The purpose of this paper is to provide insight into and methods for quantifying how poro-elastic effects may influence stress determination through micro-frac tests. Specifically, it is shown that both the fracture pressure, Pf, and the instantaneous shut-in pressure, Pisip, may increase due to poro-elastic effects. It is shown that these effects could be important formations with effective permeabilities to the injected fluid as low as 1 mD. Furthermore, it is shown that the increase in the Pf and Pisip can be less for highrates of injection than it is for low rates. High-rate injection tests can be characterized by lower values of non-dimensional times so that the change in Pf and Pisip maybe markedly less. These effects provide a plausible explanation for results of a series of micro-frac and highrate injection tests in oil sands and suggests that improved stress determination results from multiple rate injection tests.
Stress determination by means of micro-frac tests has become a common method for evaluation of stress east depth. The method has been verified by comparison with others tress measurement techniques such as overcoring [Haimson] and has proven to be a reasonably reliable and accurate method of stress measurement. However, comparative studies have been limited almost exclusively to rocks, such as granites which have negligible permeability. Micro-frac tests are now being used extensively in permeable, oil bearing formations. The purpose of these tests is to determine injection pressures required for hydraulic fracture treatments, to determine how the variation of the stress with depth may influence fracture extension, and to determine whether the stress state favours horizontal or vertical fractures. Experience has shown that micro-frac tests in oil bearing rock respond in a manner that is consistent with prior experience in nearly impermeable rocks [Gronseth and Kry, 1987]. In particular, the same techniques can be used to identify an instantaneous shut-in pressure (Pisip) which is equated to the minimum in-situ stress(s min). These similarities have given credence to the application of micro-frac testing in permeable rocks. In the past, it has been shown by several authors[Cleary, Smith, Boone and Detournay] that the magnitude of poroelastic effects can be significant. It is also well established that fracturing pressures are a function of changes in reservoir pressure that occur over long periods of time[Felsenthal and Ferrell, Salz]. Detournay et al. have investigated the case of a pressurized fracture under plane strain conditions with a fixed length. They have also include discussion and a conceptual model of how poro-elastic effects may influence in - situ stress determination. In particular, the fracture pressure at closure, Pfoc, can exceed sn, the stress acting normal to the fracture which is typically assumed to be (smin. Boonee et al. have used a poroelastic finite element model with specialized fracture modeling capabilities to show that (1) fracture closure can be clearly identified from the pressure-time history at the wellbore and( 2) that the P foc can be significantly greater than sn.
