An interpretation of the 4D seismic response to gas exsolution and dissolution

This study examines the 4D seismic signatures of gas exsolution and dissolution in a producing hydrocarbon reservoir. The physical mechanisms for these reservoir processes are investigated using a series of simulation studies, and the primary controls on the seismic response are thus identified. It is concluded that gas can attain a steady state approximately three to six months after a major reservoir pressure change, and that in this state the seismic responds to the thickness of gas accumulations existing at either the critical and/or maximum gas saturation. Application to multiple repeated surveys over a North Sea turbidite field demonstrates the practical consequences of our findings and how this insight can be used to interpret the seismic amplitudes. Interpretation of the field example confirms a low critical gas saturation of less than 1% for the reservoir rocks. However, it is not found possible to quantify the maximum gas saturation using the 4D seismic amplitude only. Quantitative estimates of the volumes of gas liberated and dissolved in the oil are found possible by integrating material balance principles with the seismic response. It is shown that the insights gained from this application can be generalised to other moderate to high permeability hydrocarbon reservoirs.