Abstract:

The paper presents three-dimensional coupled poroelastic analysis of multiple fracture propagation from horizontal wells. A 3D numerical model is developed using a combination of displacement discontinuity method for the hydro-mechanical response of porous rocks and finite element method for the fracture fluid flow simulation. The fracture propagation is analyzed based on the linear elastic fracture mechanics approach. The reservoir rock mass is assumed homogenous and isotropic with constant poroelastic physical properties and the fracturing fluid is assumed incompressible and shows Newtonian behavior. Numerical examples of multiple fracture propagation from the horizontal wells are presented. The results show that along with reservoir rock properties and in-situ stress state, the spacing among fractures has a strong influence on the evolution of multiple fracture propagation. Finally, simulation of refracturing of un-depleted zones between closely-spaced horizontal wells is presented. The results demonstrate the change of stresses and pore pressure due to reservoir depletion mainly depends on the stress anisotropy, reservoir poroelastic properties, and the timing of infill well stimulation or production duration.

Introduction

Horizontal well stimulation usually involves creating cluster of multiple fractures in stages along the wellbore using different well completion techniques. These multiple fractures stages generate large contact surface area with the reservoir which results in increase of permeability and hence, increases the oil and gas production, and the heat energy production in case of the geothermal systems. The multistage fracturing of a single or multiple horizontal wells is usually carried out either in simultaneous or sequential manner. The multiple fracturing creation process is controlled by the mechanical interaction or “stress shadowing” among multiple fractures, in-situ stress anisotropy, and the net applied fluid pressure. The term stress shadow represents variation of stresses in the region surrounding a pressurized fracture, which is a function of applied fluid pressure. The spacing between the fractures and the horizontal wells, and the in-situ stress contrast play an important role in the mechanical interaction (Sesetty and Ghassemi, 2015 & 2016).

Several numerical models have been developed to simulate hydraulic fracturing process in the oil and gas reservoirs based on the elastic rock mass assumptions. The poroelastic effects on the hydraulic fracturing process are generally ignored. The rational is that the hydraulic fracture simulation time-scale is such that these effects have not time to develop or the magnitude of these effects are small enough to be neglected. However, in case of low permeability reservoirs (e.g., tight oil and shale gas reservoir), where hydraulic fracturing is carried out using relatively low viscosity fluids and high injection rates, the coupled poroelastic phenomenon such as changes in the rock deformation due to diffusion of the pore pressure and pressure induced by mechanical deformation of the solid rock need to be incorporated for better understanding of the fracturing mechanism which often involves rock failure and/or reactivation of natural fractures. These poroelastic effects on the hydraulic fracturing process are included using the back-stress and back-pressure concept (Cheng and Detournay, 1988; Vandamme et al., 1989; Vandamme and Roegiers, 1990; Clifton and Wang, 1991; Ghassemi and Roegiers, 1996).

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