Hydraulic fracturing is an important and prevalent process both in the natural environment and industrial applications. At the same time, field hydraulic fracturing tests data provide valuable information regarding the mechanical and hydraulic behaviours of the reservoir formation. By history-matching the field bottom-hole-pressure versus time curve from hydraulic fracturing tests, a set of field-validated geomechanical models can be obtained, which is an important asset for any further works on utilizing geomechanics to enhance the injection and production performance. This paper presents a 3-dimensinal finite element model for history matching the complete bottomhole- pressure versus time curve generated during hydraulic fracturing tests considering the injection rate as input. To stimulate the hydraulic fracturing process in unconsolidated sands formation, a poro-elasto-plastic constitutive model together with a strain-induced anisotropic fill permeability model are formulated and implemented into a 3D finite element geomechanical simulator. Unlike the conventional simulation of hydraulic fracturing in hard rock, hydraulic fracture in unconsolidated sands reservoir is stimulated as a large area of dilation zone or a net or micro-cracks, inside which the effective stresses are low and hydraulic conductivities are high. It is shown the proposed numerical model can successfully capture the hydraulic fracture initiation and propagation in unconsolidated sands formation and matches the field pressure versus time curve very well.
Hydraulic fracturing can be broadly defined as a process by which a fracture initiates and propagates due to hydraulic loading (i.e., pressure) applied by a fluid inside the fracture(1). Fractures in the earth's crust are desired for a variety of reasons, including enhanced oil and gas recovery, re-injection of drilling or other environmentally sensitive wastes, measurement of in situ stresses, geothermal energy recovery, and enhanced well water production(2). Although hydraulic fracturing in hard rock has been comprehensively studied both experimentally and numerically, some fundamental mechanisms of hydraulic fracturing in unconsolidated sands have not been well understood. Experimental data clearly show that fracturing in unconsolidated sands is significantly different than those encountered in hard rock. Unconsolidated sands do not exhibit elastic-brittle behavior. In addition, unconsolidated sands have very low tensile and shear strengths at low effective stresses as well as relatively large fluid leak-off(3–5). Based on other researchers' work(3–7) on the fundamental mechanisms of hydraulic fracturing in unconsolidated sands, this paper presents the constitutive modeling and numerical simulation of hydraulic fracturing in unconsolidated sands within the framework of continuum mechanics. Numerical experiments show that this approach has special advantages in numerical modeling of large scale field problems, such as the disposal of waste cutting fluid, and micro/mini fracture tests in unconsolidated sands formation.
Even in its most basic form, hydraulic fracturing in unconsolidated sands is a complex process to model, as it involves the coupling of at least three processes:
the solid matrix deformation and failure induced by the pore fluid pressure;
the flow of fluid within the fracture and solid matrix; and
the fracture initiation and propagation after the failure of formation.