Inversion of Seismic Cross-hole Data from Dom Joao Field, Reconcavo Basin, Brazil

Abstract:

Seismic Tomography was incorporated in Reservoir Geophysics with the intention of providing high-resolution images of regions in the Earth’s subsurface that are characterized as potential reservoirs. In this work, seismic traveltime tomography in the transmission mode was applied to real data from the Dom Joao Field, Reconcavo Basin, State of Bahia, Brazil. This basin represents a landmark of oil exploration in Brazil and has been intensively studied since the 1950s. Today, the Reconcavo Basin is still the principal oil producer in the State of Bahia, but there is a demand for new technologies, especially for mature fields, to improve hydrocarbon recovery. Acoustic ray tracing for the computation of traveltimes was used for forward modeling, and the conjugate gradient algorithm with regularization through derivative matrices was used as an inverse procedure. The tomographic inversion is an ill-posed problem because the existence, uniqueness and stability conditions are not completely satisfied. Mathematical techniques are commonly used with the intention of providing better conditioning for the numerical solution of such problems. The input data in the system are the traveltimes between the sources and the receivers and the distances propagated by each ray connecting such sources and receivers. Both inputs are provided through acoustic forward modeling. The linear system is regularized by derivative matrices derived to minimize the instability. This regularization procedure has a crucially important parameter called regularization parameter lambda. For the selection of lambda we used the L-module, which is based on the well-known L-curve. The estimated tomograms were consistent with previous geological knowledge of the area and the P-wave velocity range was consistent. The results showed that traveltime tomography is feasible for the characterization of reservoirs with a high rate of vertical change, similar to the Dom Joao Field.