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
Figures & Tables
“This book, prepared for use with the first SEG / EAGE Distinguished Instructor Short Course, discusses Â"time-lapse seismicÂ" and enables geoscientists to assess the value and risk of this new technology. It covers the rationale and driving forces behind time-lapse seismic by examining the limitations of existing methods of tracking fluid flow between wells. It examines those reservoir properties that change with time and what can be observed on seismic data over elapsed time. The repeatability of seismic data and the use of Â"legacyÂ" data sets are discussed, along with a review of the seismic data acquisition schemes and data processing requirements for time-lapse analysis. The rock-physics foundation for data analysis and interpretation options also are described. A selection of industry case histories illustrates many of these points. The reader will gain an understanding of key success factors, key calibration requirements, and key uncertainties of time-lapse seismic in reservoir management.”