Characteristics of pores and fractures in tight gas sandstone reservoirs and the role of reservoir control:A case study of Lower Permian Shihezi Formation in northeastern Ordos Basin
Research on the characteristics of pore fractures in tight gas sandstone reservoirs and the factors influencing them can help to improve predictions of sweet spots in unconventional reservoirs.This study comprehensively investigates the pore-fracture characteristics of dense gas sandstone reservoirs in the Lower Shihezi Formation from the Upper Paleozoic,in the S area of the northeastern Ordos Basin,using a combination of macroscopic and microscopic observations.The target layer was found to contain residual intergranular pores,intergranular and intragranular dissolution pores,and intercrystalline pores and microcracks.Microscopic observations of the samples revealed that the increasing edge of quartz in the two stages improved the stress bearing capacity of the skeleton.The enlarged edge of quartz was found to have undergone alkaline dissolution.The physical properties of the tight gas sandstone reservoirs improved significantly under the action of alkaline and acid-superimposed dissolution.Furthermore,its physical properties improved with the increase in the quartz content.Pores and fractures at different scales developed in the tight sandstone of the target layer,which more easily led to fractures due to the concentration of stress around them,such that a large number of microcracks related to the pores were formed,including intragrae and edge microfractures.The edge microfractures usually had good directivity while the intragrae microfractures had poor directivity.The degree of fracture development was positively correlated with the cumulative thickness,porosity,and permeability of the sand body.When the density of fractures increased from 0.6 to 1.2 fractures/m,the porosity of the tight gas reservoir increased by approximately 1.2％,while its permeability prominently increased by about 0.14×10-3 μm2.Local shear rupture significantly improved the physical properties of the tight gas reservoir,while a sound positive correlation was observed between the degree of fracture development and the coefficient of energy storage(Esc).The pores and fractures of the tight sandstone gas reservoir in the area were closely related,and the coupling relationship between them was significantly influenced by the physical properties,particle size,and sorting of the reservoir.
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