This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 137936, ’Relationship Between the Hydraulic Fracture and Observed Microseismicity in the Bossier Sands, Texas,’ by A. Guest, SPE, and A. Settari, SPE, University of Calgary, prepared for the 2010 Canadian Unconventional Resources & International Petroleum Conference, Calgary, 19-21 October. The paper has not been peer reviewed.

There is an increased effort to improve our understanding of hydrofracturing by use of microseismic monitoring and analysis. A numerical method was developed to predict microseismicity occurring during hydrofracturing and the influence of fracturing on the permeability of the reservoir. The core of this technique is a representative pre-fractured volume of the reservoir that deforms locally and allows the coalescence of deformation as the stress re-equilibrates.  

Introduction

Hydraulic fracturing is used to increase the effective permeability of a producing formation and, thus, increase the production of hydrocarbons. Even though microseismicity provides a reasonable estimate of fracture parameters, our understanding of the relationship between microseismic events and the induced hydraulic fracture is not complete.

Hydrofracturing of the Bossier sandstone in east Texas was studied. Early on, hydrofracturing was modeled assuming a single fracture propagating from the treatment well, with geomechanical permeability changes determined from changes of fluid pressure or stress. Reservoir properties were matched to fit the bottomhole pressure for the hydrofracturing and production for an 80-day period such that the permeability used in this model reasonably represents the permeability of the reservoir. A technique was developed to simulate the occurrence of microseismic events during hydrofracturing by numerical modeling of fracture self-localization. This technique forms fractures the size of the grid cell and allows coalescence of neighboring fractures, depending on the evolution of stress in the reservoir. The analysis of the deformation modes (seismic-moment tensors) of such fractures showed that such deformation, even though locally and temporally variable, reflects the overall macroscopic deformation that would be caused by classical hydrofracture propagation.

Technique

To solve for the deformation associated with hydrofracturing, mechanical equations and equations for fluid motion in porous media must be solved.  The equations are coupled through pressure and permeability. In this paper, because the calculation of permeability for this technique is still not fully resolved, the mechanical and fluid-flow solutions are coupled in only one way—by treating the pressure function, generated by the flow model with pressure-dependent permeability and a hydraulic fracture, as input into the mechanical equations. Then, the local 2D geomechanical solution is based on the constitutive and geometrical equations, and mechanical equilibrium equations, which are detailed in the full-length paper. The fracture formation is solved by use of damage theory. The damage initiates when the Mohr-Coulomb or tensile-failure criteria are satisfied locally.

The heterogeneity of the material can be introduced through a random function following the Weibull distribution. The heterogeneity is input for strength of the material and Young’s modulus.

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