We present a 4D Geomechanics workflow that can be used to rapidly screen fields for geomechanical changes occurring during production and estimate the associated time-shifts that you would expect to see in 4D seismic data. This workflow gives a first-order estimate of the reservoir and overburden deformation occurring due to pressure changes in the reservoir. This estimate can be used to model the velocity changes occurring between repeat seismic surveys and the magnitude of the accompanying 4D time-shift signal. We give an example from a case study that shows how this workflow can be used for geomechanical screening and 4D seismic feasibility studies.
All reservoirs experience some amount of deformation during production. Reducing the pore pressure in a reservoir increases the effective stress on the reservoir rock, causing the rock to compact. The amount of compaction depends on the magnitude of the pressure drop and the compressibility of the reservoir rock. Conversely, a pressure increase during injection will cause the reservoir to expand. These changes at the reservoir level propagate into the overburden causing it to deform too. However, the amount of deformation at the seabed is always less than the amount of deformation at depth due to support from the sideburden. As a result, the overburden experiences unloading when the reservoir is compacting or compression when the reservoir expands. Seismic velocities are sensitive to these changes. Extension of the overburden causes microcracks to open up, reducing seismic velocities. Compaction causes hardening of grain contacts as they are pushed together, increasing seismic velocities. Although these velocity changes may be small, their cumulative effect over hundreds or thousands of meters combined with the change in path length to the reservoir can lead to measurable travel-time changes between repeat seismic surveys as illustrated in Figure 1.
In general, reservoirs that experience significant geomechanical changes during production can be grouped into three categories: 1) compressible chalks such as the Valhall reservoir in the North Sea which has undergone more than 10m of compaction since production started in 1982 (Kristiansen & Plischke, 2010), 2) unconsolidated turbidite sands such as Holstein and Genesis in the Gulf of Mexico (Ebaid et al, 2008; Hodgson et al, 2007), and 3) high pressure reservoirs such as Elgin, Franklin and Shearwater in the North Sea (de Gennaro et al, 2008; Staples et al, 2007).