\ Evolution of the plastic zone near a microfracture: a numericdl simulation and its implications on in situ stress measurement ' Depnrtmerzt of Civil Eqii~eering, Tlze Uraiversity of Alberta, Ed~nor~torz, AB T6G 2G1, Cat~nrln SAMUEL S. SHEN Depnrttnertt of Mntlze~mtics, The University of Alberta, Erl~r~ontort, AB T6G 2G1, Canndri AND HAIBING CHENG Deprtrnent of Civil Eltgirzeerirzg, Utziversit~l of Waterloo, Waterloo, ON N2L 3G1, Cartncln Received May 5, 1993 Accepted June 2, 1994 The instantaneous shut-in pressure has been used to estimate the far-field in situ minimum principal stress during microfracture testing. A plastic zone can be induced near the fracture. Because of the early plastic (irreversible) defor- mation induced near the fracture, the irreversible deformation near the fracture surface reduces the fract~~re pressure and generates a discrepancy between the far-field minimum stress and the fracture closure pressure, which has been identified as the minimum in situ stress in the past. In this paper, a finite-element numerical model is used to investigate this discrepancy due to the plastic deformation near a fracture. It is concluded that a plastic zone can be generated near a hydraulic fracture in poorly consolidated media. The fracture closure pressure can be much smaller than the minimum in sitt~ stress due to the irreversible deformation generated near the fract~lre during the active frac- turing period. Thus, one cannot use the conventional hydraulic-fracturing technique to interpret the minifracture tests in a poorly consolidated medium such as oil sand. ., ' Key worc1.s: microfracturing, in situ stress measurements, oil sands, plasticity, numerical sirnulition. La pression de fermeture instantanie a CtC utilisCe au cours d'essais de microfracture p o y ~ evaluer la contrainte principale minimum in situ dans un massif. Une zone plastique peut &tre induite p r b de 1% fracture. Par suite de la dCformation plastique (irrkversible) prCcoce induite prks de la fracture, la dCformati'on irreversible prks de la surface de la fracture rCduit la pression de fracturation et produit une inconsistance entre la contrainte minimum dans ie mas- sif et la pression de fermeture de la fracture, qui a CtC identifiCe comme Ctant la contrainte minimum in situ dans le passC. Dans cet article, un modkle nutnkrique en ClCments finis est utilisi pour Ctudier I'inconsistance due h la dCformation plastique prks de la fracture. L'on conclut qu'une zone plastiq~~e peut &tre gCnkrCe prks d'ttne frac- ture hydraulique dans un milieu faiblement consolidC. La pression de fermeture de la fracture peut &trebea~~coup plus faible que la contrainte minimum in situ 2 cause de la dCSormation irrkversible gCnCrCe prks de la fracture durant la pCriode active de fracturation. Ainsi, I'on ne peut pas ~~tiliser la technique conventionnelle de fracturation hydraulique pour interpreter les essais de minifracturation dans un milieu faiblement consolid6 tel que le sable bitumineux. Mots clts : microfracture, mesures de la contrainte in situ, sables bitumineux, plasticitC, sin~~~latioli numkrique. [Traduit par la ridaction] Can. Geotech. J. 31, 779-787 (1994) Introduction The use of microfracture tests to determine in situ stress profiles has become a standard part of the hydraulic-fracturing treatment and design process. A conventional microfracture test consists of several consecutive short pumping cycles (usually 3-5) to pressurize a preselected interval of the well so that the adjacent rock formation may be fractured. Each cycle is followed by a shut-in period of about 15-20 mi11 (Nolte 1979; Gronseth and Kry 1983; Boone et al. 1991n). A typical schematic pressure-time record is given in Fig. 1. During the microfracture tests ~mder our consideration, the pump was sh~tt off once the fracture was opened. The pressure that had built LIP due to the p~~mping process declined rapidly because of the fluid dissipation into the rock formation and backflow of the fluid into the wellbore. The fracture continued to propagate after shut-in, and the 'present addrcss: Shcll Canada Ltd., P.O. Box 2506, Calgary, AB T2P 3S6, Canada. fracture ~~ltimately closed (Boone et al. 19910). Conven- tionally, it is assumed that right at the moment of the frac- ture closure the pressure inside the fracture is equal to or slightly higher than the far-field stress normal to the fracture. Hence the minimum in situ stress in hard-rock regions can be estimated from the instantaneous shut-in pressure, extracted from the post-shut-in pressure curves obtained from the fracturing test (Nolte 1979; Gronseth and Kry 1983; Boone et al. 1991a). Conventionally, the minimum in situ stress oh has been evaluated by where p, is the shut-in pressure, and Ao is the induced stress on the fracture surface, which is assumed to be negligible according to the conventional stress-measurement theory. Many factors, however, can make significant contributions to the induced stress ACT, including the poroelastic effect (Boone 1989; Boone et al. 19910) and the plastic deforma- tion; the latter is subjected to study in this paper. Can. Geotech. J. Downloaded from www.nrcresearchpress.com by Depository Services Program on 06/11/11 For personal use only.