Experimental Study On The Effect Of Drying On Gas Seepage In Low-permeability Sandstone Reservoirs

Low permeability and tight sandstone gas reservoir have become one of the important fields of exploration and development of unconventional natural gas resources in the world.It is predicted that the production of tight gas will account for one third of the total production of natural gas in China in 2020 and become one of the main forces of natural gas supply in China.Therefore,the efficient development of compact gas is of great strategic significance for alleviating the contradiction between supply and demand of natural gas and reducing the dependence of oil and gas on foreign countries.Comparing with conventional gas reservoirs,low permeability tight gas reservoirs have unique engineering geological characteristics,which are characterized by low porosity,low permeability,prominent capillary phenomenon,rich viscous minerals and high water-saturation.These characteristics make the low permeability sandstone gas reservoir have high damage potential,and it is more difficult to develop than conventional gas reservoir.The characteristics of small pore throat radius and high clay mineral content in low permeability gas reservoirs easily lead to formation water film blocking gas flow passage and causing water lock damage.If appropriate methods can be found to "dry"low permeability sandstone reservoirs and make large number of pore throats occupied by water film become gas flow channels again,which can reduce formation water lock damage and improve production efficiency and economic benefits of low permeability gas reservoirs.Based on the idea of "drying",this paper studies the problems of reducing water saturation and improving gas percolation ability of low permeability tight gas reservoirs.The work and achievements of this paper mainly include the following aspects:(1)The basic formulation of drying agent suspension system was determined based on experiments.The dispersant agent is anhydrous ethanol,and the two optional thickeners are polyvinylpyrrolidone(PVP)and carbomer.The rheological properties of the two thickeners were tested,and the concentration range of the thickeners was determined for the subsequent core drying experiments.(2)The drying seepage experiment of low permeability sandstone cores in laboratory was carried out.The drying agent suspension system was improved based on the problems in the experiment and the feedback of the experiment results.The drying effect of three drying agent suspensions was tested aggregately.Compared with the experimental results of blank control group,it is proved that the three drying agent systems can reduce the water saturation of core and improve the gas permeability of cores to a certain extent.Compared with the irreducible water state,the average ratio of water saturation reduction of the three desiccant systems is 27.8%,23.5%and 28.6%,respectively.The gas permeability increases by 16.9%,72%and 251%compared with the bound water state.(3)Drying efficiency is affected by experimental parameters and core physical properties.Under the same experimental parameters,the ratio of gas permeability before and after core drying is positively correlated with the irreducible water saturation of core to a certain extent,and the increase of permeability is positively correlated with the original permeability of core.For cores with similar physical properties and the same experimental parameters,there is no significant difference in the ability of drying agent to reduce water saturation.(4)The water phase occurrence state and water saturation change of low penneability reservoir core before and after drying were studied by nuclear magnetic resonance technology.(NMR).Core T2 spectrum and nuclear magnetic resonance imaging results show that the desiccant system can effectively reduce water saturation in core,has obvious desiccation effect on water content in macropore and micropore of core.The drying effect analysis of micro pore throat layer of reservoir is consistent with the results of drying seepage experiment.The above work confirms that the drying agent system can reduce the water saturation of low permeability reservoir to a certain extent and improve the gas seepage ability of low permeability gas reservoir.