Study On Liquid Nitrogen Compound Fracturing In Coalbed Methane Wells

Most coalbed methane reservoirs in China are low permeable.Fracturing is the main stimulation technology for coalbed methane exploitation.However,there are many problems in conventional hydraulic fracturing,such as serious fluid loss,short fracturing fractures,poor proppant placement,and difficult flowback of fracturing fluid.Therefore,a novel method of liquid nitrogen-nitrogen gas-water compound fracturing to improve the fracturing effect of coal seam is proposed in this study.First,nitrogen is injected into the wellbore to drive the bottom hole water into the coal seam.Then,liquid nitrogen is injected to fracture and freeze the coal seam.Then nitrogen is injected again to drive the liquid nitrogen deeply into the fracture to increase the frozen area and prevent subsequent sand-carrying fluids from freezing in the borehole.Finally,conventional hydraulic fracturing is performed,and sand-carrying fluid is injected to prop the fractures.In order to reveal the mechanism of liquid nitrogen compound fracturing in coal seams and explore the feasibility of applying liquid nitrogen compound fracturing to improve the fracturing effect of coalbed methane wells,a combination of laboratory experiments,theoretical analysis and numerical simulation was used to study the mechanical properties of coal under liquid nitrogen freezing,characteristics of liquid nitrogen flow and heat transfer in fractures,fracture formation mechanism of liquid nitrogen compound fracturing,adaptability evaluation of liquid nitrogen compound fracturing technology and sand carrying method selection of liquid nitrogen compound fracturing.The main achievements are as follows:1.The physical and mechanical properties of different types of coal before and under liquid nitrogen freezing were tested,and the strengthening and damage mechanisms of water-saturated coal under liquid nitrogen freezing were revealed.The results show that liquid nitrogen freezing can strengthen and damage coal simultaneously.And the pore water inside the saturated coal freezes into ice also can strengthen and damage the coal simultaneously.In addition,the larger the rock porosity,the more significant the strengthening effect of liquid nitrogen freezing.2.The flow and heat transfer model of liquid nitrogen in fractures was established,and the heat transfer characteristics of liquid nitrogen in fractured fractures were obtained.The results show that nitrogen at the fracture tip is in a supercritical state during the process of fracture extension with liquid nitrogen,and the temperature and pressure of the supercritical nitrogen at the fracture tip are similar to the reservoir temperature and fracture extension pressure.In addition,increasing injection rate is the best way to increase the freezing area of liquid nitrogen fracturing under the same injection volume.3.Experiments of water fracturing,liquid nitrogen fracturing and liquid nitrogen compound fracturing under true triaxial stress were carried out to reveal the fracturing mechanism of liquid nitrogen compound fracturing.The results show that the compound fracturing has experienced two stages: liquid nitrogen fracturing and water fracturing.Compared with water fracturing and liquid nitrogen fracturing,compound fracturing can produce more complex and highly conductive fracture networks.4.Experiments of water fracturing,liquid nitrogen fracturing and liquid nitrogen compound fracturing on different coal-rank coals were carried out.And the adaptability of liquid nitrogen compound fracturing in coal reservoirs with different physical properties were revealed.The results show that the development degree of natural fractures and mechanical properties of coal are the main factors affecting the effect of compound fracturing.Liquid nitrogen compound fracturing can greatly improve fracture complexity in reservoirs with physical properties similar to Qinshui high rank coal and Guangsheng low rank coal and form a complex fracture network,and increase the production of single-well.5.The migration model of sand-carrying fluid in coal fractured fractures was established.The necessity of water carrying sand in liquid nitrogen compound fracturing is analyzed.The results show that the sand carrying capacity of liquid nitrogen is lower than that of water.Increasing flow velocity,reducing proppant concentration,reducing proppant particle size and reducing proppant density can improve the sand carrying effect of liquid nitrogen.Taking the actual fracturing operation as an example,if liquid nitrogen is used as the sand carrying liquid,the displacement should exceed 9 m3/min to achieve the same sand carrying effect as water.The field conditions cannot reach the displacement of liquid nitrogen carrying sand.Therefore,conventional hydraulic fracturing to improve fracture width and proppant placement after liquid nitrogen fracturing is the preferred method.The research results reveal the mechanism of liquid nitrogen compound fracturing,and explore the feasibility of liquid nitrogen compound fracturing in coal seam.It is expected to provide a feasible technology for efficient development of coalbed methane.