Study On Seepage Characteristics And Productivity Evaluation Method Of An Ultra-deep Carbonate Gas Reservoir

Ultra-deep oil and gas resources(burial depths over 6000 m)are considered to be one of the most important development areas for the oil industry in the future,and are also crucial to ensuring China’s energy security.Ultra-deep carbonate rocks have undergone early tectonic action and deep hydrothermal dissolution and reformation,its multi-scale pores,throats,and fractures form different types of reservoirs and its lithology,physical properties,and reservoir types are completely different from those of shallow and middle carbonate reservoirs.The complex pore-throat structure space of ultra-deep carbonate rocks leads to the complexity of multi-type reservoir seepage laws in the production process of gas reservoirs,and the large differences in productivity evaluation methods with medium and shallow layers.In addition,the formation conditions of high temperature and high stress also determine that the seepage law of gas reservoirs and the change law of reservoir structure during development are different from those of medium and shallow gas reservoirs.Aiming at the geological characteristics of multi-scale pore development,high temperature and high stress changes in ultra-deep carbonate rocks,this paper studies the multi-scale pore throat structure,seepage characteristics under formation conditions,the effect of high temperature and high stress on storage capacity and permeability,and the productivity evaluation of multi-type reservoirs at the mine scale to solve the problems of reservoir heterogeneity,high temperature,high stress and capacity forecasting encountered in the development of ultra-deep carbonate gas reservoirs based on the experimental method of joint characterization and the improved capacity evaluation model.First,the pore throat types and lithological characteristics of ultra-deep carbonate gas reservoirs were obtained based on rock sample surface observation,casting thin sections and scanning electron microscopy;the high-pressure mercury intrusion experiment were used to characterize the micropore distribution and characteristics of rock samples;surface observation,casting thin sections and high-pressure mercury intrusion were combined to characterize the full pore distribution from micropores to macropores in the target reservoir.Using multi-scale CT scanning technology,the pore throat structure parameters were extracted from the results of multi-scale CT scanning,the reservoirs are divided into four types: pore,pore-cavity,fracture-cavity and fracture-pore according to the pore-throat structure parameters obtained from multi-scale CT scanning and physical parameters of the rock samples,and the "digital core method" is established to correct the capillary pressure curve obtained by the high-pressure mercury injection experiment to characterize the centimeter-micron pore-throat structure of the target reservoir rock sample across scales.Secondly,the gas single-phase seepage experiment,the gas-water two-phase seepage experiment,the stress sensitivity experiment and the elastic energy experiment under the formation conditions were carried out on the multi-type reservoirs of the ultra-deep carbonate gas reservoir,and the single-phase seepage law,gas-water two-phase seepage law,stress-sensitive characteristics and elastic energy characteristics of the four types of reservoirs was obtained.The seepage characteristic parameters such as high-speed non-Darcy coefficient,gas-water co-permeability interval,stress-sensitive coefficient,elastic energy coefficient,and et al.were extracted in each seepage experiment result.The seepage characteristic parameters were combined with the degree of development of pores,fractures and caves to analyze the seepage laws of various types of reservoirs.Thirdly,CT scanning was used to analyze the pore space of rock samples of different reservoir types and quantitatively study the pore throat distribution,connectivity and the development situation of fractures and caves of rock samples of different reservoir types before and after stress sensitivity and fracturing.Triaxial mechanical experiments were applied to ultra-deep carbonate gas reservoirs to study rock mechanical parameters of rock samples of different reservoir types.The influence law of temperature on single-phase seepage and gas-water two-phase seepage flow in ultra-deep carbonate gas reservoirs were studied and the high temperature and high pressure interfacial tension experiment device was constructed to study the influence law of temperature and pressure on the interfacial tension of gas and water.Finally,the conventional productivity evaluation method was improved,and a productivity model considering the radial flow characteristics of the reservoir was established.Through the improved radial flow productivity simulation method and similar criteria,the lower limit of physical properties which is closer to the production characteristics of target reservoir with low porosity and low permeability but high single well productivity is obtained.And the correlation between pore-throat structure parameters and seepage characteristic parameters of multi-type reservoirs obtained from pore-throat structure characterization and seepage experiments under high temperature and high pressure conditions were quantitatively studied to further reveal the effect of pore-fracture-cavity structure on the seepage of multi-type reservoirs in ultra-deep carbonate gas reservoirs.