Gravity Methods: Useful Techniques for Reservoir Surveillance

Abstract

Understanding the movement of gas in oil reservoirs is important because premature breakthrough or high gas-oil ratios significantly increase costs and can reduce ultimate recovery. The Prudhoe Bay and Kuparuk River oil fields, located on the North Slope of Alaska, have an expanding gas cap and a gas storage area, respectively. Significant gas movement in both reservoirs has occurred, since discovery, and the vertical and areal extent are not as well defined as desired. The current methods of monitoring are expensive and increasingly less reliable because of well conditions. In a search for better methods to monitor gas movement, borehole and surface gravity techniques were identified. The Borehole Gravity Meter (BHGM), because of its large radius of investigation, can predict the true gas-oil contact, irrespective of a localized gas cone and other near wellbore completion problems. Bypassed lenses of oil and intervals of gas under running oil can be identified. The data from the BHGM can provide the vertical distribution of gas and oil at a well. To better understand the areal distribution of gas, high resolution surface gravity data can potentially be used. As the gas cap propagates outward and/or downward, a negative gravity anomaly is created due to the displacement of oil by gas. The results show that significant gravity anomalies have occurred in the Prudhoe Bay and Kuparuk River oil fields due to gas replacing oil. The gravity anomalies can be measured with the borehole gravity and in certain situations with surface gravity instruments. New surveying techniques are needed to obtain increased vertical and lateral resolution for reservoirs buried as deep as the North Slope fields. Global Positioning Systems (GPS) is an economically viable method for obtaining accurate surface positions for locating surface gravity stations. Surface gravity data resolution can be improved further by stripping and downward continuation. The newly developed elevator sonde for the borehole gravity meter, may significantly improve the vertical resolution of the tool in deep deviated wells. Additional developments to improve deviation and size limitations of the BHGM, data acquisition and processing of surface gravity, and testing of GPS in Arctic latitudes are required. Plans to field test these techniques and provide data to design new techniques and tools are on going.