Quantitative Characterization And Prediction Of Total Porosity Of Shale Under Stratigraphic Conditions

The exploration and development of shale gas has become an important research direction for the exploration and development of global oil and gas resources.Accelerating the exploration and development of shale gas resources has become the common choice of the major shale gas resources countries and regions in the world.Due to the late start of shale gas exploration and development in China,the degree of research is relatively low,and it is urgent to establish the theory and technology of shale gas exploration and development applicable to geological features of China based on domestic geological features.Quantitative characterization of key parameters in shale gas reservoirs has been the goal of many scholars in China.The shale total porosity is an important parameter for evaluating shale reservoir capacity and shale gas resources,and is also a resource investigate and conduct basic research in constituencies.Accurate porosity on shale gas reservoir needs to be obtained through laboratory testing.Testing reservoir porosity in the laboratory is not limited by drilling cores,and due to long test times and high costs,generally only the target layer is tested.The porosity of the whole interval is determined by the established characterization model of porosity and the well logging data.Therefore,the accuracy of predicting or obtaining the porosity is mainly based on the quantitative characterization model of the established porosity.The establishment of a quantitative characterization model of porosity is a key scientific issue.Through the quantitative characterization of the shale gas porosity,it is possible to use the well logging data to predict the porosity quickly and accurately,which makes it possible to carry out porosity profile and plane prediction based on geophysical means.This paper takes the natural shale in the Fuling area and laboratory artificial core samples as the research object.Through the core test and logging data analysis of shale in the Fuling area,combined with Wyllie time average equation,Gassmann model and Voigt model,a set of total porosity in the Fuling area quantitatively characterization model was established using velocity.Using the 3D seismic post-stack inversion technique to predict the total porosity of Fuling area.In addition,the natural shale core samples in Fuling area were taking geochemical test,physical property test and XRD experiment.The pore characteristics and mineralogical characteristics of natural shale in the Fuling area were deterined,and the diagenetic simulation instruments independently developed were used to produce a series of artificial core samples simulating natural shale.By testing the artificial core samples,the petrophysical characterization equation of formation velocity was established.Combined with the abundant well logging data and core test data in the Fuling area,the total porosity petrophysical characterization model of Fuling area was established,and the total porosity in the Fuling area was predicted.The main achievements and understandings of the paper are as follows:1)Through argon ion scanning scanning electron microscopy and sheet observation,it is found that the pore types of shale in the Fuling area mainly include inorganic pores,organic pores and micro-cracks.Among them,inorganic pores mainly include intercrystalline pores and intergranular pores of clay minerals and intragranular pores.Organic pores belong to the pores formed by the organic matter in the late thermal evolution process,as long as they are distributed in the organic matter.The micro-cracks mainly include the bedding fractures and the structural fractures,and the bedding fractures are mainly developed in the stratum development section,and the slats are easily formed between the rigid minerals and the plastic minerals.Through CT scanning,it is found that the shale in the Fuling area is mostly developed with nano-micron pores,which are mainly composed of circular tubular nano-scale pores in organic matter.The pore morphology is mat-shaped,flaky,tubular,medium connectivity,and the shape of the throat is needle-shaped,partially flaky,plaque-like,with 30 nm medium-sized nanopores.The shale micropores?pores with diameter<2nm?were characterized by CO₂ adsorption,the mesopores of 2-50nm were characterized by N₂ adsorption,and the macropores of diameter>50nm were characterized by mercury intrusion method.The combination of three methods was used to characterise pore size in the Fuling area.The shale pore size is mainly distributed between 4-50 nm,the partial pore diameter is less than 1 nm,and there are also large pore developments larger than 200 nm.2)The organic matter pores in the Fuling area are most developed.In typical logging,the organic matter pores gradually decrease from bottom to top,and the inorganic pores increase gradually,while the crumb pores and clay pores gradually increase from bottom to top.The porosity ofJY1 well was measured using the CMS-300 automatic core measurement system.Porosity test data statistics show that the porosity of JY1 well is between 1.17%and 7.98%,with the average value of 4.61%,and the distribution of porosity generally shows the top and bottom layers are high and the middle is low.Through the permeability test,it is found that the permeability of shale in the Fuling area also has a three-stage segmentation feature.The horizontal permeability of the first and third layers are slightly higher than the horizontal permeability of the second layer.The characteristics from the statistical results of matrix permeability,there are also three-stage segmentation features,the average substrate permeability of first layer is 0.341mD,and the average substrate permeability of the second layer is 0.092mD,the average substrate permeability of the third layer is 0.256 mD,which also presents a three-stage feature of"top and bottom layers are high and middle layer is low".3)Quartz content in shale is generally greater than 40%,up to 70%,feldspar content is between 8%and 15%,calcite and dolomite content is generally greater than 10%,and total clay minerals are generally ranged 45%from 60%.It is mainly composed of I/S mixed layer and illite,with a brittle mineral content of 50%-80%,and a small amount of pyrite and hematite.4)Based on the well logging data and core test data of natural cores in the Fuling area,the velocity prediction model of total porosity was established by combining Wyllie time average equation,Gassmann model and Voigt model.Based on the good response relationship between well logging data and geophysical data in the Fuling area,the total porosity of Fuling area was accurately predicted in the longitudinal direction by well logging data.In addition,combined with the 3D seismic post-stack inversion technique,the planar prediction of total porosity in the Fuling area was finally realized.5)A group of artificial cores were prepared by simulating natural shale by studying the pore characteristics,petrophysical properties and mineralogical characteristics of natural shale in the Fuling area.In the preparation process,three groups of artificial cores were prepared by controlling the mineral composition and content,mineral particle size,confining pressure,temperature,pore pressure and other parameters.The mineral components of Group A were the same,and the particle size was controlled to ensure the porosity of group A varies between 0-15%;the content of quartz in group B is the same as that of clay,and the content of feldspar and organic matter is different.By controlling particle size and confining pressure,the porosity of artificial core is about 4%;the content of quartz and clay minerals in group C is different.The content of feldspar and organic matter is the same.By controlling the particle size and confining pressure,the porosity of artificial core is about 4%.The artificial core sample was subjected to the overpressure acoustic wave test.Through the analysis of the test data,the response templates and influence factors of single parameter total porosity,quartz content,clay mineral content,organic carbon content,confining pressure,pore fluid type and formation velocity were established.Using normalization to unify the dimensions of the impact factor and establishes a characterization model of the formation velocity.Combined with the abundant well logging data and core test data in the Fuling area,the total porosity petrophysical characterization model of Fuling area was finally established,which realized the prediction of total porosity in the Fuling area,which provided a new idea and method for the quantification of shale in the future.