Possible Contribution of 4-D Microgravity to Reservoir Monitoring

Jan Mrlina
Geophysical Institute ASCR, Prague, Czech Republic
[email protected]

Monitoring of fluids in reservoirs has become an essential tool for the control of the development of hydrocarbon fields. Repeated microgravity measurements can determine gas-water or gas-oil contact displacement in time. Gravity modelling was performed to test its effectiveness in the case of gas-water movement in reservoirs (pumping, water-flooding, etc.). Several aspects of the gas-saturated reservoir were changed in the model including its size, depth, porosity and the density contrasts between fluids. The process of gas or steam injecting/pumping was also simulated. It was found that such processes can be observed by time-lapse (4-D) microgravity, but the success depends on local conditions. As an example, a water-flood process in a semi-horizontal 20-m-thick reservoir layer with 10% porosity was modeled. The results proved the feasibility of detecting the gravity signal caused by the moving flood-front even at a depth of about 2,000 m.

However, in some real cases, the conditions may differ from the model’s assumptions; for example the gas-water density contrast can vary due to pressure and salinity. Therefore, each proposed application of the method has to be preceded by a feasibility study based on 2.5-D, 3-D and 4-D gravity modelling using all available local parameters. Another application is the monitoring of CO₂ sequestration. With respect to the low-amplitudes for the expected gravity signals, the measurements have to be highly accurate, and the vertical displacements have to be controlled. The monitoring system could be significantly improved by the application of repeated borehole gravity measurements. The efficiency of the technique has already been proved by time-lapse surveys in Alaska, France, the Netherlands, Italy and elsewhere, where the gravity response to fluid movement far exceeded the signal confidence level.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain