flexible granulars over shear loads were observed. • Strong force chains shear strength was calculated based on the Hertz contact model. • Strong force chains proportion evolution was obtained by the G2 method. • Flexible particles transform macro-mechanics of plugging zone from brittle to ductile. [ABSTRACT FROM AUTHOR]

Drilling fluid circulation loss is referred to as "the most severe problem in drilling of oil and gas wells", as a routine solution, loss circulation materials are added to fill any fractures and vugs that may have been created during the drilling process. An alternative technology to control drilling fluid loss is "Wellbore Strengthening" as a set of engineering design solutions to seal and plug near-wellbore fractures efficiently. This review presents polymeric systems used as loss circulation materials (LCMs) or in wellbore strengthening applications. Moreover, factors that affect the strength of the polymeric formulations, compatibility with drilling fluid and additives, rheological properties and thermal stability are discussed. In addition to that, the review highlights the use of nano-materials in loss circulation polymeric systems. The performance evaluation methods used to evaluate the polymer gel properties are reviewed and the main characteristic of successful LCM formulations are highlighted. Polymers applied in other oil and gas operations, such as hydraulic fracturing and cementing, that have a potential for use in LCM applications are also included in this review. Image 1 • The paper reviews polymeric formulations that have been used in loss circulation treatments. • The potential of adopting polymeric systems that were previously applied in petroleum production operations is addressed. • Selection guidelines of polymeric systems for loss circulation prevention and wellbore strengthening are provided. • The possibility of using nanoparticles to improve the properties of the polymeric formulations is discussed. • The important laboratory characterization techniques used in evaluating these formulations are highlighted. [ABSTRACT FROM AUTHOR]

Determining the width of an induced hydraulic fracture is the first step for applying wellbore strengthening and hydraulic fracturing techniques. However, current 2-D analytical solutions obtained from the plane strain assumption may have large uncertainties when the fracture height is small. To solve this problem, a 3-D finite element method (FEM) is used to model wellbore strengthening and calculate the fracture width. Comparisons show that the 2-D plane strain solution is the asymptote of the 3-D FEM solution. Therefore, the 2-D solution may overestimate the fracture width. This indicates that the 2-D solution may not be applicable in 3-D conditions. Based on the FEM modeling, a new 3-D semi-analytical solution for determining the fracture width is proposed, which accounts for the effects of 3-D fracture dimensions, stress anisotropy and borehole inclination. Compared to the 2-D solution, this new 3-D semi-analytical solution predicts a smaller fracture width. This implies that the 2-D-based old design for wellbore strengthening may overestimate the fracture width, which can be reduced using the proposed 3-D solution. It also allows an easy way to calculate the fracture width in complex geometrical and geological conditions. This solution has been verified against 3-D finite element calculations for field applications. [ABSTRACT FROM AUTHOR]

Fracture-based wellbore strengthening is a widely used preventive technique for lost circulation control. However, there were limited experimental studies on wellbore strengthening with an anisotropic stress state. In this paper, we describe an experimental investigation of fracture-based wellbore strengthening on cubic Berea sandstone samples (size of 12 in 3) using a large-scale true triaxial cell. The true triaxial cell allows fracture containment to simulate wellbore strengthening, which was not available using a traditional small-scale traixial cell. We used drill cuttings and Chevron loss prevention material (LPM) as wellbore strengthening materials (WSM). Three independent stresses were applied on the rock samples and bi-wing fractures were generated. The final fracture reopening pressure (FROP) exceeded the formation breakdown pressure (FBP) after plugging the WSM. Further comparison between the experimental results and modeling results from a numerical model shows a good match for the injection pressure profile. • Experimental investigation of fracture-based wellbore strengthening was conducted on a large-scale true triaxial cell. • Fracture reopening pressures were higher than formation breakdown pressures. • Results of induced and plugged fractures were shown. • Stress redistribution was observed with plugging of wellbore strengthening materials. [ABSTRACT FROM AUTHOR]

Abstract Wellbore strengthening (WBS) methods are broadly applied in the field to combat lost circulation by plugging drilling induced fractures using wellbore strengthening materials (WSM). While changes in formation temperature during drilling have been identified as an important factor in wellbore stability considerations, the performance of WBS in a non-isothermal setting is unclear and remains as a barrier to successful designs of the types and sizes of WSMs. To understand the problem, a 3D finite element analysis was conducted to investigate the state of stresses in near-wellbore region from the interaction between induced thermal stresses and fractures based on different WBS plugging mechanisms. The model developed in this study coupled fluid and heat flow in fractured thermal-poroelastic rock formations and enabled flow restrictions in the fracture for simulating wellbore strengthening processes. By implementing different WBS mechanisms in the simulations, the induced thermal stresses from fluid leakage into the formations were analyzed, and the effectiveness of different WBS strategies in preventing fracture propagation under thermal effects was evaluated. The results disclosed how stresses redistribute around a fractured wellbore as non-isothermal mud invasion and WSM plugging take places. They show that temperature decreases of formation rock due to mud invasion can significantly facilitate fracture propagation and diminish the effectiveness of WBS. Such information is important to the successful management of lost circulation by taking thermal effects into consideration for better WSM design and WBS implementations. Highlights • A new non-isothermal wellbore strengthening model was developed. • Stress intensity factor was used to evaluate the effect of induced thermal stresses ona fracture tip. • Mud loss through induced fractures intensifies the redistribution of thermal stresses in near-wellbore region. • Fracture plugging in wellbore strengthening alters pore pressure and temperature distribution in formation. [ABSTRACT FROM AUTHOR]

Shale gas formations often exhibit well-wall instability during drilling, particularly when using water-based drilling fluids, due to the high water sensitivity of the rock and the propensity for fractures to collapse. In this paper, our study delved into the impacts of saikosaponins (SS) on enhancing the anti-collapse efficacy of water-based drilling fluids for shale gas. Our experimental findings revealed that the incorporation of 3% SS into the water-based drilling fluid led to a 71.7% reduction in bentonite swelling and a significant increase in shale rolling recovery to 98.9%. Furthermore, the water-based drilling fluid enhanced with SS could maintain a shale strength at 96.8% of its original value. Moreover, the inclusion of SS elevated the dynamic shear force of the drilling fluid, bolstered its chip-carrying capacity, and reduced its filtration loss. This is attributed to SS's ability to adhere to the shale surface via hydrogen bonding, resulting in the formation of a hydrophobic film that impedes water molecules from infiltrating the shale. Simultaneously, SS's unique multi-hydroxyl structure endowed it with robust adhesion, forming a reinforcing layer on the shale surface to fortify the wall of the well. Consequently, SS effectively heightens the anti-collapse capabilities of drilling fluids. Moreover, SS as a plant-extracted ingredient is environmentally friendly and holds immense promise for the eco-conscious advancement of shale gas. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract Lost circulation events and mud filtration in drilling operations have been investigated over the years because they lead to non-drilling time (NDT) and increase the overall well cost. Solutions have been offered from the addition of fluid loss control additives, the use of lost circulation materials (LCM), advanced pills, to novel solutions and technologies such as wellbore strengthening, casing while drilling (CwD), and managed pressure drilling (MPD). However, lithology complexities, geothermal and geochemical effects, and wellbore drilling dynamics continue to push the boundaries to enhance available preventative and mitigation strategies. In this study, pore and fracture-scale mud invasions are investigated and the results are used in characterizing filter cake wellbore strengthening. The methods used have been described in detail in the paper. The theoretical and experimental investigations revealed that wellbore temperature, drill string rotary speed, fracture width, rock permeability and porosity, LCM type, and LCM concentration are critical factors influencing filter cake evolution. SEM image and elemental maps of selected samples showed near-wellbore pore plugging by granular LCM. More than a 50% reduction in losses through a 2000 μm fracture width was recorded and concentration thresholds were established for the granular and fibrous LCM's, based on dynamic wellbore conditions. The numerical results showed that the rock permeability and filter cake permeability profiles largely control the changes in a wellbore hoop stress profile. This integrated approach will provide extensive details which will help inform decision making and select critical parameters for drilling different lithologies, loss prone zones, and achieving wellbore strengthening. Highlights • Granular and fibrous LCM concentration thresholds based on operating conditions. • Microscopic analysis of filter cake evolution in sandstone and carbonate rocks. • Rock and filter cake permeability impact on hoop stress profile. • Filter cake evolution and stability in dynamic conditions over 2000 μm fracture width. [ABSTRACT FROM AUTHOR]

Abstract A wellbore strengthening model for CBM drilling was analytically established based on the fracture mechanics, considering the cleat system. In this model, the stress field induced by the multi-cleats opened was firstly solved based on the fracture mechanics and the principle of the linear superposition. Especially, a failure criterion was introduced to judge whether the cleat was opening or closed. Then, the total stress was obtained by superimposing the stresses induced by the drilling fluid pressure, the formation pressure, the in-situ stress and the opened cleat. More importantly, the cleat-tip stress intensity factors before and after strengthening were calculated and compared to elaborate the validity of the wellbore strengthening. The results show that the cleat-tip stress intensity factors decreases obviously after the wellbore strengthening. Meanwhile, the analytical model shows an agreement to the finite element results for the stress distribution and the cleat-tip stress intensity factor. Finally, the parametric analysis was performed to reveal that: the smaller distance from the location of LCMs to the wellbore wall and the higher sealing efficiency of LCMs are positive for the wellbore strengthening; the larger number of cleats and the longer length of the opened cleat are harmful to the wellbore strengthening. This research has a contribution on establishing the wellbore strengthening model with multi-cleats. Highlights • Wellbore strengthening model is proposed considering complex distribution of cleat system. • The results of analytical model show a good agreement to those of finite element model. • Wellbore strengthening technology was verified by comparing stress intensity factor. • The parametric analysis on the cleat-tip stress intensity factor is performed. [ABSTRACT FROM AUTHOR]

Fracture-based wellbore strengthening techniques are preventive methods that can reduce the cost of lost circulation and non-productive time. The mud weight window can be extended by plugging fractures with wellbore strengthening materials (WSM) in the near-wellbore region. To maximize the strengthening effect, accurate fracture geometry prediction is of critical importance to the design of WSM. This paper presents a novel, coupled fluid flow and fracture mechanics model for wellbore strengthening applications that accounts for near-wellbore-induced fracture behavior. For fluid flow, mass conservation is considered and momentum conservation is examined; the latter shows that pressure loss with near-wellbore fracturing is low. Thus, we can neglect the pressure drop in the fractures and assume the fluid pressure inside the fractures is equal to the wellbore pressure. The pressure-width relationship (rock elastic deformation) and stress intensity factor are obtained by a dislocation-based approach. For the fracture propagation criterion, the calculated stress intensity factor is compared with fracture toughness at each time step. The stress intensity factor and fracture reopening pressure (FROP) are verified with Tada's model and Feng's model, respectively. Then, simulation results are compared with the large leak-off solutions of the Perkins-Kern-Nordgren (PKN) fracture model. The simulation results reveal that the PKN model overestimates the fracture mouth width, fracture length, and wellbore pressure. Furthermore, the simulation results of wellbore pressure show a different trend. Therefore, we cannot directly use the PKN model to design wellbore strengthening applications. The main reason is the presence of wellbore can generate near-wellbore effects that cannot be disregarded. Finally, we conduct a comprehensive parametric study (i.e., fracture toughness, Young's modulus, Poisson's ratio, horizontal stress ratio, and permeability) on wellbore strengthening fracturing. The proposed model is useful for wellbore strengthening applications using the intentionally induced fractures (i.e., near-wellbore fracturing). Particle size distribution (PSD) of WSM can be designed based on the simulated fracture geometry. No complex model mesh generation or assignment of boundary conditions are needed, which are commonly used in finite element simulation or other numerical methods. The proposed model can also be used to optimize wellbore strengthening operations by performing sensitivity analysis. [ABSTRACT FROM AUTHOR]

Although a large volume of mudcake filtration test data is available in the literature, effects of mudcake on wellbore strengthening cannot be quantified without incorporating the data into a stress-analysis model. Traditional models for determining fracture initiation pressure (FIP) either consider a wellbore with an impermeable mudcake or with no mudcake at all. An analytical model considering permeable mudcake is proposed in this paper. The model can predict pore pressure and stress profiles around the wellbore, and consequently the FIP, for different mudcake thickness, permeability, and strength. Numerical examples are provided to illustrate the effects of these mudcake parameters. The results show that a low-permeability mudcake enhances FIP, mainly through restricting fluid seepage and pore pressure increase in the near-wellbore region, rather than by mudcake strength. Fluid loss pressure (FLP) should be distinguished from FIP when a mudcake is present on the wellbore wall. Fracture may occur behind the mudcake at FIP without mudcake rupture. The small effect of mudcake strength on FIP does not mean its effect on FLP is small too. Mudcake strength may play an important role in maintaining integrity of the wellbore once a fracture has initiated behind the mudcake. [ABSTRACT FROM AUTHOR]

Lost circulation is a serious problem which always exists in the petroleum industry. Wellbore strengthening by lost circulation materials (LCMs) is a commonly applied method for mitigating lost circulation. This paper presents a hydraulic fracturing apparatus to investigate the effect of material type, concentration, and particle size distribution (PSD) of LCMs on wellbore strengthening behavior. In addition, the characteristics of pressure curves in the fracturing process are analyzed in detail. The results showed that the fracture pressure of the artificial core can be increased by LCMs, and there exists an optimum concentration of LCMs for the maximum wellbore strengthening effect. The LCMs with wide PSD can significantly increase the fracture pressure. However, some LCMs cannot increase or even decrease the fracture pressure; this is resulting from the LCMs with relatively single PSD that makes the quality of mud cake worse. The representative pressure curve in the fracturing process by drilling fluids with LCMs was divided into five parts: the initial cake formation stage, elastic plastic deformation stage, crack stability development stage, crack instability development stage, and unstable plugging stage. The actual fracturing curves were divided into four typical types due to missing of some stages compared with the representative pressure curve. In order to strengthen the wellbore in effective, good LCMs should be chosen to improve the maximum pressure in the elastic plastic deformation stage, extend the stable time of pressure bearing in the crack stability development stage, and control the crack instability development stage. [ABSTRACT FROM AUTHOR]

Drilling in complex geological settings often possesses significant risk for unplanned events that could potentially impede already cost-demanding operations. Lost circulation, a major challenge in well construction, refers to the loss of drilling fluid into formation during drilling operations. When excessive wellbore pressure appears, lost circulation is induced by tensile failure or reopening of natural fractures at the wellbore. Over years of research efforts and field practices, wellbore strengthening techniques have been successfully applied in the field to mitigate lost circulation and have proved effective in extending the drilling margin to access undrillable formations. In fact, wellbore strengthening contributes additional resistance to fractures so that an equivalent circulating density higher than the estimated fracture gradient can be exerted on the wellbore. In this study, a fully coupled hydraulic fracture propagation model based on the cohesive zone model is presented. By implementing the model, an extensive parametric study is conducted to investigate factors involved in lost circulation. The parametric influences emphasizing the mass balance within the fracture reveal mechanisms of lost circulation mitigation. Simulation studies on wellbore strengthening are conducted in two parts, hoop stress enhancement and fracture resistance enhancement. First, a near-wellbore stress analysis characterizes wellbore mechanical responses during lost circulation. The results show elevated hoop stress during fracture width development, which validates the hypothesis of hoop stress enhancement. Also, beneficial influences from poroelastic effect and high rock stiffness are demonstrated. Then, a novel method to simulate fracture sealing is introduced to quantify fracture gradient extension for field practices. With this method, a case study on fracture sealing investigates the roles of sealing permeability and sealing length. The results show inhibition of fracture repropagation and conclude that fracture tip protection is achieved through fracture sealing and fracture fluid dissipation. From the case study, operational insights on wellbore strengthening design are derived. [ABSTRACT FROM AUTHOR]

The principle of damage mechanics was adopted in order to solve the lost circulation problem when drilled low bearing capacity formation. According to quadratic nominal stress criterion and B-K criterion, cohesive element with zero thickness was inserted into fracture to simulate fracture initiation and propagation. A 3D fluid-solid coupling finite element model was established to analyze circumferential stress distribution of wellbore, hoop stress distribution of crack and geometry of fracture. Results show that closed fracture surface is still in the stage of complete fracture under the action of crustal stress and crack tip reopens after plugging particle failed. The fracture is difficult to widen for filling more plugging materials in a short time while the lost circulation rate and permeability are low. The wellbore strengthening treatment will makes the fracture become short and narrow under a larger filtration coefficient, but mud cake quality has an insignificant influence on fracture geometry when filtration coefficient increases to a certain value. The simulation results were supported by providing some cases of real wellbore condition. [ABSTRACT FROM AUTHOR]

Fracture-based wellbore strengthening techniques have been widely used in the oil industry to reduce the cost of lost circulation, especially in depleted reservoirs. Accurate prediction of induced-fracture geometry is of critical importance for best particle size distribution (PSD) selection of wellbore strengthening materials (WSM). Conventional parametric analyses mainly focused on rock properties, well conditions and WSM plug location. Thus, the deficiency of ignoring time and fluid dynamics may result in erroneous operations. In this paper, a dynamic fracture model based on the dislocation method is employed to qualitatively characterize the influence of controllable parameters on fracture propagation and fracture reopening pressure (FROP). Fracture propagation length and profile are obtained for each parameter with different values. It is found that fluid injection rate, fluid viscosity, fluid injection time, wellbore inclination and wellbore radius have an inverse relationship with FROP. On the other hand, fracture plug width has a positive relationship with FROP. Finally, a procedure for determining optimal wellbore strengthening operations by manipulating the controllable parameters is developed based on the dynamic fracture model. [ABSTRACT FROM AUTHOR]

Nanomaterials are engineered materials with at least one dimension in the range of 1-100 nm. Nanofluids--nanoscale colloidal suspensions containing various nanomaterials--have distinctive properties and offer unprecedented potential for various sectors such as the energy, cosmetic, aerospace and biomedical industries. Due to their unique physico-chemical properties, nanoparticles are considered as very good candidates for smart drilling fluid formulation, i.e., fluids with tailor-made rheological and filtration properties. However, due to the great risk of adapting new technologies, their application in oil and gas industry is not, to date, fully implemented. Over the last few years, several researchers have examined the use of various nanoparticles, from commercial to custom made particles, to formulate drilling fluids with enhanced properties that can withstand extreme downhole environments, particularly at high pressure and high temperature (HP/HT) conditions. This article summarizes the recent progress made on the use of nanoparticles as additives in drilling fluids in order to give such fluids optimal rheological and filtration characteristics, increase shale stability and achieve wellbore strengthening. Type, size and shape of nanoparticles, volumetric concentration, addition of different surfactants and application of an external magnetic field are factors that are critically evaluated and are discussed in this article. The results obtained from various studies show that nanoparticles have a great potential to be used as drilling fluid additives in order to overcome stern drilling problems. However, there are still challenges that should be addressed in order to take full advantage of the capabilities of such particles. Finally the paper identifies and discusses opportunities for future research. [ABSTRACT FROM AUTHOR]

One of the most critical aspects in the drilling operation is to reduce the nonproductive time and to avoid the borehole instability issues such as kicks, blow outs, lost circulation, stuck pipe, and breakouts. To investigate these problems, one has to understand the formation properties, fluid hydraulics, and the basic mechanics behind drilling a well. In the previous research on this field, the factors were widely discussed and results obtained were related to the formation properties. However, while considering the stresses in the wellbore, the mechanical factors such as the RPM and contact of casing at different positions in wellbore have usually been neglected. In furtherance to this study, the importance of thermal condition, fluid loss, and filter cake formation study cannot be out ruled. This work includes a new insight toward understanding the stress redistribution due to pipe contact by the wellbore and smear mechanism. Additionally, it presents the numerical analysis of influence of casing contact and downhole thermal conditions using the finite-element analysis. The classical equations used to obtain the wellbore stresses include very few parameters such as the far-field stresses, pore pressure, and wellbore geometry. They do not consider the influence of casing contact while drilling, mud-cake permeability, and elastic and inelastic properties of the formation. To take into account the effects of these parameters, finite-element analysis is carried out considering the above-mentioned parameters in various scenarios. The main objective of these simulations is to investigate the hypothesis of the increase in hoop stress considering casing contact with regard to formation stresses orientation. The study of different cases shows the variation of a few hundred psi of hoop stress. However, the thermal effect on the near-wellbore stress regions can be important for drilling in deep water and other complex drilling environments. To see the thermal effect, this study develops a thermoporoelastic model. It is found that there is decrease in radial stress and hoop stress in near-wellbore region with time. This reduction will have a considerable impact on fracture initiation pressure in the near-wellbore region. Also, the smearing effect will be influenced by stress changes due to change in temperature. [ABSTRACT FROM AUTHOR]

While drilling through depleted or partially depleted reservoirs, one may encounter a series of problems (e.g., lost circulation, non-productive time, etc.) due to narrow mud weight window (MWW). Fracture-based strengthening techniques used in the industry effectively increase fracture reopening pressure (FROP) and ultimately reduce the cost of associated problems. However, traditional analytical and numerical studies using these techniques do not consider the time effect and usually ignore the fluid dynamics. Thus, these deficiencies may result in inaccurate wellbore strengthening operations if no proper fracture diagnostic techniques are available to acquire the real-time fracture geometry. In this paper, a quasi-static, dislocation-based fracture model is extended using fluid mass conservation with leak-off. A fixed dimensionless fracture coordinate system is employed and a numerical simulation procedure is developed. The model is capable of predicting real-time fracture geometry (both fracture width and length) from given wellbore conditions. Hence, it could provide optimal particle size distribution (PSD) selection for wellbore strengthening applications. Two case studies are performed and results reveal different fluid controlling mechanisms during the fracture propagation, which are fluid storage in fractures and fluid leak-off to surrounding formation. Drilling through low permeability reservoirs (e.g., tight gas reservoirs or shale reservoirs) is different from drilling in conventional reservoirs because fast propagation of induced fractures may interact with natural fractures and result in severe lost circulation. This model is of critical importance in designing wellbore strengthening operations during drilling. [ABSTRACT FROM AUTHOR]

A new experimental set-up was designed to carry out high-pressure borehole fracturing tests on cylindrical rock samples. The experimental set-up offers full control over borehole, confining, and pore pressures. Fracturing experiments were conducted on cylindrical Berea sandstone samples. Several injection cycles were carried out on each rock sample to measure the Fracture Initiation Pressure (FIP) and the stable Fracture Propagation Pressure (FPP) at various confining pressures. The measured FIP values were compared with Hubbert and Willis’ model, and Rummel's model. The stable FPPs were measured using synthetic based mud (SBM) with and without lost circulation material (LCM), and compared with the large-scale fracturing experiments conducted at the Drilling Engineering Association (DEA) 13 investigation. Our study shows that initiation of a drilling induced fracture (DIF) is best characterized by a fracture mechanics approach such as Rummel's model. The stable FPP value changes linearly with the minimum horizontal stress (S hmin ), and it is independent of the maximum horizontal stress (S hmax ) or the vertical stress (S v ). Also, we show that adding LCM to the drilling fluids significantly enhances the stable FPP. [ABSTRACT FROM AUTHOR]

• Development of a rupture apparatus to quantify the resistance offered by the filter cake. • A relationship is obtained between the effective pressure and fracture width. • Denser and angular barite particles increase the rupture resistance of cake by about 45%. • Lower concentration of xanthan gum within the filter cake leads to a poorly cohesive system. • Porosity of the filter cake reduces by 10 to 20 % with pressure and filtration time. A novel apparatus is used to quantify the filtration characteristics of water-based drilling fluids and rupture behaviour of external filter cake. Under static filtration, the filtration pressure (400 to 800 kPa), time (15 to 60 min), the particle size distribution of the weighing agent (D 50 : 11 to 136 µm) relative to the pore size distribution of the substrate (S 50 : 14 and 65 µm) and the type of weighing agent (CaCO 3 or BaSO 4) play an important role in governing the porosity, thickness, and the rupture resistance of filter cake. With an increase in D 50 of mud, the normalised rupture resistance reduces by about 20% irrespective of the increase in the external filter cake thickness. Further, the viscosity of the filtrate increases with D 50 indicating the retention of less xanthan gum in the filter cake. The denser barite particles form a thinner and tighter cake, which exhibits higher resistance to rupture. [ABSTRACT FROM AUTHOR]

Drilling fluid losses are challenging to prevent or mitigate during drilling. Lost circulation treatments are applied to stop losses using a corrective approach or to hinder losses using a preventive approach, also known as wellbore strengthening. The key factors that must be determined when treating losses are the fracture width and the required particle size. The fracture width is often estimated using analytical fracture models. In this paper, five different fracture width models were analyzed and compared to hydraulic fracturing experiments. The hydraulic experiments were conducted on impermeable concrete cores to investigate the effect of adding lost circulation materials (LCM) to enhance the fracture breakdown pressure and the fracture re-opening pressure. The results showed that adding LCM blends enhanced the breakdown pressure up to 18% and the re-opening pressure up to 210%. The cores that were fractured with fluid-containing solids had a larger fracture width compared to the fractured cores using base fluid. Comparing the measured fracture widths from experiments with analytical models showed a discrepancy in the fracture width estimation for all of the models and the experiments, except for one model. [ABSTRACT FROM AUTHOR]

In this paper, we have experimentally studied the impact of particle size distribution (PSD) on the fracture sealing capability of lost circulation material (LCM) blends. Our primary aim was to determine the PSD which maximizes the Wellbore Strengthening (WBS) benefits obtained from fracture sealing. High-pressure borehole fracturing experiments were conducted on Berea sandstone samples under atmospheric pore pressure and various confining pressures to investigate the WBS effects of several LCM blends. Post-fracturing methods such as Computerized Axial Tomography (CAT) scan and thin-section imaging were used to investigate the geometry of induced fractures and formed seals within them. Based on the conducted experiments and post-fracturing analyses, we have evaluated and re-assessed well-known theories applicable to the design of LCM blends, such as the one-third rule, the ideal packing theory, and the Vickers criteria. Our experiments indicate that for any rock with a given set of rock strength and failure parameters, there exists an optimum PSD to maximize WBS benefits. Optimum PSD appears to be of primary importance, almost independent of LCM type. In addition, we have shown that the optimum PSD should have a bimodal structure, with sufficient concentrations of properly sized fine and coarse particles. Although the one-third rule, the ideal packing theory, and the Vickers criteria may provide some basic PSD guidelines, these theories are mainly empirical relationships based on conventional particle plugging experiments. As shown here, they do not properly represent the physics of fracture sealing. To remedy this situation, we are introducing a new family of design curves for optimum PSD, based on the underlying physics of fracture sealing observed in the WBS experiments. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive experimental evaluation to investigate the effects of adding iron-based and calcium-based nanoparticles (NPs) to nonaqueous drilling fluids (NAFs) as a fluid loss additive and for wellbore strengthening applications in permeable formations. API standard high-pressure-high-temperature (HPHT) filter press in conjunction with ceramic disks is used to quantify fluid loss reduction. Hydraulic fracturing experiments are carried out to measure fracturing and re-opening pressures. A significant enhancement in both filtration and strengthening was achieved by means of in situ prepared NPs. Our results demonstrate that filtration reduction is essential for successful wellbore strengthening; however, excessive reduction could affect the strengthening negatively. [ABSTRACT FROM AUTHOR]

Wellbore stability has plagued oil industry for decades. Inclusion of the mud in drilling and the effect of mud cake build up incorporate very complex chemical, thermal, mechanical, and physical phenomena. It is very difficult to quantify all these phenomena in one model. The after effects of mud cake buildup, its permeability and variation in thickness with time alter the actual stress profile of the formation. To see the impact of the whole mechanism, a combination of laboratoiy studies and numerical modeling is needed. This paper includes the procedure and results on stress profiles in near wellbore region based on laboratoiy studies of mud cake buildup in high pressure and high temperature environment using permeability plug apparatus (PPA). The damaged formation zone is very susceptible to drilling fluid and results in alteration of existing pore pressure and fracture pressure. This paper presents integrated experimental and analytical solutions for wellbore strengthening due to mud cake plastering. Conducting experiments on rock core disks has provided more realistic results which can resemble tofield conditions. The experimental work here provides an insight to effect of mud cake build up at high pressure and high temperature conditions using a heterogeneous filtration medium prepared from different sandstone cores. Results were used in the analytical model to see the effect of stresses in the formation. The primary objective is to investigate the wellbore hoop stress changes due toformation of filler cake by mud plastering using the analytical models built upon the laboratoiy results. The models developed in this work provide insights to quantify on wellbore plastering effects by mud cake build up. [ABSTRACT FROM AUTHOR]

Wellbore strengthening is a commonly applied method for mitigating lost circulation during drilling challenging formations with narrow drilling mud weight windows. This technique artificially increases the wellbore's maximum sustainable pressure by bridging drilling induced or natural fractures with lost circulation material (LCM). Although a number of successful applications have been reported in the drilling industry, the effects of several parameters are still not thoroughly understood. This paper presents a finite-element model to investigate the effects of these parameters on the evolution of near-wellbore hoop stress and fracture width in wellbore strengthening. Results show that the hoop stress around the wellbore and along the fracture can be substantially increased by bridging the fracture with LCM. A desirable LCM bridge should have low enough permeability to ensure there is no fluid flow across it. The best place to bridge the fracture, from the hoop-stress-engagement point of view, is the fracture mouth near wellbore wall. Fluid leak-off through the wellbore wall and fracture surfaces affects both hoop stress and fracture width distributions. Young's modulus has a significant influence on fracture width distribution, whereas Poisson's ratio has a very small influence. Finally, several useful field implications for wellbore strengthening are provided. [ABSTRACT FROM AUTHOR]

Wellbore strengthening is an effective method to mitigate lost circulation, a major non-productive-time drilling complication, which utilizes lost circulation materials (LCMs) to bridge small preexisting, drilling-induced, or natural fractures on the wellbore wall. Although a number of successful field applications have been reported, the fundamental mechanisms of wellbore strengthening are still not fully understood. Based on linear elastic fracture mechanics, this paper illustrates an analytical solution for investigating geomechanical aspects of wellbore strengthening operations. The proposed solution (1) provides a fast procedure to predict fracture-initiation pressure before and after bridging a preexisting fracture in wellbore strengthening; (2) considers effects of wellbore-fracture geometry, in-situ stress anisotropy, and LCM bridge location. The solution is validated by comparison with available fracture mechanics examples, then used to investigate and quantify the effect of several parameters on wellbore strengthening with a sensitivity study. Results show that the wellbore can be effectively strengthened by enhancing fracture-initiation pressure with wellbore strengthening operations, but the magnitude of strengthening is affected by LCM bridge location, in-situ stress anisotropy, and formation pore pressure. The proposed solution illustrates how wellbore strengthening works, and provides useful considerations for field operations. [ABSTRACT FROM AUTHOR]

The oil and gas industries have made efforts in the development of technologies and techniques in well drilling. However, as new sources of hydrocarbons are found in reservoirs, increasingly remote and geologically complex, the industry continues to handle new challenges. A new challenge faced by drilling companies in Brazil is to combat lost circulation in limestone formations naturally fractured that is a common scenario in drilling the pre-salt wells. Combating loss by the proper use of wellbore strengthening materials (WSM) and lost circulation material (LCM) is fundamental to a successful drilling. The main objective of this work is to develop an experimental study that aims to evaluate the effectiveness of different particulate materials in combating lost circulation in fractures. An experimental apparatus was proposed to physically simulate the fluid flow in fractures of 2 mm, 5 mm and 10 mm of thickness. The length of those fractures is approximately 1.5 m. In this work we studied the fluid flow in fractures as pure water, polymers solution in water and suspension of many kinds of limestone as particulate material commonly employed as loss control materials. Those fluids were characterized with the obtention of rheology curves, density, concentration of solids, particle size distribution and particle analyzing images. We evaluated the influence of different polymers and particulate material on the rheology. We studied the fluid flow and its capability of sealing the fracture with different operating conditions such as flow rate, pump pressure, solids concentration, solid form (granules, flakes and laminar) and particle size distribution in the fluid flow. For suspensions that did not seal the fractures, we determined the pressure drop as a function of the flow rate. [ABSTRACT FROM AUTHOR]

This paper presents finite-element simulation for hydraulic fracture's initiation, propagation, and sealing in the near wellbore region. A full fluid solid coupling module is developed by using pore pressure cohesive elements. The main objective of this study is to investigate the hypothesis of wellbore hoop stress increase by fracture sealing. Anisotropic stress state has been used with assignment of individual criteria for fracture initiation and propagation. Our results demonstrate that fracture sealing in "wellbore strengthening" cannot increase the wellbore hoop stress beyond its upper limit when no fractures exist. However, this will help to restore part or all of the wellbore hoop stress lost during fracture propagation. [ABSTRACT FROM AUTHOR]

One of the most challenging issues during drilling operations is lost circulation, which can cause several problems that could lead to increasing the non-productive time (NPT) and drilling cost. Wellbore strengthening techniques have been applied as a well-known approach to increase the mud weight window by improving mud cake properties or adding lost circulation materials (LCMs) to the drilling fluid. The success-to-failure ratio of remedial or preventive solutions during the field operation is highly dependent on the appropriate selection of wellbore strengthening strategy. Initial studies, which have often focused on trial and error and operational practices, have not provided a clear understanding of fundamental mechanisms and performance of LCMs in strengthening of a formation. Hence, in the last decades, several analytical and numerical studies, as well as experimental evaluations have been performed by numerous researchers to simulate the fluid loss process and LCM effectiveness. Conducting a comprehensive and well-designed experimental investigation can be a more applicable and cost-effective approach to select proper wellbore strengthening method and recommend the best drilling fluid formulation to treat the loss zone. The aim of this paper is to present an overall review on the various published experimental investigations, to assess the different aspects of lost circulation and wellbore strengthening theory. This extensive literature review collects limitations, advances, and differing opinions from experts. It also broadens the scope for future work and helps in solving industrial problems from an operational point of view. • The manuscript provides a review about lost circulation and wellbore strengthening on the various experimental researches. • The conceptual models used to describe wellbore strengthening are presented and discussed in details. • This manuscript will serve as very helpful detailed review paper for researchers in the area of wellbore strengthening. • It broadens the scope for future works and helps in solving industrial problems from an operational point of view. [ABSTRACT FROM AUTHOR]