Safe Mud Weight Research Articles

Quantitative risk assessment for Optimum Mud weight window design: A case study

Deterministic approach in selecting mud weight window at pre-drill condition has uncertainties for wellbore stability analysis. Quantitative Risk Assessment (QRA) technique has been successfully used in pre-drill condition addressing the variability of input parameters like, principal stresses and rock properties. The output of this process recommends the mud weight window with probability of drilling success which can avoid the wellbore collapse and lost circulation events. This kind of stochastic approach to predict safe mud weight window can assure stable wellbore with significant cost effectiveness related to drilling success.A workflow to perform QRAis demonstrated which starts with derivation of stresses in deviated wellbore. Seismic velocity data has been used to estimate pore pressure and validated with formation pressure obtained from reservoir characterization instrument data for a study well located in east coast of India. Mogi-Coulomb failure criteria have been used to estimate required mud weights to avoid shear failure. Mud weights required to avoid lost circulation has also been calculated. Then QRA has been accomplished to predict safe mud weight window sat specific depth within 14 ¾” hole and borehole section (12 ¼”). The results have been validated with real data set and based on that exercise, mechanical earth model and uncertainty associated with input parameters for study well has been established. A feasibility study has been performed to check the viability of drilling a near horizontal (75ο inclination) well along the reservoir unit with azimuth of 250ο. The wellbore collapse and lost circulation curves intersect near 35% probability of success point. The results of QRA indicate that the inclination and azimuth would not be favorable for successful drilling in this scenario. Based on these QRA, recommendations have been made in data acquisition plan for future drilling to reduce the variability in input parameters and successful completion of well.

Assessing borehole stability in bedding-parallel strata: Validity of three models

Bedding planes have crucial influence on borehole stability and there have been several models accounting for this kind of effect. However, few articles have assessed the difference or validity of these models, especially for the common-used elastic models that consider anisotropic strength of bedding rocks. Three models (including an empirical one), named Model A, Model B and Model C, are illustrated and compared here. With simplification and modification, calculation programs are developed to figure out the similarities and differences of collapse area and collapse pressure that determined by different models. Then, according to the field data, the applicability and validity of each model are carefully analyzed. It reveals that the results predicted by Model A are relatively reasonable compared to the other two models; the predictions of Model B can explain the collapse area extends perpendicularly to the beddings around a borehole, but cannot give the accurate collapse pressure in many cases because of the exchange of principal stresses on the borehole wall; the predictions of Model C are also not precise enough for its only taking the effects of beddings into account without the in-situ stress. Besides, the fracture pressure for boreholes of arbitrary orientations considering strength anisotropy of rocks is further studied, which is incorporated to gain a recommended model for the calculating of the safe mud weight window.

Determination of safe mud weight window based on well logging data using artificial intelligence

ABSTRACTIdentification of different stresses applied to the environment surrounding wellbore via different processes, and combining these data with mechanical parameters of common formations in hydrocarbon reservoirs comprise a key for addressing a wide range of costly problems and issues in the oil industry. In the present research, first, an attempt was made to construct mechanical earth model based on well logging data, elastic moduli of rock, and appropriate failure criteria for the final purpose of calculating and determining safe mud weight window (SMWW). Finally, appropriate artificial intelligence and machine-learning algorithms were used to establish a relationship between well logging data and SMWW, which could be used to calculate and predict SMWW without using associated relationships with the mechanical earth model. This might end up with a decreased number of required parameters for calculating SMWW, including uniaxial compressive strength. In the present research, the learning process was conducted using datasets from three wells, two of which provided training data, with the other one used as testing data. The prepared model was finally used to predict corresponding pressures to SMWW and baseline pressures for hydraulic fracturing operation. The model gave a coefficient of determination of 0.93 when applied to the testing data using support vector regression algorithm with radial basis function kernel, indicating large capabilities of this algorithm in predicting non-foreseen data.

Selection of failure criteria for estimation of safe mud weights in a tight gas sand reservoir

In an undisturbed state, a stress field present in the earth consists of virgin stress in the rock. A change in this stress occurs due to drilling the borehole and disturbing the original stress by either introducing drilling fluids or by getting reservoir flow in the borehole. This change in stress leads to a classical problem of well bore instability. To resolve wellbore instability, evaluation of rock mechanical properties and in-situ stresses is essential. The output of this study is evaluation of safe mud weights which is critical to safe drilling. If the mud weight used for drilling is higher than those predicted by the failure criteria, the mud percolates into the formation, causing tensile failure (fracture stress) which induces fluid losses. Conversely, the lower mud weight results in shear failure (collapse stress) of rock leading to borehole breakout and collapse of wellbore. Three types of failure criteria i.e. Mohr-Coulomb, Mogi-Coulomb and Modified Lade are considered in this study. The stress around the wellbore is obtained using Kirsch equation. The input parameters required for estimating the safe mud weights from these failure criteria are rock mechanical properties, friction angle and cohesion which are estimated from well log data. The rock mechanical properties like Poisson's ratio and Uniaxial Compressive Strength are computed using sonic derived compressional and shear velocities. Internal friction angle is obtained using gamma ray log and is validated with core data before it is used in further analysis. The failure criteria have been applied to two wells located in a field in Gulf of Oman. The analyses show that the Mohr-Coulomb overestimates the mud weight while the Modified Lade criterion underestimates it. The result for a tight gas sand reservoir suggests that the Mogi-Coulomb predicts better mud weight values that are in agreement to those measured values chosen for drilling.

The Maximum-Allowable Well Depth While Drilling of Extended-Reach Wells Targeting to Offshore Depleted Reservoirs

In depleted offshore reservoirs, pore pressure declines and consequently horizontal in-situ stresses decrease as well. This causes a very limited well depth for extended-reach drilling targeting to offshore depleted reservoirs. In this paper, based on analyzing the safe mud weight window of the depleted offshore reservoirs, a model of predicting the Maximum Allowable Measured Depth (MAMD) for extended-reach drilling targeting to offshore depleted reservoirs is developed. Meanwhile, the numerical method of the model is proposed, and the key affecting factors of the MAMD are also investigated. The results show the pore pressure depletion has obvious effects on the MAMD. With the depletion of pore pressure, the safe mud weight window appears narrower and even disappears, consequently the predicted MAMD becomes shorter. For a normal regime depositional environment in the depleted reservoirs, it may be impossible to drill with conventional drilling method in the nearby directions of the maximum horizontal in-situ stress, while it may be much safer and attain a long MAMD when drilling in the directions near the minimum horizontal in-situ stress. Moreover, the MAMD will decrease with the increase of Poisson’s ratio and Biot’s parameter, and its response to Poisson’s ratio is more obvious. For a specific target depleted reservoir, the extended-reach drilling with a high borehole inclination may have a longer MAMD than that with a low borehole inclination. This paper presents a method for promoting the design of extended-reach drilling targeting to offshore depleted reservoirs.

A Comprehensive Wellbore Stability Model Considering Poroelastic and Thermal Effects for Inclined Wellbores in Deepwater Drilling

Exploring and developing oil and gas in deepwater field is an important trend of the oil and gas industry. Development of deepwater oil and gas fields from a platform always requires a number of directional wells or extended reach wells targeting to different depth of water in various azimuth. Drilling of these wells is mostly associated with a series of wellbore instability problems that are not encountered in onshore or shallow water drilling. In the past decades, a number of studies on wellbore stability have been conducted. However, few of the models are specific for wellbore stability of the inclined deepwater wellbores. In this work, a comprehensive wellbore stability model considering poroelastic and thermal effects for inclined wellbores in deepwater drilling is developed. The numerical method of the model is also presented. The study shows that for a strike-slip stress regime, the wellbore with a low inclination poses more risk of wellbore instability than the wellbore with a high inclination. It also shows that cooling the wellbore will stabilize the wellbore while excessive cooling could cause wellbore fracturing, and the poroelastic effect could narrow the safe mud weight window. The highest wellbore collapse pressure gradients at all of the analyzed directions are obtained when poroelastic effect is taken into account meanwhile the lowest wellbore fracture pressure gradients at all of the analyzed directions are obtained when both of poroelastic effect and thermal effect are taken into account. For safe drilling in deepwater, both of thermal and poroelastic effects are preferably considered to estimate wellbore stability. The model provides a practical tool to predict the stability of inclined wellbores in deepwater drilling.

The performance of stochastic designs in wellbore drilling operations

Wellbore drilling operations frequently entail the combination of a wide range of variables. This is underpinned by the numerous factors that must be considered in order to ensure safety and productivity. The heterogeneity and sometimes unpredictable behaviour of underground systems increases the sensitivity of drilling activities. Quite often the operating parameters are set to certify effective and efficient working processes. However, failings in the management of drilling and operating conditions sometimes result in catastrophes such as well collapse or fluid loss. This study investigates the hypothesis that optimising drilling parameters, for instance mud pressure, is crucial if the margin of safe operating conditions is to be properly defined. This was conducted via two main stages: first a deterministic analysis—where the operating conditions are predicted by conventional modelling procedures—and then a probabilistic analysis via stochastic simulations—where a window of optimised operation conditions can be obtained. The outcome of additional stochastic analyses can be used to improve results derived from deterministic models. The incorporation of stochastic techniques in the evaluation of wellbore instability indicates that margins of the safe mud weight window are adjustable and can be extended considerably beyond the limits of deterministic predictions. The safe mud window is influenced and hence can also be amended based on the degree of uncertainty and the permissible level of confidence. The refinement of results from deterministic analyses by additional stochastic simulations is vital if a more accurate and reliable representation of safe in situ and operating conditions is to be obtained during wellbore operations.

Evaluating Single-Parameter parabolic failure criterion in wellbore stability analysis

Shear (breakout) and tensile (breakdown) rock failures are the most common wellbore instability challenges that may occur during drilling operations. In order to prevent these problems, stress concentration on borehole wall should be calculated using an accurate mechanical earth model (MEM). A comprehensive MEM should incorporate a failure criterion in order to predict the safe mud window. Thus far, several failure criteria have been utilized and discussed for the wellbore stability analysis in literature, but there is not any commonly accepted one in petroleum industry that generates the most reliable results.In this study, first, Single-Parameter parabolic failure criterion was evaluated by reproducing unconfined compressive strength (UCS) from available triaxial tests. The results were then compared with measured uniaxial tests as well. It was found that Single-Parameter parabolic failure criterion is more accurate in reproducing UCS values than Mohr-Coulomb and Hoek-Brown. In the next step, Single-Parameter parabolic failure criterion was used in mechanical earth modeling of a real case study in Persian Gulf, Iran. Results indicated that Single-Parameter parabolic failure criterion overestimates the rock strength in a very low confining pressures, making it inapplicable for effective confining pressures of zero or very low. It also reflects higher breakout limits in very high UCS values which led to a misleading narrower safe mud weight window compared to other failure criteria.

Reservoir Geomechanics Parameters Estimation Using Well Logs and Seismic Reflection Data: Insight from Sinjhoro Field, Lower Indus Basin, Pakistan

Nowadays, the failure to understand a hydrocarbon field’s geomechanics signifies an expensive risk in several complex drilling, well completion, and exploitation processes. Thus, the precise estimation of reservoir geomechanical parameters can lessen that risk and provide benefits all the way through oil and gas field’s lifespan. Several factors like compaction of sediments, lithology characteristics, overburden, hydrocarbon and capillary entry pressures add substantial effects to cause of abnormal pressure and resultantly affect the well design and stability. The aim of current study is to compute geomechanical properties such as overburden pressure and vertical effective stress and finally to define the lower and upper bounds (breakout and breakdown limits) of safe mud weight window by determining the pore pressure and fracture gradient for pre-drill planning in future exploration and real time monitoring in the Sinjhoro Field, the Lower Indus Basin of Pakistan. Current studies examined the wireline logs data of four wells drilled in the Cretaceous reservoir sands and seismic reflection data-based P wave velocity functions and describes a workflow for reliable estimation of geomechanical parameters. Our results revealed that high pore pressure (5000–7000 Psi) is predicated at reservoir zone and it increases in southeast directions. On the basis of pore pressure and fracture pressure, both lower (5388.89 Psi) and upper (6572 Psi) bounds for safe mud weight window are defined for safe drilling and well stability that will help future exploration and development in the Sinjhoro field. Both overburden pressure and vertical effective stress increase uniformly with depth but spatial variations are nonlinear and increasing in northwest directions. The present work will support the petroleum industry by reducing the borehole drilling cost with no compromising on environment and safety concerns.

Full waveform acoustic data as an aid in reducing uncertainty of mud window design in the absence of leak-off test

Creating a mechanical earth model (MEM) during well planning, and real-time revision has proven to be extremely valuable to reach the total depth of well safely with least instability problems. One of the major components of MEM is determining horizontal stresses with reasonable accuracy. Leak-off and minifrac tests are commonly used for calibrating horizontal stresses. However, these tests are not performed in many oil and gas wellbores since the execution of such tests is expensive, time-consuming and may adversely impact the integrity of a wellbore.In this study, we presented a methodology to accurately estimate the magnitudes and directions of horizontal stresses without using any leak-off test data. In this methodology, full waveform acoustic data is acquired after drilling and utilized in order to calibrate maximum horizontal stress. The presented methodology was applied to develop an MEM in a wellbore with no leak-off test data. Processing of full waveform acoustic data resulted in three far-field shear moduli. Then based on the acoustoelastic effect, maximum horizontal stress was calibrated. Moreover, maximum horizontal stress direction was detected using this methodology through the whole wellbore path. The application of this methodology resulted in constraining the MEM and increasing the accuracy of the calculated horizontal stresses, accordingly a more reliable safe mud weight window was predicted. This demonstrates that the presented methodology is a reliable approach to analyze wellbore stability in the absence of leak-off test.

Kicks Controlling Techniques Efficiency in Term of Time

Kicks are the result of under balance drilling operation. Time consumed to control the kicks will be different in depending on the controlling technique. Drilling fluid considered as a first barrier to control formation pressure and well kicks. Any advance in drilling fluids leads to more controlled operation in term of time. This paper will follow the general increasing profile of pressure before entering the reservoir. Both methods of well controlling technique; circulating techniques and non-circulating have been implemented in many oil blocks. The process of designing and casing selection, setting depth and many other issues is predominately dependent on the utilization of accurate values of formation pressure. Formation pressures used to design safe mud weights to overcome fracturing the formation and prevent well kicks. Hence the emphasis has been placed on the practical utilization of kicks pressure near the reservoir. The presented relationships will help the engineer to better understand lithological columns and reduce potential hole problems during the kick appearance. Selecting the best well controlling practical method can lead to not harming the reservoir and more production later. Changes in some drilling fluid properties have been proposed with increasing the depth without damaging the reservoir. Suggestions in relation to the casing setting point of the intermediate section are also proposed. Standard equations with proper modification for gases and safety margin have been proposed for the future drilling operation in oil fields above the reservoir.

Finite element analysis for inclined wellbore stability of transversely iso-tropic rock with HMCD coupling based on weak plane strength criterion

The finite element analysis (FEA) technology by hydraulic-mechanical-chemical-damage (HMCD) coupling is proposed in this paper for inclined wellbore stability analysis of water-sensitive and laminated rock, developed basing on the recently established FEA technology for transversely isotropic rock with hydraulic-mechanical-damage (HMD) coupling. The chemical activity of the drilling fluid is considered as phenomenological hydration behavior, the moisture content and parameters of rock considering hydration could be determined with time. The finite element (FE) solutions of numerical wellbore model considering the chemical activity of drilling fluid, damage tensor calculation and weak plane strength criterion for transversely isotropic rock are developed for researching the wellbore failure characteristics and computing the time-dependent collapse and fracture pressure of laminated rock as shale reservoirs. A three-dimensional FE model and elastic solid deformation and seepage flow coupled equations are developed, and the damage tensor calculation technology for transversely isotropic rock are realized by introducing effect of the hydration and the stress state under the current load. The proposed method utilizing weak plane strength criterion fully reflects the strength parameters in rock matrix and weak plane. To the end, an effective and reliable numerically three-step FEA strategy is well established for wellbore stability analysis. Numerical examples are given to show that the proposed method can establish efficient and applicable FE model and be suitable for analyzing the timedependent solutions of pore pressure and stresses, and the evolution region considering the hydration surrounding wellbore, furthermore to compute the collapse cycling time and the safe mud weight for collapse and fracture pressure of transversely isotropic rock.

Finite element analysis for wellbore stability of transversely isotropic rock with hydraulic-mechanical-damage coupling

The finite element analysis (FEA) technology by hydraulic-mechanical-damage (HMD) coupling is proposed in this paper for wellbore stability analysis of transversely isotropic rock, developed basing on the recently established FEA technology for isotropic rock. The finite element (FE) solutions of numerical wellbore model, damage tensor calculation and Pariseau strength criterion for transversely isotropic rock are developed for researching the wellbore failure characteristics and computing the collapse and fracture pressure of laminated rock as shale reservoirs. The classic Biot constitutive for rock as porous medium is introduced to establish a set of FE equations coupling with elastic solid deformation and seepage flow. To be in accord with the inclined wellbore situation, the coordinate transformation for global, wellbore, in-situ stress and transversely isotropic formation coordinate systems is established for describing the in-situ stress field and the results in laminated rock. To be in accord with the practical situation, a three-dimensional FE model is developed, in which several other auxiliary technologies are comprehensively utilized, e.g., the typical Weibull distribution function for heterogeneous material description and adaptive technology for mesh refinement. The damage tensor calculation technology for transversely isotropic rock are realized from the well-developed continuum damage variable of isotropic rock. The rock is subsequently developed into a novel conceptual and practical model considering the stress and permeability with the damage. The proposed method utilizing Pariseau strength criterion fully reflects the strength parameters parallel or perpendicular to bedding of the transversely isotropic rock. To this end, an effective and reliable numerically three-step FEA strategy is well established. Numerical examples are given to show that the proposed method can establish efficient and applicable FE model and be suitable for analyzing the state of pore pressure and stress surrounding wellbore, furthermore to demonstrate the effectiveness and reliability of the instability analysis of wellbore failure region and the safe mud weight computation for collapse and fracture pressure of transversely isotropic rock.

Strength varying laws of rock soaked with supercritical CO2 drilling fluid and the prediction of safe mud weight window

AbstractDrilling with supercritical CO2 (SC‐CO2) is an important new technology in petroleum engineering. The technology has many unique advantages for increasing the penetration rate and reducing formation damage, but it is a disadvantage for the wellbore stability. Therefore, an experimental technique has been developed to determine the safe mud weight window for drilling with SC‐CO2. More specifically, a special experimental apparatus has been designed to mimic the in situ temperature and pressure conditions during drilling operations. Different experiments were conducted using core samples soaked in SC‐CO2 and water‐based and oil‐based drilling fluids, respectively. Once the experimental measurements regressed, an empirical correlation, including time, temperature and pressure factors, was obtained to describe the compressive strength of the SC‐CO2‐soaked core samples. Further modifications were then made to the formulations used to calculate the densities of the drilling fluids at the corresponding burst and collapse pressure, respectively. In addition, the effects of dynamic cohesion, internal friction angle, and tensile strength on the wellbore stability were determined. Compared to the conventional water‐based and oil‐based drilling fluids, the SC‐CO2 drilling fluid performs better with respect to wellbore stability and has a wider safe mud weight window. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd.

A methodology for wellbore stability analysis in anisotropic formations: A case study from the Canning Basin, Western Australia

Stability analysis of directional wells has been a long standing issue due to complexity of geological settings and changes in the stress coordinate system when wells are being deviated. Although there have been many studies developing strategies to estimate the safe mud weight based on wellbore geometry and anisotropy of subsurface structures, the issue of instability in deviated wellbores has been widely experienced in many fields around the world, especially in Malaysia and Australia. The aim of this paper is to present a workflow for estimation of geomechanical parameters and stress states of boreholes drilled directionally into anisotropic formations. A case study from one of the wells drilled into tight shales was brought into attention to evaluate the application of the proposed methodology. The results obtained based on the interpretations of data and reports of similar incidents in the field indicated that the well was deviated due to drilling parallel to the direction of bedding planes, causing creation of blocky, and tabular cuttings with parallel surfaces at shakers. The Horizontal Transverse Isotropic (HTI) model was assumed and the ANNIE model, developed exclusively for shales, was considered for estimation of stiffness parameters required to fully characterize shale formations. Determination of the safe mud weight window using two well-known failure criteria with proven applications revealed that a three-dimensional failure criterion should have been applied to prevent the instability of the wellbore wall. In fact, following the underestimation of shear failure provided by the Mohr-Coulomb criterion, a 13 ppg mud would have been selected to save the well, which in turn could result in enlargements of the well and significant increase of completion costs. Although some practical conclusions are provided, more studies are required to evaluate the application of the proposed methodology in other wells worldwide.

Causes and deterrent of SWF in deep water of Mississippi prodelta areas

Shallow water flow (SWF) represents a costly drilling challenge in deep water. Establishing the differential pressure between the sand and the shale beds is the main objective. This is because most of the SWFs take place while penetrating the shale-sand boundaries. During compaction, sand's formation water influxes rapidly upward, whereas shale's dewatering is very sluggish. The differential pressure (ΔP) value ranges from 670 psi (1.2 ppg) to 50 psi (0.2 ppg) at variable depths. All ΔP's show high values at depth between 1000 ft and 3000 ft below mudline at any water depth, which is where most SWF takes place. ΔP noticeably increases near the mudline at greater water depth and can be the reason for the occasional conductor and wellhead sinking in the extra-deep water. Choosing correct mud-up values (ΔP) at depth is essential to combat SWF and avoid loss of circulation. The safe mud weight is calculated at different water depth and below the mudline depth. Algorithm and tabulations are systematically presented. Case histories from the Gulf of Mexico, where the upper Pleistocene-recent depositional fan was and is still active, are utilized.

Mechanical Earth Modeling for a vertical well drilled in a naturally fractured tight carbonate gas reservoir in the Persian Gulf

Abstract In this study, a comprehensive integrated geomechanical analysis has been performed for the construction of a one-dimensional Mechanical Earth Model (MEM) for a vertical well drilled in a naturally fractured tight carbonate gas reservoir of Oxfordian age in the Persian Gulf. The critical points touched upon in this manuscript are wellbore stability based on regional stress trends, rock deformation and mechanical stratigraphy and fracture study based on core samples and image logs for different lithologies. In addition to distribution of properties such as porosity and density, the model also incorporates pore pressures, state of stress, and rock mechanical properties associated with different faulting regimes. The MEM constructed for this particular well drilled in a tectonically complex HPHT region utilizes the dynamic data available in the form of logs, and using correlations, converts it to mechanical properties. The static data obtained from the core samples (for mainly tight limestone, organic rich argillaceous limestone and shale) are correlated to the properties obtained from the log analysis. With these correlations continuous profiles for the rock strength parameters are constructed to formulate equations which are used for the entire depth of the well. Based on these formation properties, regional stress trends and inferences drawn from wellbore breakouts, a comprehensive analysis for wellbore stability is carried out. Integrating the information about breakouts and natural fractures obtained from image logs, the stress concentration around the wellbore is studied to devise a safe mud weight window for the stability of the well.

Wellbore stability analysis and well path optimization based on the breakout width model and Mogi–Coulomb criterion

Abstract The failure criterion is a very important aspect of the wellbore stability analysis. But the Mohr–Coulomb criterion, a most commonly used failure criterion, is usually too conservative, due to the influence of the intermediate principal stress is not taken into account. Furthermore, the safe mud weight (MW) predicted by the conventional method is usually too high, due to the safe MW is predicted to yield no shear failure around the borehole. To solve the above problems, a semi-analytic model of wellbore stability analysis was proposed based on the stress distribution model, failure criteria and the breakout width model. The Mogi–Coulomb criterion, a new true-triaxial failure criterion, is involved in this semi-analytic model to determine the shear failure. The computational models of the lowest and highest safe MW are derived for different failure mode. The numerical method and flowchart of this semi-analytic model is provided, and a computer program is also developed in Matlab software. The impacts of the failure criteria, stress regimes and breakout width had been systematically analyzed. The optimal well path had been optimized for different stress regimes. The results showed that the lowest safe MW predicted by Mogi-Coulomb criterion is always lower than the Mohr-Coulomb criterion. The most stable drilling direction is along the minimum horizontal stress for the NF and NF–SS stress regimes, while it’s along the maximum horizontal stress in the SS, SS–RF and RF stress regimes. If the breakout width model is used, the lowest safe MW is always lower than the traditional method. The optimal well path and the wellbore stability are different between traditional and new methods. The most stable path is always the horizontal and deviated well for the traditional method, instead, it’s always the vertical well for this new method, it’s more consistent with the actual situation. These results also illustrated that an acceptable breakout is usually allowed to occur in the actual borehole, and the borehole still has enough arc to support the wellbore wall and to prevent collapse accident. Finally, both of the traditional and new methods are applied in the field case studies. The results showed that the lowest safe MW predicted by the integration of the breakout width model and Mogi–Coulomb criterion is closest to the real MW, instead, the lowest safe MW predicted by the traditional methods has exceeded the actual MW. It also illustrated the traditional method is too conservative in predicting wellbore stability, while this new method is more accurate for the integration of the breakout width model and MG–C criterion. This research can provide support to optimize the well path and MW in the drilling practices.

Application of quantitative risk assessment in wellbore stability analysis

Abstract Elastic and strength parameters, together with pore pressure and in-situ stresses are key parameters required to be known for determination of safe mud weight window (MWW) in vertical wellbores. Estimation of these parameters, however, is subjected to wide uncertainties mainly due to lack of adequate calibration information including lab and field test data. While there are literatures on the applications of probabilistic and risk analysis on wellbore stability evaluation, limited numbers of publications report on the impact of the chosen failure criteria in estimation of safe MWW under uncertain condition. In this study, data corresponding to a wellbore located in south part of Iran was analyzed using quantitative risk assessment to consider the effect of uncertainty on estimation of safe MWW using different failure criteria. The results indicated that Mogi–Coulomb and Hoek–Brown are more robust against the uncertainty of input parameters and mud weight used for this wellbore could have slightly been increased to reduce the shear failure of the borehole wall. The uncertainty in the input data might also be very critical for casing design when only a simple margin together with pore and fracture pressures are used to select the grade of the casing against burst or collapse loads. It was also noted based on sensitivity analysis that the maximum horizontal stress is the most effective parameter in estimation of MWW. This emphasizes the importance of a reliable estimation of in-situ stresses for safe drilling. The results presented here are based on a single case study, and further studies are still required to get any ultimate conclusion.

The role of moisture adsorption in wellbore stability of shale formations: Mechanism and modeling

Understanding the interaction of drilling fluid with a shale formation is a critical step to properly describe pore pressure distribution, which directly affects wellbore stability. In this study, the moisture adsorption to shale surfaces is investigated to identify the proper isotherm type curve. It is shown that the moisture content of shale is correlated with water activity using a multilayer adsorption theory. It is found that the GAB model (developed by Guggenheim, Anderson and De Boer) fairly well describes the adsorption process for the selected shale types. Accordingly, the moisture content is correlated with water activity through the GAB model, which is incorporated into the transport equations. The adsorption parameter signifies the characteristic of a specific shale that interacts with aqueous fluids and can be introduced as an index to characterize different shale formations.In this study, the constitutive equations are generalized to consider the case of non-ideal solutions. The coupled transport equations are solved using a finite difference method and numerical computations are conducted to predict the stability of the wellbore. Having developed a transient model that predicts the instantaneous moisture content around the wellbore, it is possible to update the rock compressive strength as a function of its moisture content using available empirical correlations in the literature. The results of this analysis indicate that the range of safe mud weight reduces substantially due to moisture adsorption. Comparisons of several compressive failure criteria indicate that stability reduction of the wellbore due to moisture transport is a common pattern based on the selected criteria. The results of this investigation assist in drilling fluid optimization and address wellbore stability issues in troublesome shale formations.