Seismic response to injected gas into reservoir for storage and enhanced oil recovery purposes

Natural gas is accumulated in the reservoirs as either separate gas reservoir or the gas cap in an oil reservoir. Besides, gas is also injected into a hydrocarbon reservoir for IOR/EOR or gas storage purposes. Due to the reservoir heterogeneity or fault pattern in reservoir, gas could move to unplanned parts of the reservoir or could even be leaked, which in turn, deviates from the purpose of the gas injection. To overcome this problem and to monitor the fate of injected gas, 4D seismic data has recently been employed by oil and gas companies. 4D seismic, that is indeed, the repeated 3D seismic through the time has been recently revealed to be a successful tool for this purpose. However, there has been reported some challenges about the quantitative estimation of injected gas using 4D seismic data. The source of this challenge is mainly due to the non-linear response of elastic properties of saturated rock versus gas saturation. Once the gas is injected into core plug in the laboratory, the compressional velocity is significantly decreased for a few percents of gas saturation. Nonetheless, for higher gas saturation variation, not a considerable change is observed in compressional velocity. Because of this extremely non-linear behaviour, some researchers have concluded that the quantification of gas response is not possible using seismic data. In this research, it is tried to understand the reservoir scale gas distribution that is found to be different from the laboratory scale. Gas is migrated towards the upper part of the reservoir due to the gravity effect. It is quickly reached at a fixed gas saturation that is around maximum gas saturation (1-Swir). Continuation of gas injection increases gas thickness from top to base of reservoir, while gas saturation is practically fixed. Therefore, unlike the laboratory scale, the only variable on the reservoir scale would be the gas thickness, and not gas saturation. This is the key observation that would assist to understand proper 3D and 4D seismic response to injected gas. Two main 4D seismic attributes are chosen in this paper to understand those responses. The response of time shift and amplitude change were derived analytically and investigated numerically. The variety of reservoir models with different thickness and heterogeneities were made to analyze the seismic response. It can be concluded that for the medium to high-quality reservoirs, seismic response to the injected gas is simply linear; therefore, 4D seismic is still a powerful tool to quantitatively estimate the volume, distribution and migration path of the injected gas. It is proposed to continue this research to understand the seismic response on low quality (permeability and porosity) reservoirs.