Effects of Stress on Fracture Properties of Naturally Fractured Reservoirs

Abstract

fax 01-972-952-9435. AbstractThis 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.

An Alternative Approach for Well Test Analysis and Production Performance in Tight Gas Reservoirs Considering Stress Dependent Permeability

“…Three cases are considered. For the two values of permeability, the IPR have been constructed by using the simple gas well deliverability equation (4). The third one has been generated using them′ p and k = 0.7 mD represent the stress sensitive case.…”

“…Developing trends of the permeability reduction vs. increasing effective stress is recommended as far as future performance modeling is concerned. 4. At low values of flowing bottom-hole pressures, net effective stress is high and significant loss in permeability;hence loss in productivity may be experienced.…”

“…Substituting the value of k(p) from equation (3) in equation (4) and simplifying, we will get: (complete derivation is presented in Appendix). …”

“…Where σ n is the net effective stress, σ ob is the overburden pressure, p p is the pore pressure of the formation and α is the Biot's coefficient which is usually assumed to be unity [4]. As soon as gas production begins from tight reservoirs, the reservoir pressure declines which causes an increase in the net effective stress as depicted by (1).…”