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

Abstract

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 μGal 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 μGals: 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.