Over Four Decades' Experience With Gas Dewatering at Naturally Fractured Gas Reservoirs in South-Sichuan, China, What Have We Learned?

Abstract

Early water breakthrough occurs frequently in naturally fractured gas reservoirs. To minimize the impact on gas production, gas dewatering technology has been applied successfully since 1969 in South-Sichuan gas sector, China. During past over 40 years, gas dewatering technology has been utilized in 326 naturally fractured gas pools, including pilot tests at individual wells and field wide applications in entire gas reservoirs at both initial geopressured and late pressure-depleted development phases, forming systematic and mature practices. This paper presents and analyzes five typical, innovative dewatering practices: 1). Water and gas production simultaneously from updip wells in massive gas reservoirs with weak edge water. 2). Gas production in updip wells while water production from downdip wells with strong bottom water drive. 3). Water and gas production simultaneously in downdip wells in geopressured formation below gas/water contact. 4). Combination of water/gas co-production in downdip wells near gas/water contact and pure gas production in updip wells 5). Newly drilled horizontal wells located below gas/water contact are used for water and gas co-production, in order to bring updip, prolific, and watered-out gas wells back production. The results are that production decline trends in gas pools at different stages have been arrested and flattened out, offsetting previous double-digit decline. The best field case is plateau production rate sustained for eight years without additional drilling, with peak off-take rate of above 8%, relatively stabilized flowing wellhead pressures, gas flow rates and gas/water ratios. The paper also addresses some of the lessons learned and the criteria of the optimal practice for selecting and designing gas dewatering technology.