| Chinese CBM extraction rate and utilization rate are all low, which, on the one hand, is related to the Chinese gas resources characteristics of low pressure, low permeability and low saturation characteristics, On the other hand, is because that methane extraction technologies adapting for Chinese coal bed characteristics are still needed to be further researched although in recent years the introduction of foreign successful coal bed methane extraction technology has played a role in some extent. So, coal seam gas extraction technologies improvement is of great significance. The contradiction between mining and tunneling, mining and extraction, and among mining, extraction and production area are the main contradiction of underground extraction method. Coupled with the need of intensive drilling, requires that workers can finish "efficient" drilling."Efficient" embodied in two aspects, namely, short drilling cycle and high pore forming rate. The current mine gas extraction hole drilling in loose coal seam generally adopt underbalanced drilling. Air/atomized drilling method usually be used, which greatly shortening drilling cycle, and meet engineering needs. Underbalanced drilling technology is the most promising way of drilling CBM hole for its good ability in maintaining the stability of hole wall. But that is limited. In using gas drilling, the equivalent fluid column pressure inside the wellbore approximate to zero. When the tensile strength of the formation is equal to the formation pore pressure, the borehole wall stability condition is that the tensile strength can balance the formation pore pressure. If the rock tensile strength is greater than the formation pore pressure, wall stretching caving would not happen, otherwise it will happen and hole collapse. That is why, for loose coal seam section, underbalanced drilling technology itself can not maintain hole wall stable. Research results show that in loose coal, using under-balanced drilling hole, abnormal hole ratio is generally high, especially when drilling in loose period, due to the instability of hole wall, including hole collapse, Sucked pipes, pressed pipes, dropped pipes, et al, which cause drilling problems make it sometimes even higher than90%, which can be described as drill ten nine is abnormal.The main measures to solve the instability of hole wall are casing and using mud. Using mud to protect wall process generally includes grouting, pulling out drill pipes, cleaning drill pipes, waiting for condensation and sweeping hole. For the general drilling fluid, such as cement slurry, the setting time can be up to a dozen or more hours. Coal science research institute of hunan province developed a drag type casing method, targeted for protecting the necking section, which has been applied successfully. But it is restrict in use for the three reasons followed. Firstly, when meet coal bed with too much gas, that may be increase gas accumulation and cause outburst finally. Secondly, it is difficult to ensure casing timely in loose coal seam. Thirdly, that can only protect only one section of hole wall, which not fit for distance drilling with several sections of loose coal bed.In National Natural Science Foundation of China (41002046) youth fund support, the study intends to solve the problem of hole wall instability of loose section in CBM drilling through the way of Grouting while Drilling. Grouting while Drilling is a way of segmented grouting, which is grouting in the process of drilling to reinforce loose coal hole wall when needed. Grouting while Drilling is one of the methods of protecting wall with mud. But it doesn’t need pulling out drill pipes, cleaning drill pipes, waiting for condensation and sweeping hole, which spend too much working time. Grouting while Drilling could reduce working hours and maintain the advantage of flexible retaining wall with drilling fluid which is fit for realize on-demand protecting wall. So it is more suitable for "efficient" drilling requirements.Grouting while Drilling should guarantee that a certain amount of drilling fluid could be injected to target location and could consolidate to the suitable strength in a short time to achieve the goal of protecting hole wall. Therefore, there are three key technologies of Grouping with Drilling used in CBM:(1) the seepage law of drilling fluid in coal wall of hole, which determine the seepage depth of grout, guarantee the consolidation layer thickness and provide the theoretical basis for the subsequent study of drilling fluid formulas and tool used in realizing Grouping with Drilling;(2) drilling fluid formulas, whose consolidation time could be controlled, which have a good flow properties before grouting and could achieve a suitable strength after that, and could break gel itself to reduce the influence of coal seam gas several days after drilling finished;(3) drilling tool used in Grouting while Drilling, which should be able to perceive "normal drilling" and "grouting "signals from the driller, and perform the corresponding action according to the signal.Study focus on basic principle and basic methods of above key technologies, in order to looking for the breakthrough point for Grouting while Drilling used in CBM drilling in loose section, to lay a certain foundation for its in-depth and practical research, and lay a certain foundation for the other Grouting while Drilling to Protecting wall technologies at the same time.In the process of research, innovative research mainly reflects as the follows. Apply Grouting while Drilling Protecting Wall technology to solve hole accident caused by instability of the hole wall occurred in the process of underground CBM extraction drilling. Seepage grouting theory was applied to study the slurry seepage law in the process of under-ground CBM extraction driiling in loose period. Put forward the feasible sodium silicate drilling fluid formulas could be fit for that environment and cleared its nature, found its gel breaking performance; Performance testing used in Grouting while Drilling test tank is developed to simulate Grouting while Drilling process and observe the grouting seepage law; Grouting while drilling tool is developed, which only uses mechanical signal recognition and enforcement agencies.This study mainly finished the following work:1. Seepage law in the process of under-ground CBM extraction driiling in loose period is deduced.(1)Make reasonable assumptions according to the rheological characteristic of the drilling fluid, deduce infiltration grout diffusion partial differential equation and discretize it using implicit difference scheme. Give the initial conditions and boundary conditions, and use Thomas method to solve equations. Analyze the convergence and stability of the solution of difference scheme.(2) Based on the mathematical model above, calculate an engineering example. Solve the finite difference equations in Fortran language, analyze the infiltration grouting effect in different grouting pressure, and the grouting pressure values is gotten finally.Engineering example calculation is based on the actual geological data of Jincheng Sihe coal mine. The results show that with the increase of grout diffusion radius, osmotic grouting pressure decreases, when less than5mm grout diffusion radius, grouting pressure decrease rate is larger; When the grouting pressure P0≤1Mpa, and d>5mm, grout diffusion ability is reduced; Slurry pressure P0>1Mpa, grout diffusion effect is good, could protect the hole wall; Po>1Mpa, grout diffusion effects were similar, which means that the key factor deciding the grout penetration effect is the permeability of coal seam.(3) Analyze the effect from the initial pressure, slurry viscosity, GSI and the formation of broken with Fluent.Fluent simulation results show that if see the coal and rock as porous media, even for the modulus of2.9-3.0, viscosity of1.75Pa·S sodium silicate, and0.5MPa of CBM initial pressure, grouting depth is still more than30mm. the grouting depth are similar between coal damage and strong damage, crushing coal and pulverized coal theoretically. This is different from the mathematical model calculation results.2. Grouting while Drilling fluid(1) After summary, experiment, comparison and selection of the formula studied, determine the two formula that can be used for Grouting while drilling, Three acetic acid glyceride4%+20-40%sodium silicate and1,4butyl inner fat4%+20-40%sodium silicate. When encounter hole accidents in the process of drilling, add PH stabilizer extend the initial setting time. When meet high crushing degree, low permeability formation condition, surfactants are necessary for improve the permeability of the slurry.(2) Test the performance of above drilling fluid through a traditional test instruments and Grouting while Drilling performance testing experiment slot designed, including consolidation strength, penetration depth and gel breaking performance, etc., and analyze the advantages and disadvantages of the application of the drilling fluid in Grouting while Drilling. The experimental results from the traditional test instrument show that coal consolidation strength is lower after45 minutes and the formula is restricted in the field application, but after72hours, edge along the coal grains gradually appear a large number of cracks, which means that could break gel itself, and it is conducive to connect gas channels naturally.Developed a Grouting while drilling performance testing slot to observe permeability of the slurry, and study the feasibility of the grouting under the wind pressure, which include that whether the drilling fluid could be inject to hole wall by wind pressure to meet the depth demanded that can protect hole wall. The testing slot with soil pressure box installed in different depth can help observe pressure curve along with the change of time. Seepage process of slurry also can be directly observed through the window by the naked eye. Experiments show that:1) it is helpful under the action of wind for injecting slurry to coal seam;2) infiltration depth within five minutes in coal sample, which particle diameter is less than0.2mm, is more than100mm from the output results, when wind pressure is0.5MPa without confining pressure, which means that Grouting while Drilling helped by wind pressure is feasible according to the field experience that depth of10mm is enough;3) each layer corresponding liquid pressure value go higher with the increase of particle diameter of coal sample from the output of the soil pressure box;4)the pressures from soil pressure box are smaller than ideal value when the wind pressure is constant, which could be the result of the increasing viscous resistance along with the death deeper that effect the slurry seepage.(3)Dynamic viscosity of the above drilling fluid has been checked, through which the classification of the drilling fluid is determined. Through the parameter analysis, determine the best length of time to use it. It shows weak expansion plastic fluid state in the beginning20minutes after preparation, and the slurry viscosity is about2mPa. s, similar to water. Then the viscosity increase quickly in the next20to40minutes.40-45minutes later, it is freeze if the drilling fluid is in static, otherwise it shows suspended particle state and the viscosity will no longer be changed. These show that if PH stabilizer is not be added, the optimal grouting time is within20minutes. In the process of Grouting while Drilling, the drilling fluid that still not be injected into coal seam could not consolidate because it is flow, which is helpful for washing pipe by wind blowing after the grouting is over.3. Grouting while Drilling tool(1) Determine the design principle of Grouting while Drilling tool after analyzing the downhole working conditions and the necessary requirements of the existing conditions from several design schemes. The signal transmission is pure mechanical. Circulating medium-atomizing air is used as power, through which reverse circulation to transfer "normal drilling" and "start grouting" signals, trigger the drilling grouting tools, and realize the switch. When grouting is needed, short reverse circulation is done to open spray channel. Then start the positive cycle to spray fluid that could meet the requirements of protecting the holewall. When it is finished, by short reverse circulation again, open the normal channel to control the normal drilling began, which needs positive cycle.(2) Study a pure mechanical way to transfer "grouting" signal and "normal drilling" signal and design an actuator for pressure control. The one-way valve senses signal, and ratchet transposition mechanism designed to switch the function performs signals.(3) Study the atomized spray method for Grouting while Drilling, designed a swirl atomizing nozzle, which adapt the above drilling fluid, downhole pressure, and pump pressure very well.(4) With the grouting while drilling tools casing as the research object, the strength of the casing is analyzed and designed, the thickness of the casing how much they can bear on peripheral and internal pressure is determined; Under30MPa pressure, to guarantee the normal work of the grouting while drilling tools, the scope of its internal clearance which should be reserved is identified, In the torsional analysis using Prandtl analyzed stress function method and the method of material mechanics respectively, results show that the sleeve smaller end is the stress concentration area, and compared with the material mechanics, elastic mechanics calculation results are more conservative. The difference between the two results:when non simply connected region is given, Prandtl stress function is not accurate enough.(5) The grouting while drilling tools are designed and processed according to the results of theoretical analysis, and its basic performance has been tested. The experimental results show that reverse wind can control the switching of the signals of "injection" and "normal drilling" and the channel is smooth after that, so the drilling tool design can meet the requirements of Grouting while Drilling.4. The technology of Grouting while DrillingThe technology of the grouting while drilling is investigated preliminarily, including the determination method of grouting parameters, process execution of the grouting while drilling technology.The results can not only lay certain foundation for the in-depth and practical study of Grouting while Drilling used in underground coal seam gas drainage drilling, but also can be used as the basis of other conditions that Grouting while Drilling is helpful. |
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