Development of a Coupled Dual Continuum and Discrete Fracture Model for the Simulation of Unconventional Reservoirs

Abstract

Accurate modeling of hydrocarbon production is a necessary, yet challenging step for economic exploitation of unconventional resources. One of the main challenges is to model flow to a horizontal well from a complex network of hydraulic and natural fractures. Many unconventional reservoirs comprise well-developed natural fracture networks with multiple orientations and complex hydraulic fracture patterns based on microseismic data. Conventional dual porosity and dual permeability models are not adequate for modeling these complex networks of natural and hydraulic fractures. Also, it is neither practical nor advantageous to model a large number of pre-existing fractures with a discrete fracture model. Therefore, an appropriate approach to model production from low-permeability reservoirs is to perform discrete fracture modeling for hydraulic fractures and employ a dual continuum approach for numerous natural fractures. We have developed a coupled dual continuum and discrete fracture method to simulate production from unconventional oil and gas reservoirs. Large-scale hydraulic fractures (macro-fractures) are modeled explicitly using a discrete fracture model, called EDFM, and numerous small-scale natural fractures (micro-fractures) are modeled using a dual continuum approach. The hybrid model includes three domains: matrix, discrete-fracture, and continuum-fracture domains. A systematic approach is devised to calculate transport parameters between all three domains. Moreover, EDFM allows for not only transverse and longitudinal hydraulic fractures but also macro-fractures of any orientation. Thus, the coupled model provides an effective and reliable environment to improve stimulation designs and completion strategies. We present several examples in this study to show the applicability, robustness, and performance of the hybrid method for the simulation of unconventional oil and gas reservoirs. We examine multi-stage hydraulic fractures with multiple configurations in the presence of numerous pre-existing fractures. Simulations show a noticeable contribution from natural fracture networks on total production. Furthermore, for the tight oil reservoir examined in this study, the stimulation scheme with longer hydraulic fractures improves cumulative oil production compared to the scheme with larger number of shorter hydraulic fractures. We also examine production from a tight gas reservoir wherein hydraulic fractures partially penetrate the formation height. Simulations indicate that inefficient fracture treatment can result in significant loss of production.

An embedded fracture modeling framework for simulation of hydraulic fracturing and shear stimulation

“…The EFM approach has been verified by previous authors for a range of physical scenarios of practical interest, including black oil [21], geothermal [15], unconventional gas [26], and injection-induced seismicity [16,29]. Here, a numerical example is shown to demonstrate the validity of the present model.…”

“…Moinfar et al [26] incorporated a treatment for calculating fracture permeability evolution due to changes in effective stress within an EFM framework, but did not include a formal treatment for geomechanics. Karvounis et al [16] extended their model to include a proxy geomechanical model based on changes in pore pressure to investigate injection-induced seismicity.…”

“…In this manner, shear stimulation is able to propagate at a rate related to the enhanced fracture transmissivity, rather than the initial fracture transmissivity. Considering this mechanism, models that employ simplified geomechanical models based purely on pressure diffusion, such as those presented by Moinfar et al [26] or Karvounis et al [16], may not be sufficient to predict reservoir response realistically in many injection and production scenarios of practical interest. The numerical simulation results presented in this section demonstrate that the EFM approach can be applied to solve coupled fluid flow and geomechanics problems with nonlinear fracture permeability evolution.…”

“…Previous authors have applied EFM to investigate geomechanical effects in fractured reservoirs, but this work has been limited in scope to simplified models that embody the geomechanics into empirical relationships [16,26]. In this work, the reservoir model introduced by McClure [23] and McClure and Horne [24] was extended to incorporate the EFM strategy in order to combine the effects of matrix-fracture mass exchange and a rigorous treatment of geomechanics under a unified framework.…”

The multiscale restriction smoothed basis method for fractured porous media (F-MsRSB)

“…Physical properties inside fractures and their length scales can be very di↵erent from those of the surrounding rock, adding significantly to the computational challenges, specially once realistic length scales and complex fracture network maps are considered. As a result, a variety of modelling approaches and numerical methods for di↵erent types of fractured reservoirs have been proposed [32][33][34][35][36][37][38][39][40][41][42][43][43][44][45][46][47]. Among them, the embedded fracture modelling approach [1, 37-39, 48, 49] benefits from independent grids for fracture and matrix, a promising approach for naturally fractured reservoirs and also for cases with dynamic fracture creations and closure of, e.g., geothermal systems.…”