Pore-scale heterogeneity of tight gas sandstone: Origins and impacts

Abstract

Pore-scale heterogeneity attracts attentions in tight gas sandstone as it strongly affects the hydrocarbon deliverability. However, its controls and origins remain obscure owing to the intricate physical and chemical interplays in tight sandstone. Moreover, its effects on fluid propagation are rarely discussed. This paper aims to investigate controls and origins of pore-scale heterogeneity and its impacts on fluid accessibility by integrated experiments incorporating fractal, petrologic, and fluid accessibility analyses. The dominant tri-fractal structures with wide fractal dimension ranges suggest intense pore-scale heterogeneities of tight sandstone. Pore-scale heterogeneities originate from pore attributes. The heterogeneities of large-sized and medium-sized pores are mainly determined by quantities of macropore and mesopore, respectively, while that of small-sized pore is dominated by the size of small-micro pore and content of intercrystalline pore. The system heterogeneity is a coupled expression of the morphology, geometry, and genesis of different-sized pores. Mineralogy and petrology exert complex influences. The rigid grain is generally favorable for pore-scale homogeneities, but carbonate is on opposite. Greater clay content reduces the heterogeneity of small-sized pore, increases that of medium-sized pore, but increases that of large-sized pore before 5% and then reduces it. Greater grain size and better sorting bring reductions to the heterogeneity of small-sized pore but increases to those of medium- and large-sized pores. The heterogeneities of large-sized pore, medium-sized pore, and pore system are susceptible to pore fillings, while that of small-sized pore is susceptible to matrix grains. The fluid accessibility differences in the medium- and large-sized pores, affected by the pore-scale heterogeneity variations, are responsible for system fluid mobility distinctions. The influences of heterogeneity on fluid accessibility increase with pore size but accessibility of small-sized pore is barely not affected by its heterogeneity owing to clay hydrophillia. The Coates model is improved by calibrating fluid accessibility and considering system heterogeneity.