Experimental Research Of Degradable Drilling Mud In Soft Coal Seam Drilling

Coalbed methane (CBM), refers to firedamp which is occurred in coal seam with methane as a main component, mainly absorbed on the matrix of coal particle surface, partially dissociate in coal pores or dissolved in coalbed water. CBM in China has characteristics of porous and fractured reservoir, low permeability, low reservoir pressure and low gas saturation, etc.The geological structure of our country is complicated, the forming age of coal is more, wide distribution, the characteristics of coal reservoir is large different. In general coal can be simply divided into hard coal seam and soft coal seam. To soft coal seam, because of the coal and gas outburst, coal seam is too soft and mechanical strength is low, and so on, using water or air and other conventional powder row drilling way appearing when the hole collapse, drill and spray holes and other issues, drilling hole is more difficult, gas extraction rate is low.Soft coal seam CBM development is one of the most severe challenges our country facing, protecting technology when drilling hole in such coal seam is the key one of the problems. In addition, to extract coal seam gas is one of the main purposes in drilling,so the requirements for drilling circulating medium has good reservoir protection effects, as far as possible not to harm of coal reservoir, i.e. not jam desorption of coalbed methane release channel.Drilling fluid is one of the key technologies in the realization of borehole protecting and reservoir protection. So we put forward research idea of a kind of environment friendly degradable drilling fluid,namely hole stability when drilling can be maintained, while in the drilling work ended, it can be automatically degradation and restoration of the coalbed methane desorption diffusion channels, realize dual purpose of retaining hole and reservoir protection.The first chapter briefly introduces the current technology of coal seam gas drilling and completion of coal bed gas in actual production and drilling fluid. Besides, it introduces the selected topic background, the significance of the research content of the summary, and put forward the key technique.In the second chapter, the basic characteristics of CBM are systematically summarized, the physical property of Jin-3coal seam are analyzed. Jin-3coal is anthracite, with crack size of1～10μm, almost containing no clay mineral with strong hydrability. It has a porosity of about10%, and gas permeability of0.3～0.5mD.The third chapter has analyzed the reservoir damage factors in coal seam gas drilling and performance requirements of application of drilling fluid. Coal reservoir drilling damage roots in six factors, including blockage of solid particles in drilling fluid, interactions between coal and drilling fluid polymer, coal matrix swelling, pressure sensitivity, water locking damage, and incompatibility of external fluid. The drilling fluid applied to CBM horizontal wells should meet five requirements, i.e., hole cleaning, borehole stabilization, hole lubricating reducing resistance, filtration control and reservoir protection.The fourth chapter analyses the temporary mechanism of plugging drilling fluid, such as the early adoption of calcium carbonate, polymer and the joint action of them to achieve plugging, the latter way of enzymatic hydrolysis and acidolysis to disassemble the plugging, while the acidification of coal and rock will occur during acid solution.The fifth chapter optimizes a set of degradable mud applied to the seam gas in horizontal wells, namely1000ml+6.5gCMC+10gDFD+12gCaCO3, and disassemble the plugging with JBR enzyme and5%dilute hydrochloric acid in stages. Performance test shows that, this set of drilling fluid has a lower specific gravity and water loss, good shear dilution. Swelling tests show that it has certain inhibitory effect to the expansion of coal samples. In this set of drilling fluid, solid phase particle size distribution region is wide. It has good stability and lubricity. In static condition, it has good suspension effect to cinder below20heads.The sixth chapter is the focus of this paper. It tells about evaluation methods of reservoir protection degradation. This chapter firstly has systematic summary and evaluation of the effect of the polymer biology enzyme degradation. There are eight methods:viscosity attenuation method, mud cake cleaning method, core permeability recovery method, determination of reduced sugar, carbon dioxide emissions, determination of dissolved oxygen determination method, laser particle size analysis method, mold method. Then apply the viscosity attenuation method to the evaluation of temporary plugging drilling fluid degradation. Research indicates that more than75%can be broken in4hours. The mud clearance Experimental results show that, temporary plugged slurry in the condition of enzymatic hydrolysis acid solution in2hours+2hours, can be completely degraded, and both of them are indispensable. Laser particle size analysis shows that as the degradation of the slurry, particle size of median will drop from437μm to15μm. Finally, use this set of formula to have degradation test on undisturbed coal samples and artificial coal sample. The results show that only the enzymatic acid solution can achieve completely degraded.The seventh chapter of the paper puts forward the main innovations and further points need improvments.The main innovation points of this paper involve as follows.(1) On the basis of adopting conventional biological enzyme decomposable drilling fluid, this paper puts forward a new composite degradable drilling fluid of enzymatic acid solution.(2) According to the optical principle, I apply the spectrophotometer which typically used for water analysis to evaluate stability of drilling fluid containing a large amount of solid particle.(3) In line with the principle that the fluid column pressure will change during suspended slag process, I made a device to evaluate the drill slag suspended effect of drilling fluid, and set up a set of evaluation criteria.(4) Apply the laser particle size analyzer to the evaluation of degradation effect of drilling fluid degradation.(5)Propose a rule that heating can increase the permeability of coal and rock, moreover, have further research on it through experiments.