Water Movement Surveillance with High Resolution Surface Gravity and GPS; A Model Study with Field Test Results

Abstract

J.L. Brady, SPE, ARCO Alaska Inc., D.S. Wolcott, SPE, Stephen Dean Consulting, P. H. Daggett, ARCO Alaska, Inc., J. F. Ferguson, J. L. Hare, C.L.V. Aiken, and M. Balde, University of Texas at Dallas, J.E. Seibert, EDCON Inc., G. Mader, National Geodetic Survey Abstract A unique method to monitor gas cap water movement in an Arctic environment has been developed and tested. The novel surveillance technique for monitoring the water movement had to be developed given the very limited number of wells that penetrate the gas cap. Conventional fluid monitoring techniques require drilling numerous observation wells to adequately monitor water movement. Modeling studies indicate that density changes associated with water replacing gas can be detected using high-resolution surface gravity measurements. Modeling gravity effects of water movement mass distribution, mass balance and water front detection are discussed. A test of the gravity meter and essential high precision station positioning under typical Arctic winter conditions is evaluated using the Global Positioning System (GPS). Modeling results have shown the general shape of the water front can be detected using surface gravity with Gal precision. With high precision gravity measurements, greater than 90% of the increased water can be accounted for in the resulting gravity anomaly. These estimates include reasonable assumptions concerning the noise level in the measurements of both the gravity data and the location data using the Global Positioning System (GPS). The average water front can be reliably detected within a producing well spacing (approximately 2000 ft (610 m)). Surface gravity and GPS data were gathered over the Arctic Ocean in typical winter conditions (-44 F) accurately enough to monitor the water movement (surface gravity = 5 Gals: GPS = 1 cm elevation). In fact, this was the first high precision gravity survey using a GPS antenna as an integral part of the surface gravity meter housing. Introduction This study evaluates a novel surveillance technique to monitor water movement in the gas cap of a North Slope oil field. The novel surveillance technique uses surface gravity measurements to define water and gas movements. The primary problem with monitoring gas cap water movement using traditional techniques is the sparcity of monitor wells and the lack of producing wells in the gas cap. Since a gas pipeline has not been constructed to market the gas from the North Slope reservoirs, it is re-injected to aid in reservoir pressure maintenance. This results in very few wells in the gas cap area that can be used for water monitoring. Distance between some monitoring wells is greater than 10,000 ft (3048 m). The major concern for monitoring the water movement is to ensure that water added in the gas cap does not prematurely flow down dip into the oil producing portions of the field. Surface gravity instruments measure the Earth's gravitational field at a specific point. With a grid of these measurements local structural traps, stratigraphic traps or fluid movement can be identified from surrounding regional geology providing there is a sufficient density contrast. The surface gravity technique can be applied to any field depending upon the reservoir thickness and size, depth of burial, and the density contrast between the fluids. The surface gravity technique requires that several time lapse gravity surveys be made over the life of the field. The first survey should be performed prior to any change in the fluid volumes to obtain baseline data. Future gravity surveys can be subtracted from the baseline survey to obtain the gravity anomaly associated with the change in fluid volumes. The technique assumes that any other subsurface or near surface gravity anomalies changing with time fall within a tolerable noise level from which the fluid gravity signal can be extracted, or that they be accounted for through direct measurement or modeling. The large reservoir evaluated in this study is buried at approximately 8200 ft (2500 m) and has gas-water density contrast of 0.13 g/cc as the formation gas is replaced by water. P. 381