Integrating Laboratory Testing and Numerical Modelling for a Giant Maturing Carbonate Field in UAE — II. Coupled Geomechanical Modelling of Stacked Reservoir Intervals

A mature field located in a gently dipping structure onshore Abu Dhabi has multiple stacked oil and gas reservoirs experiencing different levels of depletion. The average reservoir pressure in some of these intervals had declined from the early production years to the present day by more than 2000 psi. Coupled geomechanical modelling is, therefore, of the greatest value to predict the stress paths in producing reservoir units, using the concept of effective stress. This paper examines the implications for long-term field management—focusing primarily on estimating the potential for reservoir compaction and predicting field subsidence. This paper takes the work reported in Noufal et al. (2020) one step further by integrating the results of a comprehensive geomechanical laboratory characterization study designed to assess the potential geomechanical changes in the stacked reservoirs from pre-production conditions to abandonment. This paper adopts a geomechanical modelling approach integrating a wide array of data—including prestack seismic inversion outputs and dynamic reservoir simulation results. This study comprised four phases. After the completion of rock mechanics testing, the first modelling phase examined geomechanics on a fine scale around individual wells. The goal of the second phase was to build 4D mechanical earth models (4D MEMs) by incorporating 14 reservoir models—resulting in one of the largest 4D MEMs ever built worldwide. The third phase involved determining the present-day stress state—matching calibrated post-production 1D MEMs and interpreted stress features. Lastly, the resulting model was used for field management and formation stimulation applications. The 4D geomechanical modelling results indicated stress changes in the order of several MPa in magnitude compared with the pre-production stress state, and some changes in stress orientations, especially in the vicinity of faults. This was validated using well images and direct stress measurements, indicating the ability of the 4D MEM to capture the changes in stress magnitudes and orientations caused by depletion. In the computed results, the 4D MEM captures the onset of pore collapse and its accelerating response as observed in the laboratory tests conducted on cores taken from different reservoir units. Pore collapse is predicted in later production years in areas with high porosity, and it is localized. The model highlights the influence of stress changes on porosity and permeability changes over time, thus providing insights into the planning of infill drilling and water injection. Qualitatively, the results provide invaluable insights into delineating potential sweet spots for stimulation by hydraulic fracturing.

An experimental approach on calcarenite reservoir dilation factor

The time of flight of a plane wavefront generated from an acoustic pulse is expected to decrease when the medium length between the wave emitter and receiver is shortened. This simple idea is extrapolated to the case of reservoir compaction in order to obtain a geophysical parameter R (dilation factor) that relates the rock deformation to the variation of time of flight (also called time-lapse time shift in 4D seismics) or acoustic velocity of a plane wave propagating in the same direction of deformation. Interpretation of a few laboratory compressive tests with simultaneous ultrasonic acquisition, performed on oil-saturated calcarenite samples, are presented and discussed. The samples were subjected to several stress regimes and simultaneous ultrasonic acquisitions. Despite the formerly ultrasonic acquisition rate limitations, it was possible to obtain R values for various lateral-vertical stress ratios for each sample's linear and nonlinear stress-strain trends.

Incorporating dwelling mounds into induced seismic risk analysis for the Groningen gas field in the Netherlands

In order to inform decision-making regarding measures to mitigate the impact of induced seismicity in the Groningen gas field in the Netherlands, a comprehensive seismic risk model has been developed. Starting with gas production scenarios and the consequent reservoir compaction, the model generates synthetic earthquake catalogues which are deployed in Monte Carlo analyses, predicting ground motions at a buried reference rock horizon that are combined with nonlinear amplification factors to estimate response spectral accelerations at the surface. These motions are combined with fragility functions defined for the exposed buildings throughout the region to estimate damage levels, which in turn are transformed to risk in terms of injury through consequence functions. Several older and potentially vulnerable buildings are located on dwelling mounds that were constructed from soils and organic material as a flood defence. These anthropogenic structures are not included in the soil profile models used to develop the amplification factors and hence their influence has not been included in the risk analyses to date. To address this gap in the model, concerted studies have been identified to characterize the dwelling mounds. These include new shear-wave velocity measurements that have enabled dynamic site response analyses to determine the modification of ground shaking due to the presence of the mound. A scheme has then been developed to incorporate the dwelling mounds into the risk calculations, which included an assessment of whether the soil-structure interaction effects for buildings founded on the mounds required modification of the seismic fragility functions.

Stress-based forecasting of induced seismicity with instantaneous earthquake failure functions: Applications to the Groningen Gas Reservoir.

The Groningen gas field is a natural laboratory to test stress-based forecasting models of induced seismicity due to the detailed knowledge of the reservoir geometry and production history, as well as the availability of surface subsidence measurements and high quality seismicity data. A specific feature of that case example is the exponential rise of seismicity that was detected nearly 30 years after the onset of production. In this study, the subsurface is represented as a homogeneous isotropic linear poroelastic half-space subject to stress changes in three-dimensional space due to reservoir compaction and pore pressure variations. The reservoir is represented with cuboidal strain volumes. Stress changes within and outside the reservoir are calculated using a simple convolution with semi-analytical Green functions. The uniaxial compressibility of the reservoir is spatially variable and constrained with surface subsidence data. Coulomb stress changes are maximum near the top and bottom of the reservoir where the reservoir is offset by faults. To assess earthquake probability, we use the standard Mohr-Coulomb failure criterion assuming instantaneous nucleation and a non-critical initial stress. The distribution of initial strength excess, the difference between the initial Coulomb stress and the critical Coulomb stress at failure, is treated as a stochastic variable and estimated from the observations. We calculate stress changes since the onset of gas production. The lag and exponential onset of seismicity are well reproduced assuming either a a generalized Pareto distribution of initial strength excess, which can represent the tail of any distribution, or a Gaussian distribution, to describe both the tail and body of the distribution. This representation allows to test if the induced seismicity at Groningen has transitioned to the steady-state where seismicity rate is proportional to the stressing rate. Our results indicate that the system has not yet reached such a steady-state regime. The forecast is robust to uncertainties about the ability of the model to represent accurately the physical processes. It does require in particular a priori knowledge of the faults that can be activated. The method presented here is in principle applicable to induced seismicity in any setting provided deformation and seismicity data are available to calibrate the model.

Shallow Gas Reservoir Development in Offshore Field, Myanmar: Tapping New Reserves with Novel Approach

Discovered on the shallowest formation in Myanmar offshore field at 500 meters subsea, this reservoir is perhaps one of the most challenging reservoirs to develop in many aspects such as; risk of fracking to seabed when performing sand control completion, cap rock integrity and risk of breaching due to completion and production activities, reservoir compaction, and depletion-induced subsidence. Generally, the producing reservoirs currently developed in this field sits between 700 to 2500 meter subsea, mTVDss. Cased Hole Gravel Pack (CHGP) as sand control completion method is selected to develop the reservoir from 700 to 1650 mTVDss. None of the shallow reservoirs (shallower than 700 mTVDss approximately) has been developed in the field before, due to some technical challenges previously mentioned. Owing to these reasons, reservoir engineer and well completion team initiated feasibility study focusing on advanced Geomechanical modeling and alternative way of sand control completion combined with full project risk assessment, ultimately, to unlock huge gas reserves trapped in this field. The reservoir is finally developed with infill well and new completion technique ever been used in the company. To develop this shallow reservoir, infill well drilling with sand control completion is required. The technical analysis on the following problems was comprehensively performed to ensure that the reservoir was feasible, doable and viable to develop. Reservoir compaction and subsidence occurring with stress and pressure changes associated with depletions would not create potential hazard to production facilities. Cap-rock is stable with no breaching over entire life of reservoir depletion. No potential fault is reactivated upon depletion. Sand control completion is able to be performed safely with well-confined fracpack (risk of frac growth to seabed). Upon depletion, integrity of casing and cement is acceptable when reservoir is compacted. Full risk assessment aspects of completion operation are scrutinized. These problems were mainly analyzed using coupled 3D Geomechanical model focusing on this shallow reservoir in the area of this particular wellhead platform. Briefly speaking, the 3D Geomechanical model was coupled with reservoir pressure depletion to find stress and displacement of reservoir rock and casing due to production. The methodology is called one-way coupled modeling. To be more precise, the pre-production stress of the reservoir at initial pressure was determined and used to calculate subsequent stress change from depletion (production). Pressure depletion will increase effective stress and hence create deformation of reservoir rock which may induce underground subsidence and casing integrity. On this study, four stress-steps of pressure depletion were computed i.e. initial pressure, 25% depletion, 50% depletion and 75% depletion. On each step, stress equilibrium was simulated using finite element software. This project makes the pending development of shallow reservoir in this field doable and viable. All risks associated with well completion and production-induced depletion were deliberately reviewed and mitigated. Based on this study, the most critical risk is gas leak through seabed due to sand control completion activity (CHGP). Apart from this, the other risks such as seabed subsidence, cap-rock breaching, fault reactivation, and casing integrity upon compaction were consciously addressed, reviewed and prevented. The major risk on sand control completion was finally mitigated. The conventional extension pack was avoided and replaced with the completion technique, a so-called circulating pack. Circulating Pack is one of CHGP technique where the pumping rate and pumping pressure maintained below fracture extension rate and fracture extension pressure. This pumping rate and pumping pressure will not introduce the fracture in the formation but still able to carry proppants and place them in the annular between screen and casing to provide sand control means. Although the sand control performance of circulating pack is not up to High Rate Water Pack (HRWP) or Extension Pack, together with control of minimum drawdown and production rate will enhance the sand control performance and prolong production life. Ultimately, unlock the potential in this shallow reservoir. The well has finally been successfully completed under tailor-made design and real-time data acquisition. The reservoir has been producing successfully with the rate of about 5 MMSCFD with good flowing wellhead pressure at 590 psi similar to the design. Ultimately, this alternative approach enables the development of this shallow reservoir where the new reserves of 20 BSCF has been added to the project. This project can be a good lesson for future development of other shallow reservoirs worldwide.

Integrated Reservoir Geomechanics Approach for Reservoir Management

An integrated 3D dynamic reservoir geomechanics model can provide a diverse 3D view of depletion-injection-induced field stress changes and the resulting deformation of both reservoir and overburden formations at various field locations. It enables the assessment of reservoir compaction, platform site subsidence, fault reactivation and caprock integrity associated with multiple production and injection reservoirs of the field. We demonstrated this integrated approach for a study field located in the South China Sea, Malaysia, which is planned for water injection for pressure support and EOR scheme thereafter. Reservoir fluid containment during water injection is an important concern because of the intensive geologic faulting and fracturing in the collapsed anticlinal structure, with some faults extending from the reservoirs to shallow depths at or close to the seafloor. Over 30 simulations were done, and most input parameters were systematically varied to gain insight in their effect on result that was of most interest to us: The tendency of fault slip as a function of our operation-induced variations in pore pressure in the reservoir rocks bounding the fault, both during depletion and injection. The results showed that depletion actually reduces the risk of fault slip and of the overburden, while injection-induced increase in pore fluid pressure will lead to a significant increase in the risk of fault slip. Overall, while depletion appears to stabilize the fault and injection appears to destabilize the fault, no fault slip is predicted to occur, not even after a 900psi increase in pore pressure above the pore pressure levels at maximum depletion. We present the model results to demonstrate why depletion and injection have such different effects on fault slip tendency. The interpretation of these geomechanical model results have potential applications beyond the study field, especially for fields with a similar geology and development plan. This is a novel application of 3D dynamic reservoir geomechanics model that cannot be obtained from 1D analytical models alone.

Investigation of Stress Arching Above Depleting Hydrocarbon Reservoirs and Its Effect on the Compaction Drive Mechanism

In this paper, the compression drive mechanism, which can be considered as the most related oil drive mechanism with the reservoir geomechanical properties, has been investigated. The constant total vertical stress on the reservoir and uniaxial reservoir compaction with zero lateral strain are two main assumptions in the conventional reservoir compaction modeling. These assumptions are not considering the stress arching, which leads to a reduction in the total vertical stress and also affects the distribution of vertical and horizontal stresses. In this paper, it was tried to investigate the effects of three keys elastic parameters, including Young's modulus, Poisson’s ratio, and Biot coefficient on the reservoirs stress paths and compaction drive mechanism using numerical modeling with ABAQUS software. Based on the modeling results the reservoirs stress paths is very different from simplifying assumption in uniaxial compaction models while the total compaction of reservoir obtained using numerical modeling and uniaxial model in some cases can be equal. Also, among the three geomechanical parameters, the ratio of Young's modulus of the reservoir and surrounding rocks has a significant effect on the reservoir stress paths and compaction. The results of modeling indicate when a reservoir rock is softer than its surrounding environment, due to the arching occurrence, the total vertical stress applied to the reservoir rock reduced even to half of the initial vertical stress, which leads to the oil recovery by the compression drive mechanism will be less than 50% of the predicted value.

Multitemporal SAR interferometry for monitoring of ground deformations caused by hydrocarbon production in an arid environment: Case studies from the Sultanate of Oman

Time-lapse interferometric synthetic aperture radar (InSAR) remote sensing methods of surface deformation have proven their use in desert environments. The data are acquired frequently without the need to send personnel or equipment into the field. The quality and accuracy of the data is very high. The spatial resolution of the data is excellent and matches that of surface seismic data. These characteristics make the data well suited for a variety of time-lapse monitoring tasks. In this study, we describe the accuracy of the InSAR technique relative to other measurements such as the global positioning system and precise leveling (acquired at known stable locations). We illustrate two case studies of differing natures. In one case, gas production leads to reservoir compaction, which is tracked as surface subsidence with time using frequent InSAR data. The results are used to map zones of increased deformation and identify areas with localized changes. These insights are being used to influence decisions on new wells and well interventions, to provide support for management of facility integrity, and to advise building code and material selection that can withstand the expected rate of deformation. The second case of a shallow steam flood illustrates the use of InSAR data to identify areas of surface uplift following thermal expansion of the reservoir. These data are also used to support the monitoring of the steam chamber growth and confinement in the reservoir. The information from InSAR will become more valuable for reservoir management when the steam chamber matures and conventional downhole data acquisition consequently becomes challenging. In summary, oil and gas fields located in arid environments lend themselves well to remote sensing using the InSAR technique because (1) they are sizeable (from tens to hundreds of square kilometers); (2) they are free from vegetation, snow cover, and most atmospheric distortions, although cloud and pollution can affect the data quality; and (3) they benefit from highly repeatable long-term regular monitoring.