Fluid pressure arrival-time tomography: Estimation and assessment in the presence of inequality constraints with an application to production at the Krechba field, Algeria

摘要

Deformation in the overburden proves useful in deducing spatial and temporal changes in the volume of a producing reservoir. Based on these changes, we have estimated diffu-sive traveltimes associated with the transient flow due to pro-duction, and then, as the solution of a linear inverse problem, the effective permeability of the reservoir. An advantage of the approach based on traveltimes, as opposed to one based on the amplitude of surface deformation, is that it is much less sensitive to the exact geomechanical properties of the reser-voir and overburden. Inequalities constrain the inversion, un-der the assumption that the fluid production only results in pore volume decreases within the reservoir. The formulation has been applied to satellite-based estimates of deformation in the material overlying a thin gas production zone at the Krechba field in Algeria. The peak displacement after three years of gas production is found to be approximately 0.5 cm, overlying the eastern margin of the anticlinal structure defin-ing the gas field. Using data from 15 irregularly spaced imag-es of range change, we have calculated the diffusive travel-times associated with the startup of a gas production well. The inequality constraints were incorporated into the esti-mates of model parameter resolution and covariance, im-proving the resolution by roughly 30% to 40%.