Estimation of horizontal stress magnitudes using sonic data from vertical and deviated wellbores in a depleted reservoir

Abstract

AbstractChanges in the reservoir rock stresses caused by large variations in the pore pressure play an important role in maintaining reservoir mechanical stability and therefore are important input to drilling and completion decisions. Producer wells are often designed with complex trajectories that include both nearly vertical and deviated sections to maximize hydrocarbon productivity. A new inversion algorithm estimates the maximum and minimum horizontal stresses in the formation using radial profiles of the three shear moduli in the two axial and cross-sectional planes of a vertical or deviated borehole. This algorithm inverts differences in the far-field shear moduli with the two difference equations obtained from radial profiles of the dipole shear moduli in the two axial planes between near and far radial positions from the borehole surface. Borehole stresses are related to the far-field principal stresses in terms of borehole azimuth and deviation. Difference equations for the shear moduli and corresponding stresses use radial positions away from the borehole surface that do not exhibit any plastic deformation. Borehole sonic data acquired from a vertical and deviated wellbores have been inverted to obtain formation stress magnitudes for a given change in the reservoir pressure in a depleted, highly faulted, high-pressure and high-temperature North Sea field. Estimates of the minimum horizontal stress magnitudes from this inversion algorithm are consistent with an independent estimate obtained from a standard minifrac test.