Abstract
Six hydraulic fracturing (HF) experiments were conducted in situ at the Grimsel Test Site (GTS), Switzerland, using two boreholes drilled in sparsely fractured crystalline rock. High spatial and temporal resolution monitoring of fracture fluid pressure and strain improve our understanding of fracturing dynamics during and directly following high-pressure fluid injection. In three out of the six experiments, a shear-thinning fluid with an initial static viscosity approximately 30 times higher than water was used to understand the importance of fracture leak-off better. Diagnostic analyses of the shut-in phases were used to determine the minimum principal stress magnitude for the fracture closure cycles, yielding an estimate of the effective instantaneous shut-in pressure (effective ISIP) 4.49±0.22 MPa. The jacking pressure of the hydraulic fracture was measured during the pressure-controlled step-test. A new method was developed using the uniaxial Fibre-Bragg Grating strain signals to estimate the jacking pressure, which agrees with the traditional flow versus pressure method. The technique has the advantage of observing the behavior of natural fractures next to the injection interval. The experiments can be divided into two groups depending on the injection location (i.e., South or North to a brittle-ductile S3 shear zone). The experiments executed South of this zone have a jacking pressure above the effective ISIP. The proximity to the S3 shear zone and the complex geological structure led to near-wellbore tortuosity and heterogeneous stress effects masking the jacking pressure. In comparison, the experiments North of the S3 shear zone has a jacking pressure below the effective ISIP. This is an effect related to shear dislocation and fracture opening. Both processes can occur almost synchronously and provide new insights into the complicated mixed-mode deformation processes triggered by high-pressure injection.
Highlights
The dynamic injection pressure response monitored during hydraulic fracturing treatments in fractured reservoirs contains information on reservoir hydraulic and geomechanical properties which are, in turn, critical to describe and predict hydromechanical processes
This study extends the analysis of presented results from in-situ hy draulic fracturing experiments performed at the Grimsel Test Site (GTS) in Switzerland,[28] which were executed in the framework of the In-situ Stimulation and Circulation (ISC) project.[29]
The experiments performed next to the S1 zone indicate a connection from the hydraulic fracture towards the fracture set associated with the S1 zone, which is further disconnected to the tunnel (Fig. 1c)
Summary
The dynamic injection pressure response monitored during hydraulic fracturing treatments in fractured reservoirs contains information on reservoir hydraulic and geomechanical properties which are, in turn, critical to describe and predict hydromechanical processes. Histori cally, the transient pressure analyses (TPA) that are used as a diagnostic tool relied entirely on analytical models of fluid flow, without hydromechanical coupling. The first and most straightforward solution to constant fluid injection/withdrawal, which governs radial flow in a porous medium, was introduced by Theis[1] for groundwater flow. The TPA was extended to geothermal wells, including the two-phase flow of water and steam, adsorption of steam and pres surized reservoirs (more in Zarrouk & McLean[3]). Barker[5] introduced the generalized radial flow model for fractured reservoirs, which was extended to fractal fracture networks.[6,7]
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