EAGE Middle East Geomechanics Workshop: Lessons Learned & New Frontiers

Land surface subsidence is a catastrophic phenomenon that leads to a loss of lives and infrastructures. In arid climates, such as the United Arab Emirates (UAE), water resources are limited and rely on groundwater as the main water resource for domestic and agricultural sectors. The increasing demand for water in the country results in overexploiting the groundwater resources which can be the main cause for land surface subsidence. Monitoring of land surface subsidence can be achieved by various techniques such as leveling survey, Global Positioning System (GPS), and Interferometric Synthetic Aperture Radar (InSAR) techniques. In this study, the InSAR technique has been implemented with the Sentinel-1A images to monitor land surface subsidence over the agricultural areas in the UAE between 2015 and 2019. The results showed significant ground subsidence in spatial accordance with agricultural farms. Furthermore, the detected subsidence has been confirmed by field observation of infrastructure failures over the deformed area.

Overburden stress evolution induced by reservoir pore pressure depletion must be correctly estimated to assess the failure risk, wellbore stability, and feasibility of 4D seismic monitoring. Reservoir shape and mechanical contrast between reservoir and burden layers are known to have a significant impact on the overburden stress evolution. A detailed investigation of those impacts, however, has not been performed partly since a time-consuming geomechanical simulation is a common approach in the oil and gas industry. The recently introduced generalized Geertsma solution allows those impacts to be quickly analysed. Here, a parametric study was conducted to investigate the impact of (1) mechanical contrast between reservoir and burden layers and (2) reservoir aspect ratio (thickness/radius) on the overburden stress path using the generalized Geertsma solution. The results show the importance of correctly accounting for these factors in drilling operations and 4D seismic interpretations. Pore pressure depletion of a reservoir sandwiched between significantly stiffer layers, for example, will induce horizontal stress reduction in overburden rock and this would change the upper limit of safe mud weight window.

Understanding of fracture behaviour within the near wellbore region is critical to loss mitigation in fractured reservoirs. Linking near wellbore fracture behaviour with deviations in loss behaviour for selected wells is the purpose of this paper.

A 3-D finite element field model was developed first for the whole field. A high-resolution, near wellbore, concentric grid was then generated. Initial stresses, mechanical properties and fractures were assigned for analysing different conditions of pressure and temperature around the well.

For the case of temperature diffusion, the near wellbore region experiences a drop in the effective circumferential and radial stress, which means some of the fractures become critically-stressed. When temperature reduction and pressure increase are combined, both radial and circumferential effective stresses decline, so pre-existing fractures become more unstable. If lower mud weights are applied within the wellbore, effective wellbore stresses are reduced and fractures that intersect the well become still more critical.

The loss mechanism on well A17 is attributed to pre-existing closed and partially-cemented fractures that have been opened/sheared by lower mud temperatures and reduced mud weights. Dilatant slip of these fractures can provide increased volume for wellbore fluids to penetrate the formation and to potentially connect with other natural fractures.

This paper focuses on rock strength modelling improved by combining triaxial compressive tests with the Schmidt hammer method and well log data on the basis of core samples comprising several lithological types (sandstones, mudstones, coal, shales). The proposed methodology provides cost-free strength measurements without the destruction of the core. The Schmidt Hammer method is an application that enhances the characterization of rock heterogeneity along the sampled section and for adjustment of mud weight window in the next section by better calibration of the mechanical earth model. The research was used in building geomechanical models, as well as in subsequent well and mud weight designs.

A carbonate, light oil field in PDO is planned for development using Gas Oil Gravity Assisted (GOGD). The dome shaped structure of the field is characterized by a naturally fractured carbonate reservoir. Cap rock integrity and fault reactivation are potential Geomechanical risks associated with fluid injection, which can induce leakage pathways resulting in out of zone injection. To minimize these risks and ensure a successful execution and operation of eth field a geomechanical analysis using 3D modeling was carried out to assess and define the key operational envelope to mitigate the risks. This paper discusses the modeling approach and results.

The key objective was to analyse the induced stresses when the gas is injected at the crest of the field, increasing the pressure by 2000 kPa in the depleted reservoir formation. A Finite Element (FE) geomechanical modelling was carried out to simulate the stresses in the reservoir and caprock. The induced stresses were used to assess the slip potential of the faults interpreted on Seismic data. Mohr-Coulomb failure criterion was used to determine the risk on shear failure in the caprock and on the faults as a function of rock properties and stresses for a range of proposed development injection scenarios.

Results of the geomechanical assessment quantified the risk of breaching caprock during the injection to be low with the given injection pressure constraint. The model results indicate that the maximum operating bottom hole pressure gradient to avoid the risk of fault instability (NE/SW) of cap rock (if injection is proximal to the fault) should be limited between 10.6 kPa/m to 12 kPa/m TVD bdf , and is a function of well location.

Robust front-end loading of data, integrating the available data and the uncertainties allowed generation of a robust model providing key inputs for injection pressure optimization and mitigating the potential risks for the full-field development, whilst ensuring safe operations.

Successfully creating multiple hydraulic fractures in horizontal wells is critical for an optimal unconventional shale gas production. In order to optimize the stimulation of those wells a reliable model is required that will consider the complex physics involved in the fracturing process like the mechanical interaction of the propagating fractures with the fluid dynamics of the injected fluid.

In this paper, a fully coupled hydro-mechanical finite-element model is employed to simulate the multiple fracture propagation, potentially non-planar fractures, in a low permeability reservoir with low horizontal stress ratio. Zero thickness interface elements were used to modeling the initiation and propagation of fractures based on a non-linear fracture energy constitutive law.

A cluster spacing analysis is performed to investigate their effect on fracture geometry and on the stress changes induced by fracture growth. The results shows that the horizontal stress anisotropy, stress-shadow effects and the reservoir permeability dominate the multiple-fracture geometry. Mechanical and hydraulic interference between fractures increase when cluster spacing is reduced and a non-planar fracture propagation can be induced after the first stage due to high stress shadow effects.

New high speed geomechanical technology and an enhanced workflow have been implemented in a carbonate field case to produce mechanical and strength property distributions and pore pressure and stress magnitudes in 3D. The new modelling approach which provides very fast outputs uses both analytical and numerical techniques at high resolution based on seismic interpretations. The solutions compare favourably against measurements, drilling report information, core tests and have been used to check new planned well trajectories and Discrete Fracture Network (DFN) characterisation.

Faults are known for representing the geologic past, behaving as energy release zones or weak points during earthquakes, acting as hydrocarbon traps, producing reservoir compartmentalization, as well as for creating problems during drilling and hydraulic fracturing, and also during production and injection operations. Those are just a few reasons why the presence of faults must be accounted for in any geomechanics analysis. In this paper we describe the equivalent material approach, a method to physically include faults in the mechanical earth model (MEM), to not only understand their behavior but specially for assessing the stress field changes associated with them. This technique has been widely implemented in the industry, reaching satisfactory results when used to explain the nature of stress rotations, mud losses, fault integrity issues, hydraulic fracturing behavior, and even fluid flow and migration in faulted reservoirs. Some of these cases will be briefly presented with the only objective of highlighting the importance of including the fault material in the 3D-MEM construction to better assess their effects in the field development plans and future exploratory campaigns. The different cases show how faults affect the stress rotations and magnitudes, especially after field depletion. They also show how faults affect the mud window, and the trajectory and placement of future wells during the field development plan.

A proper field development plan require an accurate management of well placement respect the in-situ stresses. Understanding the stress status is a key factor toward any successful geomechanics model, which will guide the process of designing wells and integrate development, plans to minimize cost and maximize production. This paper represents a review of the present modelling ways of the in-situ stress variation laterally and vertically using the invented artificial intelligent tool that will assure geomechanics applications such as hydraulic fracking design, wellbore stability, well placement, and the mechanical behavior of the reservoir.

Reservoir stresses are conducted from borehole image interpretation identifying drilling induced fractures and breakouts manually, depending principally on the experience and usually impacted by inconsistencies due to biased or unexperienced interpreters. Therefore, a robust automatic or semiautomatic approach was established to reduce time, manual efficiency and consistency. The current invention adding to the above geological and structural features, the calculation of the porosity and connectivity.

The tool was successfully deployed to address a variety of challenging problems in BHI, and results showed an accurate detection of diverse and complex set of image attributes without manual intervention. The tool was tested and piloted for many wells, which concluded results exceeding 98% for full interpretation along with the analysis in less than one hour for an interval of 1000ft. This study describes and compare the different strategies of the far field from faults and well oriented from logs to predict the depth variation of stress within a layered rock formation and the impact on the stress orientation applied for Geomechanics modeling. The predictive strategies are based on well log data and in some cases on in situ stress measurements, combined with the weight of the overburden rock, the pore pressure, the depth variation in rock properties, and tectonic effects. These analyses were performed for stress profile construction and to calibrate the magnitudes of the horizontal stresses. To determine stress directions from automated borehole image interpretation data for subsequent comparison with stress directions inferred from the geomechanical models, a workflow was used based to infer Andersonian tectonic regime from stress analysis and comparing with the 1D Geomechanics models. In addition to detect SHmax and Shmin direction from breakouts or hydraulic fractures, borehole ovality, seismic anisotropy (including shear wave splitting etc.) and oriented cores. On top of that, interpret Shmin (at any specific depth) from extended leak-off tests or from the hydraulic fracture propagation pressure derived from mud-weight integration. Comparing the results with those from the fault plane solutions in restored sections, the intermediate principle stress can be calculated from the ratio of the least principal to intermediate stress as inferred from the tensile compressive transition in borehole wall.

Finally, the presented approach can optimize placing wells correctly, hydraulic fractures applications and provide safe mud window.