| The research area in this thesis was Hancheng coalbed methane(CBM) field located in the southeast margin of Ordos Basin. Based on the theories of CBM development geology, coal geology, coal petrology, rock mechanics, percolation mechanics and analog simulation, through theoretical analysis, data statistics, sample testing, simulation experiment and production practice, the study on the producing process, microscopic features, generation simulation and control measures of solid fines with coal fines as the main type in CBM development was conducted. On the basis of the identification of thickness, buried depth, component content, coal bed texture and surrounding rock type of target coal seams of CBM development in this research area, the genetic mechanisms and layers, microscopic compositions and morphology, main controlling factors and resulted formation damage and procution failures of solid fines in coal reservoirs were revealed. The simulation experiments of solid fines generation in coal reservoirs were carried out and the effects of the development of tectonically deformed coal, water-rock interactions and interlaminar different pressure drop on solid fines generation were analyzed. The dual features of differences in space and stages in time duration for solid fines generation and migration in coal reservoirs were obtained. The system of comprehensive prevention and control measures for solid fines in coal reservoirs was established, and the quantitative method of indicating coal fines related downhole failures by monitoring dynamometer card was proposed.According to the properties testing of coal petrography and data statistics of logging interpretation in this research area, the reservoir properties of target coal seams for CBM development such as component content, thickness, buried depth, coal bed texture and surrounding rock type were identified. The target coal seams were 3# coal seam of Shanxi Formation, 5# and 11# coal seam of Taiyuan Formation. Based on the quantitative compositions statistics of 17 coal samples, the organic macerals of the three sets of target coal seams mainly included vitrinite, followed by inertinite. The inorganic minerals mainly were clay minerals and contained a small amount of pyrite. Based on the data statistics of logging interpretation of 64 CBM wells, for 3# coal seam, the average thickness was 2.2m which mainly developed medium-thickness coal seam, the average buried depth was 548 m, the coal bed texture was simple, the coal roof mainly were mudstone, sandy mudstone, muddy sandstone and sandstone, the coal floor mainly were mudstone, carbon mudstone and sandy mudstone. And for 5# coal seam, the average thickness was 3.2m which mainly developed medium-thickness and thick coal seam, the average buried depth was 576 m, the development degree of coal gangue was high, the coal roof mainly were fine sandstone, mudstone, sandy mudstone and muddy sandstone, the coal floor mainly were mudstone. However, for 11# coal seam, the average thickness was 7.6m which mainly developed thick coal seam, the average buried depth was 619 m, the coal bed texture was complex and two or three layers of coal gangue were widely developed, the coal roof mainly were limestone, mudstone, carbon mudstone and siltstone, the coal floor mainly were mudstone.Based on the layer structural features of coal measures, the theoretical analysis of solid fines generation and migration in coal reservoirs was conducted. The research indicated that the generation of solid fines in coal reservoirs was a complicated process which covered multiple components, multiple formations as well as multiple factors. It was considered that the genetic mechanisms of solid fines in coal reservoirs include mecnanical mechanism and chemical mechanism. The nature of mecnanical mechanism was the mechanical stress damage and pressure variation induction in coal seam, coal gangue and surrounding rock of coal caused by drilling engineering, perforating operation, hydraulic fracturing and drainage gas recovery. The nature of chemical mechanism was the water-rock interactions between sensitive components in coal seam, coal gangue as well as surrounding rock of coal and incompatible external fluids. The potential productive sources of solid fines in coal reservoirs included coal seam, coal gangue, coal roof and floor, and then the types of solid fines mainly were coal fines, clay fines and sand fines. Based on the quantitative compositions analysis of coal fines collected from 11 CBM wells, the components of solid fines with coal fines as the main type in coal reservoirs included clay minerals, vitrinite, inertinite, pyrite and quartz, which average percentages were 50.9%, 24.5%, 18.2%, 6.3% and 0.1%, respectively. The influencing factors of solid fines generation and migration in coal reservoirs included static geological factors of coal reservoir properties and dynamic engineering factors of CBM development. The former determined the material basis and productive sources of solid fines in the ways of rock components, coal bed texture, tectonically deformed coal development and lithologic combination of coal roof and floor. The latter mainly was physical and chemical damage on coal seam, coal gangue and surrounding rock of coal in different CBM development processes. The generation and migration of solid fines in coal reservoirs could result in the reduction of formation permeability and occurrence of equipment failures.The microscopic components and structures of in situ coal, mudstone and clastic rock were observed by scanning electron microscope. The occurrence mode and output position of solid fines in in situ formation were analyzed. It was concluded that the stress sensitivity of vitrinite and fluid sensitivity of clay minerals could aggravate the production of solid fines in coal reservoirs. The tension fractures and shear fractures that formed under the tectonic stresses could cause coal structure failures, with the detachment and migration of fracture fillings, then solid fines could be produced. The research suggested that coal gangue and surrounding rock with clay minerals as the main compoments could generate clay fines or coal slime under water-rock interactions when the external fluids were injected into coal reservoirs such as drilling fluid and fracturing fluid. It was considered that drilling grinding, fracturing destruction, drainage erosion as well as the interlaminar different pressure drop would lead to multiple damage effects on clastic rock skeleton structure, which also exacerbated solid fines generation and migration.Through the physical simulation experiment of coal fines generation under fluid state, the effect of the development of tectonically deformed coal on the generation of coal fines was identified. The experimental coal samples respectively were undeformed coal and granulated coal collected from 3#, 5# and 11# coal seam. Acid etching fracture conductivity instrument was used as the experimental apparatus. Based on the preparation of moulded coal sample, quartz sands packing layer and displacement solution, two sets of simulation experiments were conducted. The first one kept the experimental conditions at a constant confining pressure and increasing displacement velocity. The second one kept the experimental conditions at a constant displacement velocity and increasing confining pressure. The results showed that at the same experimental condition, the generating intensity of coal fines in undeformed coal was low and granulated coal could produce more fines than undeformed coal. Under the stable confining pressure and increasing displacement velocity, the increased times of coal fines stage production of 3# granulated coal were 4.3 times and 7.6 times, respectively. However, the corresponding results of 3# undeformed coal were only 50% and 20%. Therefore, granulated coal was more sensitive to the variation of displacement velocity. The increase of displacement intensity could cause more serious structure damage on granulated coal, then more fines would be generated. Under the stable displacement velocity and increasing confining pressure, the yields of coal fines in 3# granulated coal showed a decreasing trend while there was no remarkable trend for 3# undeformed coal. The reason was the fluctuation of confining pressure could cause the fractures plugging or even closing, and then coal fines were blocked in granulated coal due to its complicated fractures. Therefore, the variation of confining pressure severely affected granulated coal. The particle size of coal fines was analyzed by laser particle size analyzer. The result showed that the coal fines generated in undeformed coal were mainly original tiny coal fines and big secondary coal fines. Therefore, coal fines generated in undeformed coal had a wider particle size distribution range. The fluid flow and stress variation could cause the complex fractures in granulated coal shrink or close, which resulted in the migration of tiny coal fines and the retention of large-size coal fines. Therefore, the coal fines generated in granulated coal had a narrow particle size distribution range. The micro-morphology of coal fines were observed by scanning electron microscope. The results indicated that coal fines generated in undeformed coal was mainly columnar and lamellar, and the roundness was angular. But for granulated coal, most of coal fines were mainly sub-rounded and rounded. By using oil-immersed reflective polarizing microscope, the compositions analysis of the produced coal fines were conducted, which indicated that there was no appreciable impact of tectonically deformed coal development on the compositions of coal fines. Due to the fluid sensitivity of clay minerals and stress sensitivity of vitrinite, coal fines contained more clay minerals and vitrinite with the increase of displacement velocity and confining pressure.Through the simulation experiment of water-rock interactions for solid fines generation, the effect of water-rock interactions between external fluid and rock on the generation of solid fines in coal reservoirs was identified. The experimental core sample was coal sample with the development of clay interburden. Core flooding rig system in single phase fluid flow was used as experimental apparatus. The external fluid was simulated by using KCl solution with different concentrations of 0.4%, 1.2% and 2.0%. The migration channels of fluid were simulated by paving glass beads on the surface of core sample. The result indicated that the concentration of KCl solution determined the intensity of water-rock interactions, the degree of water sensitivity and velocity sensitivity of clay minerals and the productive process of solid fines. The lower the concentration of KCl solution, the more serious the detachment and dispersion of clay mineral grains, and the more generation and further migration of solid fines. The result caused by the generation and migration of solid fines was the permeability decline of monolayer glass beads. The concentration increase of KCl solution could weaken the generating intensity of solid fines, which caused the decrease of fracture plugging and formation damage. With the concentration of KCl solution increased from 0.4% to 1.2% and 2.0%, the decreasing range of permeability increased from 30% to 28% and 14%.Through the simulation experiment of differential pressure induction for solid fines generation, the effect of interlaminar different pressure drop during CBM production on the generation of solid fines in coal reservoirs was identified. The experimental interburden samples were mudstone and siltstone, which were collected in the north of Surat Basin, Australia. And core flooding rig system in single phase gas flow was used as experimental apparatus. Based on the preparation of epoxy mounted interburden slice, 8 sets of core flooding tests in low and high differential pressures of nitrogen gas were carried out. The results indicated that interburden samples could produce solid fines under the gas phase differential pressure. The reason was the interlaminar pressure drop could cause driving force in the interburden sample, and then the force could lead to the occurrence of microscopic failures in rock structure and instability of mineral grains, which resulted in the production of solid fines with gas flow. Compared with low differential pressure, the mean particle diameter of solid fines generated under high differential pressure was smaller, which might be relevant to the cementation types of mineral grains and their binding forces. With the generation of solid fines, the permeability of interburden samples increased, while the variation trends were affected by the lithology and thickness of interburden samples as well as differential pressure intensity. Compared with siltstone, there was the higher sensitivity to the increase of differential pressure in mudstone. When the differential pressure increased from 0.5MPa to 4.0MPa, the increasing range of permeability in mudstone increased from 50% to 1.7 times while the increasing range of permeability in siltstone was smaller. With the increase of differential pressure, the bigger the thickness of interburden sample, the higher the damage degree of pressure drop surface, which caused more solid fines generation and permeability remarkable increase.Under the comprehensive influence of coal reservoir properties and CBM development engineering, there were dual features of differences in space and stages in time duration for the generation and migration of solid fines in coal reservoirs. Based on the dual features, the system of prevention and control measures was established, which included geological prediction and prevention, reservoir reconstruction remission, production control, technology and equipment management. According to the CBM development practice in Hancheng area, the feasibility of control coal fines generation and migration by monitoring dynamometer card was discussed. The comparative analysis of shape features in typical dynamometer cards of coal fines related downhole failures was conducted. The quantitative parameters of forecast coal fines related downhole failures by monitoring dynamometer card was proposed, which were R, unload line lag range and θ, load line deviation angle. According to the value of R and θ, the grades of relevant downhole failures could be divided. This method was conducive to the identification of solid fines aggregation in CBM wells and the adoption of effective control measures. With the production instances in the research area, the application effects of monitoring dynamometer card in poor drainage and stuck pump were evaluated. |
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