沁水盆地南部高煤阶煤层气井“变速排采-低恒套压”管控方法

为了提高煤层气井排采管控的科学性,以沁水盆地南部3号煤为研究对象,基于储层气-水运移产出过程和相对渗透率特征,探讨了煤储层气水产出控制机理及其影响因素,通过开展不同尺度裂隙系统内气水运移实验,分析了各类气水产出影响因素的影响模式及主要作用阶段。以降低气水运移影响因素造成的储层伤害、减小各排采阶段渗透率损失为主要目的,建立了适应于沁水盆地南部高煤阶煤层气井的"变速排采-低恒套压"排采控制方法。研究表明,气水产出依次通过基质孔隙、微观裂隙、宏观裂隙和人工裂缝,期间受到毛细管力、有效应力、启动压力和气水相渗等4要素耦合控制,压裂增压后地层毛细阻力明显增大、排水降压后有效应力会导致裂缝闭合、启动压力使气体产出滞后、气水相渗影响流态的稳定。当气井处在不同的排采阶段时,影响排采效率的主控影响因素各不相同。可将煤层气井降压产气过程依据储层压力(Pc)、临界解吸压力(Pde)、见气压力(Pjq)与井底流压(Pjd)的关系划分为4个阶段,认为PcPjd时需要以0. 1 MPa/d的降压速度快速排采以迅速克服毛细管力,降低水敏伤害; PjxPjdPc时需要以0. 05 MPa/d的降压速度排采,避免裂缝过早闭合,降低应力伤害; PjqPjdPde时需要以0. 02 MPa/d的降压速度缓慢排采,减小气对水的抑制作用; PjdPjq时采用0. 01 MPa/d的降压速度提产,同时保持套压不高于流压的一半,保持一定压差克服启动压力。在沁水盆地南部樊庄-郑庄区块应用"变速排采-低恒套压"排采控制方法,对比邻近相同地质条件、开发技术的井,相同流压时日产气量提高至原方法的1. 4倍,日产水量提高至原方法的2倍,排采500 d后的累产气量增加近25%。4个排采阶段单位压降产水量均高于传统排采管控方法。

“Variable speed drainage-low casing pressure” control method of high rank CBM wells in South Qinshui Basin

In order to improve the scientificity of CBM drainage and production,take Fanzhuang block of south Qinshui basin as a study case,based on the gas-water transport process and relative permeability characteristics of No.3 coal reservoir,the controlling mechanism and influencing factors of gas-water productive process in coal reservoir are discussed,and the influencing models and main stages of various factors affecting gas-water production are analyzed by carrying out experiments on gas-water migration in different scale fracture systems.For reducing reservoir damage caused by factors affecting gas-water migration and permeability loss in each drainage stage,the “Variable speed drainage-low casing pressure” control method is used for Qinnan block.The research shows that the gas and water products are successively passed through matrix pores,microscopic pores,macroscopic fractures and artificial fractures,during the passing period,the capillary force,effective stress,trigger pressure and the relative permeability of the four elements coupling control the process.In particular,the resistance of the capillary force increases significantly after fracturing,the increase of effective stress leads to crack closure,starting pressure makes gas output lag,and relative permeability affects the stability of flow regime.The main influencing factors of drainage and production efficiency are different in each production stages.Based on the differences above,the process of CBM well pressure reduction and gas production can be divided into four stages according to the relationship between reservoir pressure(Pc),critical desorption pressure(Pde),gas sight pressure(Pjq) and flow pressure(Pjd).When Pc