A computational model is presented which accounts for poroelastic effects, due to fluid migration in the reservoir, during hydraulic fracturing. For massive hydraulic fracturing in a high leakoff formation, the large fluid losses and long injection times cause significant changes in the in-situ stress and the pore pressure of the reservoir. The current model provides a new advanced capability for simulating the evolution of such fractures.

To model the poroelastic effects due to fluid injection, without temperature changes, the reservoir is considered to be a saturated, linear, poroelastic solid. The resulting three-dimensional poroelasticity problem is reduced to a pair of two-dimensional coupled surface-integral equations for the unknown crack opening and leakoff rates [Kurashige and Clifton, 1991]. By introducing commonly used leakoff relations, these equations are uncoupled to obtain an integral equation for crack opening in the poroelastic formation. The difference in pressure required for opening a crack in the poroelastic material versus a purely elastic material is due to the change in the in-situ stress (i.e., backstress) caused by the hydraulic fracturing treatment. These backstresses are evaluated by introducing approximations for the time histories of the crack openings and leakoff which appear in the integrals. These approximations allow the integrations with respect to time to be carried out explicitly in the back-stress calculations. Expressions to account for the poroelastic changes in in-situ stresses and pore pressures are presented. Implementation of this model in a three-dimensional hydraulic fracturing simulator is described. Example simulations are presented and discussed.

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