A Study of Geomechanical Effects on Time-lapse Seismics

Abstract

Time-lapse seismics is known to be a very effective monitoring technique for the subsurface fluid movement and saturation changes, as well as for geomechanical phenomena [Snieder et al., 2007]. The integration of seismic and reservoir engineering is now becoming state-of-theart [Boutte, 2007] while the number of applications is steadily increasing [Staples et al., 2006]. Among the future challenges to the use of time-lapse seismics is the integration with geomechanics [Landro, 2006]. The improvement of time-lapse seismic technology [e.g. Tang et al., 2007, Aarre et al., 2007] allows for better and more accurate data acquisition, that in turn allows to “see” effects previously difficult to detect. The effects of geomechanics on time-lapse seismic data have been described in detail by a number of publications [Hatchell and Bourne, 2005; Sayers and Schutjens, 2007; Cox and Hatchell, 2008; Kristiansen and Plischke, 2010]. The overall impact of reservoir exploitation on the changes in seismic response includes the following aspects: (1) Fluid saturation effects, that are based upon: (a) dependence of density on fluid saturation; and (b) dependence of bulk moduli on fluid saturation (Gassmann, 1951). This is the key effect sought in time-lapse seismics, as it allows remote monitoring of the fluid migration in the reservoir. Mainly, two effects are sought in data hopefully depending on the above saturation changes, i.e.: - time shifts, i.e. changes in reflector location in time as a consequence of changes in velocity, and mainly: - impedance changes, i.e. reflectivity changes, as impedance is the product of velocity and density, both changing with fluid content. (2) Pressure (effective stress) effect: this is the first, well known geomechanical effect, often referred to in the literature as pressure effect, but it is actually a dependence on effective stress. It is generally observed that the velocity decrease is very strong in presence of effective stress decrease (expansion), while velocity increase is relatively mild under stress increase (compaction) [e.g. Hatchell and Bourne, 2005]. This asymmetric behaviour is often explained in terms of crack opening under stress release conditions.