An offshore team recognized the need to examine oil reservoir compaction and deformation before engaging in a campaign to drill several injector/producer pairs, however, the time needed to build a detailed finite element model was not factored in prior to the start of drilling. Further, the process of actively updating the finite element model as new well information became available was not feasible within the drilling time frame. The team investigated alternative approaches for assessing subsurface changes based on readily available information, such as their current reservoir simulation model, and found that the 4D Geomechanics workflow in CoViz 4D offered a rapid process for screening their reservoir for potential geomechanical changes that might impact drilling operations.
The CoViz 4D Geomechanics workflow is based on Geertsma’s analytical method for calculating the deformation around a compacting oil and gas reservoir (Geertsma, 1973), combined with the R factor approach suggested by Hatchell and Bourne (2005) for calculating the associated seismic velocity changes. The Geertsma approach assumes a homogeneous, linear elastic half-space described by a constant – Poisson’s ratio. This approach ignores heterogeneities in the overburden, instead concentrating on the largest geomechanical changes which occur closest to the reservoir.
Figure 1: CoViz 4D Geomechanics workflow input and output grids, and their signs and meanings.
Outputs of the workflow include a new adjusted velocity model that accounts for small velocity changes as the overburden deforms. Although small, these changes accumulate over thousands of meters to produce measurable differences in the travel time to the reservoir over the period between repeat seismic surveys.
Figure 2: Changes in the reservoir simulation model cell thickness are shown along with the time-shifted velocity model. Data used with permission of owner.
CoViz 4D, a data visualization and analytics software from Dynamic Graphics, Inc., gives geologists, geophysicists, and reservoir engineers the ability to easily access and combine all relevant data associated with subsurface environments. Powerful analytic capabilities enable users to explore data relationships, analyze the accuracy of depth conversion of 3D seismic, and visualize seismic well ties and velocity models to facilitate decisions that positively impact profit and reduce operational risk. To learn more about how our software can reduce infill drilling risk by streamlining analysis of subsurface geomechanical changes, contact our team.
