Research On Porosity Measurement Methods Of Shale Gas Reservoirs

Shale gas reservoir has the characteristics of complex mineral composition and wettability,very low porosity and extremely low permeability,which makes the porosity measurement of shale core difficult.A large number of literatures research revealed that the lack of standard for shale sample pretreatment and the disunity of measurement parameters make the great difference of the porosity measurement results in different laboratory.In order to improve the accuracy of the porosity measurement results of shale gas reservoirs,in this paper,the cores of the downhole shale gas reservoir in the Lower Silurian Longmaxi formation in the Jiaoshiba area,southeastern Sichuan basin,are selected as the research object.The influence factors of the porosity measurement results were analyzed,such as core pretreatment(extracting,drying temperature,and crushing mesh),parameter setting(measuring pressure,balance time)and measurement method.Finally,the optimized pretreatment method and parameter setting standard are put forward,and the reasons for the difference in the results of different measurement methods are clarified.The results of the paper show that:(1)For pretreatment method,it is suggested that the drying temperature should be set at 110?.This temperature can effectively remove free water and irreducible water in shale.And the higher temperature will destroy the pore structure of shale and make the measurement result larger.It is suggested that there is no need to extract organic matter before the porosity measurement of high maturity shale core in the Longmaxi formation.Extraction may cause the expansion of clay minerals,block the pores and the organic was partial dissolved.Organic matter migrate with the movement of organic reagents in the pores and occupy a large amount of pore space,which makes core porosity reduction.It is suggested that the size range of the crushed grain is distributed between 4 mesh and 60 mesh.When the particle size is larger than 5mm(4 mesh),the measured porosity increases with the decrease of the particle size;When the particle size is less than 5mm,the measurement value of porosity tends to be stable.According to the size distribution of shale particle size,it is considered that 4 mesh to 60 mesh is the best particle size for porosity measurement.(2)For measurement parameter setting,considering the interaction between injection pressure and test time,it is suggested that the optimum injection pressure of plunger core gas is 2MPa,and the equilibration time is about 20min.For crushed samples,the injection pressure between 0.6MPa and 0.8MPa is recommended,and the equilibration time is about 10min to 15min.(3)There are great differences in porosity measurement between different porosity measurement methods.The porosity measured by oil using GIP method,NMR method,KIP method and DLP method is obviously smaller than that by water using WIP method,NMR method and GRI method.According to the understanding of pore connectivity of shale gas reservoir,the GIP method,NMR pore measurement method,KIP method and DLP method mainly measure the connected pores in shale gas reservoirs,which can be considered as effective pores.The process of saturated water and the crushing samples will help to communicate non-connected pores.The porosity measured by WIP method,NMR and GRI method can be regarded as total porosity.The porosity of NMR porosity is larger than that of WIP method,and there are differences in porosity between them.The low echo interval NMR method may detect the hydroxyl(OH-)of clay,so that the porosity of NMR water porosity is larger than that of WIP method.In this paper,the core porosity measurement of shale gas reservoir is systematically studied.The pretreatment and measurement parameters of different state shale core samples were clarified and the results of different methods of porosity measurement were contrast,which provides a reference for the measurement of the porosity of the core in the shale gas reservoir.