Coupled Geomechanics and Flow Simulation for Time-lapse Seismic Modeling

Friday, February 11, 2000 - 11:00am - 12:00pm
Keller 3-180
Susan Minkoff (Sandia National Laboratories)
Joint work with Charles M. Stone, J. Guadalupe Arguello, Sandia National Labs, Steve Bryant, Joe Eaton, Malgo Peszynska, Mary Wheeler, University of Texas at Austin.

Time-lapse seismic feasibility studies for compactible reservoirs such as Ekofisk in the North Sea require coupled flow simulation and geomechanical deformation modeling. We present an algorithm for 2-way coupling of flow and geomechanics and indicate what impact the coupled code has on calculation of seismic velocities and density. Each of the simulators --- the reservoir simulator from the University of Texas and the geomechanics code from Sandia National Labs --- was developed under the US DOE's ACTI program. The codes use 3D finite element discretizations and can be run in parallel. The flow simulator can account for faults and multiple flow models in a single simulation. The geomechanics code models contacts and large, inelastic deformations. We modify the black oil system to include changes in the reservoir geology (porosity) during a single simulation while still maintaining conservation of mass. Modifications to the geomechanics code allow changes in pore pressure to be included in the total stress calculation. The geomechanics code produces volumetric strain-induced porosity updates for the flow simulator. By allowing the two simulators to have different spatial grids and to take different time steps, we are able to account for physical differences in resolution between flow and geomechanics. A plastic cap constitutive model provides a realistic mechanism for capturing subsidence due to production-induced pressure drops. We demonstrate the coupled code with a synthetic example based on a diatomite reservoir in the Belridge Field, California. Ten years of primary production results in about 10 feet of subsidence at the wells. Correspondingly, we see a 2-4% change in reservoir porosity during the simulation. Calculation of saturated rock seismic velocities and density using the coupled simulation are more realistic than calculation of elastic rock properties from flow only.