煤层气井合层排采控制方法

以铁法盆地大兴井田合层排采工程井DT31井和沁水盆地南部单层排采井QS1井排水产气特征数据为基础,分析了合层排采井各排采控制阶段的流体相态特征与单层排采的异同,总结了合层排采中的层间干扰因素及排采工艺中存在的问题,探讨了单位井底流压降幅的产水量和套压作为排采控制指标的控制方法及原理。研究结果表明:不同液面深度下的单位井底流压降幅的产水量可指导制定合排期间的排水强度;合采井深部产层的临界解吸压力液面深度与顶部产层埋深重合,不适宜合层排采;憋压阶段套压的最大值主要受产层顶板埋深和初期排水降液面阶段的总压降值限制。控压产气期,采用阶梯式降套压法,同时需控制套压瞬时降幅和日降幅以防储层激动,合采井在控压产气和控压稳产阶段设置一个最小套压可以缓解产气期间液面深度与浅部产层埋深接近或重合引起的矛盾。

Drainage control of single vertical well with multi-hydraulic fracturing layers for coalbed methane development

The drainage control method was studied for multi-hydraulic fracturing layers coalbed methane (CBM) extraction.The research was based on several vertical wells，the first was single vertical well with four hydraulic fracturing layers，such as well DT31，located in Daxin Coal mine，Tiefa Basin in northeast China，the other was well QS1 with one hydraulic fracturing layer in the south of Qinshui Basin.The reservoir characteristic parameters of well DT31 and well QS1 were analyzed，including reservoir thickness，gas content，gas resource，reservoir pressure etc.The drainage data of water and gas production rates were all studied in various extraction stages.The principle of the interference was discussed while various hydraulic fracturing layers were developed in a vertical well based on the difference of the fluid phase characteristics from start to finish.The water production rate within 0.01 MPa drop of the bottom well pressure (termed Wpp) and the casing pressure were presented as two key control indices during several hydraulic fracturing layers drainage.The results indicate that the water production rate for several hydraulic fracturing layers drainage can be formulated according to the curve of Wpp with water surface depth.The two reservoirs are not available for drainage in a well while the water surface depth corresponding to the critical desorption pressure of the deep reservoir and the buried depth of the shallow reservoir coincide.It is essential to control the casing pressure after gas production begin.The maximum casing pressure during the process of pressure-holding is determined by two values，the first is the buried depth of the top coal seam roof in the shallow reservoir，and the second is the sum of the pressure drop in the process of the initial water drainage for liquid level decrease in wellbore.The casing pressure should be reduced only when the curve of the casing pressure and gas production rate are all on rise at the stage of gas production under casing pressure and downhole pressure control.The instantaneous drop of the casing pressure should also be limited in a maximum value to prevent reservoir permeability weakening.The results also indicate that if the casing pressure is maintained greater than a setting minimum during the process of gas production and stable yield under casing pressure and downhole pressure control，it would help to achieve an optimal drainage efficiency in a single well.