Adapting High Permeability Leakoff Analysis to Low Permeability Sands for Estimating Reservoir Engineering Parameters

Abstract

Abstract The modified Mayerhofer method has been proposed for estimating permeability from the pressure falloff data in moderate and high permeability reservoirs before hydraulic fracture closure following a diagnostic fracture injection test. Applying the modified Mayerhofer method in low permeability sands, however, requires understanding of the closure mechanism, which is identified with G-function derivative analysis of the before-closure pressure falloff data. This paper demonstrates how G-function derivative analysis and the modified Mayerhofer method are used in conjunction to estimate reservoir permeability in low permeability reservoirs. Numerous applications of G-function derivative analysis have shown the characteristic closure mechanisms—normal, pressure-dependent leakoff from fissure opening, fracture-height recession, fracture-tip extension, and changing compliance—all result in distinctive specialized plots using the modified Mayerhofer method. When the two methods are used in conjunction, G-function derivative analysis provides a means for identifying the falloff data that can be used to estimate permeability and fracture-face resistance without violating the assumptions of the modified Mayerhofer method. Field cases are included to demonstrate that reasonable estimates of reservoir permeability in low permeability reservoirs often can be obtained from the before-closure pressure falloff following a diagnostic fracture injection test. Introduction Valk and Economides1 published the modified Mayerhofer method for estimating permeability in moderate and high permeability reservoirs from the before-closure pressure falloff data following a diagnostic fracture injection test. The modified Mayerhofer method is based on the technique proposed by Mayerhofer, et al.,2 and differs from conventional pressure decline analysis in that the problem is formulated in terms of permeability and fracture face resistance as opposed to leakoff coefficient and spurt loss. Before-closure pressure falloff analysis techniques are beneficial for low permeability reservoirs since the shut-in time requirements are substantially lower than the time required for after-closure pressure falloff analysis. Nolte, Maniere, and Owens,3 however, have noted that fracture extension and fracture recession during closure limit the applicability of before-closure pressure falloff analysis techniques. Nolte, et al.,3 suggest that after-closure analysis of pseudolinear and pseudoradial flow regimes are superior methods for estimating reservoir parameters, but in low permeability reservoirs, the time required to achieve pseudolinear and pseudoradial flow following a fracture injection test can be excessive. G-function derivative analysis was recently proposed for identifying the leakoff mechanism—normal, pressure-dependent leakoff from fissure opening, fracture-height recession, fracture-tip extension, and changing compliance—from the pressure falloff following a diagnostic injection test. G-function derivative analysis also provides a method for identifying the falloff data that can be used to estimate permeability and fracture-face resistance without violating assumptions of the modified Mayerhofer method. The objective of this paper is to demonstrate how G-function derivative analysis and the modified Mayerhofer method are used in conjunction to estimate permeability in low-permeability reservoirs. Additionally, field cases are included to demonstrate that reasonable estimates of reservoir permeability can often be obtained from the before-closure pressure falloff data in low permeability reservoirs.