Discrete Modeling of Natural and Hydraulic Fractures in Shale-Gas Reservoirs

Abstract

Interconnections of hydraulic fractures and pre-existing natural fractures provide key channels for shale gas to flow at economic rates. Micro-seismic mapping has proved that the resulting fracture system is much more complex compared to most conventional reservoirs due to the massive multistage, multi-cluster hydraulic fracturing stimulations. It becomes crucially important to develop advanced approaches to model such a complex system to better understand the recovery mechanisms and to optimize stimulation and development plans of shale gas reservoirs.Recent advances in geological interpretations and micro-seismic mapping enable realistic modeling of hydraulic and pre-existing fracture network. Consequently, it is possible, to certain extent, to represent actual large-scale fracture distribution in reservoir modeling and simulation of shale gas development. In this paper, we apply the Discrete Fracture Modeling (DFM) approach to represent large-scale fractures individually and explicitly. This entails highly constrained unstructured gridding and construction of a connection (transmissibility) list of all neighboring cells. The geo-mechanical impact on micro-fracture system is modeled by "effective media", in which the rock permeability is sensitive to the stress change induced by hydraulic fracturing and pressure drawdown.Simulations have been performed based on the detailed modeling of an actual shale gas reservoir with considering various mechanisms including adsorption/desorption, matrix-fracture transfer, and non-Darcy effects. Sensitivity studies by varying the production rate, pressure and hydraulic fracture parameters could be onducted to provide guidance on optimizing stimulation and production designs.We have proposed and implemented an innovative simulation workflow that effectively captures multi-scale and multi-physics flow behavior caused by complex fracture system and geo-mechanical impact. A field-scale case study demonstrates that the proposed workflow certainly improves predictability in development of shale gas reservoirs.

The Impact of the Geometry of the Effective Propped Volume on the Economic Performance of Shale Gas Well Production

“…The orientation can also vary from being perpendicular to the direction of the well to following the direction of the well. The EPV is considered an ellipsoidal volume, a geometric characteristic indicated in [15,[17][18][19][20][21][22][23][24][25]. Other geometries are not considered.…”

“…These processes are called hydrofracturing [13,14]. According to microseismic imaging a shale gas formation subject to hydraulic fracturing develops damage zones or fracture volumes, Effective Propped Volume (EPV), with a fundamentally ellipsoidal geometry [15][16][17][18][19][20][21][22][23][24][25]. Furthermore, externally to this ellipsoidal zone, a fracturing zone called Stimulated Recovery Volume (SRV) is produced, generally represented by a prism in the technical and scientific literature [26][27][28].…”

A Workflow for Integrated Geological Modeling for Shale Gas Reservoirs: A Case Study of the Fuling Shale Gas Reservoir in the Sichuan Basin, China

“…The conventional simulation of HFs requires a number of assumptions, e.g., that the shale formation is homogenous or the existence of NFs is not considered and that the HFs are two-winged planar fractures that are distributed symmetrically on the two sides of the well [44,45]. However, both hydraulic fracturing tests and microseismic monitoring data show that the distribution of HFs is neither symmetric nor regular [46].…”

Role of Gas Viscosity for Shale Gas Percolation

“…Thirdly, patterns of shale gas migration play an important role in describing shale gas percolation accurately. It is found that the classic Darcy percolation law cannot accurately describe shale gas behavior especially in nanopores and under different gas states so that desorption, diffusion and slippage, viscous flow of shale gas in pores, Knudsen diffusion and surface diffusion of adsorbed gas, adsorption/desorption, matrix-fracture transfer, and non-Darcy effects [15][16][17][18][19][20] are introduced to describe the percolation law of shale gas; then, the percolation effect between matrix and fractures through percolation velocity, reservoir pressure, and production was analyzed. Fourthly, the improvement of the mathematical calculation method is another key point, such as the lattice Boltzmann method (LBM), finite element method, and finite volume method [14,21,22] which are introduced to improve the accuracy of gas percolation representation considering adsorptive/cohesive forces between the molecules and fracture network distribution.…”