Abstract:

Unconventional reservoirs are often characterized by a multicontinuum stimulated formation with complex propped and unpropped fracture networks that occur over a wide range of length-scales and geometries. Coupled numerical simulation of geomechanics and flow and transport in such formations can lead to an improved understanding of production from these systems. Computational efficiency and accuracy are both critical for modern simulation workflows for engineering design. A mixed continuous-discrete fracture discretization approach is proposed whereby a single Cartesian mesh is applied, and discrete representations are embedded to capture primary fractures, whereas continuous representations may be used for well-connected natural fractures. The geomechanics is approximated using a hybrid extended finite element method (XFEM) for discrete fractures and a dual porosity-poroelasticity treatment for the continuous representations. The flow and transport in discrete fractures are approximated using an embedded-discrete-fracture model (EDFM), whereas the continuous representation is through a dual porosity treatment. In addition to validation and mesh refinement results, a number of computational examples are presented illustrating the first-order effects of the coupled physics in realistic models.

Introduction

Production from unconventional resources such as tight and shale-gas reservoirs involves the coupling of a number of complex multiphase flow and geomechanical phenomena. Unconventional reservoirs are comprised of an ultra-low permeability fractured matrix that may consist of multiple continua. Natural fractures form highly connected networks with a wide range of length scales. The fractures are primarily supported by pore pressure. Horizontal wells stimulated by hydraulic fractures that emanate from the wellbore along clusters within multiple stages. The fractures are supported by pore pressure as well as proppants that are delivered during the completions process. The proppant concentration may vary along fractures.

Empirical observations from field practice suggest that geomechanical effects may play a first-order role in production from unconventional reservoirs. It is frequently observed that the production rate in tight gas reservoirs declines rapidly in a manner that is believed to be closely related to the evolution of fracture aperture and permeability with time (Huang and Ghassemi 2012). Proppant embedment and pore pressure-structure interactions can have a significant impact particularly within the interplay between offset wells. Numerical simulation of the coupled process in such formations can lead to an improved understanding of this interplay.

Numerous models have been proposed to capture the effects of a range of complex constitutive relations for flow and transport including confinement and non-Darcy effects (Jiang and Younis, 2015; Wu et al., 2013). Models for that resolve the geomechanics in gas production systems are scarce due to the complexity of treatment for the coupling to flow and due to the presence of discontinuities in the form of discrete fractures. The influence of conductivity loss associated with discrete hydraulic fractures has also not been well studied. Therefore, coupled processes between matrix deformation and fluid flow are important to predict fracture permeability change and reservoir production behavior (Huang and Ghassemi 2012).

This content is only available via PDF.
You can access this article if you purchase or spend a download.