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Background Due to advancements in horizontal drilling and completions technology, wells in some basins are now being completed in areas where conventional production has existed for 50+ years. This paper will focus on the Midland Basin, where wells in the Spraberry, Dean, and Wolfcamp formations have been produced since the 1960s and earlier in some cases. As a result, current unconventional wells in these formations will invariably deal with depletion. In the past, freshwater consisting of friction reducer and a foaming agent, along with nitrogen, was used to lighten clean-out fluids to a fluid density that allows circulation of these wells. The use of nitrogen, while necessary, can lead to variable fluid quality, increase clean-out times, add expense, and ultimately reduce time-to-market for these wells. Lightweight mud systems (LMS) are not new to the oilfield but relatively new to post-frac clean-out operations applications. The system consists of a proprietary water-based mud additive that reduces mud density by creating microbubbles encapsulating air in a multi-layer shell. This technology can reduce fluid weights by up to 1.9 ppg from the original base weight. The base fluid for the LMS may be freshwater, produced water, or brine. The fluids possess a low shear rate viscosity that allows good hole cleaning without risking formation invasion. The system may be used with nitrogen if needed for extreme cases. No degradation of mud motor performance has been observed when using the LMS. This paper will offer 3 case studies where the LMS provided operational and economic benefits in post-frac clean-out operations.

Wellbore instability is caused by the radical change in the mechanical strength as well as chemical and physical alterations when exposed to drilling fluids. Unscheduled events that are related to wellbore instability account for more than 10% of the drilling cost, which is estimated to one billion dollars per annual cost. Understanding shale-drilling fluid interaction plays a key role in minimizing drilling problems in unconventional resources. The need for providing a more suitable drilling fluid system for drilling operations in unconventional resources is growing. The major consequences of improper fluid selection include bit-balling, hole sloughing, and most importantly low drilling rates. This study will evaluate the effect of different inhibitive drilling fluid systems on shale drilling performance. The mud systems use in this study include cesium formate, KCl based mud systems, and a conventional water-based mud. The conventional water-based mud is used as reference fluid. An innovative HPHT drilling simulator is used to perform real-time drilling operations on cylindrical shale core samples obtained from the Tuscaloosa Marine Shale (TMS). The process involved simulating real-time drilling operation at downhole conditions where different fluids served as drilling fluid. A comparative analysis of the impact of each drilling fluid systems on the drilling performance as then performed. The analysis focused on drilling parameters such as rate of penetration, torque, and friction factor during drilling. This paper evaluates the compatibility between different fluid systems and shale formations. The paper also provides an overview of the effect of the inhibitive mud systems on drilling performance. The inhibitive muds systems (KCl based fluids and cesium formate) provided improved drilling performance compared to conventional water-based mud.

Wellbore instability is caused by the radical change in the mechanical strength as well as chemical and physical alterations when exposed to drilling fluids. A set of unexpected events associated with wellbore instability in shales account for more than 10% of drilling cost, which is estimated to one billion dollars per annum. Understanding shale-drilling fluid interaction plays a key role in minimizing drilling problems in unconventional resources. The need for efficient inhibitive drilling fluid system for drilling operations in unconventional resources is growing. This study analyzes different drilling fluid systems and their compatibility in unconventional drilling to improve wellbore stability. A set of inhibitive drilling muds including cesium formate, potassium formate, and diesel-based mud were tested on shale samples with drilling concerns due to high-clay content. An innovative high-pressure high temperature (HPHT) drilling simulator set-up was used to test the mud systems. The results from the test provides reliable data that will be used to capture more effective drilling fluid systems for treating reactive shales and optimizing unconventional drilling. This paper describes the use of an innovative drilling simulator for testing inhibitive mud systems for reactive shale. The effectiveness of inhibitive muds in high-clay shale was investigated. Their impact on a combination of problems, such high torque and drag, high friction factor, and lubricity was also assessed. Finally, the paper evaluates the sealing ability of some designed lost circulation material (LCM) muds in a high pressure high temperature environment.

Drilling mud is the most important part in drilling activity. Drilling could work fluently, safely, and economically on fluency by system and condition of drilling mud. It means the mud system and the physical properties of the slurry conform to the required specifications. There are some kinds of drilling mud that can be used in oil and gas drilling operation, such as water base mud and oil base mud. In terms of economical objective, water base mud is usually used in drilling process.The purpose of this study is to conduct laboratory research of the effect of various temperatures on the characteristic of mud system of low solid mud by adding biopolymer and bentonite extender. This research uses roller oven method as a medium for simulation to condition the mud as close as possible to the condition in wellbore to see the change of physical properties of sludge at various temperatures.The result found is that the higher the temperature, the lower the drilling mud physical properties such as density, viscosity, plastic viscosity, yield point, dial reading 600 RPM, dial reading 300 RPM, and gel strength. However, it is found that not all the physical properties decrease.

The two most important features of drilling rigs based on the core drilling principles are the drill set rotating at high speeds and the very precise spacing between the pipe and the well wall. Formation pieces that swell, flow into the well, spill or collapse during the drilling of problematic formations mix into the mud or narrow the annulus and cause well problems by restraining the rotation of the drill string and circulation. The spilled formation pieces cause an excessive solid material increase in the mud structure and due to the effect of centrifugal force, the solids adhere to the inner wall of the pipes near the surface and form a cake. Excessive cake thickness in the pipes obstruct the descent of the core barrel inner tube catcher into the well. During the removal of the inner tube, a vacuum occurs in the well due to the mud discharged from the rod and the problematic formations are poured into the annulus and block the core between the core barrel and the well wall. In this study, the performance of mud systems determined by trial and error method against the problems encountered in unconsolidated claystone, mudstone (shale), siltstone and sand - pebble stones in core coal exploration drillings in Çorum - Amasya region were evaluated. Monitoring and regular improvement of the rheological values of the mud compositions, changing the mud composition according to the well conditions, keeping the gel strength high and increasing its density with salt was found to play an important role in keeping the unconsolidated formations physically balanced. Salt mud also minimized the formation of cake in the pipe.

Abstract This paper describes the use of an innovative high-performance water-based drilling fluid system in south Europe. The novel drilling fluid allowed to successfully drill a sidetrack in a deep, high pressure-high temperature (HP-HT) well, in an area where low toxicity oil-based drilling fluids had been used in the past, due to the adverse drilling conditions. Comprehensive laboratory testing carried out to customize the drilling fluid formulation during the well planning phase, was complemented by further testing conducted during drilling operations. An obvious advantage of this system is its simplicity. It is composed mostly of liquid additives that allow a quick and easy preparation at rig site. The system is also compatible with the full range of mud densities, while exhibiting consistently superior lubricity characteristics. A tailored version of the system for this sidetrack well in southern Europe, required a heavy weighted fluid (up to 2.06 SG) to provide sufficient hydrostatic pressure for well control. The formulation included a high temperature rheology modifier that improved fluid stability at temperatures up to 160°C at total depth. A high performance lubricant was added to the system to improve drilling rate, while using an advanced steerable drilling system. Lubricity coefficients were monitored and recorded daily at rig site using a standard industry accepted ring and block type lubricity tester. The lubricity values fell in the range traditionally associated with oil-based mud usage. The successful sidetrack operations were completed with very good performances. Introduction Drilling fluids can influence all processes of well construction and operation, since incompatibility between the selected fluid and geological conditions yields many undesirable consequences in terms of drilling issues, quality of the production zone and economical performances. This is even more "true" when operating in complex drilling conditions, such as those described in this paper. In particular, one of the first applications of an innovative high performance water-based mud (HPWBM) outside the United States - it had been previously used only once in Europe - is reported herein, which made it possible to successfully drill a challenging sidetrack within a deep, high pressure-high temperature (HP-HT) well, in hard and tectonically-stressed formations. The HPWBM, optimized to the specific field conditions, met several standards for effectiveness: high lubricity, stability at high temperatures, little tixotropy, resistance to solids contamination. The fluid represented a big step forward in reducing the environmental impact, while providing performances comparable to those of low-toxicity oil-based muds used in the area for the last 20 years, as the only means to overcome the difficult drilling conditions. The reservoir is characterized by a white light brown dolomite of the Triassic age, and has two main hydrocarbon zones, an upper and a lower one. The sidetrack's target was the upper reservoir, which is an isolated fault block in the west area of the field resulting from a series of tectonic discontinuities, that had remained undrained by the previous operations. The depth of target reservoir was between 5500 and 6000 m, with a temperature of up to 160°C and pressures over 1000 kg/cm2 at total depth. In the offset wells, these demanding conditions, combined with the high formation hardness and the significant tectonic stresses, had leaded to such drilling issues as high torque, low rate of penetration (ROP) and wellbore instability. Instead, in the case described in this paper, the usage of the HPWBM, in combination with an advanced steerable drilling system, enabled the sidetrack to be successfully executed without incidents.

Many productive carbonates show complex porosity systems with widely varying proportions of primary porosity (from the matrix) and secondary porosity (from vugs and open fractures). Until now vugs present at the surface of the borehole could be quantified only using micro-resistivity images logged in water based mud. For the first time, Schlumberger will present textural analysis using ultrasonic images. The image analysis, not dependent on a specific type of mud, is based on the variation of acoustic amplitudes within the geological formation. From amplitude histograms, the vug porosity quantification is then computed through: • Breakout / Background removal • Normalization of acoustic amplitude histograms • Determination of an amplitude threshold that limits host acoustic amplitude of the matrix from lowest acoustic amplitudes for vugs. • Calibration with the total porosity log. The technique presented here is applied and validated in a carbonate reservoir of Shell Brazil, logged in a well filled with oil based mud. The new vug porosity (0.2inc vertical resolution) captures better the variation of porosity of the formations than the conventional logs. This paper will also present different applications of the vug porosity: • To calibrate the volume of macropores derived from NMR. • To construct a Reservoir Rock classification scheme and lithofacies

A new wire-line formation resistivity imaging instrument employing a ‘two-electrode’ measurement configuration has been developed for application in low-resistivity formations drilled with non-conductive (oil-based) mud. A detailed explanation of the instrument and measurement principles is provided. Modeled synthetic responses and field examples are shown. The high-spatial resolution of a ‘two-electrode’ arrangement has been well documented for nearly 25 years. The lower formation resistivity limit in oil-based mud boreholes for prior devices often precluded realizing their benefits in many important offshore deep water plays, such as Malaysia, Gulf of Mexico and elsewhere. The new instrument employs multi-frequency impedance measurements to overcome this limitation. The impedance measurements produce two image logs: a real-part impedance image that is calibrated to yield formation resistivity and an imaginary-part impedance image that conveys information about image quality. The instrument employs six individually-articulated pads each containing ten sensor electrodes that provide a 75% surface coverage in an 8½ inch borehole. Typical spatial-resolution is better than one-half-inch vertically and one-eighth inch azimuthally. Pad-to-pad depth off-set is less than 6 inches, assisting to maintain borehole coverage in the presence of common instrument rotation during logging. Field examples from typical logging environments found in offshore Malaysia are presented. Detailed geologic features such as thin-beds, slumping, cross-bedding, faulting and fracturing in low resistivity depositional environments are clearly resolved.

Hundreds of wells have been drilled since the early 1900's at the Putumayo basin in southwest Colombia. ECOPETROL (NOC) used traditional mud systems: lignosulphonates, potassium chloride, and finally lime or gypsum mud systems, all of them designed to manage exposure to anhydrite, a common mineral in the drilled formations. The calcium contamination adversely affects mud system properties, resulting in hole instability and frequent stuck pipe events. Because of these issues, the operator considered using oil base mud (OBM) in a very environmental sensitive area. The OBM would generate undesirable waste and there were uncertainties about its performance, since few wells in the area had been drilled with OBM and the results were not very successful. Therefore, for drilling operations on the ECOPETROL Sucumbíos No. 5 well, a completely new high performance water base mud (HPWBM) was proposed and accepted. The new chemistry eliminated the use of bentonite and included a new flocculation inhibition mechanism, as well as new and powerful polymers. The system demonstrated that calcium and anhydrite were not a problem any more and the well was drilled in record time. More importantly, to the operator was able to evaluate the productive formations by running electrical logs, an operation that was normally cancelled because of severe hole problems. Casings strings were set and cemented per the well program and the well was completed as planned. This paper presents the design state of the HPWBM, its performance while drilling and charts and graphics comparing the changes against the historical values recorded in the area.

The popularity of formate-brine drilling muds continues to increase as a result of the benefits in the drilling and completion stages of well construction, as well as the lessened environmental impact when compared to some other muds. Growth in use of these muds has caused operators and service companies to re-examine environmental correction algorithms because of the substantial effects these mud systems can have on nuclear logs. In addition, because formate muds generally contain lower solids than conventional muds, they can be strongly invasive. Thus, the influence of formate muds on LWD logging measurements includes both borehole environmental effects to be corrected and formation invasion effects that affect log interpretation. A combination of laboratory experiments and Monte Carlo simulations have been used to develop a better understanding of the influences formate muds have on LWD gamma-ray, density, PE, and neutron porosity logs. Several hundred lab experiments and numerical simulations have been conducted in this investigation to fully characterize LWD responses to formate muds for families of sensors comprising two collar sizes used in a large range of hole sizes. The results were used to develop borehole corrections for the nuclear logs. Formate brines are formulated to achieve a desired fluid density by using a mixture of dissolved compounds: sodium, potassium, and cesium formate. In some instances, the mixture results in a fluid with a substantially lower-than-normal hydrogen index. The mud may also have a larger-than-normal potassium concentration. Because invasion consists of nearly the whole formate mud, correct log evaluation in porous, permeable formations requires interpretation techniques that are beyond the scope of customary borehole corrections. We have taken lab measurements to attempt to quantify the effect of this invasion on LWD tool responses.

Summary Squeezing salts have been a major problem to operating companies worldwide for many years. Most previous solutions to the problem have been aimed at treating the symptoms rather than the cause. Generally, these have been based on running casings capable of withstanding the nonuniform salt loadings because satisfactory cementations have been virtually impossible to achieve because of large washouts in the salt layers. The solution is to use standard oilfield equipment to drill a gauge hole through the squeezing salts, thus providing an optimum cementation environment. Various methods were used to try to resolve the problem; the last was the use of thick-walled casings in conjunction with saturated potassium/ magnesium drilling fluids to reduce salt washout. Unfortunately, washouts still occurred because the muds could not be saturated at the surface to downhole conditions. Now, the drilling fluid is heated on the surface, enabling the required level of salt concentration to be achieved. The heating system, comprising a boiler and heat ex-changer from a conventional well-test package, has been used on eight wells to date. In each case, a gauge hole was drilled through the squeezing-salt sections and, subsequently, successful cementations were performed.

Diutan biopolymer has recently been introduced for drilling with coiled tubing on the North Slope of Alaska. Diutan biopolymer has replaced a xanthan biopolymer based mud system when drilling new sidetrack laterals from existing wells and also in well servicing with non-rig coiled tubing operations. This change in fluid systems was made by replacing the existing xanthan biopolymer used in the solids free mud system with a diutan biopolymer. The new diutan based solids free mud has shown improvements in several areas thus providing a significant performance advantage. Coiled tubing drilling (CTD) has been conducted in Alaska since 1994 with over 600 sidetrack laterals drilled to date. For the majority of these wells a solids-free mud system was used to drill in either an overbalanced or managed- pressure drilling mode. The solids-free mud used was based on a xanthan biopolymer. In 2009 planning for more difficult well candidates suggested that the existing xanthan drilling fluid systems would be inadequate for drilling certain wells without exceeding acceptable working pressures in the coiled tubing or surface equipment. The higher pump pressures expected, along with acceptable coiled tubing design parameters associated with this higher pressure demonstrated the need to modify the drilling fluid. The standard solids-free mud was modified by replacing the xanthan biopolymer with diutan biopolymer. This new system demonstrated a 20% reduction in pump pressure, better hole cleaning, a higher tolerance to the pH changes related to cementing operations and longer fluid system life. This paper will document the development of the diutan reservoir drilling fluid (RDF) system, including laboratory testing, field testing, a comparison to the xanthan based RDF, and the results of drilling actual wells. This new drilling fluid system has been in use now since July of 2009 and is the preferred drilling fluid used for coiled tubing drilling and well servicing in Alaska.

With increased concerns over the environmental issues regarding the use of oil-based mud (OBM)s, drilling companies in Iran are moving towards implementing less harmful water-based fluids. Due to low toxicity to the environment, effective shale inhibition and considerable cost savings, water-based glycol muds have the highest prospects as alternatives to OBMs in Iranian oilfields. However, designing water-based muds for drilling low-pressure shales may involve a compromise between mud weight optimization and overall environmental and economic advantages offered by these systems. The present study will focus on optimizing the weight of a glycol mud by emulsifying oil in the system and comparing its performance, environmental compatibility and cost with OBMs used in drilling low-pressure zones in Iranian oilfields. Properties of glycol solutions in the presence of oil were also evaluated. An emulsified glycol mud and an OBM with densities of 60 PCF were prepared and compared for their rheology and shale recovery. The shale recovery tests were conducted using shale samples from Maroon oilfield. Possible effects of contamination on the performance of the system were also studied. The glycol system was then treated to recover its diesel content. Overall, the laboratory data were convincing. With very low filtration rates, the emulsified glycol system is a candidate to replace OBMs in reservoir sections. Shale recoveries were comparable to those of OBMs. Compared with a similar OBM, considerable cost savings can be made and the system is more environmentally friendly. However, rheology was more difficult to control and the system was susceptible to contamination.

Актуальность исследования обусловлена необходимостью снижения стоимости используемых в буровых растворах полисахаридных реагентов, а соответственно и стоимости строительства скважины. Это направление исследований позволит определить области применения реагентов на основе карбоксиметилированного крахмала в современных рецептурах буровых растворов. Цель исследования: изучение свойств современных рецептур буровых растворов с использованием в качестве понизителя фильтрации карбоксиметилкрахмала и низковязкой полианионной целлюлозы, оценка устойчивости буровых растворов с использованием данных реагентов к воздействию солей, температур и биологической деструкции. Объекты исследования: современные системы полимер-глинистых, минерализованных полимер-глинистых и биополимерных буровых растворов, содержащих понизители фильтрации на основе карбоксиметилированной целлюлозы и крахмала. Методы: инструментальные методы определения параметров буровых растворов по ГОСТ 33213-2014 и РД 39-00147001~773~2004 (фильтрационные и реологические свойства); свойства полисахаридных реагентов, согласно ASTM D1439-15. Результаты. Проведены сравнительные исследования полимер-глинистых и биополимерных буровых растворов с использованием карбоксиметилкрахмала и низковязкой полианионной целлюлозы в качестве понизителей фильтрации. Показано, что буровые растворы с использованием низковязкой полианионной целлюлозы обладают большей пластической вязкостью и динамическим напряжением, однако структурно-механические свойства буровых растворов выше при введении карбоксиметилкрахмала. Последний, как понизитель фильтрации, обладает низкой эффективностью в минерализованных буровых растворах, при этом в условиях низких температур обеспечивает фактически равную по сравнению с низковязкой полианионной целлюлозой фильтрацию в полимер-глинистых и биополимерных растворах. Установлено, что карбоксиметилкрахмал может использоваться как альтернатива низковязкой полианионной целлюлозе в полимер-глинистых и биополимерных буровых растворах при бурении скважин с умеренными забойными температурами.

Shales present lower mechanical and drillability challenges, when compared to other common lithology types such as sandstones and carbonates1,2. However the penetration rates (ROP) of drill bits, especially that of PDC bits, do not always reflect this characterization. Research efforts, focusing on shale drillability, have now revealed that bit performance in shales is much more dependent on factors such as depositional depths, mud types and mud weights than it is on hardness and/or abrasiveness3,4. The characteristics and/or magnitudes of these factors, that have adverse effects on bit performance, will be referred to as "negative performance factors" (NPF) in this paper. PDC bits usually drill twice as fast as roller cone (RC) bits in Shales. However, this ROP trend reverses when both bit types are exposed to NPF environments. This paper will identify the causes of poor PDC bit performance when exposed to the influences of NPF environments. It will discuss an innovative technology which has specifically been developed to improve the ROPs of PDC bits in shales, especially when drilled at great depths with weighted water based mud systems. Laboratory and field data, showing the technology's positive effects on PDC bit performance will be presented. Specific laboratory tests, designed to identify operational conditions that enhance PDC bit performance in such applications, will also be discussed.

Abstract Swelling behaviour of three different mudrocks (London Clay, Oxford Clay and Fuller's Earth) of varying composition, geological ages, degree of lithification, organic matter and. expandable clay contents have been investigated in the presence of water-based mud systems (KCIICMC, Polyamine, Polyamine, + glycol and PHPA + glycol) plus water to identify the factors controlling the swelling behaviour of shales. The results show, that the degree of swelling is not directly correlatable with the proportion of swelling clay minerals in the mudrocks. Based on the experimental results, the swelling behaviour of the mudrocks depends on the composition, geological history and degree of, lithification of the mudrocks. Well lithified smectite-rich mudrock was found to show less swelling than less well lithified mudrocks with lesser amounts of swelling clays. Organic matter rich mudrock showed a greater degree of swelling than those with little organic matter, suggesting that organic matter may also influence the swelling behaviour. This investigation pro' vi des a better understanding of the swelling behaviour of mudrocks by introducing the concept of the combined action of physico-chemical and mechanical swelling processes. Introduction Swelling behaviour of different rock-fluid combinations is very important in predicting different drilling problems and determining the proper mud system to stabilize the troublesome shaley borehole environment. This characteristic of rocks is a complex function of the rock type and the mud systems and is one of the main causes of sloughing, heaving, caving and progressive hole enlargement problems in the shaley section of a borehole environment, 3). Swelling causes easy detachment of clays or shale particles both from the borehole wall and the cuttings, incorporating into the mud system as additional solid materials, and thus produces considerable changes in the rheological properties of the mud, increasing risk of differential sticking, induced fracturing and mud treatment cost. It is widely believed that smectite clay content of the mudrocks is the main culprit for excessive swelling of shale materials. In the context of drilling, this criterion of shale swelling is often unable to explain the unusual swelling behaviour of some mudrocks having little or no swelling clays. On the other hand it was found that some smectite' rich shale formations(2) have very little swelling potential compared to shales having low expandable clay content. his illustrates the role of other factors in controlling the swelling behaviour of mudrocks. Bearing this in mind an investigation was carried out to determine the factors governing the swelling behaviour of mudrocks. TABLE 1: Composition of the clay fraction Three different mudrocks and four different drilling mud systems plus fresh water were used for the study. The mudrocks are London Clay representing the Upper Tertiary mudrock, Oxford Clay representing the Upper Jurassic mudrock and Fuller's Earth representing the Lower Tertiary mudrock encountered in the North Sea. Some commonly used water-based muds such as KCI/CMC, Polyamine, Polyamine + glycol, water and a newly designed water-based mud system were used to assess the swelling behaviour and factors governing the behaviour of the mudrocks.

A new generation calcium chloride (CaCl2)/polymer-base drilling fluid system has been developed and successfully applied in the deepwater Gulf of Mexico. The system was developed specifically to address the myriad of operational problems associated with drilling the highly reactive, gumbo-laden clays found throughout the Gulf of Mexico. This paper describes the evolution of these systems and the lessons learned during this experience, including mixing and shearing procedures to eliminate the shaker screen blinding seen on wells drilled with first generation CaCl2 systems. The authors will present the application of the newest generation CaCl2 system, which employs a uniquely engineered encapsulating polymer. Among the field trials to be reviewed is one from a newly built drillship, which used the system to drill its debut well in the deepwater Gulf of Mexico. Further, the paper examines the performance of this new system compared with other water-based systems traditionally used to drill similar intervals. Experience from several wells employing calcium chloride/polymer systems and evolutionary modifications to improve their efficiency will be discussed. These sequential improvements have allowed the development of a decision tree to determine optimum applicability.

Degradation of pressurized topside static mud equipment is a very complex process. Inspection is a helpful tool to monitor degradation and helps reduce the number of critical failures. If it is left undetected and unchecked, it can lead to leakage resulting in accidents. To set up an effective inspection program, the concepts of Risk-Based Inspection (RBI) can be utilized. RBI helps to develop an optimum inspection program by evaluating the probability of failure (PoF) and consequence of failure (CoF), and combining them to estimate risk. The parts that have risk higher than the acceptable limit are then prioritized for inspection. This thesis studies the probability of failure in static equipment in pressurized mud systems on an offshore drilling installation, due to different degradation mechanisms and its influencing factors. Det Norske Veritas (DNV), suggests a number of models to estimate internal and external degradation. By analyzing inspection data from the industry, it is observed that the models can at times be inconvenient to use when the degradation process is complex. For example, it is difficult to develop a simple, yet reliable, model that can accurately predict rate of degradation in situations where corrosion and erosion are simultaneously taking place. In static equipment in high pressurized mud systems, inspection have shown that the main reason for internal degradation is high amounts of solids in the fluid, high velocity, presence of seawater, and corrosive chemicals. DNV-RP-G101 does not present any model for this situation. For external corrosion, the current models presented in DNV-RP-G101 can be used to analyze inspection data. In an old installation, the accurate quantitative records are often not available due to a number of reasons, like difficulty in measuring, old data management system, unsystematic inspections or lost records. Thus, it often becomes difficult to develop any qualitative model. On the other hand, the inspection and maintenance engineers have extensive experience that may be utilized for developing effective subjective models. In this thesis, based on some simplified parameters, a methodology for evaluating probability of failure is established. The methodology is divided into external and internal degradation, where the subjective judgments are more evident for internal degradation. Simplified flowcharts and tables are developed to easily evaluate probability of failure.

Master's thesis in Petroleum engineering Drilling fluid is an essential part of drilling operation. The main functions of the drilling fluid are to transport cutting, to maintain well pressure and cooling formation and drill-bit. The detail knowledge of drilling fluid is very important to design safe and proper drilling operations. This thesis presents the characterization and performance evaluation of 70/30 and 90/10 Oil Water Ratio of Oil Based Mud systems. The characterization is through direct experimental measurements and the performance is through simulation and experimental studies as well.

With the growth in popularity of synthetic-based mud (SBM) systems in Australia, and the emergence of more advanced wireline logging tools, there is an expectation that better hole conditions will give rise to better value- for-money evaluation programs. This is certainly true from the drilling cost perspective, where a logging job can be completed without need for 'wiper' trips to recondition the hole. However, in order to extract maximum value from the latest generation of logging tools, careful attention needs to be applied to the formation evaluation program.There have been some unexpected results from the wireline data quality viewpoint, with some positive results mixed with outcomes that fell below expectation, both linked to the complex nature of the synthetic-based mud recipes. Contrary to intuition, good quality electrical images may be obtained in synthetic-based muds, and some mud additives used to stop filtrate loss can seriously degrade the quality of acoustic images that are traditionally run in SBMs. Nuclear Magnetic Resonance works well in SBMs, especially where invasion is kept to a minimum. Holes drilled with SBMs do not normally display borehole ellipticity, which may be used to infer tectonic stress directions. However, oriented cross dipole shear-weave logging is able to detect azimuthal anisotropy, which may be related to tectonic stress, in holes with a circular profile.There are trade-offs using different SBM recipes, which will impact on formation evaluation programs. Close consultation with the logging contractor and mud company, as far ahead of the drilling of the well as possible is recommended to ensure a satisfactory outcome.