Abstract
In order to meet the growing energy demand worldwide, the technology of hydraulic fracturing for shale gas extraction has been rapidly developed and applied in recent years. However, man-made fluid injection has also caused several problems of high societal concern such as induced earthquakes. The focus of this paper is on the process of induced fault activation in shale reservoirs caused by hydraulic stimulation activities. We constructed a two-dimensional numerical model representing the horizontal cross-section of a faulted reservoir and simulated important coupled hydro-mechanical processes during fluid injection. We parameterized our model with the site investigation data of an actual shale gas project including in-situ stress, rock properties and injection protocol, etc. We represented the natural fault zone as a geological composite involving a fault core and a damage zone consisting of a set of subsidiary fractures parallel to the fault strike. We coupled the solid deformation and fault displacement with Darcy-type fluid flow based on poroelasticity principles and hydro-mechanical constitutive relationships. The heterogeneous nature of the permeability of the shale formation is mimicked by a random field model governed by a log-normal probability density function and directional correlation lengths, such that we can explore the impact of reservoir heterogeneity on the fault slip behaviour. In addition, we also studied the effects of fault length, dip angle and injection point-to-fault distance on the fault slip. Specifically, we analysed the link between fault slip and fluid flow field, permitting the capture of fluid movement in the heterogeneous faulted reservoir, the temporal and spatial evolution of preferential flow channels and their consequences on induced fault slip. The results of this research have important implications for understanding the triggering processes and mechanisms of fault slip during shale gas exploitation as well as many other injection-related engineering activities.
Highlights
The development of effective reservoir injection technologies has recently become the focus of many geoengineering disciplines, including petroleum recovery, shale gas production and carbon sequestration[1]
Rutqvist et al [8] presented a 2-D numerical model to simulate the potential consequences of fault reactivation during shale gas hydraulic fracturing operations
We first analysed the effects of the position of fluid injection and the correlation length of reservoir permeability field on the fault reactivation behaviour
Summary
The development of effective reservoir injection technologies has recently become the focus of many geoengineering disciplines, including petroleum recovery, shale gas production and carbon sequestration[1]. Significant efforts have been devoted to studying the induced fault slip and seismicity, it is still not clear how the injected fluid migrates in the subsurface; the injection point is sometimes quite far away from the seismogenic fault, but is still able to trigger the fault to slip. It is known in the hydrogeology literature that crustal formations are usually highly heterogeneous accommodating channelized fluid flow [13]. We develop a coupled hydro-mechanical model in the context of the finite element method to analyse how reservoir heterogeneity affects the flow pathways of injected fluids and fault slip behaviour.
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