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Oil Reservoir Compaction; Risk Reduction for Drilling Infill Wells

During production, North Sea chalk reservoirs are commonly prone to severe compaction due to reduction in pore pressure—this increases the load on the rock matrix, resulting in compaction. In some extreme cases the result is seabed subsidence on the order of 10 meters. This magnitude of subsidence causes challenges to the existing production infrastructure, including the integrity of the drilled wells, and leads to geomechanical changes in the subsurface which can impact seismic travel times. The longer than expected travel times for the seismic waves to reach the reservoir results in the shifting of target locations for infill drilling. The impact of geomechanical changes on seismic travel times is well known and documented (e.g., Barkved and Kristiansen, 2005; Kritiansen et al.,2005; Staples et al., 2007; van Gestel et al., 2013; Aoife Toomey et al., 2017). To understand and counter these effects, one possible solution is to build a finite element model describing the deformation of the reservoir and overburden. However, the building of such a model is time consuming and requires significant expert knowledge.

An offshore team recognized the need to examine compaction and deformation in their reservoir 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 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.

This first order approach, using a reservoir simulation model (which was history matched against production data), highlights if a more detailed finite element analysis is needed, and saves time and expense if it is not. Furthermore because of the ease of the workflow, a Geomechanics specialist is not required, and due to the fast calculation time, new well information can be included, as necessary.

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.

Using the CoViz 4D Geomechanics workflow, the team successfully updated the velocity profile for the field, thus providing more accurate horizons which could be used in the process of targeting and landing wells. As each well was drilled, the reservoir model was updated, and the latest information was included in the next iteration of the 4D Geomechanics study – this resulted in increased accuracy of well placement.

See the CoViz 4D Geomechanics page for more information.

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