| Hot dry rock is one kind of clean and renewable,hot rock energy which contains no or very little water or steam inside has broad application prospect in our country.Temporary plugging and diversion fracturing technology is a new technical means to improve the complexity of fracture network in hot dry rock and the efficiency of heat extraction.Relevant studies show that fracture surface roughness and rock temperature have significant effects on the transport and plugging process of temporary plugging agent in the fracture,but the influence law is not clear.Therefore,it is of great significance to study the transport and plugging rule of temporary plugging agent in rough fracture of hot dry rock reservoir under high temperature to improve the thermal mining ability of hot dry rock.In this paper,through core testing of water-saturated hot dry rock,numerical simulation of transient plugging agent migration and high temperature temporary plugging experiment,the physical and mechanical properties of fluid-saturated hot dry rock under high temperature,migration law of transient plugging agent in rough fracture and plugging characteristics of temporary plugging agent in rough fracture are systematically studied.The main achievements are as follows:（1）The effects of saturation fluids on the physical properties of the hot dry rock at high temperature are studied through experiments:the imbibition rate increases gradually with increase of heat treatment temperature.At each temperature,more distilled water is imbibed into the hot dry rock samples than nano emulsion.Thermal diffusion coefficient of dry samples at normal temperature,those of saturated nano emulsion samples reduce by 11.67%and 13.21%at600℃,and those of saturated distilled water samples reduce by 4.01%and 4.18%.The fracture morphology of saturated distilled water samples shows‘H’and‘W’shaped distribution,and that of saturated nano emulsion samples shows a network distribution.However,the fracture morphology of two saturated fluid samples is more complex than that of dry samples.The softening effect of distilled water on rock samples is10.32%higher than that of saturated nano emulsion.Poisson’s ratio of saturated nano emulsion samples reduces more than that of saturated distilled water samples.Comprehensive analysis of 3D CT scanning imaging and NMR in the fracture zone show that as temperature increases from 200℃to 600℃,the artificial fractures in the fracture zone obtained by CT scanning increases by about 33%.With increase of temperature,the amplitude of T2 spectrum curve of NMR experiment increases,and the advancement velocity of saturated fluid front increases,and the range of fluid sweep is enlarged.The area enclosed by T2 curve of and x-axis for samples saturated with distilled water at 600℃increases by 8.58 times compared with that of dry samples.This indicates that high temperature and fluid enhance the complexity of artificial fractures in the hot dry rock samples.（2）Transport law of temporary plugging agent in rough fracture:The relative position between the injection point and fracture inlet surface has an important influence on the migration mechanism of temporary plugging agents in the fracture.When the injection point was at the upper injection port,the velocity,force,and temperature of temporary plugging agents reached their highest values.The upper injection port was followed by the lower injection port and the middle injection port.In addition,the particles in the upper and lower main inlets diffused in a single wing shape,and the particles in the middle inlet diffused in a double wing shape.The larger the injection angle was,the more significant the changes in temporary plugging agents’velocity,force,and temperature were.Depending on the injection angle,the particle temperature boundary was arc-shaped,obtuse-angle-shaped,or acute-angle-shaped.As the wall temperature increased,the average temperature of particles gradually increased but the proportion of actual average temperature of particles gradually decreased.The velocity distribution of fluid and particles was either a clump distribution or a sheet distribution.When the fracture surface roughness increases by10.44 as measured by the joint roughness coefficient,the particle force and particle temperature increase by 12.0%and 37.8%,respectively.When the fracture surface temperature increases by 200 K,the particle force and particle temperature increase by 88.2%and 14.4%,respectively.（3）Temporary plugging mechanism of rough fracture in hot dry rock at high temperature:the fracture surface roughness of hot dry rock is a major factor affecting the fracture plugging performance of the temporary plugging agent mixture.Under the same conditions,higher fracture surface roughness corresponds to an easier formation of stable temporary plugging bodies.The combination of fibers and temporary plugging agent particles of different concentrations can effectively plug fractures with a width of 4 mm.However,the amount of required temporary plugging agent mixture increases by 82.5%when the temperature is doubled in value under the same temporary plugging agent particle concentration.To a certain extent,the increased temporary plugging agent particle concentration counteracts the influence of temperature on the plugging capacity of the temporary plugging agent mixture.Considering the economic factor,1.5%is the optimal temporary plugging agent particle concentration.The temporary plugging bodies formed by cylindrical particles are more stable than those formed by spherical particles.When temporary plugging agent particles in two morphologies are mixed to plug fractures,spherical particles play a dominant role,yield a small volume of discharged fluids,thus offering high plugging efficiency. |
PDF Full Text Request