In-Situ Determination of Residual Gas Saturation by Injection and Production of Brine

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract A new in-situ procedure has been developed for measuring the residual gas saturation in watered-out zones of gas reservoirs. Gas-free formation brine is injected into a well penetrating the water-invaded zone to dissolve the penetrating the water-invaded zone to dissolve the residual gas from a region around the wellbore. The brine is then produced from the well. The volume of brine containing no dissolved gas is then determined by sampling the brine and analyzing for gas content. The original residual gas saturation can be calculated from these data. The procedure has been confirmed by computer simulation and by laboratory tests on a long glass-bead pack. Two full-scale field tests have also been conducted; the interpretation of one of these tests is discussed in detail. Introduction Water invasion is a significant drive mechanism in many natural gas reservoirs. The residual gas saturation left behind the water front is an important parameter required for accurate prediction of recoverable reserves, frontal advance velocity, and optimal production scheduling. The traditional method for determining residual gas saturation is laboratory waterflooding of core plugs. With that method there is usually some uncertainty as to whether the core samples used are truly representative of the entire reservoir zone, or whether the displacement process is representative of that within the reservoir. Pressure coring has been used successfully to measure residual gas saturations, but it is a very expensive procedure which requires that a new well be drilled in an already-depleted portion of the reservoir. In theory, neutron logging methods could be used in existing water-invaded wells to measure the residual saturation. However, cased-hole logging tools sample a relatively small volume of reservoir, and local wellbore effects might produce results that are not typical of general produce results that are not typical of general reservoir conditions. In the new method described here, a relatively large volume of reservoir around a well is observed. Brine containing no gas is injected into the well, dissolving the residual gas within a circular zone around the wellbore. Brine is then produced from the well, and the concentration of dissolved gas in the brine is measured during production. The original residual gas saturation in the zone can be calculated from the amount of brine produced before dissolved gas appears. This paper describes the theory of the method, results from laboratory and field tests, and results from test simulations using mathematical models. The test accuracy and range of applicability determined from field experience are reported.