基于恒速压汞技术研究页岩气储层孔隙结构:以湘西北地区五峰组页岩为例

页岩气储层孔隙结构是页岩气富集成藏、储层评价和优选有利区的关键参数,区分孔隙和喉道是表征页岩气储层孔隙结构的关键。本文选择4块具有不同渗透率的湘西北地区奥陶系五峰组页岩为研究对象,基于恒速压汞实验讨论孔隙和喉道的大小、分布特征及其相互关系以及与渗透率的联系。结果表明:具有不同渗透率的页岩样品表现为较为相近的孔径分布特征但差异较大的喉道分布特征。页岩样品渗透率的大小与孔隙半径没有明显相关关系;喉道大小及其分布特征是控制低渗储层孔隙结构的关键要素之一。渗透率较低页岩样品的喉道以喉道半径小且集中分布为特征,而渗透率较大页岩样品的喉道以喉道半径大呈分散分布但主要以大喉道为特征。喉道特征是研究页岩气储层储集空间和吸附能力的重要部分,在以后对页岩气的勘探开发中应特别注意及重视。

Pore structure of shale gas reservoirs revealed by constant-speed mercury injection experiments: A case study of Wufeng Formation shale from northwestern Hunan Province

The purpose of this paper is to study the generation mechanism and model of gas reservoirs in tight sandstone of Upper Paleozoic in the Linxing area, Ordos Basin. Through logging data interpretation, fluid inclusion measurement and paleofluid pressure recovery, the types and distribution characteristics of gas reservoirs are summarized, the accumulation periods, migration dynamics and transport system are analyzed, and the formation mechanism and model of the reservoirs are clarified for this area. The study shows that the main gas-bearing strata of the Upper Paleozoic in the Linxing area are Taiyuan Formation, Shanxi Formation and Lower Shihezi Formation, and the distribution of gas reservoirs is controlled by source-reservoir-cap combination, structural location and reservoir physical properties. The Upper Paleozoic experienced two stages of accumulation. Of them, the Middle Jurassic is the first stage, during which the natural gas was dominated by CO2 with a small amount of gaseous hydrocarbon, the residual pressure of fluid in the reservoir was low, fractures did not cut through quartz particles, natural gas migration rate was low, and the accumulation scale was small. The early Cretaceous is the second stage of hydrocarbon accumulation, where natural gas was mainly methane with a small amount of heavy hydrocarbon and liquid hydrocarbon, residual pressure in reservoirs was relatively high, and cracks developed well cutting through quartz grains. Combined with faults, these cracks formed a three-dimensional network of transport, through which fluid migration rate was high, migration distance was long, and the convergence area was large. Consequently a series of large gas reservoirs were formed, which were the most important gas accumulation period in the Upper Paleozoic of the Linxing area. The strata close to the Zijinshan pluton were invaded by high-pressure fluid from late Jurassic to early Cretaceous, forming a gradually weakened reservoir forming zone centered by rock mass. The reservoir forming model of tight sandstone gas in the Linxing area can be summarized as follows: It is characterized by two stages of hydrocarbon accumulation, of which the late stage is main accumulation period. The non-hydrocarbon gas of early natural gas is dominant and the methane proportion is relatively high in late natural gas. The overpressure, followed by buoyancy, plays a critical role in gas migration. The pores and cracks are dominant in gas migration pathway and faults are auxiliary. The lithology gas pool is dominant and the structure gas pool is less.