| Tectonic coal is widely distributed in China and it contains rich coalbed methane(CBM)resources.The new fractures formed by hydraulic fracturing or hydraulic cutting technology are not only difficult to open and connect with the original pores and fractures,but also easy to be filled and blocked by tectonic coal particles.Therefore,the theory and technology of horizontal well drilling and reaming in tectonic coal to release stress to promote CBM extraction has become a new technical direction,and have gradually become one of the technical bottlenecks that restricting the large-scale development of the CBM industry.Relying on the Major Scientific Research Instrument Development Project of National Natural Science Foundation of China “The analogue experiment system for the tectonic coal insitu CBM recovery by horizontal well cavern completion and stress relief(41727801)”,this thesis carries out the research by combining experimental test,theoretical analysis,geomechanical model test and numerical simulation analysis.The physical and mechanical properties of tectonic coal are measured,the dynamic response characteristics of drilling parameters during multistage reaming are analyzed,and the geological-engineering model for multistage reaming of horizontal well in tectonic coal is established.On this basis,an experimental system of reciprocating multistage reaming of horizontal well interacting with tectonic coal seam is independently developed.The drilling characteristics,the stress evolution law of tectonic coal surrounding rock,the deformation and damage characteristics of borehole in the whole process of multistage reaming in tectonic coal are systematically studied,and the interaction mechanism between multistage drilling-reaming and tectonic coal is revealed.The main research content and results are as follows:(1)The differences of physical and mechanical properties of tectonic coal and intact coal were analyzed.The structure of intact coal was uniform and dense,while the tectonic coal was affected by tectonic stress,resulting in a developed pore-fracture system with different pore fracture forms and irregular the distribution.The ultrasonic velocity and resistivity of tectonic coal were lower than those of intact coal.The strength and deformation resistance of intact coal was significantly higher than that of tectonic coal.The peak strength and elastic modulus under uniaxial stress were9 times and 22 times higher than that of tectonic coal,and the peak strength and elastic modulus under triaxial stress were 2～3.7 times and 12～25 times higher than that of tectonic coal.The postpeak deformation of intact coal was mainly strain softening,while the post-peak deformation of tectonic coal gradually transformed from strain softening to ideal plasticity with the increase of confining pressure.(2)The dynamic response characteristics of drilling parameters under the conditions of single hole diameter drilling and multistage reaming were revealed.The influence of drilling pressure,rotary speed,bit diameter,coal strength and stress state on drilling speed and torque were analyzed through the single hole diameter drilling test.The results showed that the drilling speed increased with the increase of rotary speed and drilling pressure,and the drilling torque increased with the increase of drilling pressure,bit diameter,stress and coal strength.The variation of drilling speed and torque,acoustic emission energy and strain of tectonic coal with the gradual increase of hole diameter were analyzed through the multistage reaming test.The results showed that the variation law of drilling speed was: initial drilling stage < primary reaming stage ≈ secondary reaming stage,and the variation law of drilling torque was: initial drilling stage < primary reaming stage <secondary reaming stage.The AE energy presented a “platform” distribution in the initial drilling,primary and secondary reaming stages,and the intensity increased in turn.The strain variation amplitude was the largest near the borehole wall and gradually decreased with distance,and increased with the diameter of borehole.(3)The deformation characteristics of surrounding rock and the mechanism of borehole shrinkage in tectonic coal were explored.On this basis,the geological-engineering physical model of multistage reaming of horizontal well in tectonic coal was established,and the multistage reaming technical method was proposed.Based on the physical model and technical method,the experimental system of reciprocating multistage reaming of horizontal well interacting with tectonic coal seam was developed.(1)The deformation characteristics of surrounding rock and the mechanism of borehole shrinkage in tectonic coal were studied,and the effects of mechanical parameters such as compressive strength,elastic modulus,internal friction angle and Poisson’s ratio,in-situ stress and drilling fluid column pressure on the borehole shrinkage were analyzed;(2)The stress evolution mechanism of coal body under the condition of multistage reaming was revealed.The concentrated stress during the formation of the initial borehole damaged the coal around the borehole,changing the coal structure and reduces the ultimate bearing capacity.Therefore,when reaming the initial borehole,the damage range of the coal increases and the concentrated stress under the new equilibrium state decreases;(3)The experimental system of reciprocating multistage reaming of horizontal well interacting with tectonic coal seam can realize the simulate of coal seams with various buried depths,carry out multistage diameter-changing reaming simulation without changing bit,monitor and collect the drilling parameters(such as drilling speed,rotation speed,drilling pressure,drilling torque),as well as the stress of simulated coal seam;(4)The similar materials of tectonic coal and its roof and floor were prepared according to the similarity theory,and a geomechanical test model was made.Meanwhile,the temporary plugging film-forming drilling fluid suitable for tectonic coal was developed.(4)Based on the multistage reaming simulation test coupled with drilling fluid,the drilling parameters and cuttings transport effect of drilling fluid were explored.The results showed that the drilling torque increased successively,but the variation of drilling pressure was not obvious in the process of multistage reaming.In the initial drilling stage,the cuttings carried by the drilling fluid were normal and the backflow velocity was stable.In the initial stage of primary reaming,the cuttings transport effect of drilling fluid was poor and the backflow was blocked first and then increased.In the secondary reaming stage,the annular space between drill pipe and borehole wall increased,therefore the backflow resistance of drilling fluid decreased.Compared with the initial drilling,the annulus backflow velocity during primary reaming and secondary reaming was significantly decreased.The shape of the borehole was approximately regular cylindrical after drilling and reaming.The drilling fluid was evenly attached to the borehole wall,showing a good wall-building ability.(5)The evolution law of stress and deformation of coal around borehole under the condition of multistage reaming was revealed.There was an obvious stress redistribution in front of the borehole during drilling,and the stress in front of the bit increased first,and then gradually decreased after drilling.The stress variation of coal around the borehole showed that after the primary and secondary reaming,the stress release range gradually increased,and the concentrated stress gradually transferd to the deep part of the coal.The apparent resistivity matched well with the damage evolution law of borehole,and the damage range of stress redistribution to the coal around the borehole can be intuitively obtained through the inversion results of apparent resistivity.Through the numerical simulation of drilling and reaming of horizontal wells in coal seams in structural areas,the differences of stress distribution and damage of surrounding rock in tectonic coal and intact coal were compared.The interaction mechanism between multistage reamingdrilling and tectonic coal seam was explained.The results would be helpful to reduce the risk of borehole instability during horizontal well drilling.There are 178 figures,30 tables and 286 references in this dissertation. |
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