Determination and application of critical desorption pressure in overpressure seam of Western Guizhou

In order to reduce the gas drainage damage by the overpressure seam before desorption,it was necessary to determine the critical desorption pressure of the coalbed methane well in the overpressure seam so as to make a fine gas drainage control. The paper had a discussion on the overpressure identification method of the seam and the influences to the gas drainage control. According to the geological parameters and gas production data of the overpressure coalbed methane wells in multi seams of Songhe Block in Guizhou, the numerical model of the critical desorption pressure in the overpressure seam in the block was established. The results showed that the overpressure coal reservoir with many free gases could cause a big difference between the critical desorption pressure of the overpressure seam and the calculation value of the Langmuir’s equation. The overpressure coal seam would have a high energy and the pressure fluctuations occurred from the gas drainage process would have a high possibility to cause a Jamin Effect. The critical desorption pressure calculated by the established mathematical model could be applied to the gas drainage control of the overpressure coalbed methane well. Before the casing pressure occurred, the flow back rate was up to 37.1% and could effectively avoid the coal seam damage caused by the irrational gas drainage control. The study on the critical desorption pressure of the overpressure seam could provide the guidance to the fine gas drainage control of the coalbed methane well in this type coal seam before desorption.