Experimental And Theoretical Studies On The Feasibility Of The Ice Temporary Plugging Fracturing Of CBM Well

Hydraulic fracturing is one of the effective measures to increase production in Coalbed Methane Reservoirs at present. However, due to the special nature of coal reservoir, conventional fluid loss control and temporary plugging means for improving coalbed methane fracturing effect is not very obvious, and there are some drawbacks. This paper provides a new technology named "Ice Temporary Plugging Fracturing" (ITPF), whose mechanism is: pumping the liquid nitrogen into the formation to reduce the temperature and form ice in the coal fracture, changing the natural coalbed into frozen coalbed, temporary plugging the formation to decrease the fracturing fluid leakoff or to divert.Based on the previous research work of CBM well fracturing fluid leakoff control, temporary plugging and related fields, this paper carried out the following studies:1) the forming conditions of ice,2) the plugging capability of the ice in coal fracture,3) the influence of freezing on the coal properties,4) the influence of freezing on the fracturing treatment,5) distribution and variation of coal temperature near wellbore.The main contents presented in this paper are:1. The development of the ice temporary plugging ability test deviceA special device is designed and manufactured to test the plugging capability of the ice in coal fracture. The "Core freezing unit" of the device has a "Double chamber and dual channel" structure. It makes the simulation of core freezing process under formation condition possible and can simulate the leakoff process of fracture fluid to fracture surface. The static strength of the device is analyzed. The device satisfies the requirements of security coefficient and selecting method of allowable stress value of domestic and foreign pressure vessel standard material. The working pressure of the device could reach 20MPa.2. Study of plugging capability of the ice in coal fractureThe formation condition and plugging capability of the ice has been tested. Under the formation pressure, ice generated at about -4 degrees Centigrade. The plugging capability of the ice is extremely high and temporary plugging by ice is theoretically feasible.3. Study of influence on physical property of coal due to freezingThe coefficient of linear expansion of coal rock has beed determined. The change rule of tensile strength of saturated coal with the change of temperature has been studied. Above the freezing point, the tensile strength of coal rock sample will decline with the decrease of temperature. Below the freezing point, the tensile strength of coal rock sample will increase with the fall of temperature, yet in a slow growth. The formation of ice increases the dispersion degree of the tensile strength of coal rock sample.4. Study of influence on hydraulic fracturing due to freezingThe formation fracture pressure prediction model which takes the influence of temperature into account has been establishes. The effect of artificial formation freezing on the principal stress and fracture pressure are studied. The artificial formation freezing has a great action on reducing the principal stress and fracture pressure. By the means of the 3D stimulation design software, it has analysised the influence of different formation temperatures on the fracture shape under the same treatment process. Freezing has the most influence on the Average Fracture Height, then Secondary Fracture Length, the DFN Fracture Length and the Max Average Width at Perforations.5. Heat transfer model of the ITPF TechnologyA new physical model of liquid-nitrogen Freezing system is designed. Based on the physical model, it establishes the wellbore temperature field and heat transfer model of coal seam, both considering the heat of evaporation and the change of physical property of nitrogen caused by the phase change. What’s more, it also establishes the calculation model of mass flow and pressure in the freezing system and the definite conditions, finally determining the solving method of the model.6. The dynamic simulation programThis paper has made the demand analysis on the dynamic simulation program of the ITPF technology. The overall design and detailed design on the procedure has been conducted from the top to down. It adopts Visual Basic 6.0 to take the procedural coding and debugging. The program has the function of "simulation of the Freezing process of the ITPF technology" and "display of ice plugging and changing situation".7. Simulation and analysis of the freezing processThe program has been used to simulate the ground freezing process of the ITPF technology. The distribution and change rule of temperature in near bore coal bed has been studied. Some important parameters such as liquid nitrogen dosage, freezing time and freezing radius are acquired. The temporary plugging scope is limited. ITPF could bring better result in turnaround fracture tech than decrease the fracturing fluid leakoff.According to the investigation in this paper, the Ice Temporary Plugging Fracturing Technology is feasible, which would play an important role in instructing the design and application at the site.