Characterization Of Pore Structure And Elasticity And Permeability Properties Of Multi-scale Dolomite Reservoir

With the continuous improvement of exploration and development degrees of middle-shallow oil and gas resources,exploring the deep and ultra-deep marine oil and gas resources has become a key area of exploration and development.The journey marching into deep and ultra-deep is an important trend in China’s future oil and gas exploration,as well as a key to solve the China’s energy shortage.The dolomite reservoir of the Lower Cambrian Xiaoerbulake Formation in the Tarim Basin is presently one of the most important areas for deep and ultra-deep marine oil and gas exploration.However,due to its old age,deep bury,and suffering multi-stages diagenetic superimposition,integrated with integrated influences from sedimentary environments,diageneses,and pressure-temperature conditions,the Xiaoerbulake dolomite is well developed with multiscale pore structures and hence featured by strong heterogeneity.As such,a combinedly use of various technical tools to quantitatively characterize the multi-scale and multi-type pore structures and elasticity and permeability properties of the Xiaoerbulake dolomite reservoirs is very important for deep and ultra-deep marine oil and gas exploration in the Tarim Basin and similar places around the world.The main results and understandings obtained from this study are as follows:1.There are 13 microfacies types and 8 pore types in the studied area based on micropetrographic observations and scanning electron microscope observation,of which the residual granular fine-grained dolomite and fine-grained dolomite have good physical properties.The dissolution pores are the main reservoir spaces.2.In view that the Xiaoerbulake Formation is featured by multi-scale pore structures,an integrated complementary experimental techniques are used to quantitatively characterize the nanoscale to centimeter-scale pores.(1)A permeability prediction model has been established based on the parameters of pore structures extracted from the cast thin sections.The correlation coefficient of the prediction model is 0.78.The results of high-pressure mercury injection show that the pore structures of core samples are different from those collected from the outcrop.The core samples are dominant by nano and micro pores,whereas the micro pores are the main reservoir space in outcrop samples.(2)The pore structures of dolomite have obvious scale effects in representative elementary volume and self-similar interval.The representative elementary volume of millimeter-to micron-pore dolomite reservoir is firstly determined by medical CT scanning.Then a prediction model of representative elementary volume is established based on various parameters of pore structures.The representative elementary volume size of micron-pore-developed samples is determined by the micron CT scanning,which reveals that the representative elementary volume correlates with the pore size.(3)The self-similar interval of the sample is determined based on the fractal theory and the cumulative distribution curve of pore diameter.(4)A new method combinedly using the medical CT data and high-pressure mercury injection data is then established to obtain the full-scale pore size distribution and the total porosity of the cavernous dolomite reservoir.3.3D digital core is reconstructed based on the micron CT scanning.The elasticpermeability parameters obtained by numerical simulation are the key factors to quantitatively characterize the reservoirs.(1)Elastic parameters are obtained by numerical simulation with finite element method.The simulation results and pore structure parameters show that the aspect ratios of pores and porosity are the main factors affecting the P-velocity of the S-wave.(2)The permeability of simulate digital core is modeled both by the pore network and lattice Boltzmann methods.The modeling suggests that results from the lattice Boltzmann method can better reflect the seepage characteristics of real samples.4.Rock physical modeling is a bridge to build the micro-physical characteristics and macro-elastic properties of reservoirs.Rock physical theoretical model is an effective tool to obtain parameters of pore structures.4.Rock physical modeling is a bridge to build the micro-physical characteristics and macro-elastic properties of reservoirs.Rock physical theoretical model is an effective tool to obtain parameters of pore structures.A multi-scale rock physics model based on critical porosity is established through the parameters’ determination of multi-scale pore structure and verification of the experimental data.In order to simulate the changes of elastic parameters of deep reservoir,a multi-scale high-pressure rock physics model based on critical porosity has been established by adding pressure into the multi-scale rock physics model model.The parameters of pore structures obtained through inversion of the rock physics model would play important roles in further analysis and evaluation of reservoirs.