Closure trajectory model of pressure-preserved controller for coring in coal seam surface well

The traditional method of coal exploration is to obtain coal samples using an open coring tool and estimate the gas loss through theoretical calculations. However, the measured gas content of these coal samples is generally low, significantly impeding the development and utilization of coalbed methane and compromising the safety of coal mining operations. To meet the demands of deep coal exploration, an innovative pressure-preserved coring tool was specifically designed for the coal seam surface wells. This tool has a total length of 6.7 m and can extract a 3 m long pressure-preserved coal core sample at a single time. In order to enhance the reliability and success rate of pressure-preserved coring in deep coal seam, a closure trajectory model was constructed for the pressure-preserving controller operating in a downhole fluid environment. Meanwhile, the variation curve of the elastic torque for the triggered shrapnel of the pressure-preserved controller was obtained through experiments and theoretical derivation. To validate the accuracy of the closure trajectory model and gain insights into the flipping and closing process of the pressure-preserved control valve cover in a drilling fluid environment, laboratory experiments were conducted to trigger the closure of the pressure-preserved controller. The results indicate that the measured moving trajectory of valve bonnet in the experiments has a deviation less than 5% from the theoretical calculations, verifying the accuracy of the model. On this basis, the trigger system of pressure-preserving controller within the pressure-preserved coring tool was optimized. Besides, eight bottom-pull triggering experiments were conducted in a test well with a deep of 30 m in the bentonite-based mud environment at a density of 1.1 g/cm³ and viscosity of 60 s. In all the experiments, the pressure-preserved controller can be closed stably with a 100% success rate, and ensure no leakage for at least 350-min continuous operation at 14 MPa, thereby verifying the reliability of the trigger system of pressure-preserved controller. The model presented in this paper can accurately predict the closure trajectory of the pressure-preserved controller in the downhole, enabling the design of customized triggering systems for different drilling mud systems to enhance the success rate of pressure-preserved coring in deep coal seam.