Wellbore Wall Research Articles

ПРИНЯТИЕ РЕШЕНИЙ ПО ВЫБОРУ ОПТИМАЛЬНОЙ ПЛОТНОСТИ БУРОВОГО РАСТВОРА НА ОСНОВЕ ГЕОМЕХАНИЧЕСКОГО МОДЕЛИРОВАНИЯ И НЕЧЕТКОЙ ЛОГИКИ

In order to avoid various drilling problems associated with instability of the wellbore walls, lost circulation, etc., it is necessary to use the concept of a “safety window” of the drilling fluid. Calculation of the boundaries of the mud safety window makes it possible to justify the choice of mud density for safe drilling of wells. However, with all its advantages, it is impossible to accurately determine the boundaries of the “safety window” due to their blurriness, which requires the use of appropriate methods based on fuzzy logic and taking into account this type of heterogeneity. This article proposes an approach based on the application of the basic principles of fuzzy set theory to the choice of drilling mud density using the example of several fields in Azerbaijan, Russia and Kazakhstan. An equation for determining density was obtained, calculations were made using this expression for some fields, and the boundaries of the so-called “safety window” were assessed based on geomechanical studies.

Evaluation and Application of Wellbore Stability of Deep Oil Wells in the Southern Margin of Junggar Basin

The stability of the oil well wellbore is a prerequisite for selecting the optimal completion method. In this paper, based on experimental testing and theoretical models of rock mechanics parameters in deep oil reservoirs, the in situ stress parameters of deep oil wells are accurately predicted. On this basis, a full life cycle assessment model for wellbore and perforation casing stability was established, and the effects of pressure depletion and changes in the production pressure differential on wellbore stability and casing stability were analyzed. The research results indicate that as the formation pressure decreases, the critical collapse pressure difference around the wellbore significantly decreases. The greater the production pressure difference, the more likely the wellbore is to become unstable. Under the original formation pressure coefficient, if there is no casing, the critical failure pressure difference of the wellbore wall is 55 MPa. After cementing and perforation, when the casing is uniformly stressed and the formation pressure drops to a coefficient of 0.83, the casing will not be damaged even when the wellbore is completely emptied. At this time, there is still a certain safety production pressure difference in the perforated formation. This study can effectively guide the optimization of well completion and safe development in deep oil reservoirs.

Optimizing the heat extraction performance in the Qingfeng Karst geothermal reservoir

Faults are typically associated with hydrothermal reservoirs and may control the heat extraction of geothermal reservoirs. However, in Karst geothermal reservoirs, the dissolution of soluble rocks results in pronounced spatial porosity and permeability anisotropy (PPA), potentially weakening the dominant role of faults. The question arises: which plays a dominant role, PPA or faults? Should geothermal exploitation strategies be adjusted under the interaction of both factors? In this study, we proposed a coupled Thermal-Hydraulic-Geothermal Wellbore (THG) numerical simulation model to streamline the integration of non-isothermal flow and heat transfer processes within a 3D wellbore. Utilizing this model, we analyzed the influence of faults compared to PPA and assessed the geothermal well doublet's heat extraction performance, including thermal breakthrough time, temperature losses along the wellbore wall, and recovery factor (Rg). There are specific geothermal reservoir development conditions leading to the latest thermal breakthrough time and the maximum Rg. The results of contribution rates highlight the influence of injection rate in determining Rg. It is crucial to note that thermal breakthrough time does not necessarily increase with injection temperatures. A more suitable definition for thermal breakthrough time in the Karst geothermal reservoir has also been identified. This study provides an essential reference value for the efficient development of similar Karst geothermal reservoirs.

Hydrothermal coupling model between wellbore and permafrost for drilling in arctic cold regions

The abundant oil and natural gas resources in the Arctic cold regions have driven people to conduct drilling operations in these harsh environments. During drilling operations in Arctic permafrost regions, the linear heat source effect generated by the flow of drilling fluid in the wellbore continuously melts the surrounding permafrost, which can lead to wellhead settlement and instability of the wellbore wall, among other accidents. In order to provide guidance for drilling engineering in cold regions, there is an urgent need to study the coupled heat transfer between the wellbore and the permafrost in Arctic drilling. Due to the significant influence of hydrothermal coupling process in permafrost on temperature transfer, conventional wellbore heat transfer models are not suitable. Therefore, this paper proposes a hydrothermal coupling model between the wellbore and permafrost for drilling in Arctic cold regions and analyzes the heat transfer in the wellbore and the hydrothermal processes of the formation using numerical methods. By applying this model, the insulation effects of existing main wellbore temperature control and insulation technologies were studied. The main research findings indicate that drilling process can cause the permafrost around the wellbore to thaw, and the thawed permafrost around the wellbore presents a “horn” shape that expands from the wellhead to the bottom of permafrost layer; Without insulation measures, the volume of thawed permafrost around the wellbore can reach 315 m³ after 7 days of circulation; During the drilling process, moisture migration occurs. Under the effect of gravitational seepage, unfrozen water content in the thawed permafrsot around the wellbore increases from the wellhead to the bottom of the permafrsot layer; Vacuum insulated tubing has almost negligible impact on the temperature of the drilling fluid in the wellbore, while it can reduce the volume of thawed permafrost around the wellbore by 62.5 % to 88 % or even higher; Drilling fluid cooling systems can significantly lower the temperature of the drilling fluid in the wellbore but cannot effectively prevent the thawing of permafrost around the wellbore. Therefore, in practical drilling operations, priority should be given to using high-quality vacuum insulated tubing or combining both methods simultaneously.

Optimizing Deep Geothermal Drilling for Energy Sustainability in the Appalachian Basin

This study investigates the geological and geomechanical characteristics of the MIP 1S geothermal well in the Appalachian Basin to optimize drilling and address the wellbore stability issues encountered. Data from well logs, sidewall core analysis, and injection tests were used to derive elastic and rock strength properties, as well as stress and pore pressure profiles. A robust 1D-geomechanical model was developed and validated, correlating strongly with wellbore instability observations. This revealed significant wellbore breakout, widening the diameter from 12 ¼ inches to over 16 inches. Advanced technologies like Cerebro Force™ In-Bit Sensing were used to monitor drilling performance with high accuracy. This technology tracks critical metrics such as bit acceleration, vibration in the x, y, and z directions, Gyro RPM, stick-slip indicators, and bending on the bit. Cerebro Force™ readings identified hole drag caused by poor hole conditions, including friction between the drill string and wellbore walls and the presence of cuttings or debris. This led to higher torque and weight on bit (WOB) readings at the surface compared to downhole measurements, affecting drilling efficiency and wellbore stability. Optimal drilling parameters for future deep geothermal wells were determined based on these findings.

Migration characteristics of bubbles moving in gas–liquid countercurrent two-phase flow in an inclined pipe

While exploring and developing oil and gas, well kick and overflow accidents are inevitable. In such an accident, if the drilling bit is not at the bottom of the well, the drilling tool is blocked, or the reservoir contains toxic gases such as hydrogen sulfide, the traditional technique for well control is no longer suitable, and the bullheading method must be adopted to kill the well. However, during bullheading, the flow patterns of gas and liquid are intricate, making gas velocity hard to predict. To solve these problems, a set of visual simulation experimental device for directional well bullheading was built to find out the effects of wellbore inclination, liquid viscosity, gas and liquid velocities, and bubble size on bubble migration characteristics in gas–liquid countercurrent (gas flowing in opposite direction to the liquid). Prediction models of bubble distribution coefficient and rising velocity considering liquid viscosity, bubble size, and wellbore inclination angle were worked out. The experimental results show that small bubbles are mainly located in the center of the wellbore and large bubbles tend to approach the wellbore wall in gas–liquid countercurrent. Increasing wellbore inclination, viscosity, and the velocity of the liquid flowing against the bubbles results in a gradual decrease in Taylor bubble migration speed, which promotes the bubbles' pressing-back, as inhibiting the upward movement of large bubbles is essential for effective bullheading operations. The prediction models of distribution coefficient and bubble migration velocity have an error of less than 10% and 9. 78%, respectively.

Construction of Mechanical Earth Modeling for Mitigating Wellbore Instability in Tanuma Shaly Formation -Southern Iraqi Oil Field

Wellbore instability has become an increasing concern for horizontal and extended-reach wells, especially through shale cap rocks. This paper presents a comprehensive analysis of the factors that led to the wellbore being unstable in the Tanuma Formation, the Nasiriyah oilfield in southern Iraq. By constructing a mechanical earth model using open-hole wireline data from two drilled wells. Mohr-Coulomb, Mogi-Coulomb, and Modified Lade failure criteria are used to analyze the breakouts and determine the density of mud that is required for a safe wellbore wall. The results indicated that the Mogi-Coulomb criterion is more similar to the actual well failure than the other two criteria. The wellbore instability findings showed that the most stable wells are those that are vertical and have a low deviated angle (less than 10°), and it is advised that these wells be drilled with a mud weight of 11.3 to 14 ppg. The acceptable range for the weight of the mud for horizontal and greatly deviated wells is between 11.9 and 13.8 ppg. Consequently, the future wells development plan for the studied region is improved and leads to cost reductions.

Numerical investigation of wellbore damage due to drill string lateral vibration

During drilling operations, cyclic loading is exerted on the wellbore wall by the vibrations of the drill string. This loading could lead to rock fatigue, which in turn might result in wellbore failure. In this study, a numerical model is developed to simulate the effects of repeated loading on rock fatigue and failure. The simulation is based on an elasto-plastic constitutive model coupled with a damage mechanics approach, which allows us to examine the wellbore instability due to drill string vibrations. The model is verified with the existing data in the literature related to experiments on impact of a steel ball against a curved wall. The findings indicate that cyclic loading increases the development of plastic strain around the wellbore significantly compared to static conditions, promoting rock fatigue. Furthermore, the cyclic loading expands the radius of the yielded zone substantially, a critical factor for maintaining wellbore integrity. The proposed model can be used to evaluate the wellbore stability under repetitive loading caused by the drill string action.

Development of high-performance fiber-reinforced concrete for drilling wellbore walls in highly mineralized strata and its sulfate attack resistanceattack resistance

Metakaolin has been incorporated into high-performance fiber-reinforced concrete for wellbore wall drilling to enhance its durability in strata with highly mineralized water. This study established a benchmark, utilizing fly ash, slag powder, and metakaolin as the factors in an orthogonal test to assess the durability of concrete against sulfate attack. The range analysis and an integrated balance method were employed to optimize the mix proportion, the optimized mix proportion of high-performance concrete was determined as concrete: cement: fly ash: slag powder: metakaolin: pumping agents: gravel: sand: water: polyvinyl alcohol = 1: 0.2: 0.075: 0.05: 0.106: 2.767: 1.556: 0.371: 0.003. The apparent and microscopic morphologies before and after the erosion of both the benchmark group and optimized mix proportion group were investigated. The triaxial permeability tests were conducted on these groups under varying confining pressures to elucidate concrete permeability trends. Additionally, a damage constitutive model for concrete under a sulfate attack was formulated based on the durability tests. This study could provide valuable insights into the industrial utilization of concrete in deep shafts within highly mineralized water strata in Northwestern China.

A study of mechanism of wellbore instability by means of strain energy density

The paper presents a new analytical method based on the strain energy density to investigate the mechanism of wellbore instability. To establish the method, pre- and post-drilling states of strain energy densities were first formulated, and then have been applied to the wellbore wall using the strain energy balance. This innovative approach differentiates the instability prone zones into two distinct parts; the compressive, and the tensile zones that also known as borehole breakout and drilling-induced fracture, respectively. These two types of instabilities are related to the restored strain energy density (RESED) and the released strain energy density (RLSED), accordingly. Among the two types of instabilities, the geometrical characteristics of the compressive instable prone zone, that is, width, 2 θ b , and length, L b , were further formulated, examined and proposed. The results of the presently developed method have been compared with those that obtained by using the existing borehole breakout’s geometry. In each case, good agreement was found between the results of this study and the conventional methods. This method has further been compared and verified by using the experimental data. Finally, it has been demonstrated that how this method helps to determine the optimal mud pressure for a safe wellbore design.

Study on the Damage and Failure Process of Prefabricated Hole-Fracture Combination Defects in Shale

The rock formations in oil and gas reservoirs are predominantly composed of laminated shale, which contains various defects such as pores and fractures. These defects have a significant impact on the stability of wellbore walls. As a result, this study utilized rock samples from a specific oil and gas reservoir and introduced pre-existing pore and fracture defects within them. Uniaxial compression tests were conducted on rock specimens with varying angles between the fractures and the bedding planes. The study involved measuring and analyzing the impact of pre-existing defect morphology on shale’s mechanical properties. Additionally, through the use of digital image correlation (DIC) technology, a comprehensive strain field map was obtained, depicting the initiation, propagation, and ultimate failure of surface cracks in shale under loading conditions. This allowed for both qualitative and quantitative analysis of the connection between shale’s damage and failure processes and the evolution of strain fields. Ultimately, this research provides a theoretical basis for wellbore stability and the development of shale oil and gas fields. Conclusions drawn from the study are as follows. With an increase in fracture angle, the rock’s elastic modulus gradually increases, and both compressive strength and strain exhibit a pattern of higher values on both ends and lower values in the middle. When the fracture angle is less than 30°, significant stress concentration occurs at the tip of the fracture. When the angle exceeds 60°, stress concentration around pores dominates. In the range of 30° to 60°, there is a combined stress concentration around both pores and fractures, significantly reducing rock stability. Crack propagation is influenced by bedding planes and exhibits varying degrees of ductility, ultimately resulting in a tension-shear mixed failure. When fractures are parallel to bedding planes (angle = 0°), defects are symmetrically distributed, and stress concentration at the fracture tip dominates, leading to crack initiation from the fracture tip and eventually forming an “H”-shaped tensile failure. When fractures are perpendicular to bedding planes (angle = 90°), stress concentration around pores is greater than at the fracture tip, causing cracks to initiate around pores and eventually collapsing on one side of the fracture.

Axial Load Peristaltic Transfer Mechanism of the Drillstring to Improve Penetration Rate

Summary During the drilling of long horizontal wells, the significant frictional resistance generated by the wellbore walls poses a challenge for the drillstring to efficiently transmit load to the drill bit, which eventually reduces drilling efficiency and restricts the extension distance achievable. Inspired by the structure and movement principle of an earthworm, we propose an earthworm-like load transfer method for the drillstring to address this issue. Specifically, the proposed method involves the installation of a pulse generator and multiple vibration subs within the same drillstring, decomposes the drillstring into multiple sections and modulates it to creep like an earthworm, thus facilitating load transfer. Experimental studies and numerical simulations were conducted in this paper to explore the fundamental mechanisms of earthworm-like crawling, aiming to enhance the efficiency of load transfer within the drillstring. The experimental results suggest that adopting earthworm-like excitation can increase the load transfer efficiency of the drillstring by 36–52% compared to conventional drilling methods. However, if the drillstring experiences helical buckling, there is a significant decrease in the efficiency of load transfer. Meanwhile, a dynamic model of the drillstring, considering the 3D wellbore trajectory, multipoint excitation, Stribeck friction, and penetration rate, has been developed. The simulated results from the proposed model align well with the experimental results obtained before the drillstring buckling, with an error of less than 5%. The simulation results for a 1000-m drillstring indicate that the earthworm-like excitation significantly enhances the efficiency of load transfer compared to conventional drilling methods. This improvement is attributed to the increase in the proportion of reverse-motion drillstring segments by 35.8–40.25%, which will greatly reduce the instantaneous total vector frictional force of the entire drillstring.

Enhancing drilling operations: prioritizing wellbore integrity, formation preservation, and effective mud waste control (case study)

This study examined the issues of wellbore instability encountered during the drilling of the vertical exploration Al-Rateena-1 well, Block 25D, Al-Rawat Field, Sudan. Measurements of the wellbore diameter revealed significant damage to the formation interval between 2775 and 2925 m. This damage released approximately 800 barrels of drill cuttings from the affected zone and overlying formations. A comprehensive wellbore stability assessment was conducted to identify the root causes of instability and develop strategies to mitigate future occurrences. The assessment involved a thorough analysis of the pore, vertical, effective vertical, and minimum and maximum horizontal pressures. All of these parameters play a crucial role in maintaining wellbore integrity. This study also evaluated the potential impact of these parameters on groundwater and soil quality, leading to the development of an efficient waste management system. The safe mud weight range for the well was determined to be between 13.5 and 15.5 pounds per gallon (PPG). However, the drilling fluid used during the operation had a significantly lower density of only 10 PPG. This ultimately leads to the collapse of the wellbore wall. Examination of drilling cuttings revealed elevated levels of various heavy metals. These heavy metals included Lead (94.12 ppm), Mercury (62.87 ppm), Nickel (1403 ppm), Copper (343 ppm), Calcium (23132.72 ppm), Magnesium (3700 ppm), and Barium (16000 ppm). These elements pose a significant threat to both surface water and groundwater quality. It is possible that they also contributed to the wellbore wall collapse. It was hypothesized that the interaction between these elements and the water-based drilling fluid or its additives could have triggered the collapse event. The findings of this study highlight the importance of conducting comprehensive wellbore stability assessments before drilling. Such assessments should incorporate detailed investigations, modelling, calculations, and simulations of various parameters that influence wellbore stability. In addition, careful consideration must be given to the potential environmental impacts of drilling fluids and cuttings, particularly in areas with sensitive ecosystems. The wellbore instability in the vertical exploration Al-Rateena-1 well was primarily caused by the use of an underweight drilling fluid. This underweight drilling fluid results in the collapse of the wellbore wall. Elevated levels of heavy metals in drill cuttings suggest a potential impact on groundwater quality. It is also possible that they contributed to the wellbore instability. Comprehensive wellbore stability assessments are essential to prevent similar incidents in the future. This study provides a detailed account of wellbore instability. This includes the identification of root causes and the development of preventive measures. This study also highlights the potential environmental impact of drilling fluids and cuttings. The findings of this study can help improve drilling practices and environmental protection in the oil and gas industry.

Improving Drill-String Extended Reach Efficiency in High-Temperature and High-Pressure Conditions: An Experimental Investigation

Abstract Drill string buckling behavior is of interest to the oil and gas industries. Buckling severity increases as drilling continues and causes a lock-up condition beyond which drilling cannot commence. The friction force between the drill string and the wellbore is key in influencing drill string buckling initiation and progression. Herein, the effects of adding nano-sepiolite (NSP) as an additive to water-based drilling fluids on the friction between the drill string and wellbore wall were investigated. Various samples with different sizes and compositions of NSP were tested for their ability to reduce the coefficient of friction (COF) that delays the onset of buckling to facilitate deep drilling by providing improved lubrication characteristics. The drill string sinusoidal and helical buckling and lock-up condition and the axial force transfer (AFT) were experimentally investigated. The water-based drilling fluids’ lubricity, COF between the drill string and the wellbore wall, and the stability of the rheological properties of the NSP-modified water-based drilling fluids containing 1–6 wt% NSP were tested in a high-temperature and high-pressure (HTHP) environment utilizing an in-house experimental setup. Sepiolite in nano-form at 4 wt% and with a size distribution of 30–60 nm improved the stability of the water-based drilling fluids rheology, significantly decreased the COF of the water-based drilling fluids, and improved the drill string axial force transfer. NSP additives significantly improved the COF and the drill string AFT in the HTHP environment compared with that of other commercial drilling fluids.

Research on Wellbore Stability in Deepwater Hydrate-Bearing Formations during Drilling

Marine gas hydrate formations are characterized by considerable water depth, shallow subsea burial, loose strata, and low formation temperatures. Drilling in such formations is highly susceptible to hydrate dissociation, leading to gas invasion, wellbore instability, reservoir subsidence, and sand production, posing significant safety challenges. While previous studies have extensively explored multiphase flow dynamics between the formation and the wellbore during conventional oil and gas drilling, a clear understanding of wellbore stability under the unique conditions of gas hydrate formation drilling remains elusive. Considering the effect of gas hydrate decomposition on formation and reservoir frame deformation, a multi-field coupled mathematical model of seepage, heat transfer, phase transformation, and deformation of near-wellbore gas hydrate formation during drilling is established in this paper. Based on the well logging data of gas hydrate formation at SH2 station in the Shenhu Sea area, the finite element method is used to simulate the drilling conditions of 0.1 MPa differential pressure underbalance drilling with a borehole opening for 36 h. The study results demonstrate a significant tendency for wellbore instability during the drilling process in natural gas hydrate formations, largely due to the decomposition of hydrates. Failure along the minimum principal stress direction in the wellbore wall begins to manifest at around 24.55 h. This is accompanied by an increased displacement velocity of the wellbore wall towards the well axis in the maximum principal stress direction. By 28.07 h, plastic failure is observed around the entire circumference of the well, leading to wellbore collapse at 34.57 h. Throughout this process, the hydrate decomposition extends approximately 0.55 m, predominantly driven by temperature propagation. When hydrate decomposition is taken into account, the maximum equivalent plastic strain in the wellbore wall is found to increase by a factor of 2.1 compared to scenarios where it is not considered. These findings provide crucial insights for enhancing the safety of drilling operations in hydrate-bearing formations.

Stability of the wellbore wall during marine hydrate depressurization production based on a modified Drucker-Prager model

Different from oil and gas production, hydrate reservoirs are shallow and unconsolidated, whose mechanical properties deteriorate with hydrate decomposition, so there is a risk of wellbore wall failure during depressurization production. However, the existing researches of wellbore stability focuses on the drilling process, and the stress model used does not taking the effect of casing into account. Therefore, this paper aims to propose an analytical approach suitable for the analysis of wellbore wall stability during production. We proposed a stability analysis criterion based on a modified Drucker-Prager model, derived an effective stress model considering casing-reservoir interaction, established a multi-field coupling model reflecting the dynamic effect of hydrate decomposition, and developed a solver. An experimental system was designed to verify the models. A case study found that the stability is worst near the casing in the direction of maximum horizontal in-situ stress, relatively better in the direction of minimum horizontal in-situ stress, and progressively increase away from the casing. The stability decreases with the decrease of hydrate saturation, and the lower the saturation, the faster the stability decreases. The inhomogeneous of in-situ stress has great influence on the stability, and if it is too high, the wellbore wall will be damaged.

Extraction of biodiesel from pomelo peel and investigation of its efficiency as a lubricant in water-based drilling fluid

Abstract Reducing friction between drill string and wellbore wall is one of the key functions of drilling fluid which may result from tight holes, key seats, differential sticking, or cutting build up along the drill string. However due to growing environmental concerns, biodegradable lubricants which are also cost-efficient are appreciated and mandated in many countries. This study discusses the performance analysis of biodegradable biodiesel produced from pomelo peels and Petroleum-based lube oil as drilling fluid lubricants. The test samples comprise 700 mL of water + 10 % bentonite + 3 % barite + 0.25 % CaCO3 + 0.25 % KOH + 1 % CMC + lubricant. Two sets of lubricants were utilized to prepare 10 samples of drilling fluid, one set with petroleum-based lubricant and other with biodiesel. Essential oil was extracted from pomelo peel with the help of the Clevenger apparatus. The oil was then converted to biodiesel through the process of transesterification. Biodiesel properties were tested for the ASTM standards. Both sets of drilling fluid samples were tested for their rheological, filtration and lubricating properties and from the results obtained it can be concluded that both type of lubricating agents have almost negligible impact on rheological and filtration loss properties. However, the lubricity coefficient values drastically decreased as we increased the lubricant content, which is an indication that biodiesel extracted from pomelo peel acts as a better lubricant. The values of the lubricity coefficient for biodiesel based drilling fluid also fell well within the advised range (<0.30). We can therefore draw the conclusion that biodiesel made from pomelo peel can replace petroleum-based lubricants while also lowering the environmental concerns related to lubricants.

Simulation of Filter-Cake Formations on Vertical and Inclined Wells Under Elevated Temperature and Pressure

Summary Drilling fluids consist of colloidal suspensions that are made up of solid particles suspended in liquid. The aqueous component of the drilling mud invades a permeable formation due to differential pressure, leaving solid deposits on the drilled formation walls, forming a filter cake. Typical drilling fluids should create a thin, slick filter cake and fluid loss control to seal permeable zones as swiftly as possible. Tight holes, elevated levels of rotational resistance and friction, wellbore obstruction, fluid loss into formations, suboptimal well-log data, and reservoir formation impairment result from thick filter cakes and excessive filtration. Controlling infiltration of fluids to establish a minimal, low-porosity filter cake is frequently required to avoid drilling and production issues. In this study, we used computational fluid dynamics (CFD) to model the development of filter cakes induced by pipe rotation in vertical and inclined wellbore walls under elevated temperature and pressure during deep drilling. Multiple tests validate the basic model for application in both shallow and deep drilling operations. By use of an Eulerian-Eulerian approach to describe drilling fluids as a binary-phase system, including particles in suspension in the non-Newtonian fluid, our research investigates the power-law and Herschel-Bulkley rheological models. These models successfully predict the flow pattern of drilling fluid at the bottom region of deep drilling operations. The study further explores filter-cake formation by examining the influence of overbalanced pressure on filter-cake thickness at various pressures. Drilling simulations, incorporating pipe rotation and eccentricity effects under realistic operating parameters, accurately project filter-cake thickness, aligning closely with experimental measurements and existing CFD literature in this domain.

Effects of nanosilica on the properties of brine-base drilling fluid

In the process of drilling oil and gas wells, the shrinkage and falling of wellbore walls are often caused by the expansion of mud shale water. To date, conventional additives have been unable to plug the pore throats of shale rock with nanoscale pores and thus cannot effectively solve the problem of wellbore instability encountered in shale formation drilling. In view of this situation, the idea of using nanosilica to plug the nanopore throat of mud shale is proposed to reduce its permeability and to slow water intrusion. The influences of nanosilica on the properties of brine-base drilling fluid drilling fluid are evaluated by measuring the viscosity, filtration loss and swelling of the drilling fluid. The results of laboratory experiments show that the improvement in drilling fluid properties can only be achieved on the basis of salt resistance; that is, a salt-resistant soil slurry should be used. A concentration of 1–5% nanosilica can improve the viscosity of the drilling fluid by increasing the internal friction between particles. However, nanosilica materials are sensitive to salt concentration. Nanosilica particles can be deposited on the surface of a filter cake to block the pores of the filter paper, and the filtration loss reduction rate can reach 40.2%. Blocking the pores of the clay plays a role in preventing the clay from absorbing water and expanding. The optimal addition amount of silica is 3%, and its salt resistance can reach 16%. Considering the experimental results of filtration loss, swelling amount and cost performance, 3%NP + 4%NaCl + SWM-B is selected as the optimal formula. The results of this study can be applied to effectively improve the phenomenon of wellbore instability during drilling in shale formations, and it has important application value.

An Investigation of Wellbore Instability in Problematic Formations from Eridu Oil Field, Southern Iraq

Wellbore instability is a main challenge and contribute to suspending the drilling operations that lead to over-well time, known as Non-productive time (NPT). The cost of drilling operation can be reduced by using a geomechanical model that contributed enormously to the design of an appropriate mud weight window and determined the optimum well trajectory for the plan well. Several wellbore instability encountered while drilling operations in the Eridu oil field, such as (hole collapse, losses, caving, and tight hole). In this study, a geomechanical model (MEM) was developed by using the related data (open hole log measurements, core analysis data, drilling observation data), then these data were utilized in software (Techlog, version 2021). Two failure criteria (Mogi –Coulomb and Stassi d’ Alia failure criteria) were used for the prediction the breakdown and breakout profile; moreover, address the optimum mud window to mitigate wellbore instability-related issue for different well trajectory. The geomechanics results demonstrated that the Mogi failure criterion produced reasonable results when combined with the caliper log. Also, the Eridu oil field fault regime is divided into two regions: strike-slip faults in carbonate rocks and normal faults in clastic rocks. To avoid a majority of wellbore instability problems in shale formations, the safe mud weight range is 11-12.5 ppg. The failure analysis, especially shear failure, showed that the mud weight used previously in this field is improper to support the rock on the wellbore wall. This study’s finding could be used as a cost-effective tool when the plan to drill a direction and horizontal well in Eridu oil field..