Dual-Pore-Shape Structure In Shale And Its Application In Reservoir Characterization

After 30 years research work in American,shale oil and gas are successfully extracted from micro-/nano-pores in shale,and the shale gas revolution kicks off.Therefore,shale reservoir characterization to acquire internal micro-/nano-pore structure in shale is the important foundation to understand the storing and flow mechanisms of gas in shale,as well as the mechanical behaviors of shale rock.Shale has some complicated and unique characteristics,including high compactness,low porosity,low permeability,complicated pore structure and high sensitivity to water.It is the source bed and bearing spot in the same time as well.It needs application of experiment procedure,data analysis and numerical simulation to figure out the microscopic properties of shale.The experiments are the most direct method to reveal pore structure.Gas adsorption experiment therein is an effective way to characterize nano-pores in porous materials.However,during the processing of the adsorption isotherms,all methods intend to calculate pore size distribution and other pore information based on the assumption of uniform pore shape,which will lead to a great error when dealing with complicated pore structure in shale.This work presents a new dual-pore-shape pore size distribution calculation method based on nitrogen adsorption experiment,which can understand and characterize the nano-scale pore structure in shale.Due to the pore structure acquired,a new expression of apparent permeability is given,which connect the macroscopic and microscopic studies.Therefore,the research in this work not only improves microscopic experiment technology and makes innovation in analysis method,but also has great application value in accurate evaluation of storage and economic exploitation of shale gas.The main contents and innovative findings of this study are described as follows:1.Hundreds of shale fundamental experiments are performed to accumulate data for following research.A large amount of shale samples are collected from different shale formation in China and sent to complete several experiments.The experiments fall into three categories:one to study geo-chemical characteristics of shale,including evaluation of organic matter abundance,measurement of organic matter maturity,assessment of organic matter type,and analysis of whole-rock and clay compositions.One to study morphological characteristics of shale,including observation of apparent morphology and measurement of pore structure information.One to study the physical characteristics of shale,including measurement of permeability and adsorption capability,and mechanics properties test.The experiment data of shale basic properties are the foundation of following researches.2.A new dual pore-shape-pore size distribution calculation method is proposed.The method can effectively distinguish the cylindrical pores and slit pores under nano?scale in shale matrix,and calculate the pore size distributions of each.The method is based on the assumption that cylindrical pores and slit pores both exist in shale.According to the theory of adsorption and capillary condensation,considering the difference in hysteresis loops of cylindrical pores and slit pores,the basic calculation equation was derived to obtain pore size distributions.After verification of molecular sieve material SBA-15,the method is applied to shale rock.The result shows that the pore volume and specific surface area are reduced by 5%and 15%compared to the result of conventional single pore shape method.This finding will help to recognize the storing amount of shale gas and the ratio of free gas and adsorb gas.3.The fractal characteristics of dual-pore-shape pore structure are obtained from the transitional shale research.The new dual-pore-shape pore size distribution method is applied to study the shale samples collected from a marine-continental transitional shale reservoir in China.The results show that,there are more cylindrical pore under nano-scale in shale than slit pores,and the percentage can reach over 80%.After specifying the pore structure and fractal characteritics,it is found out that there is connection between fractal dimensions and pore shape.Cylindrical pore is the controlling factor of the surface complexity and self-similarity of larger pores,and slit pore is the key of fractal dimension of smaller pores.4.Considering multiple effectis,a dual-pore-shape apparent permeability calculation method for shale matrix is proposed.It is known that the calculation of apparent permeability should consider the influences of different effects under different pore size,including diffusion,slip regime,stress dependence,and adsorption.Whereas,the shape of pore will affect the influences of these effects,as well as instrinc permeability.Therefore,during the calculation of apparent permeability,the impacts of cylindrical pores and slit pores are discussed separately,and then the factor of pore size distributions of both are taken into account.After the comparison of different influences of different effects in both pore type and sensitivity analysis,the actual dual-pore-shape pore size distributions is also considered in apparent permeability calculation.The result of apparent permeability which only consider uniform pore size and shape has a certain error.Whereas,the result considering actual dual-pore-shape pore size distribution is more accurate and reliable.