ISRM conference, Montréal, Canada, 2015
Valentin S. Gischig, Giona Preisig
Department of Earth, Ocean, & Atmospheric sciences, UBC Vancouver, Canada
Hydraulic stimulation of deep reservoirs is an essential procedure for developing engineered geothermal systems (EGS), but also in other Geo-energy contexts. Generally the goal is enhancing permeability to such a degree that fluid flow rates within the reservoir are sufficiently high for an economically productive reservoir. Two fracture stimulation mechanisms are distinguished that may or may not occur concurrently during the hydraulic treatment: 1) During hydro-fracturing (HF) new tensile fractures are propagated from the borehole by means of a fluid pressure overcoming the minimal principle stress σ
plus the tensile strength T
of intact rock. In case that there is a pre-existing fracture set oriented normal to σ
3 and with little or no tensile strength, only σ
has to be exceeded.
Fluid injection is performed via small packed intervals for creating a stack of hydraulic fractures. 2)
During hydro-shearing (HS), over-pressure induces slip along a pre-existing fractures that are favourably oriented in the stress field for reactivation in shear. Such stimulation is usually performed in a large packed interval or in an open borehole. Which mechanisms occurs predominantly during stimulation depends on the structural properties of the rock mass, on the in-situ stress field, but also on the orientation of the discontinuities that intersect the open-hole section and thus are being pressurized. Both mechanisms also differ in how they affect fracture permeability. While permeability gained through HS is mostly irreversible due to rearrangement of asperity contacts accompanying shear dilation, permeability enhanced during HF reduces nearly reversible after pressurization unless proppant is used to ensure permanent apertures.
In this contribution, we critically discuss the pros and cons of both mechanisms regarding efficiency of enhancing permeability and the associated hazard of induced earthquakes based on literature review and numerical modelling. We first review literature that report on how much permanent permeability is gained by HF or HS, and assess it in light of the permeability required for productive EGS reservoirs. We also compile information regarding the degree of seismicity induced during HF- or HSdominated stimulation procedures, together with conceptual studies that reveal characteristics of seismicity associated with the two mechanisms. Many observations and models indicate that HF may have a higher tendency of being aseismic, while felt seismic events are usually associated with HS. In a second part, we present a hydro-mechanically coupled fracture flow model that investigates, if and to what degree the stimulation strategy can be designed such that one of the two mechanisms is evoked and dominates over the other. It shows that HS may dominate in presence of larger persistent fractures nearly optimally-oriented in the stress field, even if HF is attempted in short packed intervals. The model further demonstrates that stimulation of multiple parallel fractures may be inhibited through stress transfer from one fracture to the neighbouring ones. Stress transfer promotes the development of permeability across an anisotropic layer of the stimulated rock instead of a large volume. Hence, the study sheds light onto the feasibility of creating productive EGS reservoirs in crystalline rock at several kilometres depths, and reveals possible strategies for enhancing permeability at an acceptable level of induced seismicity.