超破裂压力注气开发煤层甲烷探讨

中国大多数煤层气藏具有低压、低渗、低饱和等特点，采用常规增产改造和降压开发技术难于奏效。文章提出了超破裂压力注气技术开发该类低渗煤层气藏的设想，将一次开采与增产改造、提高采收率紧密结合，在开发初期即实施提高采收率措施。研究表明，煤层甲烷解吸-扩散-渗流过程受多种因素影响，在煤层气开发中，一方面需要提高煤层渗透率，另一方面还必须促进甲烷高效快速解吸。超破裂压力注气开发煤层甲烷的机理主要包括降低甲烷分压诱发甲烷解吸，通过竞争吸附置换解吸甲烷，诱发微裂隙形成和天然裂缝延伸，补充能量，防止裂缝闭合和水相圈闭，降低应力敏感损害。超破裂压力注气方式包括注气吞吐和气驱，注气吞吐适用于单井开发，气驱则要形成注采井网。要针对注气开发煤层气特点，选用合理的注气和完井方式，形成注气开发煤层气配套技术，提高煤层甲烷产量和采收率。

DISCUSSION TO EXPLOIT COAL-BED METHANE BY GAS INJECTION WITH OVER-BREAKING PRESSURE

Most of coal-bed gas reservoirs characterize low pressure, low permeability, low gas saturation etc. in China. It is not successful to use the techniques of conventional stimulations and pressure-decreasing recovery. The article proposes the idea to exploit the coal-bed gas reservoirs with low permeability by gas injection with over-breaking pressure, which combines the primary recovery with the stimulation and the improvement of recovery factor closely and performs the measures to improve the recovery factor even in the initial production stage. Study shows the process of desorption-diffusion-percolation of coal-bed methane is affected by several factors. It is necessary for coal-bed gas exploitation to improve the permeability of the reservoir in one hand and promote efficient and fast desorption of the methane in the other hand. The mechanism to exploit coal-bed methane by gas injection with over-breaking pressure mainly includes decreasing the partial pressure of methane to induce methane desorption, displacing the desorbed methane by competitive sorption, forming micro-fissures and extending natural fractures, increasing energy for gas flow, preventing fracture closure and water-phase trap, and reducing stress-sensitivity damage. The mode of gas injection with over-breaking pressure includes the huff and puff of gas injection and the gas drive. The mode of huff and puff is suitable for single-well reservoir. The mode of gas drive can only be used with injection/production well pattern. It is suggested according to the characteristics of coal-bed gas exploited by gas injection, the proper modes of gas injection and well completion should be selected, and the matching techniques of coal-bed gas exploited by gas injection should be developed to improve the production and recovery factor of coal-bed methane.