Development Evaluations of Lenticular Shallow Gas Reservoirs with Bottom Water in Shengli Oilfields

Abstract

Shallow gas reservoirs are widely distributed in China and mostly occur at depths shallower than 1500 metres (4921.50 ft). These occur as lenticular and marginal reservoirs, with small and scattered reserves, with quite complex gas-water relationship and edge/bottom water in the majority of sand-bodies. Each sand-body forms an unattached and closed gas-water system, but there is limited elastic energy in the aquifer. The production performance of one of such reservoirs is as follows:Per-well initial gas rate is rather low with fast decline.The reservoir has a gas drive to weak elastic water-drive.Water incursion has obvious influences on production results.Wellhead pressurse could be used to represent the changes of formation energy, for it is impacted little by wellbore accumulated liquid.Finally, formation damage resulting from drilling, perforating etc. has noticble influences on the gas wells' performance. Thus the main factors that could decide reasonable well rates should be the following factors: Efficient utilization of formation energy, reservoir structure should not be destroyed, accumulated liquids in the wellbore should be continuously removed. In addition better development results can be achieved through initial sand control and edge/bottom water fingering/coning should be minimised. The purpose of this paper is to have a review of the parameters and the performance of the Shengli oil reservoirs. It could be used for reference for similar type of reservoirs. Introduction According to China National Standard, Natural Gas Specification (GBn 270–88),1 shallow gas reservoirs (SGR) are buried in depths less than 1,500 meters (4921.50 ft). The source of gas includes biogas, coalformed gas, oil-type gas and water-soluble gas etc. Shallow gas reservoirs are mainly distributed in Neogene & Quaternary systems in terms of horizons and are distributed widely in China with some considerable reserves in places, such as DF1–1 gas field in Yinggehai Basin (Pliocene series),2 Hangzhou Bay (Quaternary) in southeast coastland,3 Luliang Basin in Yunnan Province,4 Bohai Bay Basin in Eastern China, Songliao Basin in Northeastern China, Sichuan Basin,5 Chaidamu Basin6 and Jianghan Basin in Hubei Province etc. According to preliminary evaluation, total reserves are up to about 2–5×1012m3 (70.6–176.6 Tcf), excluding that of water-soluble gas. But only 0.3×1012m3 (10.6 Tcf) or so of original gas in place (OGIP) has been proven. Hence there is still a wide exploration scope.