D4.5 Role Of Existing Fault Segments
Creators: name: RACHEZ, Xavier
Publisher: name: Zenodo
Year: 2012
Subjects: production
exploration

description: The aim of the development of a geothermal site by hydraulic and/or chemical stimulation is to irreversibly increase the injectivity/productivity of a well by a local increase of the transmissivity of the fractures and/or faults intersecting the well and an improvement of the connectivity of the well to fracture and/or fault network. The generation of microseismicity is inherent in these methods of development of wells. However, if a large number of microearthquakes is inevitable, major events (magnitude between 2 and 3) must be studied carefully. Understanding these events may lead to propose scenarios for the stimulation reducing or eliminating them while maintaining the effectiveness of the stimulation relative to the injectivity.

This report corresponds to the deliverable D4.5 for the GEISER project. It consists in a description of the results obtained by the partners involved in sub-task 4.3, referred to as “Role of existing fault segments”. Different modeling approaches and laboratory experiments have been performed to investigate the role of fractures and fault segments as a key factor influencing induced seismicity.

Laboratory stick-slip experiments coupled with Acoustic Emission (AE) monitoring during fluid injection have been performed at partner GFZ in low porosity sandstone rock specimens, with saw-cut planes, loaded under triaxial compression. The seismic nature of the saw-cut plane slip has been investigated by recording the spatio temporal (4D) evolution of AE events and by analysing them in terms of different crack types and hypocentre locations. The friction coefficients were calculated for every hydraulically reactivation of the saw-cut plane. The reactivation of the cohesionless saw-cut planes occurred at different pore pressures, keeping constant the axial load and increasing the pore pressure values. During the stick-slip events, a stress drop was monitored together with the sliding of the saw-cut plane. Each stick-slip event was composed by multiple stress drops; however, since these events occurred under axial force control the focus was only on first stress drop (to avoid the servo effects). The experiments show that slip along the saw-cut plane was accompanied by an increased number ofAE events, all located in different patches along the saw-cut plane. New fractures, far from the saw-cut planes were not induced. Mohr-Coulomb envelopes reached sliding lines with different coefficients of static friction, which indicates a change in the roughness of the saw-cut planes (higher coefficients of static frictions, thus, more rough surfaces). Shear cracks were dominant during the sliding, while the number of tensile cracks was not negligible. The latter were related to local pore pressure perturbations and local dilatancy taking place during the sliding.

A specific continuous approach has been adopted by partner TNO in order to study the role of pore pressure changes causing fault zone reactivation and inducing seismicity during the creation of an EGS. For modeling the mechanical behavior of the fault zone  the softening ubiquitous joints model was used with random strength properties. A hyperbolic deformation with effective stress for the reversible tensile fault zone opening and a linear relationship between plastic shear strain and irreversible fault zone opening was implemented. The effective permeability of the fault zone was described by a cubic dependence on the fault zone aperture. A model based on the Soultz-sous- Forêts GPK3 injection well was created, intersected by a dominant fault zone. The model reproduced the reactivation of the fault zone due to water injection and the growth of an area with large directional permeability. The model was used to perform a sensitivity analysis of parameters like injection rates, in-situ stress regime, fault zone strength, and frictional weakening. This allowed for the evaluation of the trends of their impact on fault zone reactivation, including reactivated area, seismic moment and moment magnitudes. Next, a simplified model for the GPK3 stimulation and post shutin phase was constructed. Simulated fault zone reactivation and induced seismicity were compared with the actual fault zone reactivation (seismicity front) and monitored seismicity. For the early phase of stimulation of the GPK3 injection well, the model gives realistic results for the reactivation of the fault plane, the spatial evolution of the seismicity front and the total cumulative seismic moment generated. Results for the later phases of stimulation and the post-shut in phase can be improved by extending the model with additional fault zones. To obtain a more realistic spread in induced seismicity, a further tuning and calibration of the (spatial distribution) of the strength properties and stress drop with the actual data of the GPK3 injection phase is needed.

A Discrete Fracture Network (DFN) approach has been adopted by ARMINES. It has been developed to simulate discontinuous systems in order to provide means of evaluating forced flow and mechanical interactions away from injection wells, and thus the activation of faults and fractures which theoretically can lead to induced seismicity. This approach is based on the assumption that fluid moves through a rock mass within a system of interconnected fractures and that flow in the rock matrix is negligible by comparison. The representation of the geometry can be a superposition of deterministic or stochastic objects. These elementary objects are disc shaped. The flow rule inside the connected fractures (discs) is the so-called "cubic law" where the fracture aperture varies with normal stress, fluid pressure and thermal stress. In case of shear rupture, a dynamic-like stress drop is generated and fracture cohesion is cancelled. Therefore fracture stability is insured until pressure changes or thermal changes are enough to meet the rupture criterion again. The situations where tensile regime is obtained are not considered, as the mechanics of crack growth are not introduced in the code and also because most of the hydro-mechanical interactions start to develop at much lower regimes. Different reservoir configurations have been addressed: a 3D multi fracture system and a single fault segment with heterogeneous properties under a given stress regime. According to the models, the development of shear is controlled by pressure diffusion, and therefore in both cases hydraulic properties, contrasts in properties of the various structures and boundary conditions are governing the mechanical process. The critical conditions with regard to the occurrence of large induced seismic events are obtained in the situation of some large fracture with unknown hydraulic properties exists at an intermediate distance of the well, and when the ‘bulk’ hydraulic diffusivity of the massif is limited. In the case this structure is also of poor natural hydraulic diffusivity or locked by other fault segments,
description: FP7