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This study evaluates the application of supercritical carbon dioxide chelation extraction technology for treating heavy metal ions (Zn2+ and Cr3+) in drilling fluid waste. Through a combination of experimental and molecular dynamics simulation methods, the influence of extraction parameters (temperature, pressure, duration, and chelating agents) on the extraction efficiency are investigated. Findings show that increased duration and pressure significantly improve extraction efficiency, while temperature has a complex effect, initially increasing efficiency but plateauing and slightly decreasing at higher temperatures. The optimum extraction condition with pressure of 220 bar, temperature 348.15 K and an extraction duration of 70 min using ethylene diamine tetraacetic acid (EDTA) as the chelating agent has been determined. Importantly, molecular simulation analysis revealed that citric acid outperforms EDTA in terms of Zn2+ and Cr3+ aggregation by forming larger aggregates with greater numbers of molecules while reducing overall aggregate count. Furthermore, optimization of extraction pressure in the EDTA system shows potential benefits. These results suggest that supercritical carbon dioxide chelation extraction technology has strong potential for environmentally friendly and reliable waste management in the drilling industry. This study represents a significant step forward in developing sustainable solutions for heavy metal removal from drilling fluid waste.

Drilling fluid continuous wave(DFCW) generator is an advanced downhole instrument and has broad application prospects. The load torque acting on the DFCW generator has a great impact on the performance on the DFCW generator, and there is a lack of theory describing the load torque because of the irregularity of the rotary valve. In this paper, the drilling fluid flow area is divided into the curve flow area, the radial flow area and the axial flow area. The flow area of each part is divided into several meshes based on finite element theory, then the load torque acting on each part is calculated based on the fluid momentum conservation theory. The theoretical model is verified by combining the simulation and the experiment, and the effects of the discharge and the axial gap on the load torque are analyzed. The research results show that:(1) the theoretical model can be used to determine the load torque, (2) the amplitude of the load torque reduces as the gap decreases and increases rapidly as the discharge increases. The results can provide the theoretical guidance for the structure design of the rotary valve and the control system design of the drilling fluid continuous wave generator.

Extensive studies have been carried out on cutting rock with a PDC cutter, but cutting rock assisted by impact force is rarely studied. In this paper, cutting rock using conical and cylindrical PDC cutters assisted by impact force were researched with the explicit dynamic model. The laws of cutting rock using a cylindrical cutter assisted by impact force are the same as those of a conical cutter. There are thresholds of impact frequency and amplitude when they are single variables. When impact frequency is lower than the threshold frequency, the impact frequency is the dominant frequency in the frequency spectrum of weight on bit (WOB), and the amplitude of dominant frequency and removal volume decreases with the increase of impact frequency. When the impact frequency is higher than the threshold frequency, there is no dominant frequency in the frequency spectrum of WOB, and the removal volume behaves the same. When the impact force is lower than the threshold amplitude, there is no dominant frequency in the frequency spectrum of WOB, and it does not affect the removal volume but the removal volume is positively correlated with the impact amplitude. When the impact amplitude is higher than the threshold amplitude, the removal volume is also positively correlated with the impact amplitude, and the removal volume assisted by low-frequency (20 Hz and 40 Hz) impact force is higher. The frequency threshold and amplitude threshold of the conical cutter are smaller than those of the cylindrical cutter. Although the cutting depth and removal volume of the conical cutter are lower than those of the cylindrical cutter, the amplifications of cutting depth and removal volume of the conical cutter are higher than those of the cylindrical cutter when assisted by impact force.

Stick-slip vibration reduces the drilling rate of penetration, causes early wear of bits, and threatens the safety of downhole tools. Therefore, it is necessary to study suppression methods of stick-slip vibration to achieve efficient and safe drilling. Field tests show that the use of downhole axial impactors is helpful to mitigate stick-slip vibration and improve rock-breaking efficiency. However, there are many deficiencies in the study of how axial impact load affects stick-slip vibration of a PDC bit. In this paper, based on the two-degrees-of-freedom spring-mass-damper model and similarity theory, a laboratory experiment device for suppressing stick-slip vibration of a PDC bit under axial impact load has been developed, and systematic experimental research has been carried out. The results show that the axial impact force can suppress the stick-slip vibration by reducing the amplitude of weight on bit and torque fluctuations and by increasing the main frequency of torque. The amplitude of impact force affects the choice of the optimal back-rake angle. The impact frequency is negatively correlated with the fluctuation amplitude of the rotary speed. When the impact frequency is greater than 100 Hz, the fluctuation amplitude of the rotary speed will not decrease.

Pulsed jet significantly affects the pore pressure of deep formation and then affects the mechanical ROP. In this paper, the effect of jet pulsation on deep pore pressure in the formation is studied by the finite element method. Under the pulsed jet, the maximum relative negative pressure is along the wellbore axis, and the farther it is to the borehole axis, the smaller the relative negative pressure is. With the increase of pulse frequency, the relative negative pressure increases and the maximum point of relative negative pressure moves upward. The optimum pulse frequency is from 50 Hz to 500 Hz. There is a linear relationship between pulse amplitude and relative negative pressure.

Coalbeds are characterized by high organic matter content, presence of joints and smooth joint surface, which lead to weak bonding between coal and set cement and poor cementing quality of coalbeds, and consequently the economic and effective development of coalbed methane (CBM) is restricted seriously. In this paper, components, compositions and wetting characteristics of coal were analyzed in order to improve the water wettability of coal, the effects of different surfactants and their concentration and soaking time on the surface wetting angle of coal and interfacial bonding strength were tested and measured. The high-efficiency hydrophilic wettability modifier was optimized. The strengthened coalbed bonding prepad fluid system was developed and applied practically in the Qinshui Basin, Shanxi province. The results of laboratory tests and field applications show that the compact adsorption of surfactant containing ether and sulfate radical on the surface of coal can convert the surface wettability into strong hydrophilicity at the concentration of 0.3% within 30 s, thus improving significantly the interface bonding strength (up to 65.5%) between coal and set cement. The field application in CBM well cementing shows that the strengthened coalbed bonding prepad fluid can improve the cementing quality from unqualified level to high level, and play a significant role in improving CBM hole interval cementing quality. It is concluded that this technology provides an effective guarantee for coalbed cementing quality improvement and smooth implementation of stimulation measures like fracturing. Keywords: Coalbed methane (CBM), Coal, Well cementing, Wettability, Interfacial bonding strength, Cementing quality, Surfactant, Prepad fluid

In previous studies it has been shown that supercritical carbon dioxide (SC-CO2) jets are more efficient at rock breaking than water jets, but the corresponding systematic mechanism explanations were not available. Therefore, this paper conducts an in-depth analysis of the fluid properties with equations suggested by the American National Institute of Standards and Technology (NIST), establishes the fluid-solid-heat coupling model with the support of ANSYS, and reveals the stress responding mechanism of SC-CO2, water and nitrogen jets. When the rock is at room temperature, the jets increase the temperature of the rock and generate thermal stress. Furthermore, the temperature elevation effect decreases in the sequence of water at 70°C, nitrogen at 70°C, carbon dioxide at 70°C and water at 27°C. In comparison with water jets at 27°C, jets at 70°C in carbon dioxide increase the maximum stress range and the high stress range of rock, accounting for the low threshold pressure and large volumes of rock breaking in the current literature. For jets at the same temperature (70°C), the stress in rock decreases in the sequence of water, nitrogen and carbon dioxide, indicating that previous studies have not taken the effects of the temperature into consideration, leading to the incorrect conclusion that carbon dioxide is more efficient for rock breaking. In addition, this paper also analyzes the jet field and the stress field with respect to the formation temperature and finds a decrease in thermal stress generated by jets produced at rock temperature. Both the temperature decreasing effect and the rock breaking stress decrease in the sequence of carbon dioxide, nitrogen and water. When the difference in jet pressure decreases or the elastic modulus of the rock increase, SC-CO2 jets boast more palpable advantages over nitrogen and water jets in rock breaking. This shows that SC-CO2 jets boast the potential of effectively breaking rock in deep formations.

Currently, there is no proper method to predict the pore pressure disturbance caused by multistage fracturing in shale gas, which has challenged drilling engineering in practice, especially for the infilling well drilling within/near the fractured zones. A numerical modelling method of pore pressure redistribution around the multistage fractured horizontal wellbore was put forward based on the theory of fluid transportation in porous media. The fracture network of each stage was represented by an elliptical zone with high permeability. Five stages of fracturing were modelled simultaneously to consider the interactions among fractures. The effects of formation permeability, fracturing fluid viscosity, and pressure within the fractures on the pore pressure disturbance were numerically investigated. Modelling results indicated that the pore pressure disturbance zone expands as the permeability and/or the differential pressure increases, while it decreases when the viscosity of the fracturing fluid increases. The pore pressure disturbance level becomes weaker from the fracture tip to the far field along the main-fracture propagation direction. The pore pressure disturbance contours obviously have larger slopes with the variation of permeability than those of the differential pressure. The distances between the pore pressure disturbance contours are smaller at lower permeability and higher viscosity. The modelling results of the updated pore pressure distribution are of great importance for safe drilling. A case study of three wells within one platform showed that the modelling method could provide a reliable estimation of the pore pressure disturbance area caused by multistage fracturing.

The drill string used in drilling is in a complex motion state downhole for several kilometers. The operating attitude and eccentricity of the downhole drill string play important roles in avoiding downhole risks and correcting the output of the imaging measurement sensor while drilling (IMWD). This paper proposes a method for measuring eccentricity while drilling using two sets of caliper sensors coupled with a fiber-optic gyroscope for continuous attitude measurement, which is used to solve the problem of the quantitative measurement of complex eccentricity that changes in real-time downhole. According to the measurement and calculation methods involved in this article, we performed simulations of the attitude of the drill string near where the IMWD tool is located in the wellbore under a variety of complex downhole conditions, such as centering, eccentricity, tilt, buckling, rotation, revolution, etc. The simulation and field test results prove that the distance between the imaging while drilling sensor and the borehole wall is greatly affected by the downhole attitude and revolution. The multi-sensor layout measurement scheme and the data processing based on the above-mentioned measurement involved can push the drill collar movement and eccentricity matrix specifically studied downhole from only qualitative estimation to real-time measurement and quantitative calculation. The above measurement and data processing methods can accurately measure and identify the local operating posture of the drill string where the IMWD sensor is located, and quantitatively give the eccentric distance matrix from the measuring point to the borehole wall required for environmental correction of the IMWD sensor.

A wellbore surface is an irregular surface structure. The distribution of points on the wellbore surface measured based on the drilling diameter is not uniform. Thus, the conventional modeling method based on a point cloud cannot satisfy the needs of real-time measurement updating and wellbore display. This study proposes a spiral profile method for drilling shaft surface reconstruction. Scattered data along the drilling diameter are measured, and an inverse distance weighting cylindrical space surface algorithm with iterative interpolation is used to obtain the spiral angle and pitch of a relatively homogeneous helical contour line along the surface of the shaft. Using sets of four adjacent points in the spiral, quadrilaterals are formed, and then all obtained quadrilaterals are used to form the wellbore inner surface structure. This method can further construct the outer surface spiral contour line to advance the quadrilateral surface to the spatial hexahedron structure. The caliper and gamma measurement data obtained from the calibrated wellbore were used to verify the real-time surface reconstruction and fusion while drilling. The homogenized reconstructed surface profile is more than 99.5% similar to the actual measurement. Proved by experiment and application, this method has very high real-time performance, and the three-dimensional stereo imaging wellbore with additional gamma attributes has good visual effects.

Heat transfer along the wellbore was analyzed, and then a closed mathematical model, which fully couples the hydrostatic pressure, temperature, physical properties of CO2 and friction, was established to keep bottom-hole pressure constant during drilling process. Based on the pressure profile in wellbore achieved for a certain surface back pressure, a pressure controlling method for managed pressure drilling with supercritical carbon dioxide was presented. The influences of mass flow rate, well depth and inlet temperature on the annulus pressure profile and surface back pressure were investigated. The results show that, the pressure profile is almost in linear correlation with well depth in the annulus, which provides convenience for well control. The needed back pressure (applied by surface choke) decreases with increasing mass flow rate and decreasing well depth. The impact of inlet temperature on the annulus pressure profile, surface back pressure and flow friction is negligible. It also shows that the density of CO2 increases significantly and abruptly at a critical pressure. It is suggested that the storage pressure of CO2 in surface tank be larger than the critical pressure for a certain temperature. Key words: supercritical carbon dioxide drilling, managed pressure drilling, wellbore heat transfer, annulus pressure, surface back pressure

The low permeability reservoirs in SL Oilfield are featured by large buried depth, poor physical properties, strong diagenesis and heterogeneity, and there exist serious water sensitivity damage and water lock during the drilling process, which significantly restricts the exploration efficiency and benefits of low permeability reservoirs. In order to solve the reservoir protection problems of low permeability reservoirs, novel polymer plugging agent and water lock prevention agent were optimized and characterized in detail, and the polymer plugging agent could work synergistically with water lock prevention agent to impart reservoir protection performance due to film-forming shielding effect and low surface tension. The low damage water-based drilling fluids were also established and evaluated, and the results indicated that low damage water-based drilling fluids exhibited excellent reservoir protection performance with permeability recovery of above 90%, and filtrate surface tension is 18.5mN/m. It could be concluded that the low damage water-based drilling fluids could be priority alternatives to low permeability reservoirs exploration in SL Oilfield.

The load transfer difficulty caused by borehole wall friction severely limits the penetration rate and extended-reach limit of complex structural wells. A new friction reduction technology termed "earthworm-like drilling" is proposed in this paper to improve the load transfer of complex structural wells. A mathematical model based on a "soft-string" model is developed and solved. The results show that earthworm-like drilling is more effective than single-point vibration drilling. The amplitude and frequency of the pulse pressure and the installation position of the shakers have a substantial impact on friction reduction and load transfer. An optimization model based on the projection gradient method is developed and used to optimize the position of three shakers in a horizontal well. The results verify the feasibility and advantages of earthworm-like drilling, and establish a solid theoretical foundation for its application in oil field drilling.

Underground rocks usually have complex pore system with a variety of pore types and a wide range of pore size. The effects of pore structure on elastic wave attenuation cannot be neglected. We investigated the pore structure effects on P-wave scattering attenuation in dry rocks by pore-scale modeling based on the wave theory and the similarity principle. Our modeling results indicate that pore size, pore shape (such as aspect ratio), and pore density are important factors influencing P-wave scattering attenuation in porous rocks, and can explain the variation of scattering attenuation at the same porosity. From the perspective of scattering attenuation, porous rocks can safely suit to the long wavelength assumption when the ratio of wavelength to pore size is larger than 15. Under the long wavelength condition, the scattering attenuation coefficient increases as a power function as the pore density increases, and it increases exponentially with the increase in aspect ratio. For a certain porosity, rocks with smaller aspect ratio and/or larger pore size have stronger scattering attenuation. When the pore aspect ratio is larger than 0.5, the variation of scattering attenuation at the same porosity is dominantly caused by pore size and almost independent of the pore aspect ratio. These results lay a foundation for pore structure inversion from elastic wave responses in porous rocks.

A modified model for predicting the friction force between drill-string and borehole wall under in-plane vibrations was developed. It was found that the frictional coefficient in sliding direction decreased significantly after applying in-plane vibration on the bottom specimen. The friction reduction is due to the direction change of friction force, elastic deformation of surface asperities and the change of frictional coefficient. Normal load, surface topography, vibration direction, velocity ratio and interfacial shear factor are the main influence factors of friction force in sliding direction. Lower driving force can be realized for a pair of determinate rubbing surfaces under constant normal load by setting the driving direction along the minimum arithmetic average attack angle direction, and applying intense longitudinal vibration on the rubbing pair. The modified model can significantly improve the accuracy in predicting frictional coefficient under vibrating conditions, especially under the condition of lower velocity ratio. The results provide a theoretical gist for friction reduction technology by vibrating drill-string, and provide a reference for determination of frictional coefficient during petroleum drilling process, which has great significance for realizing digitized and intelligent drilling.

Formation permeability distribution is important in the oil industry since it is a key tool in forecasting oil production performance, a matter of primary concern to oilfield operators. However, the direct problems of reservoir analysis are limited in practice by a lack of input parameters such as reservoir vertical heterogeneity. Therefore, inverse problem modeling is important to achieve reservoir vertical heterogeneity using production profiles, which are always known for existing reservoirs. In this way, what has occurred inside a reservoir can be understood by applying reservoir output in the reverse model. Because formation permeability can frequently be represented by a normal distribution and can describe heterogeneity and water production capability, an inverse problem of formation permeability was modeled to study reservoir vertical heterogeneity and reservoir production performance based on oil-water two-fluid flow theory and optimization problems. Moreover, through establishing an objective function, we obtained an optimal solution for water production rate. Finally, one case is simulated by this model and good results achieved, which are compatible with production and experimental data.

Drill string vibration is a widely studied topic. This paper developed a real-time measurement system near the drilling bit and extracted the lateral vibration, longitudinal vibration time series of bottom rotating drill string. In order to reconstruct the phase space, we estimated the delay time with mutual information and calculated the embedding dimension through Cao’s method. Finally, the chaotic characterization of the system is analyzed by calculating the correlation dimension and the largest Lyapunov exponent. The results show that such system can exhibit positive finite-time Lyapunov exponents and a clear convergence toward the correlation dimension, which is a strong indicator for the chaotic behavior of the system. It is expected that the new dynamics found in this paper could be of potential implication to the control methods of the drill string vibration.

Based on Buckley-Leverett theory, one inverse problem model of the oil-water relative permeability was modeled and proved when the oil-water relative permeability equations obey the exponential form expression, and under the condition of the formation permeability that natural logarithm distribution always obey normal distribution, the other inverse problem model on the formation permeability was proved. These inverse problem models have been assumed in up-scaling cases to achieve the equations by minimization of objective function different between calculation water cut and real water cut, which can provide a reference for researching oil-water two-phase flow theory and reservoir numerical simulation technology.

Kinetic inhibitors have the advantages of low cost, small dosage and good effect. Thus, they have great development prospects in the field of natural gas hydrate prevention and control. However, its inhibition mechanism is not clear enough, which severely limits the development and application of kinetic inhibitors. In this paper, the hypothesis of PVCap inhibition of CH4 dissolution is verified by experiments, and the influence of its concentration and molecular weight on the inhibition effect is explored. And the following experimental results are obtained. PVCap inhibits hydrate nucleation by slowing down the dissolution of CH4 before hydrate induced nucleation. And the higher the concentration and molecular weight of PVCap, the more obvious the effect of inhibiting CH4 dissolution and the stronger the ability to inhibit hydrate nucleation and growth. [ABSTRACT FROM AUTHOR]