固井水泥浆对裂缝割理发育型煤层气储层的伤害机理

我国滇东黔西地区煤层气储层割理、裂缝发育,破裂压力低,固井水泥浆(以下简称水泥浆)容易侵入煤层气储层,造成储层伤害以及储层改造破裂压力异常升高等现象。为了揭示水泥浆伤害该类煤储层的机理,在分析煤岩的理化性能和潜在伤害方式的基础上,通过CT扫描、电镜扫描等技术手段,直观分析煤心内部污染前后裂缝、孔隙结构发育情况与水泥浆在裂缝、孔隙中的侵入、堵塞情况,进而计算得到水泥浆和裂缝在煤心中的体积占比关系,建立了水泥浆伤害煤储层的定量评价方法。研究结果表明:(1)水泥浆及其滤液在压差作用下沿煤储层的裂缝侵入储层内部,其侵入程度随裂缝、孔隙发育程度变化,裂缝、孔隙越发育,侵入程度越高;(2)侵入储层内部的水泥浆经胶结固化后,形成的水泥产物在裂缝、孔隙中致密填充,并致密覆盖煤心表面,严重堵塞煤层气流通通道,表现出固化后的水泥产物使煤心渗透率降低、煤岩抗压强度升高,使后续压裂破裂压力异常升高,影响压裂改造效果;(3)水泥浆滤液对煤岩的碱敏、速敏影响程度远小于水泥浆侵入对煤岩的伤害程度。结论认为,所建立的水泥浆污染煤储层的定量评价方法对提高煤储层固井质量、保证煤层气高效开发等都具有指导作用。

Damage mechanism of cement slurry to CBM reservoirs with developed fractures and cleats: A case study from eastern Yunnan and western Guizhou in China

The coalbed methane (CBM) reservoirs in the areas of eastern Yunnan and western Guizhou are characterized by developed cleats and fractures and low fracturing pressures, so cementing slurry ("slurry" for short) can invade into CBM reservoir easily, resulting in reservoir damage and abnormal increase of reservoir transformation fracturing pressure. In order to reveal the damage mechanisms of slurry to this type of coal reservoirs, we analyzed the physical and chemical properties and potential damage modes of coal rocks. Then, the development situations of fractures and pores before and after the coal core samples were internally contaminated and the invasion and plugging situations of slurry in fractures and pores were analyzed intuitively by means of CT scanning and scanning electron microscope (SEM), and the percentage of slurry and fractures in coal core volume was calculated. In this way, a method to quantitatively evaluate the damage of slurry to coal reservoirs was established. And the following research results were obtained. First, under the effect of differential pressure, slurry and its filtrate invade into coal reservoirs along the fractures. The invasion degree varies with the development degree of fractures and pores. The more developed the fractures and pores, the higher the invasion degree. Second, the cement products formed after the slurry in the reservoirs gets cemented and solidified fill the fractures and pores tightly and cover the surface of coal core samples densely, so CBM flowing channels are blocked severely. Consequently, the permeability of coal core samples decreases and the compressive strength of coal rocks increase, leading to the abnormal increase of subsequent fracturing pressure and impacting the fracturing stimulation effects. Third, the effect of slurry filtrate on the alkali sensitivity and speed sensitivity of coal rocks is much less than the damage degree of slurry invasion to coal rocks. In conclusion, this newly developed quantitative evaluation method for the damage of slurry to coal reservoirs is of guiding significance to improving the cement job quality of coal reservoirs and ensuring the efficient CBM development.