Pressure analysis of the hydromechanical fracture behaviour in stimulated tight sedimentary geothermal reservoirs

Abstract

Hydromechanical phenomena in fractured sediments are complex. They control the flow in stimulated tight sediments and are crucial for the exploitation of geothermal energy from such rocks. We present the analysis of a cyclic water injection/production (huff-puff) process, a promising method to extract geothermal energy from tight sedimentary reservoirs. It uses a single borehole, which considerably reduces investment costs. A huff-puff test was performed in a 3800-m deep sedimentary formation (borehole Horstberg Z1, Lower Saxony, Germany). The analysis presented herein explains the downhole pressure measurements by a simplified reservoir model containing a single vertical fracture. The model addresses the flow behaviour between the fracture and the rock matrix in a layered formation, and the coupling between fluid flow and the mechanical deformation of the fracture. The latter aspect is relevant to predict the efficiency of the geothermal reservoir because cooled regions resulting from a particular injection/production scheme can be identified. The analysis methods include, 1) the curvefitting code ODA used for a determination of different flow regimes (radial or linear), 2) an analytical solution for the calculation of the injection pressure, assuming a time-dependent fracture area, and 3) the simulator ROCMAS, which numerically solves the coupling between fluid flow and fracture deformation. Whereas each single approach is insufficient to explain the complete test data, a combination of the results yields an understanding of the flow regimes taking place during the test.

3-D geomechanical–numerical model of the contemporary crustal stress state in the Alberta Basin (Canada)

“…Bell and McLellan, 1995) in the case of reservoir stimulation by hydraulic fracturing. This is important in geothermal reservoirs (enhanced geothermal systems -EGS) (Legarth et al, 2005;Wessling et al, 2009) and issues of inadequate understanding of the spatial stress pattern (e.g. Brown, 2009;Duchane and Brown, 2002).…”

Joint estimation of transmissivity and storativity in a bedrock fracture

“…Water is stored or released from the fracture as it opens and closes in response to hydraulic stresses and from the adjacent rock matrix. Matrix flow can be ignored in low‐pressure injections or low‐conductivity rocks [ Mathias et al , 2010], suggesting that in many shallow bedrocks, storativity is a function primarily of the opening and closing of the aperture in response to fluid pressure (fracture stiffness) [ Rutqvist and Stephansson , 2003; Schweisinger et al , 2009; Wessling et al , 2009]. As both storativity and transmissivity are related to aperture, therefore, they are physically related in bedrock fractures.…”

“…As both storativity and transmissivity are related to aperture, therefore, they are physically related in bedrock fractures. Although fully coupled hydromechanical models have been able to reproduce responses from single‐ and cross‐hole hydraulic tests in bedrock [ Cappa et al , 2006, 2008; Mathias et al , 2010; Rutqvist et al , 1998; Schweisinger et al , 2009; Wessling et al , 2009], they have yet to be combined with advanced tomographic inversion techniques as described above for modeling purposes.…”

3-D-geomechanical-numerical model of the contemporary crustal stress state in the Alberta Basin

“…During hydraulic fracturing (Bell, 1996a;Haimson and Cornet, 2003;Hubbert and Willis, 1957; and leak-off tests (e.g. Li et al, 2009;White et al, 2002;Zhou, 1997), the down-hole pressure is increased up to pressure loss due to fluid leakage in the rock mass. This happens, when the hydraulic fracture splits apart the surrounding rock perpendicular to the least principal stress (σ 3 ), usually assumed to be S hmin in sedimentary basins, and therefore the fracture opens in S Hmax orientation (Fig.…”

“…The term "leak-off tests" is used variably, and can be distinguished by their aim into formation integrity tests (FIT), "classic" leak-off tests (LOT) and extended leak-off tests (XLOT) (White et al, 2002). The general method is similar, but differs in pumping cycles and the point at which the pumping is stopped.…”