4d Seismics - Principles and applications

The principles and applications of 4D Seismics are reviewed. Certain constr aints need to be met for successful 4D-projects. These constraints already come in at acquisition stage: it is mandatory to keep the acquisition geome try the same for both surveys. In marine work this often is not possible du e to the phenomenon called feathering, and the consequence is an increased uncertainty in the result, i.e., the amount of Non-Repeatable Noise (NRN) i ncreases. Fig. 1 summarizes the different tools and loops (Marschall, 1996) which form the 4D-process, and we will refer repeatedly to this figure in the following discussion. 4D Seismics in terms of processing and interpretation in principle may give two results: it may be used to determine drained/undrained compartments within a reservoir (which in case of segrega ted flow-conditions also represent the actual endpoint-saturations), or it may be used to quantify for saturations (which includes the first task by definition). The approach used is called sigma-(sigma)-approach (Marschall, 1997a, 1998a ), i.e., a two-phase procedure: first analysis, then evaluation. In this co ntext it is similar to geostatistical techniques. The basic monitoring quantity is acoustic impedance. In addition other attr ibutes may be used, as, e.g., in the case that the moving OWC-interface can be mapped directly in the differenced amplitude sections (= differenced mi grated sections): under these circumstances then the angle beta formed betw een OWC-interface and top-of-reservoir allows for the explicit calculation of the actual flow-velocity v'. The formula to do this is called "4D formul a" and will be discussed in a later section. P-wave seismics is sufficient as long as only fluids are involved, but as s oon as free gas needs to be quantified, S-wave data is needed. In addition it is important to recognize that principally two types of reservoirs exist , i.e., high-impedance reservoirs (= carbonates) and low-impedance reservoi rs (= sandstones).