Stress measurements in anisotropic rock by hydraulic fracturing

Many practical applications of rock mechanics require knowledge of the in-situ state of stress. Conventional stress measuring methods, such as borehole deformation gage and door stopper techniques, are severely limited in depth, rock quality, and stress level. Hydraulic fracturing has the potential of overcoming most of these limitations, and so far laboratory experiment and field measurements have demonstrated the soundness of the technique in intact quasi-isotropic rock. Recently an experimental program was undertaken to study the effectiveness of hydraulic fracturing in extremely anisotropic rock. Thinly foliated Martinsburg slate was hydrofractured at 4 different cleavage inclinations, under a set of 3 unequal simulated principal in-situ stresses. The results showed a predictable relationship between in-situ stresses and hydraulic fracturing pressures dependent on filiation inclination. Under normal in-situ stress conditions, and by hydrofracturing through a thin lining of Plaster of Paris, all the induced fractures were vertical and perpendicular to the direction of the smaller horizontal principal stress, just as in the isotropic case.