Key technologies for accurate potential tapping in the mid-to-late stage of stable production of Changxing Formation bioherm high-sulfur gas reservoirs with bottom water in the Yuanba Gas Field,Sichuan Basin
The Changxing Formation bioherm high-sulfur gas reservoirs with bottom water in the Yuanba Gas Field of the Sichuan Basin are buried at nearly 7 000 m,and correspond to remaining gas potential areas generally characterized by small reef bodies,thin beds,strong heterogeneity,complex gas-water relationships,and low production of vertical wells.These characteristics pose challenges to the accurate prediction of remaining potential in the mid-to-late stage of stable production of the gas reservoir,and threaten the safe drilling of ultra-deep horizontal wells and the expected production of gas wells.This paper reviews the progress made in the Yuanba Gas Field since the adjustment of its potential tapping in terms of main controlling factors of high yield and enrichment,evaluation of reef-reservoir connectivity,fine characterization of barriers and flow channels,and potential tapping strategies and supporting technologies.The key technologies suitable for the accurate potential tapping in the mid-to-late stage of stable production of gas field are developed.The following results are obtained.First,high-yield wells are mostly located at structural highs or at the top of bioherm.The development situations of reef bodies,reservoirs,barriers and flow channels and the inter-reef reservoir connectivity have significant impacts on the distribution and development potential evaluation of non-producing reserves and remaining gas.Second,if the permeability ratio of the reservoir to the barrier,Kr/Kl=60 and Kr/Kl=150,is taken as the threshold of barrier for water isolation,the barriers in this area can be classified into three types:tight,weakly permeable,and highly permeable.The highly permeable barrier has a Kr/Kl＜60,indicating a weak ability to isolate formation water.Third,by virtue of the flow unit index Ifz,the reservoirs can be roughly classified into four types.The reservoir with Ifz＜2.47,indicating a low permeability,is considered as a barrier.Fourth,the key to evaluating the connectivity between reef bodies depends on whether there are reservoirs between reefs and the permeability contrast of the reservoirs between reefs.If there are no reservoirs between reefs,the connectivity depends on the starting pressure gradient of the inter-reef reservoirs and the production pressure difference of gas wells.In conclusion,the reserves in the potential areas can be recovered efficiently by classifying these areas with reef body scale,barrier and flow channels,as well as reservoir connectivity and gas-water distribution,as the remaining gas potential evaluation criteria,and rationally selecting the recovery strategies such as new well drilling,existing borehole sidetracking and old well potential tapping,and corresponding technologies.The proposed key technologies for accurate potential tapping have been applied to the Yuanba Gas field,achieving remarkable results in 8 adjustment and potential tapping wells,with the allocated production rate exceeding 260×104m3/d and cumulative gas production rate exceeding 30×108 m3.These technologies are of important guidance and reference significance for the efficient development of similar gas reservoirs.
Yuanba Gas FieldChangxing FormationBioherm gas reservoir with bottom waterHigh yield and stable productionBarrierFlow channelConnectivityPotential tapping strategy
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