Second EAGE Workshop on Geomechanics and Energy

Induced seismicity as a result of the fluid injection to enhance formation is a recognized safety risk in geothermal projects. Development of new models to better understand the underlying triggering mechanics and co-evolution of seismicity with subsurface pressure, stress and permeability is essential both for field management and hazard assessment. Recently, we have developed a workflow to integrate stochastic descriptions of seismicity with deterministic pressure, stress and permeability evolution modeled in a reservoir simulator. The approach couples the geomechanics and flow simulator FEHM (Finite Element Heat and Mass transfer) - the deterministic component – with an Epidemic-Type Aftershock Sequence (ETAS) model that generates synthetic catalogs of microearthquakes (MEQs). The ETAS model incorporates Omori law spatiotemporal decay, a Gutenberg-Richter frequency-magnitude model, and the idea that larger events trigger more numerous aftershocks. This model could reproduce successfully some patterns observed in induced seismicity in its application to a generic hydroshearing stimulation experiment, including diffusive migration of seismicity away from the well and co-evolution of injection and seismicity rates and moment release. Our stochastic-deterministic model provides a framework for investigating the relationship between injection parameters and bulk or average properties of the induced seismicity.

We discuss the impact of in-situ stress variations and near-wellbore geomechanical complexity on the efficiency of simultaneously initiating and propagating hydraulic fractures from a horizontal well. Such a multi-stage hydraulic fracturing technique is used routinely in the development of unconventional reservoirs in order to reduce operations costs. However, the resulting production rate from the different fractures along the well has been found to vary widely for a large number of different reservoirs. We show by numerical modeling of the multi-stage fracturing process that it is extremely difficult to properly balance the flow rate entering the different fractures stimulated at once during a pumping stage. It results that some fractures not only propagate further than others but also receive more proppant, both of which ultimately impacting the production rate of each fracture. Based on numerical modeling and field evidence, we argue that the reasons for such a poor efficiency of the fluid partitioning can be related to heterogeneities in both the in-situ stress field and the tortuous near-wellbore fracture path.

A detailed numerical modelling study was carried out to represent geotechnical aspects of the Wieczorek underground coal gasification (UCG) site in Poland. A coupled thermos-mechanical numerical model was created to represent a single coal burning panel. The coal burning process was simulated by modifying the energy balance equation with an additional term related to the calorific value of coal as a source. Temperature dependent material properties were assigned to the coupled thermal-mechanical model according to published data. In the model, the burning zone spread about 7.5m laterally after 20 days of burning. Results from the coupled model were used to gauge a worst-case scenario in terms of the potential size of a formed cavity. This data was used within a less computationally expensive mechanical-only numerical model in order to evaluate the ground subsidence caused by the worst-case scenario for single and multiple UCG burning panels. The single panel burning resulted in 23mm of ground subsidence at the top of the model after long term coal burning. The ground subsidence measured at the top of the model, at the center point of the gasification arrangement, was approximately 72mm when five panels were burnt with an edge to edge panel distance of 5m; this was increased to 85mm for seven panels. The numerical modelling results have implications to the industrial application of UCG. Keywords: Numerical model, underground coal gasification, calorific value, subsidence.

When faults are present, their stability and hydraulic behaviour during production and injection can play a critical role in the development of oil and gas reservoirs. Analysing fault criticality based on a calibrated 4D geomechanical model coupled to a reservoir simulation helps to set production and injection constraints with higher confidence to avoid fault reactivation at any stage during field development. The risk of fault reactivation needs to be evaluated in many reservoirs, because of the serious consequences it may have. Not only can previously isolated compartments start communicating, fault reactivation may also cause a loss of containment and the generation of leakage pathways along faults resulting in out-of-zone losses of hydrocarbons. A geomechanical simulation coupled to a reservoir simulation provides a robust starting point to evaluate the impact of faults and fractures and their behaviour throughout the lifetime of a reservoir. Insights into fault behaviour were obtained for a case study reservoir during different stages of field development.

A numerical geomechanical model is presented to characterize the stress field at a candidate site for a nuclear waste repository in Switzerland (Zürich Nordost). Lithological formations of approximately 20 to 200 m in thickness are considered in the model through specific rock properties as individual geomechanical units. Special attention is given to the Opalinus Clay (Lower Dogger), the designated host rock of high level waste at the candidate site.The modeled stress field is calibrated against stress data from borehole breakouts and hydraulic fracturing measurements conducted within the site. In general the state of stress strongly correlates with geomechanical properties. The stiff formations show much higher stress anisotropy with higher SH magnitudes and lower Sh magnitudes than the softer formations. In particular, it is concluded that the stress field in the Opalinus Clay is not very sensitive to changes in the boundary conditions as the stiffer formations (notably the limestones of the Upper Malm and the Upper Muschelkalk) take up the far-field tectonic stresses.

The paper presents the analysis of the water retention behaviour of compacted MX80 granular bentonite when subjected to cyclic wetting and drying episodes. Bentonites are highly swelling materials and they experience strong modifications of their fabric when subjected to changes in their water content. Interestingly, the link between the evolution of the retention behaviour and the structural modifications during wetting and drying episodes has not been deeply investigated. To this regard, a new protocol has been designed to analyse the retention behaviour of swelling materials. The protocol allows to collect representative samples for microstructural investigations at different stages of the imposed hydraulic stress path. Microstructural analyses were run by combining mercury intrusion porosimetry tests and SEM observations. An irreversible modification of the retention behaviour was observed once the material approached the fully saturated state during the first wetting. The water retention capacity of the material was increased as a result of such modification. The microstructural analysis allowed relating this change in the retention properties to a strong modification of the fabric.

The geo-energy sector is nowadays bringing ahead advanced technologies such as shales gas extraction, CO2 sequestration and nuclear waste geological storage, where the exploitation of shale formations is considered. Due to the great depth involved in the mentioned applications and to the difficulties in retrieving intact samples, reconstituted shale specimens are often adopted for hydro-mechanical testing. Reconstituted and intact shales may substantially differ in their hydro-mechanical behaviour due to the particular structure of the natural material; such peculiar structure is the result of diagenesis and burial history. This paper presents and experimental campaign aimed at (i) characterizing the role of diagenesis for Opalinus Clay shale from the northern region of Switzerland and (ii) understanding how representative the behaviour of reconstituted material is respect to the one of the natural shale. The investigation program comprises a series of low and high-pressure oedometric tests. The results of the tests on the reconstituted material are compared to those on the intact one and the major aspects related to the effect of structure on the geomechanical behaviour have been highlighted. Particular attention is given to the compressibility, swelling response, permeability and secondary consolidation of the material at the reconstituted and intact states.

Geologic carbon sequestration is a promising option to reduce carbon dioxide emissions to the atmosphere and mitigate climate change. The injected CO2 will reach the storage formation at a colder temperature than that of the host rock. This cold CO2 will cool down the caprock by conduction, which will induce thermal stress reduction and pressure changes that will affect caprock stability. We analyzed thermoporomechanical response of Swiss shale and found that the effective stress changes induced by cooling do not have, in general, the potential to jeopardize the caprock sealing capacity.

High permeability fluid flow pathways are widely observed in nature, and their formation occurs on both geological and human timescales. Outcrop study as well as interpretation of seismic cross-section show clear evidences of these multi-scale vertical pipe features, where unconsolidated to loose material is present inside. Even if well documented, their formation process remains still not answered yet. We propose a physically consistent 3D two-phase model of focusing fluid flow that allow the formation of such vertical high permeability channels. Viscous or creep rheology is the key feature to explain this formation process. Our result show that the proposed mechanism triggers pipe formation where permeability increases over two orders of magnitude in impermeable shale, and with propagation speed close to 2 meters per year in these usual ceiling rocks. Our results are in good accordance with the values needed to explain the fast vertical breakthrough of CO2 plume in the layered Sleipner saline aquifer.