Hydraulic fracture reopening pressure and the estimation of maximum horizontal stress

In hydrofracture stress measurements, the magnitude of the maximum horizontal stress, S_H, is commonly estimated from the borehole pressure required to reopen an induced axial crack. Examination of the processes which govern the borehole pressure history recorded during the reopening cycle of such tests indicates two sources of error in the estimates of S_H derived using the conventional method proposed by Bredehoeft et al. [Bredehoeft JD, Wolff RG, Keys WS and Shutter E, 1976, Colorado. Geol. Soc. Amer. Bull., 87, 250–8]. The first arises from the failure to include a term arising from pressure penetration into the crack prior to reopening in the force balance acting across the mouth of the induced axial cracks. The problem can be remedied by using a modified ‘reopening equation’ which includes pressure penetration of the crack. The second source of error is more problematic and concerns the correct identification of the true reopening pressure from the borehole pressure records. Analysis of the process of reopening aided by numerical simulations shows that the true reopening pressure is generally less than the apparent (i.e. that detected) reopening pressure. The discrepancy between true and apparent reopening pressures increases with larger hydraulic compliance of the test equipment. The compliance in question refers to that of the fluid volume between the flow meter and the crack mouth(s). Simulation of a pair of 1 m high axial cracks with 2 μm residual hydraulic aperture in a 100 mm borehole, indicates that the system compliance must be reduced to 5×10⁻⁷ m³/MPa to enable the true reopening pressure to be estimated to better than 10%, at flow rate is less than 10⁻⁴ m³/s. This is several orders of magnitude less than applies to conventional hydrofracture systems, but is attainable for tests in small holes at shallow depth by making relatively minor system modifications. Tests at greater depth, however, would seem to require downhole measurement of flow at the packers. We validate our assertions with a field test in which reopening pressure was determined mechanically and hydraulically.