Pore Structure And Adsorption Capacity Of Wufeng Formation And Member 1 Of Longmaxi Formation Shale Reservoir In Pingqiao Block And Zilichang Block

Fuling shale gas field has an important commercial exploitation value in China.,whose discovery greatly promotes the development of Chinese shale gas industry.The marine shale reservoirs of rich organic matter of Upper Ordovician Wufeng Formation and Lower Silurian Member 1 of Longmaxi Formation is the main production reservoirs.The research of the pore structure characteristics and methane adsorption capacity of shale reservoir has the oretical significance and application value in shale reservoir geological evaluation and shale gas exploration and development.The thesis is to JY-B Well in Pingqiao Block and JY-C Well in Zilichang Block as the research object.Through literature research,data collection,organic geochemical analysis and X-ray diffraction quantitative,it gets whole rock mineral petrological and geochemical characteristics of shale samples.By field emission scanning electron microscopy?FE-SEM?,Image Pro-plus software,low-pressure gas adsorption?N₂ and CO₂?and mercury injection porosimetry?MIP?,the pore structure characteristics of OM pore and full aperture pore of shale samples are researched.The methan adsorption capacity of shale samples is obtained by high-pressure CH₄ adsorption.The differences among pore structure characteristics and methan adsorption capacity of every layer and two wells by comparing them with each other.In combination with characteristics of petrology and geochemistry of shale samples,it discusses the influence factors of pore structure characteristics and methane adsorption capacity.The OM of Wufeng Formation and Member 1 of Longmaxi Formation shale from research blocks is at over mature stage,the OM type belongs to?.The TOC of Wufeng Formation and the lower part of Member 1 of Longmaxi Formation shale of JY-B Well and JY-C Well is mainly between 2⁵%,as the prime layers,and the TOC of the upper part of Member 1 of Longmaxi Formation shale is between 1²%,as the subprime layers.The mineral of objective formations are mainly composed by clay minerals,quartz,a small amount of carbonate minerals and pyrite,etc.Four kinds of rock facies are mainly developed,that is,mixed siliceous shale,rich mud siliceous shale,mud/silicon mixed shale,rich siliceous mud shale.Using low-pressure gas adsorption?N₂ and CO₂?and mercury injection porosimetry?MIP?for the multi-scale representation of matrix pores and micro cracks of shale samples,the results as follows:1)The pore of shale is composed by cylinder shape,the ink bottle shape,slit shape pore and so on.2)The micropore?0²nm?and mesopore?2⁵0nm?of the prime layers are more developmental than the subprime layers.The micropore of?3?Layer is the most developmental in the prime layers,and the micropore and mesopore of?7?Layer and?8?Layer is relatively developed in the subprime layers.3)The curve of pore size distribution is multi-peak type,the preponderant average pore size of Wufeng Formation is at 0.35nm and 0.90¹5nm,the preponderant average pore size of the lower Member 1 of Longmaxi Formation is at0.35nm,0.45⁰.60nm,0.90¹5nm and 25²70nm,the preponderant average pore size of the upper Member 1 of Longmaxi Formation is at 0.80⁰.90nm and 25²70nm.4)According to MIP,the results show that the higher TOC of shale samples into the greater mercury injection content,when the pressure of mercury injection is bigger than30MPa.The connected mesopore of the prime layers is more developed than the subprime layers,and the connected macropore and microcrack are more developed than the prime layers.5)Comparing the pore structure parameters of the prime layers of the two wells,it found that the mesopore of JY-B Well is developed,and the micropore of JY-C Well is developed.JY-C Well has small pore throat of mesopore,poor connectivity,lots of closed pore and the ink bottle pore.Using FE-SEM to observe the pore type and morphology of Wufeng Formation and Member 1 of Longmaxi Formation shale,the OM pore is the most developed in matrix pore,the OM pore is observed and statistically analysised by using the Image Pro-plus software for the sem photos of OM particles to get the pore characteristics,the characteristics as follows:1)The main shape of the OM pores in Wufeng Formation is polygon with shrinking edge,the shapes of the OM pores in the lower part of Member 1of Longmaxi Formation are ellipse and near circle,while flattened elliptic,irregular subrotund and linear OM pores are observed in the upper part of Member 1 of Longmaxi Formation.2)The main development range of OM pore width is within2⁶00nm,and the quality of OM pore width within 5³0nm range are more.The OM pore of the widths of 100⁶00 nm in the shale samples of Member 1 of Longmaxi Formation are more than that in Wufeng Formation.3)The average surface OM pore rate in single grain of OM of Wufeng Formation and?3?Layer is relatively the largest in the prime layers,and the average surface OM pore rate of?7?Layer and?8?Layer is relatively large and approximate in the subprime layers.4)The OM pore shape of JY-C Well is more irregular than JY-B Well,and the pore size of JY-C Well is smaller.The influence factors of pore structure mainly come from the external factors?tectonic movement and sedimentary environment?,and internal factors?TOC,mineral composition and content?.Sedimentary environment has affected the shale lithology and mineral composition and content,the lateral structure of extrusion drape,bedding decollement and lifting effect make the original shale gas reservoir space being compressed change,thus affecting the shale pore structure.The volume of micropore and mesopore within 2¹5nm has a positive correlation to TOC,obviously.Because of the biogenic silica,with the increase of the content of siliceous,the BET specific surface area,the micropore and mesopore volume increase.The clay minerals content has a negative correlation to the micropore and mesopore volume,because the objective layers are buried deep in the study area is larger and strong diagenesis,which collapse and destroy the intercrystalline pore of clay mineral and the intergranular pore.Using the high-pressure CH₄ isothermal adsorption to test the methan adsorption capacity of shale samples.In the condition of 30?,the saturated adsorption quantity of?1?Layer and?3?Layer is the greatest,and the saturated adsorption quantity of?8?Layer is relatively great in the upper Member 1 of Longmaxi Formation,the saturated adsorption quantity of JY-C well is greater than JY-B well in the prime layers.The methane saturated adsorption quantity is positively related with TOC.Because of the biogenic silica,the methane saturated adsorption quantity increases with silica content.The methane saturated adsorption quantity is negatively related with clay minerals content.The methane saturated adsorption quantity has a positive relationship with the surface area of OM micropore and mesopore.So the higher TOC,the more organic micropores and mesopores development,the methane adsorption capacity of Wufeng Formation and Member 1 of Longmaxi Formation is greater.