低丰度致密砂岩天然气分布特征及充注过程半定量评价——以鄂尔多斯盆地定北地区上古生界为例

Distribution characteristics of low-abundance tight sandstone gas and a semiquantitative evaluation method on its charging process:A case study of the Upper Paleozoic in the Dingbei area,Ordos Basin

鄂尔多斯盆地致密砂岩气藏为天然气运移路径上的滞留气藏,气藏充注程度低、丰度低、产气量低、气水关系复杂,天然气是否有效充注、充注程度如何均不明确。为此,以鄂尔多斯盆地定北地区上古生界致密砂岩气藏为例,基于岩心、测井、地震、生产动态等资料,厘清了天然气分布特征,并综合烃源岩、砂体以及断裂分布空间组合关系,确定了天然气充注方式,再根据充注点、充注动力和阻力,确定了优势充注路径,最后开展了不同充注路径下的气水分布评价。研究结果表明:(1)上古生界致密砂岩气层主要为下二叠统太原组2段第2层(太2-2层)、第3层(太2-3层)与中二叠统下石盒子组1段,同时存在气层、气水同层以及水层,气水关系复杂,气井产量差异大,存在局部高产气井;(2)致密砂岩气藏天然气充注点可分为源储邻近充注点、近源充注点以及远源充注点3类,天然气充注方式可分为砂体源内充注、砂体叠置纵横扩散充注、储集体内断裂沟通跨砂体充注以及复合充注4类;(3)建立了天然气充注路径评价参数充注动力(P_(M))和充注阻力(P_(D)),P_(M)越小,充注动力越强,P_(D)越小,充注阻力越小,P_(M)大且P_(D)小,则充注动力损失小,从充注点到井点的充注动力最强、阻力最小,为天然气优势充注路径;(4)确定了4大类11小类气水分布类型和4类充注方式下气水分布规律,其中研究区西部源内充注、近源充注、断砂有利配置充注均可形成较高丰度的天然气富集,东西部远源充注和断砂较有利配置充注可形成较低丰度的天然气富集。结论认为,该研究成果不仅可以为该区目的层寻找充注程度更高的天然气富集区提供依据,也能够在低丰度气藏区的天然气充注优势区进行有效评价,较传统方法更加精细,更适合单个气藏的精细评价。

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 wells locally.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 (P_M) and charging drag (P_D),are determined.A larger P_(M) value implies a stronger charging momentum;a larger P_(D) value indicates a lower charging drag.When both P_(M) and the P_(D) 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 lowabundance 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 semiquantitative evaluation approach is more precise than traditional ones,and more suitable for a single gas reservoir.