The Gas Flow Mechanism And Productivity Analysis Of Transitional Shales Of Zhongmou Block In The Southern North China Basin

Marine-continental transitional shales are of vital importance to the shale gas area.The study of shale gas exploitation is relatively weak and urgent to be solved.It seriously restricts the development level of shale gas in China,especially the mechanism,regularity and mode of natural gas seepage.In this paper,the gas seepage characteristics and productivity influence factors of the marine-continental transitional shales,which are represented by the Zhongmou block in the southern North China Basin are studied,based on the integrated methods of the experimental examinations,the theoretical analysis and the numerical simulations.The production capacity prediction models under different fracturing conditions with vertical and horizontal well are established,and the fracturing programs are optimized combined with the economic indicator.(1)Based on the study of pore characteristics,gas occurrence space and migration mechanism of the Lower Permian Shanxi-Taiyuan shales,the triple-porosity media model of organic matter-inorganic matrix-fractures was established.That is,a small amount of organic pores and a large number of inorganic holes act as the basic structure,the layered sedimentary fractures act as the basic framework,and shrinkage fractures on bedding plane act as the basic characteristics with some occasional parallel structural fractures.(2)Based on the flow process of shale gas in different pore medium,the pore-permeability mathematical model and the seepage-stress coupling model for the triple porosity system of transitional shale are established.The pressure evolution in each pore medium during shale gas production is simulated by COMSL Multiphysics numerical simulation software and the gas flowing characteristics are discussed.The results show two flow patterns of ‘series flow' and ‘parallel flow' under different methane gas diffusion coefficients.(3)Based on the geological model,the influences of different reservoir parameters on oil recovery,daily gas production and cumulative gas production are studied by means of numerical simulation.The main factors affecting the gas production capacity of shale are determined by orthogonal design and multi-index analysis.Considering the relationship between main control factors and shale gas production capacity,the cumulative gas production and shale gas recovery equation under different fracturing conditions of vertical and horizontal wells are established,respectively.(4)For the target fracturing zones,the shale gas productions under different fracturing parameters are compared and analyzed,which shows that the horizontal length and the producing degree are the main parameters to determine the production capacity.In a certain fracturing condition,the increase of fracture length can effectively communicate natural cracks,thereby increasing production capacity.Taking NPV value greater than 0 and the yield rate of 8% ~ 12% as the economic evaluation index,three types of fracturing parameters are optimized for the marine-continental transitional shales.The research of this paper is helpful to grasp the law of gas seepage in the shale reservoir of the Zhongmou block in the southern North China Basin,which provides the scientific guidance for designing shale gas development programs,and provides the theoretical support for the efficient development of shale gas in China.