Hydraulic Fracturing of Naturally Fractured Tight Gas Formations

Abstract

A workflow is presented for modeling actual hydraulic fracturing jobs in tight gas formations of the Western Canada Sedimentary Basin (WCSB). Matching of hydraulic fracture length and resulting gas production from past jobs provides a learning curve that allows design optimization of future fracturing jobs.Key parameters needed for modeling the design of hydraulic fracturing jobs in these types of formations are discussed and include properties such as porosity (matrix, natural fracture and isolated or non-connected), permeability (matrix and natural fracture), water saturation (matrix and natural fracture), shear velocities (even when direct measurements are not available), Poissons ratio, shear modulus, Young modulus, Biot constant, overburden, pore pressure, and net stress. The critical evaluation of these properties provides a valid representation of the tight gas formation and what the natural gas production outcome from the hydraulic fracturing job might be. Important recommendations to achieve a correct integration of logs, well testing, production decline analysis, and 3D modeling of the hydraulic fracturing job are also presented; as wells as an understanding of the differences that can arise from each source of information. The paper incorporates petrophysical and new linear-dominated well testing and production decline analysis methods using triple porosity models for matching petrographic work and production decline; and for quantifying rock properties such as natural fractures and slot porosity, intergranular porosity and isolated non-effective porosity.It is concluded that even though each tight gas reservoir is unique and as such, should be considered as a research project by itself, the workflow presented in this study could prove to be of value in other regions of the world where tight gas formations are present.

Petrophysics of Triple-Porosity Tight Gas Reservoirs With a Link to Gas Productivity

“…The model is represented by the equation (Al-Ghamdi et al 2010;Deng 2010;Leguizamon and Aguilera 2011) where is total porosity, fraction; b is matrix block porosity scaled relative to the bulk volume of the matrix system, fraction; nc is nonconnected porosity (PHInc) scaled relative to the bulk volume of the composite system, fraction; and 2 is porosity of natural fractures (PHI 2 ) scaled relative to the bulk volume of the composite system, fraction. The development of Eq.…”

“…1 has been presented by Al-Ghamdi et al (2010) where it was used for evaluation of fractured and vuggy carbonates in the Middle East. The use of the equation was extended to evaluation of tight gas formations by Deng (2010) and Leguizamon and Aguilera (2011). The porosities in Eq.…”

“…1 and 2 is calculated from well logs and is represented by . The vug porosity ratio (v nc ), a concept introduced by Lucia (1983), was used initially for evaluation of carbonate reservoirs (Al-Ghamdi et al 2010 and has been extended successfully by Deng (2010) and Leguizamon and Aguilera (2011) for the case of isolated dissolution porosity and other types of nonconnected porosities present in tight gas sands. In this case, v nc is equal to the isolated nonconnected porosity ( nc ) divided by total porosity ().…”

“…For example, the porosity used in the simulation is effective and eliminates the nonconnected porosity. The petrophysical model also helped in the determination of advanced reservoir parameters (Green et al 2007a(Green et al , 2007b such as pressure-dependent modulus stiffness factor, pressure-dependent leakoff (PDL) coefficient and transverse-storage coefficient (TSC) as explained by Leguizamon (2011). There are preliminary indications that larger fracture porosities from the triple-porosity model correspond to larger PDL coefficients and TSCs.…”

“…This is understood as larger fracture half-lengths increasing the area of drainage in these types of reservoirs (Brohi 2011;Brohi et al 2011). This in turn leads to higher economic returns (Leguizamon and Aguilera 2011).…”