Principle and technology of coring with in-situ pressure and gas maintaining in deep coal mine

The prevention and control of coal mine gas disaster are both crucial and challenging to mine safe production.The core issue lies in the accurate determination of coal seam gas content and pressure. At present,the open or ball valve sealed coring is widely used to obtain coal samples containing gas.However,these approaches demonstrate a low coring efficiency,poor timeliness,and easy gas loss,making it hard to ensure the test data's accuracy and effectiveness,and fail to effectively evaluate coal seam risk characteristic index to guide engineering practice.This study begins with an in-depth analysis of the connotation of deep coal seam in-situ pressure,expounds on the primary goals of in-situ pressure and gas retaining coring,and recommends some methods for determining and computing in-situ gas pressure and content.In addition,based on Steinmetz solid's geometric principle,the concept of pressure and gas retaining coring tool is proposed.The multiple anti-rotation coring structure design and coiled tubing operation technology help to minimize the disturbance of coal sample during the coring process,striving for consistency between the coal seam gas content of the samples obtained and the in-situ gas content.Five types of self-triggering pressure-maintaining controllers are designed,and the relevant parameters of pressure-maintaining capacity are tested on the self-developed indoor pressure retaining characteristic test platform.The results show that as the cone angle of the pressure retaining controller increases,the effective supporting surface of the edge increases as well,and the ultimate strength goes higher.At the same time,the torsion resistance experiment of the coiled tubing is carried out.When the torque,length and outer diameter are fixed,the torsion angle of the coiled tubing gradually decreases with the increase of tube wall thickness,enhancing the torsion resistance,and resulting in reduced disturbance to the core. Finally,a prototype of in-situ pressure and gas retaining deep coal corer is developed.It is composed of a roadway drilling rig,coiled tubing equipment,and pressure and gas retaining coring device. According to the simulation and laboratory test data,the saddle pressure-maintaining controller(the maximum pressure-maintaining capacity can reach 100. 9 MPa) and the coiled tubing made of CT90 steel(with a diameter of 38.1 mm,tube thickness of 3.18 mm) have been selected.The technology of in-situ pressure and gas retaining coring has been preliminarily established.The entire system runs smoothly with flexibility during the field test and successfully obtains pressure-maintaining samples.It has been preliminarily verified that the prototype design is reasonable and the assembly is precise and reliable.The coring system comes with excellent pressure-maintaining performance,low disturbance,high efficiency,and logging capability while drilling.It is expected to break through the technical bottleneck of in-situ gas pressure and content determination.