Mud Weight Research Articles

Płuczki wiertnicze w skomplikowanych warunkach: przegląd

The review in this article focuses on various aspects of drilling extended reach (ERD) wells. Reaching extreme depths and setting world records for deviation illustrates the importance of well design and operating strategies. Studies of articles describing various locations, including Sakhalin Island in Russia and offshore Vietnam, provide insight into ERD operations. Furthermore, the challenges of drilling in specific geological conditions, such as layered sandstones and reactive clay, are considered. Particular attention is paid to issues related to wellbore instability and drilling fluid optimization. The results of technical studies highlight the key role of maintaining wellbore stability in achieving successful ERD results. The articles emphasize the importance of understanding geomechanical factors, employing optimal mud weight, lubrication, and specialized drilling fluids to counteract instability. It demonstrates that maintaining appropriate mud weights and employing specific drilling techniques are crucial for mitigating instability-related issues. The integration of mechanical and chemical approaches is advocated for effectively managing shale-related instability. The utilization of innovative materials and fluid systems is central to the successful resolution of stability-related problems. The incorporation of micronized sealing polymers in conjunction with conventional plugging materials is detailed as an effective approach to counter wellbore instability. The synergistic combination of materials, additives and mud salinity is showcased to achieve effective shale stabilization and optimize drilling time. The authors emphasize the importance of selecting the optimal composition for each well based on experience and laboratory testing and present laboratory-tested solutions that have been successfully applied in field operations. In summary, these articles collectively offer insights into a range of strategies to combat wellbore instability. They cover the use of advanced materials, innovative fluid systems, and chemical approaches to maintain wellbore stability, improve drilling efficiency, and reduce nonproductive time.

Using One-Dimensional Geomechanical Model for Determining the Safe Wellbore Orientation of Zubair Reservoir, Southern Iraq

The wellbore instability issues are firmly occurring in the Iraqi oilfield and particularly in the southern region of Iraq. It is causing a significant loss of time dealing with many types of downhole problems, as well as an increase in the operations costs because of the instability consequences. By using the well information and open-hole logging data, the One-Dimensional (1-D) geomechanical model generated with Schlumberger Techlog software (V.2015) was used to assess the mechanical stability of the wellbore. The model was calibrated and validated by using core data (Triaxial test), the formation pore pressures, and the Mini-frac test). The 1-D model was validated across the actual well conditions (i.e., the original mud weight and the actual wellbore failure) by using three criteria of failures which are Modified Lade, Mogi-Coulomb, and Mohr-Coulomb. The validation process showed that the failure criterion of Mogi-Coulomb is better than the other two criteria in presenting the actual borehole failure, and thus it can be used to develop the plans of wellbore orientation. By investigating wellbore orientation with various scenarios, ranging from a vertical trajectory to the horizontal strategy, the wellbore orientation is mainly safe (with the minimum breakout) in vertical and inclined wells up to 40º with minimum mud weight of 11.7 and 12.6 ppg. The optimum wellbore trajectory should be in the direction of minimum horizontal stress, which is North-West to South-East (130º-160º). The developed plans provide a guideline for performing stable drilling operations in the south of Iraq and the nearby areas, thus effectively contributing in reduction the overall cost.

An integrated one-dimensional geomechanical model to identify the optimal mud weight and the well trajectory for the Zubair formation

An integrated one-dimensional geomechanical model to identify the optimal mud weight and the well trajectory for the Zubair formation

Applicability of sawdust as a green additive to improve the rheological and filtration properties of water-based drilling fluid: an experimental investigation

Drilling fluid is essential to oil and gas drilling operations due to its diverse functionality. But the most commonly used drilling fluid additives are hazardous chemicals and are not biodegradable. As a result, the demand for environment-friendly additives has been raised to replace the hazardous chemical additives. Considering the rising interest toward green additive, this study examined the applicability of sawdust to be used as a biodegradable drilling fluid additive to improve the rheological and filtration properties of water-based mud. Sawdust was chosen for this study because of its high cellulose content, widespread availability, and low cost. Following the guidelines set by the American Petroleum Institute (API), we carried out laboratory experiments that encompassed three distinct concentrations of sawdust: sample 1 (with 0.25% sawdust), sample 2 (with 0.50% sawdust), and sample 3 (with 0.75% sawdust). The concentrations of sawdust were measured as a weight percentage of the total volume of the base water. Concentrations above 0.75% led to gelation issues. Results showed minimal impact on mud weight at 0.25% sawdust, while 0.50 and 0.75% concentrations caused slight weight reduction due to foam development. Plastic viscosity increased by 28.5, 42.8, and 71.4% for sample 1, sample 2, and sample 3, respectively, compared to the base mud. Sawdust-containing mud exhibited desirable gel strength and shear-thinning behavior. Moreover, sawdust significantly improved filtration properties by reducing fluid loss and mud cake thickness. Sample 2 (0.5% sawdust) performed well in terms of filtration properties. Mud cake permeability followed the trend: kBM>k0.75%sawdust>k0.25%sawdust>k0.50%sawdust\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${k}_{\\mathrm{BM}}>{k}_{0.75\\% \\mathrm{sawdust}}>{k}_{0.25\\% \\mathrm{sawdust}}>{k}_{0.50\\% \\mathrm{sawdust}}$$\\end{document}. Based on promising laboratory performance, lower concentrations of sawdust are recommended as a cost-effective and eco-friendly additive to enhance rheological and filtration properties of water-based drilling fluid systems.

Borehole stability in naturally fractured rocks with drilling mud intrusion and associated fracture strength weakening: A coupled DFN-DEM approach

Borehole instability in naturally fractured rocks poses significant challenges to drilling. Drilling mud invades the surrounding formations through natural fractures under the difference between the wellbore pressure (Pw) and pore pressure (Pp) during drilling, which may cause wellbore instability. However, the weakening of fracture strength due to mud intrusion is not considered in most existing borehole stability analyses, which may yield significant errors and misleading predictions. In addition, only limited factors were analyzed, and the fracture distribution was oversimplified. In this paper, the impacts of mud intrusion and associated fracture strength weakening on borehole stability in fractured rocks under both isotropic and anisotropic stress states are investigated using a coupled DEM (distinct element method) and DFN (discrete fracture network) method. It provides estimates of the effect of fracture strength weakening, wellbore pressure, in situ stresses, and sealing efficiency on borehole stability. The results show that mud intrusion and weakening of fracture strength can damage the borehole. This is demonstrated by the large displacement around the borehole, shear displacement on natural fractures, and the generation of fracture at shear limit. Mud intrusion reduces the shear strength of the fracture surface and leads to shear failure, which explains that the increase in mud weight may worsen borehole stability during overbalanced drilling in fractured formations. A higher in situ stress anisotropy exerts a significant influence on the mechanism of shear failure distribution around the wellbore. Moreover, the effect of sealing natural fractures on maintaining borehole stability is verified in this study, and the increase in sealing efficiency reduces the radial invasion distance of drilling mud. This study provides a directly quantitative prediction method of borehole instability in naturally fractured formations, which can consider the discrete fracture network, mud intrusion, and associated weakening of fracture strength. The information provided by the numerical approach (e.g. displacement around the borehole, shear displacement on fracture, and fracture at shear limit) is helpful for managing wellbore stability and designing wellbore-strengthening operations.

Determination of the mud weight window and optimum drilling route, using the comparison of different failure criteria: a case study

Determination of the mud weight window and optimum drilling route, using the comparison of different failure criteria: a case study

A new robust predictive model for lost circulation rate using convolutional neural network: A case study from Marun Oilfield

A major cause of some of serious issues encountered in a drilling project, including wellbore instability, formation damage, and drilling string stuck – which are known to increase non-productive time (NPT) and hence the drilling cost – is what we know as mud loss. The mud loss can be prevented or at least significantly reduced by taking proper measures beforehand provided the position and intensity of such loss can be properly predicted using an accurate predictor model. Accordingly, in this study, we used the convolutional neural network (CNN) and hybridized forms of multilayer extreme learning machine (MELM) and least square support vector machine (LSSVM) with the Cuckoo optimization algorithm (COA), particle swarm optimization (PSO), and genetic algorithm (GA) for modeling the mud loss rate based on drilling data, mud properties, and geological information of 305 drilling wells penetrating the Marun Oilfield. For this purpose, we began by a pre-processing step to attenuate the effect of noise using the Savitzky-Golay method. The whole set of available data was divided into the modeling (including 2300 data points) and the validation (including 483 data points) subsets. Next, the second generation of the non-dominated sorting genetic algorithm (NSGA-II) was applied to the modeling data to identify the most significant features for estimating the mud loss. The results showed that the prediction accuracy increased with the number of selected features, but the increase became negligible when the number of selected features exceeded 9. Accordingly, the following 9 features were selected as input to the intelligent algorithms (IAs): pump pressure, mud weight, fracture pressure, pore pressure, depth, gel 10 min/gel 10 s, fan600/fan300, flowrate, and formation type. Application of the hybrid algorithms and simple forms of LSSVM and CNN to the training data (80% of the modeling data, i.e. 1840 data points) showed that all of the models tend to underestimate the mud loss at higher mud loss rates, although the CNN exhibited lower underestimation levels. Error analysis on different models showed that the CNN provided for a significantly higher degree of accuracy, as compared to other models. The more accurate outputs of the hybrid LSSVM model than those of the simple LSSVM indicated the large potentials of metaheuristic algorithms for achieving optimal solutions. The lower error levels obtained with the CNN model in the testing phase highlighted the excellent generalizability of this model for unseen data. The more accurate predictions obtained with this model, rather than the other models, in the validation phase further proved this latter finding. Therefore, application of this method to other wells in the same field is highly recommended.

The Using of Unconventional Method depending on Well Power in the Mishrif Formation to Release Differential Stuck Pipe, Nahr Umr Oil Field, Southern Iraq

One of the main drilling problems is differential stuck pipe that lead to excessive non-productive time. The drill string usually stuck in the filter cake in permeable formations because it becomes immobile as a result of the drill string movement to connect a new pipe, make a trip, handle a drilling issue, etc. because there is a high permeability zone and a pressure difference between formation and filtering. So, without first releasing it by mechanical or chemical processes, the driller is unable to reciprocate or rotate it. Nahr Umr oil field was selected to study the problem of differential stuck pipe occurred while tripping out in the Hartha Formation and procedure steps to formulate the appropriate method to release the stuck string. Used unconventional method to free stuck pipe depend on well power in the Mishrif Formation after the conventional method failed to treat the problem. Then, the results were discussed with estimating stuck pipe depth and the force needed to release the string and comparison between the methods relative to the time and cost. Also, the result can be concluded that the appropriate mud weight is 1.27gm/cc. On the other hand, it was concluded that the unconventional method cheaper than the conventional method but it can be used if the formations contain oil or salt water only.

The Impact of Drilling Operation Parameters on the Lost Circulation Problems for Oil and Gas Wells Based on Field Data

Lost circulation, the loss of drilling fluid into holes or fractures throughout the geological formations being drilled, is the most expensive problem encountered in drilling operations for oil and gas wells. It is known that when there is a loss of circulation during the drilling process, the operation activities are delayed or stopped for curing or treatment, resulting in extra time known as non-productive time (NPT). The NPT issues during the drilling operations are the most important priorities for planning any drilling program. Therefore, the lost circulation is one of the main problems which can control the NPT. To mitigate the lost circulation problems, an integrated investigation is necessarily needed for the main parameters which enhance/downgrade the rate of lost circulation.
 Drilling through the Dammam and Hartha formation is typically accompanied with substantial nonproductive time due to severe and often entire drilling fluid losses. The majority of the curing this problem require time for preparation, placement or setting. In this research, a field study was carried out to investigate the effect of 6 drilling parameters on the rate of the lost circulation. The investigated parameters are the rate of penetration, weight on bit, stand pipe pressure, mud weight, flow rate, and the equivalent circulation density. Furthermore, the effect of lithology type on the lost circulation rate was clearly determined and correlated. In addition, a graphical representation of bit depth vs the real drilling time/date was constructed to compare the lossy wells vs non-lossy wells named frithogram. This study was based on field data for 350 wells drilled in Rumaila Oilfield, southern Iraq. The results of this study showed that if these six operating parameters are standard and controlled, the significance of lost circulation problems could be reduced. However, the main factors which are responsible for the lost circulation problems are lithology types, facies and/or diagenesis process, which mainly distinguish the carbonate formations.

A novel star polymer for regulating fluid loss in oil-based mud under high temperature conditions

Oil-Based Mud (OBM) is a highly effective drilling fluid system for challenging wellbore conditions such as water-sensitive and high-temperature drilling operations. To minimize filtration loss and achieve thin filter cake, fluid loss control additives are essential. This study introduces a star polymer as an alternative to traditional additives like gilsonite or linear polymer. The star polymer was synthesized using Reversible Addition Fragmentation Chain Transfer polymerization (RAFT) with hydrophilic and lipophilic monomers and crosslinkers. Mud properties such as rheology, fluid loss, mud cake thickness, and emulsion stability were evaluated for OBM and compared with commercially available fluid loss control additives at simulated downhole pressure and temperatures. Spectroscopy and thermogravimetric analysis were used to determine the chemical architecture and thermal stability of the star polymer. The results showed that the star polymer demonstrated thermal stability up to 225 °C, resulting in a small amount of fluid loss volume and ultra-thin filter cake for OBM systems with mud weight at 10.5 ppg and 13.4 ppg. Furthermore, the star polymer improved emulsion stability without significantly affecting plastic viscosity (PV), unlike commercially available fluid loss control additives. The results were consistent for both diesel and synthetic oil-based mud systems.

Assessing the Minimum Safe Drilling Mud Weight to Avoid Damage Zone by Frictional-Strengthening Cohesion-Weakening Models

Assessing the Minimum Safe Drilling Mud Weight to Avoid Damage Zone by Frictional-Strengthening Cohesion-Weakening Models

A machine learning approach for the prediction of pore pressure using well log data of Hikurangi Tuaheni Zone of IODP Expedition 372, New Zealand

Pore pressure (PP) information plays an important role in analysing the geomechanical properties of the reservoir and hydrocarbon field development. PP prediction is an essential requirement to ensure safe drilling operations and it is a fundamental input for well design, and mud weight estimation for wellbore stability. However, the pore pressure trend prediction in complex geological provinces is challenging particularly at oceanic slope setting, where sedimentation rate is relatively high and PP can be driven by various complex geo-processes. To overcome these difficulties, an advanced machine learning (ML) tool is implemented in combination with empirical methods. The empirical method for PP prediction is comprised of data pre-processing and model establishment stage. Eaton's method and Porosity method have been used for PP calculation of the well U1517A located at Tuaheni Landslide Complex of Hikurangi Subduction zone of IODP expedition 372. Gamma-ray, sonic travel time, bulk density and sonic derived porosity are extracted from well log data for the theoretical framework construction. The normal compaction trend (NCT) curve analysis is used to check the optimum fitting of the low permeable zone data. The statistical analysis is done using the histogram analysis and Pearson correlation coefficient matrix with PP data series to identify potential input combinations for ML-based predictive model development. The dataset is prepared and divided into two parts: Training and Testing. The PP data and well log of borehole U1517A is pre-processed to scale in between [-1, +1] to fit into the input range of the non-linear activation/transfer function of the decision tree regression model. The Decision Tree Regression (DTR) algorithm is built and compared to the model performance to predict the PP and identify the overpressure zone in Hikurangi Tuaheni Zone of IODP Expedition 372.

Geomechanics Analysis of Well Drilling Instability: A Review

Wellbore instability is a significant problem faced during drilling operations and causes loss of circulation, caving, stuck pipe, and well kick or blowout. These problems take extra time to treat and increase the Nonproductive Time (NPT). This paper aims to review the factors that influence the stability of wellbores and know the methods that have been reached to reduce them. Based on a current survey, the factors that affect the stability of the wellbore are far-field stress, rock mechanical properties, natural fractures, pore pressure, wellbore trajectory, drilling fluid chemicals, mobile formations, naturally over-pressured shale collapse, mud weight, temperature, and time. Also, the most suitable ways to reduce wellbore instability are selecting the appropriate drilling mud and maintenance, reducing the Equivalent Circulating Density (ECD) by using suitable hydraulics, selecting hole-trajectory, and compatibility of drilling fluid with the formation being drilled. Moreover, factors such as (minimizing time spent open hole, using offset-well data, and monitoring trend changes (torque, circulating pressure, drag) must be considered. As a result of this paper, wellbore instability problems can be controlled by careful study of reservoirs to find the solutions to some issues or reduce them.

1D Geomechanical Model For Wellbore Stability in Z Field, Y Well Sanga Sanga Working Area, Kutai Basin

This research is about 1D geomechanical model for wellbore stability in Z Field, Y Well Sanga Sanga Working Area, Kutai Basin where wells have been drilled. The Purpose of this Research is to analyze the stability of the well starting from knowing the stress regime that occurs, predicting the occurrence of wellbore failure, and determining safe mud weight window for next drilling. The method use in this Research is a numerical modelling method using log data and drilling data that has been obtained and then managed using Techlog Software. The result of this Research show the magnitude of mechanical properties of the rock that have been obtained, then in general the stress regime that occurs in the Z Field formation is the normal regime even though the strike slip and reverse regime are inserted at a certain depth, then based on the prediction results of failure in this well is wide breakout, which in general occurs in lithology with sandstone, finaly safe mud weight window can be estimated properly, so that it can be used for further well drilling. This research is about 1D geomechanical model for wellbore stability in Z Field, Y Well Sanga Sanga Working Area, Kutai Basin where wells have been drilled. The Purpose of this Research is to analyze the stability of the well starting from knowing the value of the mechanical properties of the formation, the stress regime that occurs, predicting the occurrence of wellbore failure, and determining safe mud weight window for next drilling. The method use in this Research is a numerical modelling method using log data and drilling data that has been obtained and then managed using Techlog Software. The result of this Research show the magnitude of mechanical properties of the rock that have been obtained, then in general the stress regime that occurs in the Z Field formation is the normal regime even though the strike slip and reverse regime are inserted at a certain depth, then based on the prediction results of failure in this well is wide breakout, which in general occurs in lithology with sandstone, finaly safe mud weight window can be estimated properly, so that it can be used for further well drilling.

A Developed Robust Model and Artificial Intelligence Techniques to Predict Drilling Fluid Density and Equivalent Circulation Density in Real Time.

When drilling deep wells, it is important to regulate the formation pressure and prevent kicks. This is achieved by controlling the equivalent circulation density (ECD), which becomes crucial in high-pressure and high-temperature wells. ECD is particularly important in formations where the pore pressure and fracture pressure are close to each other (narrow windows). However, the current methods for measuring ECD using downhole sensors can be expensive and limited by operational constraints such as high pressure and temperature. Therefore, to overcome this challenge, two novel models named ECDeffc.m and MWeffc.m were developed to predict ECD and mud weight (MW) from surface-drilling parameters, including standpipe pressure, rate of penetration, drill string rotation, and mud properties. In addition, by utilizing an artificial neural network (ANN) and a support vector machine (SVM), ECD was estimated with a correlation coefficient of 0.9947 and an average absolute percentage error of 0.23%. Meanwhile, a decision tree (DT) was employed to estimate MW with a correlation coefficient of 0.9353 and an average absolute percentage error of 1.66%. The two novel models were compared with artificial intelligence (AI) techniques to evaluate the developed models. The results proved that the two novel models were more accurate with the value obtained from pressure-while-drilling (PWD) tools. These models can be utilized during well design and while drilling operations are in progress to evaluate and monitor the appropriate mud weight and equivalent circulation density to save time and money, by eliminating the need for expensive downhole equipment and commercial software.

Comparative study of bentonite and Ikwo clay for oil-based drilling mud formulation

As the demand for energy rises, drilling activities and the use of drilling mud by oil and gas industries keep increasing, leading to large consumption and high importation cost of drilling mud materials. The study is conducted to replace high cost of drilling mud materials (bentonite) which is a burden on the companies and our economy at large, with respect to foreign exchange challenge. Locally sourced drilling mud material, clay precisely, from Ebonyi State, particularly Ikwo clay, is yet to be explored as alternative to commercial bentonite. The specific objective of this study is to analyse and compare Ikwo clay in replacement to commercial bentonite as viscosifiers for formulating oil-based drilling mud. The driving motivation is to utilize local content (Ikwo clay) which is in abundance and has remained unprocessed substitute for bentonite. The clay undergoes activation using sulfuric acid (H2SO4) following conventional procedures. XRF spectrophotometers determine chemical compositions and functional groups of bentonite and clay samples. Oil-based muds are separately formulated using bentonite and Ikwo clay additives. The mud weight, pH, apparent viscosity, plastic viscosity, yield point, and gel strength of the respective muds are evaluated. The interactive effects of the process variable on viscosities of each mud sample are determined, and the viscosity of the respective muds was optimized using the Central Composite Design (CCD) tool of Design Expert software (version 12) based on RSM. Activation of the local clay improved its properties, as results of the chemical composition revealed that Ikwo clay contains exchangeable cation like potassium ion (K+)) and an essential metal (iron (iii) ion (Fe3+)), with the presence of alumina signifying its suitability in terms of plasticity and swelling ability. The Ikwo clay mud had a higher sample mud weight than the bentonite mud, which indicates that Ikwo clay can manage formation pressure and improve wellbore stability. The optimum viscosities of the oil-based muds with bentonite and Ikwo clay additives are obtained at 15.49 cP and 18.82 cP, respectively. The optimum values are realized at a bentonite/Ikwo clay dosage of 8 wt%, a temperature of 313 K and a mixing time of 35 min. The novelty of Ikwo clay compared to imported bentonite is its ability to offer structural stability to drilling mud (Rana et al., 2020). This feature is used to control wellbore instability during drilling operations. Ikwo clay is a good fit for bentonite replacement as a viscosifier in formulating drilling mud.

Numerical Simulation of Pore Pressure Change Caused by Hydrocarbon Generation in Chezhen Sag and Its Influence on Hydrocarbon Accumulation

The pore fluid pressure is important for the generation, migration, and accumulation of hydrocarbons. The Chezhen Sag region in the Bohai Bay Basin is typically characterized by pore fluid overpressure, which is the difference between the pore fluid pressure and the hydrostatic pore pressure. The formation mechanisms of pore overpressure and the accumulation regularity of the “upper source-lower reservoir” type in this region remain unknown. In order to investigate these problems, based on the existing seismic, logging data, and regional tectonic stress environment, we established a two-dimensional finite element model to simulate the fluid–solid coupling processes in the Chegu 25 block of the Chezhen depression. We calculated the abnormal overpressure generated at the source rock during hydrocarbon generation and the processes of hydrocarbon migration and accumulation along the faults and analyzed the dynamic conditions of the hydrocarbon downward accumulation. The results showed that overpressure could accelerate the migration of hydrocarbon and improve the efficiency of hydrocarbon accumulation. When the overpressure was too large, tensile fractures and shear fractures could occur, resulting in hydrocarbon dissipation, and changing the results of the oil and gas accumulation. The overpressure at the source rock was mainly caused by hydrocarbon generation, while the overpressure at the reservoir was primarily created by unbalanced compaction. As the dominant channel of hydrocarbon migration that exists, overpressure will change the direction and path of hydrocarbon migration in the fault. Therefore, the high permeability of the fault and the existence of pore fluid overpressure can explain the “upper source-lower reservoir” hydrocarbon accumulation model strongly explained the high permeability of faults and the presence of overpressure. The simulated overpressure results were also in good agreement with the mud weight equivalent overpressure and the drill stem tests (DSTs).

Statistical analysis of past kicks and blowouts occurred in a Middle Eastern oilfield

In 2017, blowouts and then explosions occurred in a Middle Eastern oilfield. A root cause of the incident is lack of study and investigation of the past kicks and blowouts data. Therefore, as a pioneer work in the region, data gathering and analysis of past kicks and blowouts were made in the studied oilfield to learn lessons and find gaps. Out of the 149 drilled wells, a total of 117 kicks and three (3) blowouts occurred. In this work, a list of drilling parameters to be considered in data gathering and analysis were suggested as a guideline for future works elsewhere. The statistical analysis not only showed all the three exploration wells kicked which is not a surprise, but it also showed that 39 out of 146 (26.71%) also experienced kicks during reservoir drilling. The large number of kicks in development wells proved that possibility of kick occurrence in development wells is not low. In exploration wells, the predominant kick causes were gas-cut mud and insufficient mud weight which indicates the necessity of using pressure while drilling in addition to drilling rate control systems in exploration wells. However, in development wells, lost circulation was the predominant kick cause indicating the necessity of using low-weight drilling fluids and managed pressure drilling systems. The direct role of human error exists at least in 60% of kicks occurred in this field, which shows the great importance of improved drilling personnel training. Although only 3.45% kicks in development wells occurred due to improper hole fill-up during tripping, this cause should not only be deemed trivial, but it should also be taken seriously as being the cause of the blowout. The 2.56% possibility of kick conversion to blowouts and 67% risk of blowout conversion to explosion emphasize the necessity of maintaining primary well control and using efficient and early kick detection systems. Bullhead was the more commonly used method than standard well control methods; as this kill method may not always be safe, its application should be revised.

Investigation of collapse pressure in layered formations based on a continuous anisotropic rock strength criterion

Wellbore instability severely constrains the exploration and development of shale gas. In order to evaluate the impacts of anisotropy and water on wellbore instability, three different types of criteria are fitted to strength data of LMX shales with different moisture contents. A new model of transversely isotropic borehole stability considering compliance incremental tensor induced by natural fractures is proposed, then a more preferred drilling direction is performed by the new model. Results indicated that, Pariseau’s model is more attractive in predicting shale strength, including the difference of strength between vertical bedding and parallel bedding. Based on Pariseau’s model, the prediction accuracy of shale strength is improved by 33.04%. The Pariseau’ model gives a disparate collapse pressure from Jæger’s weak plane criterion, the most unstable drilling area shifts from northeast and southwest to the central area corresponding to relatively lower inclination. The collapse pressure only decreased by 0.55 MPa with considering the anisotropy of elastic parameters, but the strength criteria have a distinct influence. Compared with the results predicted by Jæger’s plane of weakness, the collapse pressure increased by 8.55 MPa using Pariseau’ model. Besides, invasion of water in bedding plane will aggravate borehole instability, especially in late drilling period, collapse pressure for the vertical wellbore increased by 6.35 MPa. Shale strength depends not only on the hydrostatic pressure and orientation angle, but also on the water content, which should be considered in mud weight design and well trajectory optimum.

Fully coupled two-phase hydro-mechanical model for wellbore stability analysis in tight gas formations considering the variation of rock mechanical parameters

Tight gas reservoirs are typically characterized by the unsaturated case that contains both water and gas fluids. However, previous wellbore stability analysis usually assumes that the porous rock is only saturated with a single fluid, thereby failing to describe the two-phase seepage behavior and the change in water content on the mechanical properties of the unsaturated rock, resulting in inaccurate mud weight predictions from wellbore stability analysis. Therefore, this paper aims to propose a fully coupled two-phase hydro-mechanical model that is available to obtain the change in water saturation and incorporates the variation of rock mechanical parameters with water saturation into the coupling relationship to assess the wellbore stability. By comparing five models representing the saturated and various unsaturated conditions, the influences of two-phase seepage and rock mechanical parameters on the evolution of pore pressure, stresses and wellbore stability are clarified. Finally, the influences of wellbore pressure, initial water saturation, porosity, and permeability on wellbore stability are discussed. The results show that the unsaturated pore pressure is influenced not only by the compressibility and viscosity of the fluid, but also by the saturation level and wellbore pressure. The difference in effective stresses is explained by the difference in poroelastic effect between saturated and unsaturated conditions. The decrease in elastic modulus and Poisson's ratio affects the effective tangential stress, while the increase in Biot's coefficient affects the effective radial stress. The prediction result of the present model that considers the combined effects of the two-phase hydro-mechanical coupling and the variation of all five mechanical parameters with water content is more consistent with the real borehole diameter curve. In the field case of the presented tight gas reservoir, the excessive overbalanced pressure is detrimental to wellbore stability, even worse than the underbalanced pressure. A near-balanced pressure may be the most beneficial for maintaining wellbore stability and protecting tight gas reservoirs. The higher risk of wellbore instability may correspond to formations with relatively high permeability, high porosity and high gas saturation.