Experimental design for teaching hydraulic fracture extension behavior in true triaxial coal rock
[Objective]As a critical component of China's energy mix,coal resources play a vital role in the rapid development of the national economy.Therefore,the safety and efficiency of mining technology is paramount.Hydraulic fracturing technology has proven to be an effective method for enhancing coalbed methane production in mining operations,finding widespread application in coal mining,shale gas mining,and various other industries.With shallow coal mines gradually depleting,there is a continual increase in the depth of coal mining each year to meet the escalating demand for energy production and consumption.Understanding the expansion mechanism of hydraulic fractures in coal rock bodies under true triaxial stress conditions is crucial for mastering hydraulic fracturing technology.This knowledge is essential for efficiently extracting gas from underground coal mines,providing valuable guidance for developing specialized core courses on gas prevention and control in mines and predicting and preventing gas disasters,providing practical value to the mining industry.[Methods]The hydraulic fracturing test is conducted using a self-developed true triaxial coal fracturing and seepage experimental system.Triaxial stress loading in the system involves pre-pressurization via oil pumps and step-by-step pressurization using constant speed and constant pressure pumps to gradually apply and release loads.Fracturing fluid is injected into the specimen through a piston driven by the booster fluid of the constant-speed and constant-pressure pump,ensuring a stable state of constant flow and pressure during the injection process.The data acquisition and control subsystem can monitor and control other subsystems in real time,independently collecting data and generating curve diagrams.This capability enables practical comparative analysis of the pressure-time curve evolution during the injection of true triaxial samples and the hydraulic fracture expansion mechanism.By acquiring injection pressure-time curves and hydraulic fracture expansion patterns during the injection of fracturing fluid,the behavioral characteristics of hydraulic fracture expansion under true triaxial stress conditions can be explored and analyzed.[Results]Throughout the experiment,the real triaxial stress state exhibits minimal fluctuation over a five-minute period,with the maximum deviation recorded at±0.02 MPa,fully meeting the experimental requirements.In the initial phase of fracturing fluid injection,as the injection point reaches full capacity,the injection pressure abruptly increases to 6.6 MPa.Subsequently,upon the initiation of hydraulic fracture cracking,the fracturing fluid follows the fracture path,leading to a rapid decline in injection pressure.Throughout the subsequent stable expansion of the hydraulic fracture,the injection pressure remains essentially constant at 2.65 MPa,gradually decreasing to 2.47 MPa.After the specimen undergoes complete fracturing,the hydraulic fracture penetrates through,leading to a substantial overflow of fracturing fluid and consequent rapid depletion of the fluid.The notable rise in injection pressure during the initiation stage of hydraulic fracture is due to the fracturing fluid at the injection point needing to exert pressure exceeding the maximum tensile strength of the specimen under the true triaxial stress state.This condition is vital for initiating the hydraulic fracture and its subsequent expansion.Once cracking begins in the hydraulic fracture,the stress field surrounding the fracture is affected by the disturbance.The expansion behavior during this phase is primarily determined by the stress distribution at the fracture tip and the mechanical properties of the specimen.[Conclusions]Building upon an independently developed experimental system for true triaxial coal rock fracturing seepage,this paper establishes a comprehensive set of experimental methods for true triaxial coal rock hydraulic fracturing.By applying the principles of hydraulic fracturing technology in underground coal mines,students can design stress state parameters for laboratory true triaxial hydraulic fracturing experiments using actual ground stress data from field engineering applications.Furthermore,they can explore the impact mechanisms of constant-flow,constant-pressure,and variable-displacement injection of fracturing fluids on the injection pressure-time curves and the morphology of hydraulic fractures.This experimental teaching approach enables students to develop a mindset oriented toward scientific research and innovation.It highlights the fusion of theory and practice,fostering enthusiasm for learning and strengthening subjective innovation capabilities.
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