Coupling Mechanism On Reservoir Damage Of Coal-bed Gas Well In QinNan

Inorder to deepen the understanding of the harm caused by CBM wells extraction in the Qinshui area, improve controlling technology of discharge and miming, and save the cost of production and management. We take Qinnan Coal-bed Gas field as a research instance, by collecting CBM wells engineering data and discharge-mining data systematically, gathering water samples and coal-rock samples in the vicinity of CBM well, expounding the production features of damage caused by CBM wells with higher, middle or lower level yield. At the same time, the paper reveals CBM wells discharge-mining reservior damage’s occurrence condition and damage degree under different production intensity by means of coal reservior’s simulation experiment about velocity sensitivity. Jam in effect and stress sensitivity, coupling analysis production features under different discharge-mining stage of the CBM wells and different discharge-mining harm, and promulgate the inner relations between velocity-sensitivity, jamin effect, stress sensitivity and CBM wells discharge-mining’s intensity, catch up with optimization craftworks for CBM wells’ discharge-mining work. The main conclustions achieved by this research are following:(l)The discrimination model model of damage mechanism in the extraction process of CBM wells was established:the velocity sensitivity effect is easy to occur in the early stage of Qinnan coal-bed gas wells, and the peak period of gas lock appears in its middle stage, and stress sensitivity effect occurs in its late stage. In the discharge and mining process, gas lock is the easiest to occur in reservoir damage of above three damages, velocity sensitivity effect is easier to occur, the last one is stress sensitivity effect.(2)Through the velocity sensitivity test, when the drainage rate is higher than8m3/d, the occurrence possibility of velocity sensitivity effect can greatly increased.8m3/d drainage rate can be used as a safe drainage boundary. And the same time, obtained four kinds of change patterns of permeability in velocity sensitivity test:﹕lowing down after the sharp decline in a certain point;②decreasing rapidly and then begining steady decline in a turning point;③after a period of time, permeability gradually become smaller, after rising slowly, and then decreasing slowly;④permeability showed a linear decrease. (3)Proved that lager drainage rate can easily decrease gas and water relative permeability. when the drainage rate is higher than8m3/d, the higher of the drainage rate, the grerater the loss of relative permeability caused by it.(4)The result of stress sensitivity experiment shows that:①ore pressure increasing (up to3.5Mpa) when the effective stress is lower, the permeability of coal reservoir improved. This improvement is more obvious when the original permeability greater.②The loss of permeability is larger before3.5Mpa effective stress, the loss of permeability is the highest when the effective stress after2.5Mpa or3.5Mpa.The stress sensitivity is strongest when the effective stress increase from2.5Mpa to9Mpa.③he permeability of coal reservoir decrease with pore pressure increase, when the pore pressure was lower than the critical pressure for permeability change (pore pressure in the process of experiment increased from0.8Mpa to3.5Mpa).Model simulation results show that the occurrence probability of stress sensitivity and velocity sensitivity effect coupling is bigger during the late of the low productivity wells.