Estimation of depletion or injection induced changes in reservoir stresses using time-lapse sonic data

Abstract

A new technique uses time-lapse borehole sonic data acquired in a vertical borehole parallel to the X3-axis to estimate changes in formation stresses caused by reservoir depletion or injection. A pre-production baseline survey acquires sonic data in an open or cased hole along with estimates of reservoir pressure, overburden and minimum horizontal stresses. After years of depletion or injection, a monitor survey acquires sonic data in an observation well. Both sonic datasets are processed to obtain the borehole Stoneley and cross-dipole dispersions. An inversion algorithm inverts the measured Stoneley dispersion to estimate the far-field shear modulus C66 in the borehole cross-sectional plane. The shear moduli C44 and C55 in the two orthogonal borehole axial planes are obtained directly from the low-frequency asymptotes of the two cross-line flexural dispersions. Differences in the three shear moduli from the baseline survey yield the maximum horizontal stress magnitude and an acoustoelastic coefficient using the estimated pore pressure, overburden and minimum horizontal stresses. The three far-field shear moduli in the three orthogonal planes are also obtained from the subsequent monitor survey. This algorithm uses the acoustoelastic coefficient from the baseline survey and the three shear moduli after depletion or injection to estimate changes in the maximum and minimum horizontal stress magnitudes caused by reservoir pressure changes.