Abstract
Hydraulic fracturing has been widely used for unconventional reservoirs especially after the technique of horizontal drilling was invented. Acidizing treatment is often incorporated as a propagation enhancement of fractures, in particular, for very tight, low-permeability carbonate-rich reservoirs. How an individual crack propagates into a stressed medium subject to fluid pressure acting on the crack surfaces and meanwhile being affected by the chemically aggressive environment is still an open question. This short paper investigates the fundamental coupled chemo-hydro-geomechanics as encountered in typical scenarios of hydraulic fracturing, with a specific focus on the role of acidizing treatment. The constitutive relations consisting of a reactive-chemo-elastic and a reactive-chemo-plastic formulation are presented, followed by the coupling with geochemical processes, namely reactive-diffusion equations. Numerical investigations of two representative cases within the chemo-elastically defined regime are presented, featuring a laboratory injection test and in-situ stress conditions of a deep geothermal reservoir, respectively. The results have demonstrated that the chemical dissolution process plays a critical role in the distribution of circumferential stress around the crack tip as well as the evolution of crack propagation, and that acidizing treatment may accelerate cracking exponentially after sufficient chemical exposure.
Highlights
The technique of hydraulic fracturing has been developed for many years and has gained enormous success world-widely and in USA, the modelling of this multiple-physics/chemical fracturing process falls far behind field applications
This study investigates a fundamental problem, i.e. how the chemically aggressive environment affects crack propagation, in particular, how an individual crack propagates into a stressed geomaterial subject to a combined chemical and mechanical load
The chemo-elastically affected zone is the centre of this study and the deviatoric strain invariant is chosen to be a proxy for the local mechanical damage
Summary
The technique of hydraulic fracturing has been developed for many years and has gained enormous success world-widely and in USA, the modelling of this multiple-physics/chemical fracturing process falls far behind field applications. This study investigates a fundamental problem, i.e. how the chemically aggressive environment affects crack propagation, in particular, how an individual crack propagates into a stressed geomaterial subject to a combined chemical and mechanical load. By the definition in fracture mechanics, a crack extends when the stress intensity (indicated by the Jintegral) is no less than the material toughness. Subcritical crack propagation can be promoted by the following means: 1) enhancing stress localization in the vicinity of the crack tip, via e.g. decreasing the energy potential for generating new surfaces of micro-cracks in the process zone around the tip; or 2) lowering the
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