Wang Xiaoliang

Abstract: Solid fines generation in coalbed methane (CBM) development can cause serious formation damage and production breakdown. The anisotropy of coal reservoirs makes fines issue more complicated. In this study, the experimental analysis of the correlation between tectonically deformed coal types and fines generation characteristics was implemented. Two samples with different coal structure types, undeformed coal and granulated coal, were collected from the same coal seam. Under single-phase fluid flow, two sets of core flooding experiments were conducted to generate fines from these samples. The yields of fines produced at varying experimental conditions were analyzed quantitatively. The characteristics of these fines were microscopically observed through the use of a laser particle size analyzer, a scanning electron microscope, and a polarizing microscope. The results indicated that tectonically deformed coal types significantly influenced the generating intensities, particle sizes and morphological features of fines. Because of the varying degrees of structural destruction, compared with undeformed coal, granulated coal contained more original tiny fines in the fractures and was more sensitive to variations of fluid flow rates and reservoir effective stress, which could intensify the generation of fines. Undeformed coal could generate fines with wider particle-size distribution ranges and larger mean particle sizes than granulated coal could. The micromorphology of fines produced from undeformed coal was mainly angular. However, for granulated coal, most of the fines were subangular and even subrounded. No appreciable impact of the development of tectonically deformed coal on the compositions of fines was found. Because of the water sensitivity of clay minerals and the stress sensitivity of vitrinite, the produced fines contained more clay minerals and vitrinite with increases of displacement velocities and confining pressures under laboratory conditions. Through well logging interpretation, the development intensity of tectonically deformed coal in three CBM wells was identified. After analysis of the concentration and mean particle size of fines collected from these wells, it was concluded that the more developed the tectonically deformed coal, the higher the fines concentrations and the smaller the fines mean particle sizes, which was consistent with the experimental results.

Abstract: The invention relates to an experimental method for simulating the generation and the migration of coal powder during coal-bed methane exploitation. The experimental method comprises the following steps of selecting experimental apparatuses; collecting coal rock samples to prepare artificial coal briquettes used in a simulation experiment; preparing a displacement solution for simulating fluid migrating in a coal-bed methane reservoir; preparing a propping agent filling layer for simulating a crack passage of a coal reservoir and a fracturing effect; designing an experimental scheme for simulating the generation and the migration of the coal powder during the coal-bed methane exploitation; collecting the coal powder in discharge liquid in the simulation experiment, and performing analysis on coal powder producing characteristics; and performing quantitative analysis on changes of the permeability of the coal briquettes under different experimental conditions. According to the experimental method disclosed by the invention, based on geological factors of the coal reservoir and engineering factors of coal-bed methane, horizon source, influence factors, producing characteristics and the like of the generation of the coal powder are analyzed, the physical stimulation of the geological characteristics of the coal reservoir and the physical stimulation of the draining and exploiting process of the coal-bed methane are realized, the law of the generation and the migration of the coal powder is obtained, and experimental data support is provided for the effective prevention and treatment of the production of the coal powder during the coal-bed methane exploitation.

Abstract: To fundamentally study the adsorption capacity, swelling effect and permeability characteristic of coal seams with and without tectonic damage, the natural coal and reconstituted coal manufactured via simulating in situ geological conditions were investigated. The results show that the reconstituted coal possesses higher adsorption equilibrium time and maximum adsorption capacity comparing to the natural coal. The multitudinous intergranular seepage paths and large specific surface area of it supply adequate opportunities and sites for adsorption of injected gas. The anisotropy swelling was observed in the natural coal, which is manifested as that the axial swelling strain surpasses the radial swelling strain. Contrarily, the swelling strain of reconstituted coal is approximated to homogeneous and isotropic variety. The natural coal possesses swelling hysteresis phenomenon in the low adsorption stage, this is because of the deformation sequence from internal swelling to volume swelling conducted in it. The permeability of natural coal and reconstituted coal decreases remarkably after being saturated CO2 and N2. Especially, the permeability sensitivity of reconstituted coal is higher than natural coal and has enormous decreasing amplitude after injected high-pressure CO2, which reveals us that weak coal seams may be the unstable areas for CO2-ECBM or CGS in deep coal seams.

Abstract: Coal and gas outbursts (hereinafter referred to as ‘outbursts’) result in serious damage and often occur in tectonically-deformed coal that is rich in gas. It has been demonstrated that the level of outburst risk declines with increasing coal moisture content. Moreover, an outburst generally lasts for mere tens of seconds and the required energy is mainly provided by the gas expansion energy. However, the gas desorption and energy release characteristics of tectonically deformed coal with different moisture contents during the first dozen seconds have not often been studied. In this study, a set of self-designed gas desorption equipment was used to carry out gas adsorption-desorption experiments in the first dozen seconds (about 13 s) on coal with different moisture contents. The results show that the drop rate of the gas pressure increases with an increase in coal moisture content, and the total amount of gas desorption and the mass flow rate of gas desorption reduce correspondingly in the gas desorption process. Moreover, the gas velocity also slows with increasing moisture content. Under different gas pressures, the total gas expansion energy (TGEE) and the total gas energy (TGE) released from the coal decrease with the increasing moisture content. Correspondingly, it takes a shorter time for coal to release 90% of the TGEE and the TGE. For all of the coal samples, the time taken for releasing 90% of the TGEE is shorter than that for releasing 90% of the TGE. Meanwhile, the ratio of TGEE in the TGE increases with the increasing moisture content. The TGEE accounts for 14–16% of the TGE released from coal samples of different moisture contents under different gas pressures.

Abstract: Both fines migration and creep can damage coal permeability during coal seam gas production. This study investigates the individual and coupled effect of fines migration and creep on coal permeability. We perform three types of flow experiments on coal samples: (1) water flow with both fines migration and creep, (2) gas flow with creep and negligible fines migration, and (3) water flow with only fines migration. During the experiments involving creep, both radial and axial strains are measured. Analytical models for fines migration and creep are used to explain the permeability decrease during experiments. After stress was applied, the coal samples creep after elastic deformation. Fines migration or creep alone causes permeability reduction by about 25%, whereas their combined effect causes about a 60% reduction. Additional fines were generated due to stress loading causing additional pore blockage, which explains the greater permeability damage when both fines migration and creep are present. An expansive radial creep was observed, which is strongly correlated with permeability reduction. Existing creep model could not match the observed expansive radial creep.

Abstract: Measurements of the coal fines production and the impact of these fines on the permeability of two coals from the Bowen Basin, Australia, were performed at different flow conditions (single-phase water or gas, two-phase water and gas) and pressure conditions. The fines collected from each coal samples ranged in size from 1 μm to 14 μm. For both coal samples, during the first 50 h, the permeability decreases from 0.005 mD and 0.048 mD by 60.9% and 85%, respectively, followed by gradual decline with fluctuations. By the end of water injection, the permeability drops by 88% and 89%, respectively. This phenomenon is attributed to the counteraction between formation damage (cleats plugging and coal fines settlement) and breakthrough of coal fines from the samples (widened cleats). It was found that coal fines volumetric production is proportional to the third power of flow velocity once the flow paths for coal fines are established. The critical flow velocities of coal fines production for both samples were also obtained. For hydrophobic coal, water-drive-gas two-phase flow introduces abrupt permeability loss due to coal fines generation and migration. Furthermore, pauses (well shut-in) in the experiments cause slight permeability drops. A comparison between the two samples indicates that narrower and less connected cleating system results in more frequent coal fines generation and migration, resulting in significant permeability fluctuations with general decreasing trend. Tortuosity of the cleats can enhance the deterioration in permeability by coal fines behaviours. This study delivers fundamental understandings of coal fines generation and migration during the CSG production process, and useful guidelines are suggested to be implemented in the field to minimize production loss induced by coal fines behaviours.

Abstract: Fines migration has posed a great challenge to gas and water production in CBM reservoirs, resulting not only in dramatic permeability reduction but also in excessive wear on equipment. The objective of this study was to investigate critical flow conditions for massive fines detachment in the dewatering phase, for the purpose of yielding an improved understanding of fines detachment mechanisms and their effective control in the field. First, fines migration experiments under saturated conditions, including effluent concentration and permeability measurements, were conducted at elevated pressure gradients on fractured coal samples with various apertures. Experimental results indicate the existence of a critical pressure gradient (CPG) for massive fines detachment. Second, a mathematical model was developed to describe single particle detachment in the fracture, accounting for the coupling effects of hydrodynamic and extended-DLVO forces. Effects of fines size and fracture aperture on fines detachment were ...