The primary controlling parameters of porosity, permeability,and seepage capability of tight gas reservoirs:a case study on Upper Paleozoic Formation in the eastern Ordos Basin,Northern China

Tight sandstone gas(hereafter"tight gas")has become a subject of unconventional gas exploration globally.The large-scale development and use of tight gas resources in the USA,in particular,facilitated the rapid rebound of natural gas production in the USA,in addition to driving the rapid development of tight gas worldwide.In the eastern Ordos Basin,the Upper Paleozoic feature includes multiple layers of gas,a shallow depth,and notable potential for exploration and development.However,the reservoirs in the area are relatively tight,exhibit strong heterogeneity,and possess a complex micropore structure,thus restricting the eff ective economic development of oil and gas.Thus,research on the primary parameters controlling pore throat structure and the seepage capability of low-permeability reservoirs will be beneficial for the effcient exploration and development of natural gas in the eastern Ordos Basin.The parameters of reservoir porosity and percolation ability,as well as permeability,were analyzed using systematic sampling of the of the Upper Paleozoic Benxi,Taiyuan,and Shanxi Formations in the eastern Ordos Basin,constant-rate mercury injection experiments,nuclear magnetic resonance analysis,and gas–water-phase experimental studies.The results indicate that reservoir porosity is controlled by the effective pore volume and number,whereas permeability is controlled by the largest throat radius,rather than the average.The effective pore volume controls the movable fluid saturation,while reservoir percolation capability is controlled by the effective pore volume,irreducible water saturation,and size of the gas–water two-phase seepage zone.

Reservoir and development characteristics of the Da'anzhai tight oil in Sichuan Basin,SW China

The oil in the Jurassic Da'anzhai reservoirs in the Sichuan Basin is unconventional tight oil,which accumulated in or near source rocks,and did not experience extensive migration in a large-scale long distance.The first submember,second submember and third submember of Da'anzhai Member are dominated by shell limestone which is widely and continuously distributed,and are typical near-source lacustrine shell limestone tight reservoirs.Complex lithology,multiple types of reservoir space and complicated pore structure are developed in these reservoirs.The effective reservoir space mainly includes micro-pores and micro-fractures with strong fabric selectivity.The petrophysics experiment reveals that the average connected matrix porosity of tight oil reservoir in Jurassic Da'anzhai Member is about 2.13%,lower than that of other tight oil reservoirs but higher than the average effective porosity(0.97%)from previous single alcohol-saturated method.According to production performance data,the Da'anzhai shell limestone reservoir is not a simple porous or fractured reservoir,but has complex porethroat-fracture association or storage-seepage mode.Because the development of fossil shells controls the development of micro-fractures,fluids are difficult to enter into but easy to escape from the reservoirs.Although the pore-throat is fine,the sorting is poor and the displacement pressure is high,the movable fluid saturation and mercury ejection efficiency of the reservoir in the Da'anzhai Member is only slightly lower than that of some storage-seepage modes,and higher than that of Oil-bearing Group 7 of Yanchang Formation in the Ordos Basin.The reservoir in the Da'anzhai Member is one of the few tight oil reservoirs with high natural productivity.The tight oil in the shell limestone of the Da'anzhai Member has great development potential,but its extensive and effective development also has some challenges,such as high seepage resistance of matrix and ineffective single development mode.The development mode of the Da'anzhai tight oil should draw lessons from the Bakken Formation in North America and Oil-bearing Gourp 7 of Yanchang Formation in the Ordos Basin,and thus,effective development technologies based on volume fracturing and fine operation for shell limestone tight oil in the Da'anzhai Member in Sichuan Basin are developed to realize the development of profit and scale.