Fluid Contact Movement Monitoring in Gas Reservoirs using Pressure Transient Data: Galaxy North Field Example.

Abstract

Abstract Pressure transient tests have been used for many years for the determination of well and reservoir parameters. Initially analyses of the tests were limited to evaluation of well performance. With the introduction of computer and pressure derivative log-log plots, numerous signatures of well reservoir and boundary responses are diagnosed and their model parameters estimated. Of recent, controlled reservoir monitoring from either permanent downhole gauges or standard bottom-hole pressure tests especially in gas reservoirs show that gas-water contact movement can be detected from the late time pressure derivative responses hence, water breakthrough time could be predicted. For a historical pressure transient tests taken at different times, the superimposition of their different pressure derivatives on the same log-log plot enable detection of advancing water front. The estimated fluid contacts from subsequent tests can be used to calculate gas volumes/reserves and predict water breakthrough time for the gas wells. This process was applied in Galaxy North field. The Field is a major domestic gas supplier for power generation in Nigeria and it started production in Dec 1963 with oil wells, while the gas wells came on-stream in 1984. Four gas wells have been drilled to in the field date. Current production is at 10Mbopd and 50MMscf/d from oil/gas and gas reservoirs. Pressure transient test are routinely carried out in the field to understand reservoir dynamics and gas wells performances and to ensure proper production planning towards uninterrupted gas supply to domestic market. The pressure transient tests carried out in Galaxy North wells between 2004 and 2013 depicted responses characteristic of upward trend (pseudo no-flow boundary). The boundary was established to be due to the effect of advancing gas/water contact. The estimated gas/water contact was compared with open-hole saturation log of an infill well drilled in June 2012 and results were close. This was used tovalidate the results of the pressure transient analyses. This paper presents guidelines and best practices on the use of classical reservoir tools in monitoring the gas water contact and the information derived could be used in estimating gas reserves and predict the future performance of the gas wells to ensure effective gas delivery to customer.