4D Gravity - fluids monitoring in reservoirs
Authors: Mrlina Jan 
Published: March 2015

Abstract
Monitoring of fluids in reservoirs became an essential tool for the development of hydrocarbon fields. Besides 4D seismic, time-lapse microgravity measurements can determine fluids contact displacement. High resolution microgravity surveys have to be conducted with geodetic control of possible vertical movement of observation sites. Modeling and simulation have been performed to evaluate gravity monitoring (4D gravity) in the case of semi-horizontal formations, large and deep Arabian-type reservoir, and a small anticline structure undergoing gas extraction and/or storage. Models were tested for various sizes of gas-saturated sections of reservoir formation, but mainly for various depths, from 500 m to 3000 m, respectively. The case of water-flooding process in semihorizontal 20 meters thick reservoir layer with 15% porosity and 100 kg/m3 density contrast at water-gas contact (WGC) was analyzed. It turned out that WGC movement can be observed by surface 4D gravity, but the resolution significantly deteriorates with depth. Combination with borehole gravity monitoring is suggested as a sophisticate solution for the future. Then the gas extraction during production, as well as a process of injecting/pumping in a small-size anticline structure was simulated in various depths (700 m to 1800 m) and density contrast of 250 kg/m3, in order to determine the limits of the technique. It was found that gravity effect of such process can be observed in given geometry depending on the thickness of the gas-saturated zone. The amplitudes of the differential gravity signal reached between 10 to 50 µGal depending on the depth of the model. The results proved the possibility of detecting the moving flood front in reservoirs by 4D gravity. At greater depth the resolution of the technique decreases, as well as in case of lower density contrast at WGC. Each intended project should be started by detailed feasibility study based on local conditions and parameters of reservoir.

Introduction
In sedimentary basins, the rocks of high porosity represent a necessary condition for fluids accumulation and migration. It is not that important which type of porosity is present, whether the fracture porosity in limestone, or the lithological porosity in sandstone. In sufficient volume the difference between dry and "wet" pores can produce significant change in bulk density, and thus also in the respective gravity signal [1]. Under favorable conditions we can perform the monitoring of fluid content and migration in the rock formation. This may typically be the case of gas-oil or gas-water filled reservoir rocks, but steam-oil just as well. The gravity signal depends on the thickness and porosity of the reservoir formation [2, 3].
Gravity units used in the following text: 1 microGal (µGal) = 10-3 mGal = 10-8 m/s2.
The effect of gas extraction could be observed during reservoir exploitation by repeated measurements of gravity within a surface network, or at least a set of reservoir oriented profiles. The contact of gas and water (WGC) is moving in relation to gas extraction or brine injection, or any similar process (e.g. steam injection). Such a contact acts as a moving density boundary. The results may be used to constrain time-lapse seismic surveys (4D seismic), or in conditions where time-lapse seismic is not relevant.