高煤阶煤层气井储层压降扩展规律及其在井网优化中的应用

Pressure drop expansion law of high rank coalbed methane reservoir and its application in well pattern optimization

为明确煤层气井开发过程中煤储层压降扩展规律,建立了考虑气水同流的压降扩展模型,开展了压降扩展速度及压降漏斗扩展形态主控因素分析,在沁水盆地南部樊庄、郑庄区块设计了5口直读压力钻孔,实时监测煤层气井生产过程中地层压力变化和传导规律,基于监测结果讨论了储层压降扩展在井网优化中的应用。研究结果表明:高煤阶煤储层在平行于最大水平主应力方向的压降扩展因压裂裂缝影响存在明显的双段性,压降横向及纵向扩展速度较快;在垂直于最大水平主应力方向的压降扩展双段性不明显且压降扩展速度较慢。甲烷解吸膨胀的弹性能对煤层气井压降扩展的影响不可忽略,储层边界的压力扩展在压力降至临界解吸前后存在“双峰”变化。在相同排采制度下,压降漏斗扩展速度及形态主要受水动力条件、渗透率、地解比及束缚水饱和度影响,水动力条件越活跃、渗透率越低、地解比越小、束缚水饱和度越大,越不利于压降横向及纵向传播,含气饱和度较低的储层进行开发时,需要适当延长排水期、扩大压降漏斗在解吸前的波及范围,才能够在解吸后获得更快的解吸效率,以换得更长的稳产时间。通过分析部署在樊庄、郑庄开发井网内的井间地层压力监测井实测数据,认为储层压降扩展存在协同降压和协同解吸2个阶段,为了实现协同增产,两井井距范围内煤储层压力必须下降至解吸压力以下。使井间储层压力全部降低至解吸压力以下的最大井距定义为“协同增产临界井距”。该井距受渗透率及应力场控制,渗透率控大小、应力场控方向。渗透率越高,临界井距越大;与最大水平主应力方向夹角越小,临界井距越大。煤层气开发过程中应考虑不同方位的“协同增产临界井距”差异,开展差异化井网井距设计,以最大化动用资源,提高采收率。

In order to clarify the law of reservoir pressure drop expansion,a pressure drop expansion model considering the same flow of gas and water is established,and the factors affecting the pressure drop expansion velocity and the shape of pressure drop funnel are analyzed.The results show that the pressure drop expansion of high coal rank coal reservoir is affected by fracturing,and the pressure drop expansion in parallel to the horizontal maximum principal stress direction is obviously double due to the influence of fracturing fracture,and the pressure drop longitudinal expansion velocity is faster.The influence of the elastic energy of methane desorption expansion on the pressure drop expansion of CBM wells cannot be ignored.The pressure expansion of reservoir boundary has a“double peak”change before and after the pressure drops to the critical desorption.Under the same drainage system,the expansion speed and shape of the pressure drop funnel are affected by hydrodynamic conditions,permeability,solution ratio and bound water saturation.The more active the hydrodynamic conditions are,the lower the permeability is,the smaller the geo solution ratio is,and the greater the irreducible water saturation is,the more unfavorable the lateral and vertical propagation of pressure drop is.When developing reservoirs with low gas saturation,it is necessary to extend the drainage period and expand the sweep range of pressure drop funnel before desorption,so as to obtain higher desorption efficiency after desorption and obtain longer stable production time.By analyzing the measured data of the inter-well formation pressure monitoring wells deployed in the development well pattern of Fanzhuang and Zhengzhuang blocks,there are two stages of reservoir pressure drop expansion:cooperative pressure reduction and cooperative desorption.In order to achieve a cooperative production increase,the coal reservoir pressure must be reduced below the desorption pressure within the range of two wells.The maximum well spacing that reduces the reservoir pressure between wells to below desorption pressure is defined as the“critical well spacing for synergistic production increase”.The well distance is controlled by permeability and stress field,and the permeability and stress field are controlled.The higher the permeability,the greater the critical well spacing,and the smaller the angle with the direction of the maximum horizontal principal stress,the greater the critical well spacing.In the process of coalbed methane development,it is necessary to consider the difference of“critical well spacing for synergetic stimulation”in different directions,and carry out differentiated well pattern and well spacing design,so as to maximize the utilization of resources and enhance oil recovery.