Experimental Study On Physical Parameters Of Shale Gas Reservoirs

Shale is hard brittle formation and easy to form cracks. And shale permeability is in nano-darcy level. Conventional core slug drilling and test methods cannot characterize the physical parameters of shale such as porosity and permeability. The rock sample is selected from the Lower Silurian Longmaxi formation marine shale in the southern region of Sichuan Basin. Shale porosity and permeability test methods are optimized to match with the shale samples and the physical characteristics of shale based on comparison of present commonly used physical properties test principle and methods, sample processing and testing conditions, etc. The research results in this paper are of great theoretical and practical significance for the correct understanding of shale physical properties, evaluation of shale reservoir, shale gas reserve calculation, implementation of the development plan and gas reservoir numerical simulation. The results are as follows:(1) The shale pore size distribution is studied combining high pressure mercury injection, low temperature nitrogen adsorption, nuclear magnetic resonance. The mercury injection and low pressure nitrogen adsorption joint pore size distribution of Longmaxi formation shale shows that the distribution of pore/throat is three peak distribution patterns, and the most of pores are macro-pores due to the artificial micro-cracks caused in the process of mercury injection experiment. NMR method can avoid the influence of man-made destruction to the test sample in the process of test. Test results show that shale pore size distribution is bimodal, and the main pores are meso-pores. The NMR can accurately measure the whole shale pore volume within the scope of pore size distribution of shale, especially the nano-scale pore volume.(2) The NMR experimental parameters for shale are optimized based on the NMR porosity measuring principle, including echo spacing, saturation pressure, formation water salinity, sample size and type of saturated fluid. The NMR is compared with the conventional GRI and low pressure nitrogen adsorption method. By comparison, the value of the NMR porosity is close to that of the GRI porosity. The NMR has advantages of simple sample preparation, nondestructive, fast, etc.(3) The experimental parameters of crushed-rock pressure-decay method are optimized, including measuring pressure, particle size and sample weight on the basis of the measuring principle of crushed-rock pressure-decay method. The test result can accurately characterizes the permeability of shale cuttings. The crushed-rock pressure-decay method has the advantages of wide sample source, simple sample preparation, measurement quickly compared with the commonly used pulsed-decay permeability method, and it can avoid the influence of stress sensitivity of shale and slippage effect.(4) A theory model for shale metrix permeability measurement is built. The multi-scale mass transfer of shale gas in the nanoscale pore and adsorption are considered in the model. Research shows that regardless of the multi-scale mass transfer of helium leads to smaller helium permeability, and the helium is suitable as the test gas of crushed-rock pressure-decay permeability method. The theoretical and accuracy of the crushed-rock pressure-decay permeability method are verified.