Geomechanical Coupled Modeling of Shear Fracturing in Non-Conventional Reservoirs
Creators: name: Nassir, Mohammad
Publisher: name: Graduate Studies
Year: 2013
Subjects: subject: Engineering--Chemical
subject: Engineering--Petroleum
subject: Fracture
subject: Geomechanics
subject: Pseudo-continuum
subject: Non-conventional Reservoirs
subject: Shear Fracturing

description: Hydraulic fracturing is an essential tool for economical development of shale gas and tight gas reservoirs. Analysis of the performance of fracturing jobs and optimization of the treatment design requires modeling which accounts for all important features of the process and ideally covers both the treatment and post-stimulation production of the well. From micro-seismic monitoring and the stimulated wells production data it is now well established that the productivity of the wells is due not only to the classical tensile single plane fracture (SPF), but to the development of an enhanced permeability region (stimulated reservoir volume or SRV) around it caused by shear fracturing and/or stimulation of existing dual porosity. The shape and size of the SRV, and the permeability enhancement in the SRV depend on both the injection process and on the geomechanics of the reservoir (i.e., development of complex fracturing). Current techniques are not able to predict the SRV dependence on fracturing job and rock mechanics parameters, which precludes any meaningful optimization. In this work we have developed a new 3-D coupled geomechanical and flow model for analysis and optimization of tight and shale gas stimulation treatments. The formulation includes the dynamic propagation of tensile (SPF) and shear fractures when the failure criteria are met. Non-fractured blocks are assumed to be of linear elastic material; whereas in the failed blocks, fractures and rock compliance matrices are homogenized to form an equivalent compliance matrix. Simple Mohr-Coulomb and tensile failure relationships were used as the criteria for detecting fracture creation. Hyperbolic function is implemented for each fracture normal deformation analysis which will be integrated into the elasto-plastic constitutive model to describe the fracture overall normal and shear deformation. The permeability enhancement during the fracturing process is computed and is the principal coupling between the flow and geomechanics. The region of enhanced permeability with respect to its initially low value presents what is called in the literature the stimulated reservoir volume. Flexibility of the code to select either tensile or shear fracturing mechanism or combination of both allows various scenarios to be examined. Different cases of 2-D and 3-D simulations are presented which demonstrate some important features of the process. First, it is found that a wide SRV can result in the case where only shear fracturing is the dominant mechanism, and its width depends on the horizontal stress contrast as expected. Second, the loss of elastic coupling due to shear failure and relatively low permeability enhancement of the growing failed region require increasing pumping pressure with time for further failed zone growth, even though the injected fluid is of low viscosity (water). Further, under high injection pressure, an efficient fracture elasto-plastic constitutive model developed drives both maximum and minimum effective stresses to zero or tensile and therefore creation of tensile fracture can be predicted simultaneously with shear fracturing. This will then provide means of modeling proppant transport in some fracturing cases. The examples also show that in order to obtain a relatively wide SRV development, the effective rock cohesion should be of low magnitude. This may be explained by the presence of microfractures and other planes of weaknesses, or by reactivation of pre-existing, sealed natural fractures. Wider SRV propagation is also contributed when the initial reservoir pressure is abnormally high in magnitude. In general, closeness of the reservoir initial conditions to shear failure surface is the key reason for a wide SRV growth. The new model is a significant step towards development of an integrated predictive tool for the optimization of shale gas development and offers a valuable insight into the (still debated) mechanics of shale stimulation. The approach, based on pseudo-continuum treatment using elasto-plasticity combined with SPF modeling has a number of advantages compared to discrete fracture network modeling which is also being pursued.