The feasibility of using time-lapse 3D seismic to monitor fluid displacements in an Abu Dhabi onshore oil field

Abstract

Time-lapse or ‘4D’ seismic has been used to directly monitor the fluid movements in some oil- and gas-reservoirs. This technique allows one to verify and where necessary to update the reservoir flow models, e.g. to incorporate ‘geological surprises’ like unexpected flow barriers or conduits. The method is based on the change of the acoustic properties (velocity, density) of the reservoir when the initial reservoir fluid in place (oil, gas, brine) is replaced by others while the reservoir is produced. Under favourable conditions this change in properties leads to a detectable change in the seismic reflection. Successes of 4D seismic have been reported, mostly from clastic reservoirs located offshore and onshore heavy-oil/tar sands reservoirs. In such reservoirs the change in seismic response with changes in reservoir fluid is relatively large. Direct hydrocarbon indicators, AVO and bright spots often work well in these reservoir types. Furthermore the good quality of modern 3D marine seismic data allows one to reliably detect these changes in many offshore cases. For the large onshore oil fields produced by ADCO, the situation is more challenging. The main reservoirs are formed by fairly hard carbonates; they are reasonably deep and consist of several thin zones with quite different flow properties due to variable porosity, permeability and fracturing. The surface is covered by sand dunes, which severely hamper the recording of high quality recorded seismic data. Some oil fields have been producing for more than 30 years and have been penetrated by several hundreds of wells, including infill horizontal drilling. However, the reservoir behaviour is still not fully predictable and sometimes wells do not perform as expected. With extensive use of water-injection for production support and gas-injection programs scheduled, a further calibration and refinement of the reservoir models would be quite welcome. The recent improvements in the quality of 3D seismic data seen in Abu Dhabi could enable the successful application of 4D seismic (and its associated benefits) for these fields. For this reason ADCO and its shareholders have carried out a feasibility study into the possibility of using 4D seismic for the seismic monitoring of several production scenarios for one of their main fields. Over this field a high-spec 96-fold 3D seismic survey was acquired during 1997 and 1998 for structural mapping. If the feasibility study were sufficiently encouraging, this survey would act as a base case for future follow-up 4D surveys at appropriate times covering areas of specific interest. The feasibility study followed a phased approach integrating several disciplines, with the results of one part being used for the other.seismic properties as a function of reservoir rock properties and fluid content were derived from well logs and models;reservoir simulations were run for several production scenarios, to compute in detail the expected fluid distribution profiles over time;from these fluid profiles and the static model rock properties, synthetic seismic response histories were computed representing sequential 4D's;a seismic noise analysis and a 3D seismic repeatability test were carried out to estimate the real-life seismic noise levels;finally these results were integrated to find out which production effects would be detectable with the measured noise. These cases should be detectable by a future time-lapse survey.