Experimental Study On Pore Structure And Single-Phase Flow Characteristics In Tight Reservoirs

With the depletion of conventional petroleum resources worldwide,tight oil has become a hot spot in the development of unconventional petroleum resources.Tight oil reserves are abundant and widely distributed in China,and are found in major basins.Tight reservoirs have a large number of nano and sub-micron pores with complex structure and the permeability is extremely low,resulting in poor pressure transmission capacity,slow flow rate and low single-well productivity.Therefore,understanding the microscopic pore structure and seepage law of tight reservoirs is the basis for its effective development.Due to these special properties of tight reservoir rocks,there are some shortcomings of the conventional pore structure research methods.With the development of technology,the advantages of nuclear magnetic resonance(NMR)technology in pore structure research are emerged.In the seepage law of tight reservoirs,the research focuses on the theoretical level,and the experimental rules are few.Based on the previous studies,a lot of experimental researches were carried out on the microscopic pore structure and seepage law of a tight reservoir in Shengli,and provides a theoretical basis for the development of tight reservoirs.In the research of pore structure,high-pressure mercury intrusion(HPMI)and NMR T₂ distributions test were carried out on a small number of samples,and an improved method for transforming the T₂ distribution to pore size distribution(PSD)was established.The NMR pore structure of other samples was obtained by this method.In the research of flow characteristic,a gas permeability testing device based on pressure decay method was designed and built,and the gas permeability eliminated slippage effect of the cores was obtained.Then,the single-phase flow characteristic in tight reservoir rock under different pressure gradients was studied by constant velocity injection method and vertical tube method.In addition,it was compared with the gas permeability.Finally,combined with centrifugation and NMR,the movable fluid in tight cores was tested,and the tight cores were evaluated from distribution of movable fluid,T₂ cutoff value and movable fluid saturation.The results show that the PSD of tight reservoir rocks is dispersed.The pores are mainly in nano-scale,while a small number of pores are in micron-scale.There are mainly two kinds of PSD curves:unimodal and bimodal,and most are unimodal.The pore size of the cores with unimodal PSD concentrate in nano-scale,and the movable fluid is concentrated in the pores at the center of the peak,and recovery is small.Samples with bimodal distribution generally have nano-pores at the left peak,while sub-micron pores and micro pores are at the right peak.Movable fluids mainly concentrate in the pores of right peak,and these bimodal samples have more moveable fluid.Experiments of flow characteristic in tight rocks show that there is a good linear relationship between flow rate and pressure difference at high or low-pressure gradients,and the fitting line passes through the origin.The permeability under different pressure gradients are very close,and the results are basically the same as gas permeability eliminated slippage effect.It shows that there is no non-Darcy phenomenon in single-phase flow in tight cores.And no threshold pressure gradient and boundary layer were found.