Comparative Study On Breakthrough Pressure And Snap-off Pressure Of Saturated Mudstone In Ordos Basin

Gas flow in low permeable rocks and sealing efficiency and ability of caprock are very important in hydrocarbon production,greenhouse gas geological storage and nuclear waste treatment.The snap-off pressure and capillary breakthrough pressure are not only important parameters for evaluating the sealing efficiency and capacity of the caprocks,but also important critical pressur studies have focused on capillary breakthrough pressure(Pc),few studies have considered the influence of snap-off pressure(Ps)or its relationship with capillary breakthrough pressure.Also,there are limies for gas flow in extremely tight reservoirs.Most previous theoreticaltations in the methods of measuring snap-off pressure in the past.The Ordos Basin in China is rich in oil and gas resources,as well as recognized as an excellent geological storage site for greenhouse gases because of its huge storage capacity in saline formation.it is very necessary to use breakthrough pressure and snap-off pressure to evaluate the sealing efficiency and capability of the Ordos caprock,and to study gas migration and dissipation in rocks.In this paper,This research uses the experimental equipment designed by the research group,a new method is presented to determine the Pc and Ps of three low-permeability mudstone core samples in the Ordos Basin using CH4.In the process of measuring the snap-off pressure,it is found that two different value of snap-off pressure can be obtained by slowly or rapidly reducing the upstream reservoir pressure.Two types of pressure can distinguish as instantaneous snap-off pressure(rapidly upstream reservoir pressure drop)and natural snap-off pressure(slowly upstream reservoir pressure drop).Their difference can be related to the change of water film thickness in channel,which affects the radius of the channel.At the same time,we get the related information of rock mineral composition,clay content and pore size distribution of core samples by X-ray diffraction and high pressure mercury injection,carbon dioxide and nitrogen isotherm adsorption.Through the analysis of these important information combined with the experimental results,it is indicated that the capillary breakthrough pressure,instantaneous snap-off pressure and natural snap-off pressure are all positively related to the proportion of micropores,mesopores and negatively related to the proportion of macropores.And it decreases with the increase of porosity.The ratio of capillary breakthrough pressure to snap-off pressure is used as parameters to evaluate the sealing efficiency of caprock.As this ratio approaching to 1,sealing efficiency turns better,resulting in the easier caprock sealing and the less gas leakage after gas breakthrough.Experiments show that the ratio of instantaneous snap-off pressure(PI)to PB of the three mudstones are 0.7-0.75,which are not affected by the pore size distribution of rocks.The ratio of natural snap-off pressure(PN)to PB is 0.34-0.55,and has a good correlation with the pore size distribution of rock.This study co MPares the breakthrough pressure and the cutoff pressure,focusing on the difference between the breakthrough pressure and the cutoff pressure,and has certain guiding significance for the related issues of gas migration and dissipation in low permeability rocks.