Rock physics—The link
This section covers predicting the seismic changes that might be observed as a result of changes in reservoir properties over time. The starting points are the simple connections between rock properties and the (zero offset) seismic reflection event, relating 1) the reflection coefficient to the acoustic impedance (AI) change at a boundary:
These must be related to primary reservoir changes (or pore fluid composition, pressure, and temperature) and to secondary effects, so we try to quantify the effect of the primary changes on reservoir rock bulk density, compressional wave velocity, and on Poisson's ratio (the ratio of the fractional transverse contraction to the fractional longitudinal extension as a volume of material is stretched). In particular, we would like to be able to predict the seismic response changes that might then occur, as in the cartoon in Fig. 3.2, both at zero offset and at larger angles of incidence.
Bulk density (Fig. 3.3) is the sum of fluid density times porosity plus the matrix density effect. Clearly, in low porosity reservoir rocks, fluid changes from brine through oil to gas will make little difference to the bulk density, so one of the first success factor requirements is the presence of a high porosity reservoir rock.
As reservoir pressure depletes, the overburden stress causes compaction with reduced reservoir porosity and increased density. The higher the initial porosity, the greater the impact of pressure depletion (Fig. 3.4). A pressure increase, however, is unlikely to produce a reverse effect.
The effect of temperature on