Distribution characteristics of low-abundance tight sandstone gas and a semi-quantitative evaluation method on its charging process:A case study of the Upper Paleozoic in the Dingbei area,Ordos Basin
Tight sandstone gas reservoirs in the Ordos Basin are stagnant gas reservoirs on the path of natural gas transportation,with low charging degree,low abundance,low gas production,and complex gas-water relationship.Gas charging in these reservoirs remains unclear in terms of both effectiveness and extent.Taking the Upper Paleozoic in the Dingbei area of the Ordos Basin as an example,this paper clarifies the distribution of tight sandstone gas using the data of core,logging,seismic and production,and determines the gas charging pattern according to the spatial configuration of source rocks,sand bodies and faults.Then,the dominant charging paths are defined based on the charging points,dynamics and resistance.Finally,the gas-water distribution is evaluated under different charging paths.The results show that the Upper Paleozoic tight sandstone gas layers in the study area are mainly found in the second and third layers of the second member of the Lower Permian Taiyuan Formation(layers of Tai 2-2 and Tai 2-3),and the first member of the Middle Permian Lower Shihezi Formation,where there are gas layers,gas-water layers and water layers,suggesting a complex gas-water relationship,which leads to greatly varying yield rates of wells,with high-yield wellslocally.For the tight sandstone gas reservoirs in the study area,three types of gas charging points are identified:near-source/reservoir charging points,near-source charging points,and far-source charging points,and four types of gas charging patterns are defined:in-source charging within sand bodies,vertical and horizontal diffusion charging in superposed sand bodies,charging crossing sandbodies communicated by fractures in the reservoir,and composite charging.Two parameters for gas charging path evaluation,charging momentum(PM)and charging drag(PD),are determined.A larger PosM value implies a stronger charging momentum;a larger PD value indicates a lower charging drag.When both PM and the PD are large,the loss of charging momentum is small,corresponding to the strongest momentum and the least drag from the charging point to the well site,which suggests a dominant gas charging path.Four types(incl.12 subtypes)of gas-water distribution and the gas-water distribution rules under four charging patterns are identified.Specifically,in the western part of the study area,in-source charging,near-source charging,and charging in a favorable fault-sandbody configuration can all result in high-abundance gas enrichment;in the eastern and western parts,far-source charging and charging in a favorable fault-sandbody configuration may induce low-abundance gas enrichment.The study results provide a basis for seeking gas-enriched zones with higher degree of charging in the target layers in the Dingbei area,and also enable an effective evaluation of favorable gas charging zones in low-abundance gas reservoirs.The proposed semi-quantitative evaluation approach is more precise than traditional ones,and more suitable for a single gas reservoir.
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