Difference In Water Uptake And Pore Structures Of Cambrian Shale Intervals In Yichang Area And Its Significance To Shale Reservoir Evaluation

Following a major breakthrough in the Ordovician Wufeng Formation-Silurian Longmaxi Formation shale gas in the Sichuan Basin,the Cambrian shale has a wide range of distribution,large thickness,and high abundance of organic matter.It is a high-over-mature marine shale in southern China.An important successor area for rock and gas exploration and development.In recent years,Cambrian shale gas exploration has obtained industrial gas flow from multiple wells in Yichang area of,Hubei province,area of Sichuan province,showing good prospects for exploration,but so far,the shale reservoirs pore characteristics and the main controlling factors are still very weak,which restricts further effective exploration and development.In this paper study,we selected the core samples from Well ZK003 of the Cambrian Shuijingtuo Formation in Yichang area and carried out the water absorption experiment,helium porosity measurment and low-temperature nitrogen adsorption pore size distribution of rock cylinders,and combined with XRD mineral composition and organic carbon content,the water uptake characteristics and pore structure characteristics of shale reservoirs in different intervals in the vertical direction are further discussed and evaluated.And the main controlling factors to the pore structure differences in organic-rich intervals and the influence of seal conditions on shale gas preservation are further investigated.In addition,with a combination of tatal gas content results of the Shuijingtuo Formation shale at the drilling site that have been reported and published in the Yichang area,a preliminary prediction is made for a favorable interval of the Cambrian shale gas exploration and development in the Yichang area.The following key points of view are obtained in this study:(1)There are significant differences in the water uptake characteristics in different shale intervals of the Lower Cambrian in Yichang area.Four divided-interval in the vertical direction are identified.The water uptake,porosity and pore structure parameters of the shale have a postitive correlation with minerals composition and TOC content.The average TOC content of the bottom first section and the fourth section which is overlain the Shuijingtuo Formation organic-rich interbveral is less than 1.0%,with the average water uptake of 3.8mg/g rock and5.4mg/g rock,and the average porosity of 1.0%and 1.5%,respectively.In contrast,the average TOC content of the organic-rich second and third shale interval is 5.0%and 3.4%,respectively,with the average uptake amount of 14.9mg/g rock and16.0mg/g rock,and the average porosity of 3.2%and 4.1%,respectively.good positive correlation between the water uptake and the porosity appears in organic-rich shale intervals.The third shale interval has a lower TOC content but higher water uptake and porosity than the second shale interval.A slightly higher,clay minerals content in the third inetrval than the secondone might attribute to the difference in pore characters.(2)The results of low-temperature nitrogen adsorption experiments show that the Cambrian Shuijingtuo Formation shale is dominated by a small-size pores,mainly micropores and mesopores.which play an important role in shale gas preservation,pore network connection.Pore volume and specific surface areas are small in the organic-lean first and fourth intervals of the Shuijingtuo Formation.In contrast,pore volume and the specific surface areas of the organic-rich second and third shale inetrvals are high,with the average surface area of 19.4m~2/g and 17.5m~2/g;and the average pore 0f 17.3ul/g and 18.1ul/g,respectively.The micropores and mesopores with pore diameters ranging from<2nm and 2?50nm are well developed,and their contribution to total porosity in the second and third organic-rich shale intervals is greater than 85%,while the contribution of macropores to the total porosity is only about 10%.Specifically,the micropores contribution in the third shale interval is smaller than that of the second one,which may be related to the organic matter type variaiyon.(3)TOC content,quartz,intergranular and interlayer pores of clay minerals,and cementation of calcareous minerals are important factors affecting the porosity,pore structure and brittleness of the Cambrian organic-rich mudstone in the Yichang area.The clay minerals content in the third shale ietrval is significantly higher than that of the second one,while the carbonate content in the third inetrval is lower than that of the second one.The TOC content shows a good positive correlation.Among them,the water uptake of the three shale inetrvals has no obvious correlation with the quartz content.The clay minerals in the second and third shale inetrvals have a positive coorelation with good specific surface area,pore volume and water uptake.A better correlation with clay mineral content in the third shale interval is acheieved.Carbonate minerals have a strong negative correlation with water uptake,specific surface area and pore volume in organic-rich shale intervals.The high content of quartz and carbonate in the Shuijingtuo Formation makes the brittleness index of the organic-rich shale reservoir greater than 68%,which indicates that TOC content and mineral composition affect water uptake.The minerals composition is one of the main factors affecting the pore difference of the second and third organic-rich shale intervals.(4)The relatively tight interbeds of marl and mudstone developed on the top of the organic-rich third inetrval shale of the Shuijingtuo Formation and the interbedded marls at the bottom play a very important role to the preservation of shale gas in the third shale interval.The poor sealing of the second shale interval floor results in high hydrocarbon expulsion efficiency,which makes the organic pores in this interval smaller.The average water-uptake porosity and total porosity of the third member of the shale are 0.4%and 0.9%greater than that of the second one,respectively.At the lithological boundary between the second oragnc-rich shale interval and the first organic-lean interval,the presence of bitumen indicates that the second shale inertval has experienced petroleum expulsion downward the lithological interface.Stronger expulsion efficiency may be one of the factors leading to the difference in the pore structure of the second and third shale intervals.(5)The third shale inetrval in the Shuijingtuo Formation has high TOC(the average TOC content of 3.4%),favorable to shale gas generation,and it has a good seal condition at its top and bottom for shale gas preservation.Inorganic pores and organic nanopores together constitute a favorable storage and pore network in the organic-rich third shale interval of the Shuijingtuo Formation.The brittleness index of this section ranges from 58.3%to 77.0%,with an average of 68.1%,showing good fracturing ability.With the combination the gas-bearing-content results reported in Well Yidi 2 in the adjacent study area,The averaged total gas content in the third shale interval is 5.5 m~3/ton rock.Therefore,it is preliminarily considered that the organic-rich third shale interval of the Shuijingtuo Formation in Yichang area,western Hubei province is of high-quality reservoir for shale gas exploration target.