Advantages of an Unstructured Unconventional Fractured Media Model Integrated Within a Multiphysics Computational Platform

Abstract

Low permeability reservoirs are currently being produced using horizontal wells and massive hydraulic fracturing operations. The design of stimulation jobs requires an integrated knowledge of the reservoir (lithology, mechanical properties, fracture properties, PVT, etc), needing calibration and scenario simulation capacities. Current tools permitting such a workflow exist, yet rarely fully integrated within a single package. In this paper we aim to show the advantages of using two new tools, presently developed as prototypes, namely an unstructured fracture model and a multiphysics coding platform designed to integrate all concepts currently under research pertaining to unconventional reservoirs. Characterization of unconventional reservoirs implies the conciliations of several scales, demanding the integration of a potentially large fracture information database. Within this "rich" database, today's models have difficulties integrating all this information. Thus, improving upon current Discrete Fracture Networks (DFN) would require many fractures to be accounted for (up to 500,000). Coupling of such DFN's to reservoir modeling packages often use up-scaling methods, resulting in models which in turn are simulated using extensions of classical dual continuum models. Current reservoir models do not integrate all physical pertinent phenomena though as being important for gas or multiphase production such as dynamic permeability varying with pressure, nonequilibrium effects, multicomponent adsorption models, diffusion effects or proper transfer functions between matrix and fractures. Using a realistic example inspired from field data, we show how the construction of a fracture model using a consistent Discrete and Deformable Fracture Networks (DDFN), tractable for multiphase flow reservoir simulations, can help describing a complex fracturing case. The use of a coding platform tailored to pertinent unconventional physics is discussed, through examples of developed multiphysics Geoscience applications. The example shows how the integration of the representation of a multistage operations through a DDFN model, using the joint characterization of a field natural fracture system and a propagating fracture network corresponding to the hydraulic fracturing process, calibrated on the BHP and microseismic cloud, is input as a specific unstructured dual discretization into a reservoir model. This explicit description of the fracture geometry is coupled to a non-discretized matrix refinement function accounting for matrix heterogeneities, well-adapted to the dynamic pressure behavior observed in such reservoirs. A generalized multiple interacting continua formulation (named "transient transfer influence function") is used within the matrix medium, allowing the simulation of a longer transition period, typical of many unconventional reservoirs, thus improving matrix contribution during hydraulic fracturing. Because the full process may include several hundred thousand fractures and approximately the same number of cells for the matrix medium, we show how run time performance is improved through a preconditioning technique which reduces the condition number of the matrix associated to the linear system, and speed up the iterative parallel linear solver convergence. The discussion of results obtained by using the integrated DDFN is extended to the potential use of an adapted computational platform which could be used for the inclusion of specific physics pertinent to unconventional reservoirs. This DDFN approach is able to computationally handle 100,000's of fractured coupled to a fluid flow simulator. The platform on which it was implemented could be extended to multiphysics problems, essential for unconventional resources.