Effects of Stress on Fracture Properties of Naturally Fractured Reservoirs

Abstract

Abstract This paper analyzes the effect of stress on the rock properties fracture and matrix compressibilities, fracture and matrix porosities, and permeability in naturally fractured reservoirs (NFRs). In NFRs, fluids are stored inside the matrix pore space and inside the fractures of the rock. The reservoir characterization parameter indicating the volumetric fraction of fluids deposited inside the fractures is the storage capacity ratio, which is function of the fracture and matrix porosity, and fracture and matrix total compressibilities. Due to the difficulty to obtain these values, in reservoir engineering computations such as pressure transient analysis and reservoir simulation, among others, it is generally assumed that the matrix and the fracture total compressibilities are equal. This induces a big uncertainty in the estimation of the storage capacity ratio and leads to a wrong estimation of the volume of fluids inside the fractured rock. Changes in pore pressure due to production or injection of fluids affect the effective reservoir in-situ stress. The mechanical behavior of the fractured rock and its effects on the rock properties permeability, porosity and compressibility in the matrix and fracture frames are analyzed using the elastic properties bulk modulus and normal compliance of the fracture. These properties can be obtained from petrophysical core analysis or multi-component seismic interpretation, and linked to pressure transient analysis through the storage capacity ratio equation. A step-by-step procedure of the analysis is presented and illustrated with an example for quantifying the effects of changes in effective stress on the fracture and matrix compressibilities. Introduction Well test analysis has been one of the most basic tools to characterize and quantify properties such as permeability, storage capacity ratio and the interporosity flow parameter in naturally fractured reservoirs. This reservoir characterization study integrates several geosciences such as: Petrophysics, Rock Mechanics, Seismic, Geophysics, Reservoir Engineering, and Production Engineering to investigate and quantify the effect of stress on several rock properties: permeability, compressibility, and porosity of naturally fractured reservoirs. Dual Porosity Models These models are used to simulate reservoir systems composed of two different types of porosity, matrix and fracture that coexist in a rock volume. It is usually assumed that the matrix consists of a set of porous rock systems that are not connected to each other, have a low transmissibility and have a high storage capacity. Furthermore, it is also assumed that the fracture system has low storage capacity, high transmissibility and it interconnects the porous media. Most dual porosity models assume that the matrix provides the fluids to the fractures, and the fractures transport the fluids to the well. As shown in Figure 1, different idealizations of the matrix/fracture geometry have been proposed such as the sugar cube model by Warren and Root 1, parallel horizontal fractures by Kazemi 2 and match-stick column models by Reiss 3. The multi-porosity model proposed by Abdassh and Ershaghi 4 is a variation of the dual porosity model, which assumes a fracture set that interacts with two groups of matrix blocks with different porosities and permeabilities.