Mechanical Earth Model Research Articles

Fracturing index-based brittleness prediction from geophysical logging data: application to Longmaxi shale

Understanding of brittle and ductile behavior in low permeability shale formation is essential for optimizing the completion and stimulation treatment in a shale play. For the quantification of brittleness in shale, several methods for estimating fracability index have been proposed in the literature. However, uncertainties still exist in the interpretation of fracability to decide where to place perforation clusters, what magnitude of fracability index mean to hydraulic fracture initiation and propagation, and how brittleness is best defined. This study aims to clarify these unresolved issues by introducing improved fracturing-index (FI) model after modifying Yuan et al. (SPE J 2017:1–9, 2007) fracability-evaluation model (Frac). Comparing with the use of Frac, the improved FI accurately predicts the brittle and ductile regions and provide a comparatively better idea about favorable fracturing sweet spots at TOC rich zones with added advantages of estimating efficient fracture initiation and propagation, and resistant to proppant embedment. In this study, mechanical rock properties were derived through acoustic logging data to develop mechanical earth models and designed brittleness templates for differentiating brittle and ductile regions in Longmaxi gas shale reservoir. The effectiveness of designed brittleness templates was verified through improved FI evaluation model and pre-existing fractures. The obtained result revealed that the points of high FI fall exactly into the predicted brittle regions and the points of low FI fall exactly into the predicted ductile regions. The applicability of the designed brittleness templates was further verified through the data from offset Well J-3, laboratory testing of several outcrop samples, and shale sample from different origin with varying composition.

Geophysical data processing, rock property inversion, and geomechanical model building in a Midland Basin development project, Midland/Ector counties, Texas

The current state of the art for development of unconventional projects integrates geology, geophysics, and engineering into a comprehensive reservoir description. To that end, we have designed a comprehensive geophysical workflow to define the structure, stratigraphy, and rock mechanics of a stacked reservoir sequence in the west-central Midland Basin. The purpose of this paper is to describe the workflow. Portions of three seismic data sets were merged and processed anisotropically, preserving the full range of azimuths. Well control consisted of 35 compressional sonic logs, eight shear sonic logs, three sonic scanners, two lateral formation microimaging tools, and three cores. Nine newly drilled laterals are on production along with a large number of vertical legacy wells. Prestack inversion produced acoustic impedance, shear impedance, and density with good accuracy, although the compressional-to-shear-velocity ratio (VP/VS) was not stable. This volume was successfully recreated via neural net, which was also used to calculate total porosity and total water saturation volumes. Poisson's ratio, Young's modulus, and brittleness were created using standard equations and the neural net VP/VS volume. Structural attributes consisted of ant track, coherence, semblance, fault probability, and Kmin and Kmax curvature. The final step in this portion of the workflow was to create a high-resolution depth conversion velocity volume. A geocellular grid was built and populated with petrophysical volumes, lithology facies, and structural attributes. Principal stresses were incorporated by first establishing 1D mechanical earth models (MEMs) where appropriate log suites were available. These 1D MEMs were extended to a 3D MEM, although confidence in this volume is not high due to known pore pressure and closure stress reductions as a result of legacy vertical well production. Calibration incorporating new data as it becomes available is an ongoing task. An informative simulation using the MEM is frac models, which define the most optimal landing points for laterals. This MEM and the simulations it can perform have easily demonstrated the value of this integrated workflow.

Introducing optimized validated meshing system for wellbore stability analysis using 3D finite element method

Finite element method (FEM) as a powerful tool for studying stress and strain status is being extensively employed in geotechnical studies. As the initial and boundary conditions, element type, and meshing system heavily affect the accuracy and precision of the results obtained from FEM, in this research we present a novel approach which is optimized and validated by the results obtained from reality. At first a mechanical earth model (MEM) was constructed using different well logging data, results of core analysis, and drilling reports for one of the central Iranian carbonate reservoirs. Then, a depth range was selected in the pay zone of a vertical well for FEM simulation. The selected depth range consists of two different zones: the upper zone with normal faulting regime and the lower zone with strike-slip faulting regime.After studying different model sizes, mesh densities, element types, and boundary conditions, a cylindrical model consist of a combination of regular and irregular hexahedral elements for far-field region, and fine-grained tetrahedral elements for near-wellbore region was obtained. Two different approaches were selected for FEM modeling, in the first approach a pre-drilled well was considered in the model, and in the second approach the model geometry before drilling without a pre-drilled well was subjected to an initial state of stress, removal of the elements, and loading which simulate the drilling process using a certain mud weight. The results of simulating the state of strains around the wellbore using this meshing system in both FEM approaches had acceptable adoption with drilling data.

Case study of wellbore stability evaluation for the Mishrif Formation, Iraq

During drilling operations for the E oilfield in the Mishrif formation in southern Iraq, stuck pipe has been identified as a significant geomechanical problem for several wells. In this study, a 1-D mechanical earth model (MEM) of the Mishrif formation is compiled based on its state of stress and rock strength parameters, and is utilized to assess the contribution of borehole collapse leading to the stuck pipe problems. The results of this study show that wells characterized by stuck pipe are drilled along azimuths which promote wellbore collapse. Three different failure criteria, the Mohr-Coulomb, Mogi-Coulomb, and Modified Lade rock failure criteria, are investigated in order to determine feasible drilling trajectories (i.e. azimuths and inclinations) and mud pressure conditions for many different wells in the Mishrif Formation. If a specific azimuth for a well cannot be altered, an optimum inclination is recommended to reduce the severity of the borehole collapse. However, as the intermediate principal in-situ stress increases the optimum drilling inclination progressively changes. The presented study shows that 1-D MEMs are an important tool to both assess and address existing wellbore stability problems and to provide guidance for future well plans for better drilling efficiency by reducing non-productive time.

Petroleum geomechanics modelling in the Eastern Mediterranean basin: analysis and application of fault stress mechanics

A fault stress analysis of a typical gas field in the Eastern Mediterranean is presented. The objective of this study is to provide estimates of thein situstresses and pore pressure for populating a regional Mechanical Earth Model and to characterize the stability of faults under current and changing reservoir conditions. The fault stability analysis is based on the Mohr-Coulomb frictional faulting theory. The verticalin situstress is estimated using seismic and density data and the bounds of the horizontal stresses were determined for different fault regimes. The pore pressure for determining the effectivein situstresses is estimated using the Bowers pore pressure prediction method. Fault stress analysis is performed in a series of calculations and the results are plotted on Mohr diagrams for shear failure. The fault stress analysis is performed on a wide range of alternative azimuth orientations forSHmaxin order to capture the uncertainty on the actual orientation. Sensitivity with respect to reservoir pore pressure change suggests that pressure reduction in the reservoir improves the fault stress stability, ignoring in the current analysis any stress arching effects. Pore pressure increase decreases the normal stress on the fault leading to increasing risk of shear failure of the critically stressed faults. The case study examines eight faults on the Aphrodite gas field with the objective to characterize if the faults are active or remain dormant under current stress conditions and how the stability may change in reservoir injection or depletion conditions.

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.

Failure criterion effect on solid production prediction and selection of completion solution

Production of fines together with reservoir fluid is called solid production. It varies from a few grams or less per ton of reservoir fluid posing only minor problems, to catastrophic amount possibly leading to erosion and complete filling of the borehole. This paper assesses solid production potential in a carbonate gas reservoir located in the south of Iran. Petrophysical logs obtained from the vertical well were employed to construct mechanical earth model. Then, two failure criteria, i.e. Mohr–Coulomb and Mogi–Coulomb, were used to investigate the potential of solid production of the well in the initial and depleted conditions of the reservoir. Using these two criteria, we estimated critical collapse pressure and compared them to the reservoir pressure. Solid production occurs if collapse pressure is greater than pore pressure. Results indicate that the two failure criteria show different estimations of solid production potential of the studied reservoir. Mohr–Coulomb failure criterion estimated solid production in both initial and depleted conditions, where Mogi–Coulomb criterion predicted no solid production in the initial condition of reservoir. Based on Mogi–Coulomb criterion, the well may not require completion solutions like perforated liner, until at least 60% of reservoir pressure was depleted which leads to decrease in operation cost and time.

A novel approach to estimate the variations in stresses and fault state due to depletion of reservoirs

Geomechanical changes may occur in reservoirs due to production from reservoirs. Study of these changes has an important role in upcoming operations. Frictional equilibrium is one of the items that should be determined during the depletion as it may vary with respect to time. Pre-existing faults and fractures will slide in regions where there is no frictional equilibrium. Hence, it may be concluded that reduction in pore pressure can initiate the sliding of faults. Whereas, it is also possible that faults will not exist after a certain time as production recovers the equilibrium. Casing shearing or lost circulation may be occurred due to faulting. In reservoirs which depletion leads to frictional equilibrium, decrease of fractures and faults leads to some variations in permeability. Hence, predicting the effect of depletion on frictional equilibrium is required in dealing with casing shearing or lost circulation in drilling of new wells. In addition, permeability modeling will be more precise during the life of reservoirs. Estimation of necessary time to create or vanish faults is vital to be successful in production from wells or drilling new wells. Achieving or loosing of equilibrium mainly depends on in situ stresses and rate of production. Estimation of the in situ stresses at the initiation state of reservoir is the key to study the state of faults. The next step is to predict the effects of depletion on in situ stresses. Different models are suggested in which decrease of horizontal stresses is predicted as function of pore pressure variation. In these models, different assumptions are made such as simplifying the poroelastic theory, ignoring the passing time, and considering the geometry of reservoir. In this article, a new model is proposed based on theory of inclusions and boundary element method. This state-of-the-art model considers the geometry of reservoir. In addition, changes of in situ are obtained as a function of time to reach to a more precise model capable of applying during the reservoir life. Finally, the model is imposed on real cases. The effect of depletion on faults is studied in reservoirs of normal and strike-slip stress regimes, and conventional and proposed models are compared. For this aim, in the first step, mechanical earth models of these two reservoirs are built using logging and coring data. Stress polygon method and poroelastic horizontal strain model are used for strike-slip and normal regimes, respectively. In reservoir 1 which is in a strike-slip stress regime, a fault is distinguished in formation microimaging (FMI) log. For this reservoir, the required time to achieve to frictional equilibrium is calculated. In the reservoir 2, the potential depth of fault sliding is analyzed and required time for faulting in that depth is predicted. The predicted time for satisfaction of frictional equilibrium using the proposed model has a significant difference with the predicted time using the previous methods. In addition, the proposed model predicts that different parts of reservoir 2 are willing for faulting during depletion. The previous model determines only one point that faulting may happen. It is necessary to use this new model to consider different important factors such as geometry and time to gain more accurate predictions.

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.

Modelling hydraulic fracturing process in one of the Iranian southwest oil reservoirs

ABSTRACTHydraulic fracturing creates a high conductivity channel within a large area of formation and bypasses any damage that may exist in the near wellbore region. Moreover, it has been one of the major well stimulation techniques to increase well production. In order to model and study the feasibility of implementing this technique in one of the Iranian Southwest oil reservoirs, an extensive literature survey was carried out. One-dimensional mechanical earth model was constructed. Stable mud window was determined and types of failures were diagnosed. Consequently, on these bases, geomechanical feasibility was confirmed and hydraulic fracturing treatment for the case study was designed. Different scenarios were defined and sensitivity analysis was conducted together with the confirmation of economic feasibility. Furthermore, channel fracturing technique was employed and it was found that using this technique can be more suitable and beneficial than conventional fracturing method.

Geomechanical modellingl of Paleozoic Shale Gas Formation: a case study from the Baltic Basin, northern Poland

The article presents the importance and position of geomechanical modelling workflow in reservoir characterization studies dedicated to unconventional shale reservoirs. We show the results of 3D geomechanical modelling carried out in an onshore area within the Baltic Basin, northern Poland, where the Silurian and Ordovician shale formations are the exploration targets. The fundamental elements of the methodology, processes, and available datasets used in the modelling are discussed. The petrophysical, elastic, and mechanic properties of the rock were applied in the modelling process, along with the principal stresses and pore pressure in the geological formation. Moreover, the main calculation methods and data requirements for the Mechanical Earth Model construction are discussed. A comprehensive 3D geomechanical model was constructed, providing important information to engineers and decision makers which allows them to optimize well placement, the direction of the horizontal section of the borehole and the parameters of hydraulic fracturing treatment. The model can identify zones of higher potential within the area of interest in terms of efficient stimulation treatment design.

Coupled reservoir and geomechanical simulation for a deep underground coal gasification project

Underground Coal Gasification (UCG) is an in-situ technology for extraction of energy from otherwise un-mineable coal seams. As coal is gasified, high temperature is generated and cavities are formed. Hence, the UCG imposes significant geomechanical changes to strata. The province of Alberta, Canada recently operated a deep UCG demonstration project, at a depth of 1400 m. The demonstration project successfully produced methane, hydrogen, and other gases. The current study aimed at conducting a sequentially coupled coal gasification and geomechanical simulation to study effects of the Alberta UCG on the coal seam and bounding seal system. A mechanical earth model was built for the test site utilizing geological layers reported for the site and under anisotropic in-situ stress magnitudes and orientations, particular to the Western Canadian Sedimentary basin. Ten chemical reactions along with their kinetics were implemented in a reservoir simulator. The Controlled Retraction Injection Point (CRIP) method was studied, in which four gasification chambers were simulated. The product gas compositions, over a period of 60 days, were in good agreement with the syngas composition measured at the demonstration project. By utilizing the coupling workflow, complex three-dimensional (3D) geometry of the UCG cavities as well as temperature and pore pressure, were passed along from the gasification module to a geomechanical simulator. This allowed simultaneous observation of geomechanical response of the strata as the gasification process advanced, syngas produced, and cavities developed.

Constraining the in-situ stresses in a tectonically active offshore basin in Eastern Mediterranean

The objective of this study is to estimate numerically the in-situ stress field in a tectonically active marine sedimentary basin. The Levantine basin in Eastern Mediterranean which is currently explored for gas and oil reservoirs is used as a case study. The study was based on elasticity, and poroelastoplasticity constitutive equations which were calibrated with material parameters derived from seismic data and correlation functions. The geometry of the Mechanical Earth Model is also constructed from seismic data. The equilibrium and constitutive equations are solved with the finite element method. The model simulates a basin area initially at rest and then loaded by horizontal motion to simulate the active tectonic movement of the plates over the geological time. The effect of boundary conditions and initial conditions are studied along with other important parameters that influence the problem such as the interface strength of fault surfaces and the plate movement. Information based on mud-density in drilled nearby wells are used to calibrate the horizontal motion and constrain the insitu stresses at the well locations and hence within a good approximation elsewhere. We found that in the active tectonic regime of East Mediterranean the horizontal stress increases in such extent that it can be compared with the vertical stress. The adhesive behavior between the walls of a fault can cause interlocking resulting to even larger horizontal stress values. Smaller stresses are generated in sliding conditions between the fault surfaces due to the dissipated energy in the sliding process. Finally, the stress distributions become highly complex with stress rotation and arching close to the fault area.

Understanding Completion Performance: Software-Guided Work Flows and Models

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 179172, “Understanding Completion Performance in Niobrara-Codell Reservoirs Through the Use of Innovative Software-Guided Work Flows and Models,” by K.J. Wallace, Encana Oil and Gas; P. Reyes Aguirre, Schlumberger; E. Jinks and T.H. Yotter, Encana Oil and Gas; and Raj Malpani and Felipe Silva, Schlumberger, prepared for the 2016 SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 9–11 February. The paper has not been peer reviewed. This paper describes a comprehensive field study of eight horizontal wells deployed in the stacked Niobrara and Codell reservoirs in the Wattenberg Field (Denver-Julesburg Basin). The overall goal was to understand the geometry of the hydraulic fractures (propped) and the producing volume with respect to completions design, target reservoirs, and well spacing. Through this understanding, the asset can be developed more effectively and economically. Field Description The Wattenberg Field is a basin-center petroleum accumulation located northeast of Denver, from which hydrocarbons have been extracted over the last 50 years. Development in the Wattenberg Field began in 1970 from vertical J-Sandstone wells, with production of the Niobrara following in 1976. In 2009, horizontal development began in the Niobrara formation. The Upper Cretaceous sequence from Codell to Niobrara is the current focus of horizontal development in the Wattenberg Field. Petrophysical Model A robust 3D mechanical Earth model (MEM) provides the framework for data integration and physical simulations. In this work flow, the MEM calculates or approximates a number of input parameters. Each of these variables is defined both spatially and stratigraphically and is housed in a 3D geocellular model. Because many of the input calculations are dependent on other variables, a set of variable relationship definitions keeps the model dynamic so that, during the tuning process, all initially established data relationships are honored.

A novel model for wellbore stability analysis during reservoir depletion

It is not common to build a geomechanical model for depleted reservoirs as logging and coring are costly and time consuming in such reservoirs. On the other hand, in regular analysis of wellbore stability, the effect of time is completely ignored. As a result, with time there will be some errors in the evaluation of wellbore stability in depleted reservoirs. In order to determine the optimum wellbore trajectory during the reservoir life, it is necessary to have a model which can estimate the rock properties based on the first full logging suite and also consider the depletion effect. In this study, a novel model is proposed which combines a mechanical earth model, borehole circumferential stresses, Mogi-Coulomb failure criteria and simulation of pore pressure variation near wellbore to determine the optimum well trajectory during drilling and production in a condensate offshore reservoir. As a main output from the application of the new model, it was found that the most stable wellbore trajectory changed after 18–27 years of production. This critical output from this study raises the need to consider the possibility of changing the designed well trajectory over the life of the reservoir to maintain wellbore stability and optimize the drilling and production operations.

Data integration, reservoir response, and application

The microseismic activity observed in and around a geologic formation undergoing carbon dioxide (CO2) injection is a combination of natural, or “background”, microseismicity plus that activity which is induced by injection operations. Since injection pressure within storage target formations are maintained safely below fracture pressure this induced activity typically originates at natural pre-existing zones of mechanical weakness presented by structural or stratigraphic features. The combination of mechanical properties and in situ stresses dictate the focal mechanism for microseismic emissions, an understanding of which facilitates the use of observed microseismicity for regulatory compliance and project management.Under favorable conditions microseismic activity may be unambiguously correlated with structural and/or stratigraphic features directly observed in seismic data, thus providing strong constraints to interpretation of observed microseismicity for focal mechanisms. However, in many cases, such as at the Illinois Basin–Decatur Project (IBDP), this direct correlation is elusive and other indirect support is required. Analysis of microseismicity at IBDP has been performed within the context of the integrated reservoir and mechanical earth models developed as part of the site characterization and monitoring program. The IBDP integrated modeling workflow involved continuous and geotechnically consistent data integration for geologic modeling, calibrated flow simulation, three-dimensional (3D) mechanical earth model, and coupled hydro-mechanical simulation. Using the coupled model, scenario-based forward modeling of microseismicity was performed for hypothetical focal mechanisms inferred from observed data.The experience gained at IBDP illustrates the importance of integrated modeling in the interpretation of microseismic activity for focal mechanisms and provides valuable insights into critical data gaps which could be the target of future basic research efforts.

Technology Focus: Seismic Applications (March 2016)

Technology Focus From time to time, I am asked to address general audiences. The mission is to describe what we do in the seismic business. Typically, the first slide I show is a prenatal ultrasound display of my daughter. I explain that using reflected sound waves to create such an image is precisely what we do with the Earth. The world’s first reflection seismic field tests were conducted near Oklahoma City in 1921, and, ever since then, the industry has endeavored to improve that seismic imaging process. So, indeed, one of the papers selected for this Technology Focus section and one of the papers recommended for additional reading deal with case histories in which imaging is improved through better velocity-model building. In the first paper, integration of microgravity data, resistivity measurements, and seismic is the key in the onshore case history from Qatar. In the first additional-reading paper, full-waveform inversion of seismic travel times and amplitudes is the key in the case history from offshore Australia. Advances in seismic applications are not just confined to imaging, though. In another additional-reading paper, amplitudes are used for identifying porous zones in an otherwise tight-sandstone gas reservoir in Oman. And, in the third additional-reading paper, the authors use the amplitudes in 4D analyses for identifying new reservoir drive mechanisms in a field offshore Brunei. Both imaging and amplitude inversion benefit greatly from broader-band data. Therefore, this is the topic of the second paper presented. In this example from offshore Malaysia, the marriage of a new acquisition technique with new processing algorithms yields broader frequency content, enabling more-accurate estimates of gas in place to be derived. But even further advances in seismic applications are taking place now, especially in the world of unconventional resources. The authors of the third selected paper discuss the integration of miscroseismic data with 3D-seismic attributes, well-log data, and completion data to understand the geomechanical rock properties in the Midland basin of Texas. This information was important for planning the spacing of new wells. I hope this journey is successful in showing that substantial advances continue to be made globally in seismic applications. But, perhaps even more importantly, I hope the journey shows that some of the more exciting advances are actually arising from the integration of seismic with other technologies and that, while imaging may still be king in seismic (and in some fields of medicine), the applications of seismic to building 3D mechanical Earth models, for example, are gaining prominence. JPT Recommended additional reading at OnePetro: www.onepetro.org. IPTC 17905 High-Resolution Anisotropic Earth-Model Building on Conventional Seismic Data Using Full-Waveform Inversion: A Case Study Offshore Australia by Bee Jik Lim, Schlumberger, et al. SPE 177552 Seismic Reservoir-Quality Prediction, Khazzan Field, Oman by T. Chris Stiteler, BP, et al. IPTC 18491 4D Seismic in Stacked Reservoirs—From Puzzles to Insights on Production Drive Mechanisms by Denis Kiyashchenko, BSP, et al.

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.

Development of a mechanical earth model in an Iranian off-shore gas field

Wellbore instability is a quite common event during drilling, and causes many problems such as stuck pipe and lost circulation. It is primarily due to the inadequate understanding of the rock properties, pore pressure, and earth stress environment prior to well construction. This study aims to use the existing relevant logs, drilling, and other data from offset wells to construct a precise mechanical earth model (MEM) describing the pore pressure, stress magnitudes and orientations, and formation mechanical properties of South Pars Gas field. Since the core test data, MDT/XPT data, and LOT/XLOT data were not available to calibrate the developed model, each component of the model was determined using a range of existing methods and relations, and then the wellbore instability was analyzed based on the developed MEM and the Mogi-Coulomb failure criterion. The predicted incidents such as the lost circulation and tight hole were then compared with the caliper log and reported drilling events to determine the consistency of the model. Since the stability analysis based on the developed MEM had the most agreement with the caliper log and reported drilling events, the equations presented by Eaton and Zoback had good estimations of the pore pressure and rock strengths. Also the estimated horizontal stresses were precise enough to enable the constructed MEM to predict the wellbore instabilities. The stress regime in the field of study was strike-slip, which is frequently specified in the industrial technical reports of the studied field. Finally, it was concluded that the Mogi-Coulomb failure criterion minimized the conservative nature of the mud pressure prediction due to the consideration the strengthening effect of the intermediate stress.

Geomechanical-Evaluation Enabled Successful Stimulation of a High-Pressure/High-Temperature Tight Gas Reservoir in Western China

SummaryThe Keshen reservoir in China is a deep, tight-gas-sandstone reservoir under high tectonic stress with reservoir pressure at more than 16,000 psi (110 MPa) and temperatures up to 165°C. Development wells for this field are in excess of 6500 m in true vertical depth (TVD). Stimulation is required to provide sufficiently high production rates that compensate for the high cost of drilling and completing wells. Hydraulic-fracture design and execution must be optimal to ensure economic production.To effectively stimulate a more-than-200-m-thick sandstone reservoir with consistently high performance, it is necessary to understand the mechanical behavior of the reservoir, especially mechanical properties and in-situ stresses because the two control the creation and propagation of each hydraulic fracture. The mechanical behavior is complicated by high tectonic stresses, significant compaction, and high overpressure.To gain an in-depth understanding of the mechanical properties and in-situ stresses of the Keshen reservoir, an integrated geomechanical evaluation was conducted. The evaluation used the core from two wells, KS205 and KS207, and log data obtained from 15 wells including the wells with core evaluation in the field. A laboratory-testing program to investigate the mechanical behavior of the reservoir sandstone under realistic in-situ stresses, pore pressures, and temperature was performed. The description of mechanical behavior obtained from the laboratory testing was used to calibrate and augment mechanical Earth models (MEMs) constructed from well-log data. The reliability of the completed MEMs was validated through comparison between wellbore-stability predictions with observation of borehole failure from the borehole-microresistivity image.The geomechanics information was delivered to the stimulation-engineering team. Hydraulic-fracture design and execution was conducted on the basis of this information. The outcome of hydraulic fracturing was very encouraging.This study demonstrated that successful stimulation of a tight reservoir in high pressure/high temperature (HP/HT) relies on integrated geomechanical analysis.