| Coal is the main energy source in China and has long-term demand and irreplaceable characteristics.Methane gas is an associated gas of coal,which is both a major source of mine disaster and a clean energy source.At present,gas drilling has become the main way to prevent coal mine gas disasters and develop underground gas resources.However,as the depth of coal seam exploitation in China reaches the geological stage of low permeability,softness and high ground stress,the problem of deterioration of gas drilling is becoming more and more prominent,which seriously restricts the efficient extraction and large-scale economic utilization in coal mines in China.Field engineering shows that the borehole sealing performance is the main technical index of the efficiency,concentration and utilization rate of the gas drilling.It is a comprehensive characterization of the time effect between the gas emission rate and gas extraction concentration and gas extraction allowable concentration controlled by the leakage amount of the seal section.Therefore,it is necessary to further study the change law of gas drilling seal with time and improve its control technology,which is of great engineering value and social significance for improving gas drilling efficiency,extraction concentration and utilization rate.In this thesis,the theoretical derivation,experimental determination,numerical analysis,example verification,material invention and field text are integrally applied to fissure formation and evolution,in the borehole seal section,as well as leakage characteristics.Meanwhile,systematic and in-depth study on time-varying law of gas emission along with control mechanism of sealing and phase change gel technology has been utilized separately.To clarify the cause of gas drainage drilling theory fundamentally together with proposing a method of sealing the borehole with the phase change gel materials,at the same time,which has been verified efficaciously by field application.The specific research results are as follows:(1)Based on the plastic softening,expansion and creep properties of coal and rock,unified strength theory is employed to analyze the mechanical deformation model of viscoelastic,plastic-softening and plastic-flow,so as to obtain the damage radius and the minimum borehole sealing radius plus analytical solution of the sealing radius theoretically.Combining with the physical and mechanical parameters of coal and rock,the impact of the stress environment,mechanical parameters and time factors on the above theoretical values is analyzed numerically and the deformation damage of roadway and surrounding rock is mainly generated at the beginning of its formation.The intermediate principal stress coefficient and the mechanical strength of coal and rock have the main control effect on the mining radius,while the softening and expanding capacity of rock has obvious influence on the surrounding rock under unsupported conditions,the average value is about 5%.At the same time,based on the structural characteristics of the borehole seal,the concept and classification of the heterogeneous structure of the borehole seal section are proposed,and the heterogeneous structure of the borehole seal section under the condition of typical layered heterostructure is theoretically and numerically simulated.The damage characteristics are analyzed,and the damage of the fracture structure of the borehole seal is obtained.The radial fracture damage and interface friction damage caused by the uncoordinated deformation of the heterogeneous materials are mainly caused by the radial stress damage and the drilling of the plastic flow zone.The lower the strength of the sealing material,the more the cracks in the seal segment heterogeneous structure develop over time.(2)Based on the contact-type layered heterostructure model of the borehole seal section,the gas seepage field of the borehole seal is analyzed.The Darcy seepage law is used to derive the gas leakage control equation of the borehole seal section under the plastic damage condition of the coal wall and the surrounding rock.The research shows that the change of air leakage with time mainly occurs in the initial stage of gas drilling,reaching a stable value in about 10 days.When the plastic flow area of the surrounding rock is completely sealed,the leakage of the drilling seal section can be controlled above 60%.(3)By analyzing the characteristics of borehole gas flow field and using Darcy seepage theory,the steady-state and unsteady gas flow control equations under the superimposed flow field of homogeneous coal seam are derived.Based on the gas storage parameters of 3~#coal seam of Gaohe coal mine,numerical analysis shows that the gas emission quantity of borehole is divided into four stages:rapid decay period,fast decay period,slow decay period and stable period.With the increase of gas pressure and gas permeability,the span of each decay period is prolonged,and the amount of surge during the steady period is increased.At the same time,compared with reducing the suction negative pressure and increasing the diameter of the drill hole,the gas emission can be increased by 2.7 times for each 100m increase in the length of the drill hole,indicating that the increase of the length of the drill hole can effectively increase the gas amount per unit time.(4)According to the concept and calculation method of borehole sealing degree,the evaluation model of borehole sealing performance is proposed,and it is pointed out that the attenuation of gas concentration in borehole extraction is a direct reflection of the deterioration of borehole sealing performance.Combined with the time-effect governing equations of the leakage amount of the borehole seal section and the gas emission quantity of the borehole,the time-varying control equation of the gas drainage concentration of the borehole is derived,and the decrease of the gas concentration of the borehole extraction is mainly divided into rapid decline period,slow down period and stable period.When the sealing rate reaches 50%,the gas concentration can be increased to more than 25%under the same coal seam conditions.Based on the relationship between the theoretical leakage of the borehole and the actual air leakage,a drilling seal quality evaluation model was established.After analyzing the environmental characteristics of the sealing hole,the sealing grouting parameters and the physical and mechanical properties of the existing drilling sealing material,a novel phase change gel(PCG)drilling seal theory using the"phase transition"properties of gel materials to meet the requirements of sealing materials at different stages of drilling is proposed.(5)The basic formula of the phase change gel sealing material was determined by screening and modifying the water-soluble polymer material.Material performance tests show that PCG materials are superior to current drilling sealing materials in terms of permeability,cementation,degradability and cost.Based on the typical difficult-drilling coal seams in the north and south wings of 3~#coal seam of Shanxi Gaohe coal mine,phase change gel sealing technology was applied to the gas extraction drilling under the corresponding conditions.Compared with the"two plugs and one injection"sealing technology of expanded cement,the average amount of drilling gas extraction is increased by 1.5 times and the extraction concentration is increased by 2 to 4 times.The phase change gel has obvious effect on improving the sealing performance of the borehole.The results of the thesis systematically studied the control mechanism of the borehole seal,and applied the new phase change gel drilling seal material to improve the borehole sealing performance.It provides scientific theoretical guidance and feasible and effective technical means for the current low gas drilling concentration and low utilization efficiency.The results have important engineering value and social significance for realizing the gas“safety-resource-environment”sustainable development strategy. |
PDF Full Text Request