Research On Tight Gas Reservoirs Evaluation And Classification Of Benxi Formation-Shanxi Formation In Eastern Ordos Basin

With the continuous development of natural gas exploration in recent years, the eastern ordos basin area gradually becomes the important expand area of natural gas exploitation in our country, which tight gas reservoirs of the Upper Paleozoic Benxi formation-Shanxi formation generally have poor reservoirs properties, with complex micro-pore structure and seepage flow factors affecting, which strongly restraint the effective economic oil and gas exploitation. Therefore, it is very important to research the reservoirs to conduct a comprehensive and systematically, to identify the evaluation method for tight gas reservoirs in the study area, and to establish reservoirs classification evaluation standard in the later exploration and other areas tight gas reservoirs evaluation.In this paper, reservoirs characteristics are analyzed and summarized by means of the casting thin sections identification, Scanning electron microscopy, X-ray diffraction whole rock and clay minerals experiments, at the same time reservoirs microscopic pore structure characteristics are confirmed by conventional mercury Penetration, the Constsp Mercury Penetration experiment, the Nuclear Magnetic Resonance (NMR) and other tests, meanwhile reservoirs seepage flow characteristics and ability are quantitative analyzed by Gas-water relative permeability experiment, Gas-water two phase displacement experiment and low permeability core start-up pressure gradient experiment.According to the research of reservoirs basic characteristics, tight gas reservoirs sand structure, microscopic pore structure, seepage flow characteristics and the rock mechanical characteristics these four kinds evaluation methods can be ascertained. Through the reservoirs sand body structure evaluation method, it can be summarized that there have box-type, bell-type, funnel-type and finger-type these four kinds of typical sand body structure. Based on the distribution rule of the sand body structure pattern on the section and plane, it can be drawn that the physical property of box-type and bell-type sand body are better, which mainly distribute in the main channel and the channel flanks, and usually indicate the better reservoirs. On the contrary, the funnel-type and finger-type sand body generally indicate the poorer reservoirs.Through the reservoirs microscopic pore structure evaluation method, ultimately flow zone indicator, the maximum throat radius and movable fluid saturation these three kinds of evaluation parameters can be determined. First, a new parameters that named flow zone indicator FZI can be summarized, which can quantitative characterization the morphological characteristics of capillary pressure curve, at the same time the main control parameters of tight gas reservoirs porosity, permeability can be determined by Constsp Mercury Penetration experiment, and then through Nuclear Magnetic Resonance experimental determination of movable fluid saturation, which can be analyzed by microscopic pore structure.According to the reservoirs seepage flow characteristic evaluation method, displacement types, the range of gas-water two phase seepage zone and start-up pressure gradient these three kinds of evaluation parameters can be determined. At first, it can be obtained qualitatively that there have four kinds displacement types in research area through Gas-water two phase displacement experiment; Next, through redefining gas-water relative permeability curve and analyzing low permeability core start-up pressure gradient experiment in the study area, optimizing the parameters of quantitative evaluation of reservoirs seepage ability.By maens of the rock mechanics characteristics evaluation method, the mineral brittleness index and mechanical brittleness index can be confirmed. Through the calculation of mineral composition and rock mechanics parameters analyzed rock mineral composition and rock mechanics parameters, which are complementary to each other, it can be analyzed that evaluation of the reservoirs classification evaluation should be combined mineral brittleness index and mechanics brittleness index.According to tight gas reservoirs sand structure, microscopic pore structure, seepage flow characteristics and the rock mechanical characteristics these four kinds evaluation methods, it can be ascertained that reservoirs sand structure, gas water displacement type these two qualitative characterization parameters and the largest throat radius, flow zone indicator, movable fluid saturation, the range of gas-water two phase seepage zone, start-up pressure gradient and brittleness index these six important evaluation parameters of tight gas reservoirs, combined with mathematics methods, it can be fitted out two comprehensive parameters, which are tight gas reservoirs six units classification coefficient See and reservoirs comprehensive evaluation index REI, finally combined with the conventional classification parameters, tight gas reservoirs classification evaluation standard of the Upper Paleozoic in eastern Ordos Basin can be established quantitatively, which can be classified into four types from good to bad in the study area.