Interpretation of shut-in pressure in hydrofracturing pressure-time records using numerical modeling

A definite shut-in pressure in hydrofracturing techniques is needed for obtaining the correct information on the in-situ stress regime in a rock mass. The relation between the behavior of hydraulically induced fractures and the condition of remote stress is considered to be the major reasons of an ambiguous shut-in pressure in hydrofracturing pressure-time history curves. This paper describes the results of a series of numerical analyses carried out using a code based on the distinct element method, to compare several methods for determining the shut-in pressure during hydro-fracturing. The fully coupling of hydraulic and mechanical analysis was applied, and the effects of four different discontinuity geometries in numerical modeling have been investigated for this purpose. The effects of different remote stress regimes and different physical properties on hydraulic fracture propagation have been also analyzed. Several methods for obtaining shut-in pressure from the ambiguous shut-in curves have been applied to all the numerical models. The graphical intersection methods, such as(P vs. t) method,(P vs. log(t)) method,(log(P) vs. log(t)) method, give smaller values of the shut-in pressure than the statistical method,(dP/dt vs. P). Care should be taken in selecting a method for determining the shut-in pressure, because there can be existence of a stress anomaly around the well bore and fracturing from the wellbore by a constant flow rate may have a more complicate mechanism. (C) 2011 Elsevier Ltd. All rights reserved.