Abstract The current study aimed to investigate the degree of pollution from drilling mud on different scales of fractures in coal reservoirs. Drilling mud for field utilization was prepared in the laboratory and different scales of fractures were generated in coal and rock samples before and after being polluted by the drilling fluid and observed using scanning electron (SEM) and an optical microscope. Using Monte Carlo modelling, the networks of the millimeter (mm), micron- and nano-meter sized fractures, before and after pollution were separately determined. After programming, seepage models of mm, micron- and nano-meter sized fractures were separately constructed using Matlab software. The degrees of pollution of the drilling fluid on the different scales of fractures were calculated according to the seepage principle and compared with the measured permeability of the polluted boreholes. The micron- and nano-sized fractures were more sensitive to drilling fluids compared to the mm-sized and larger fractures. When the original permeability of the coal reservoir was low, different scales of fractures were all greatly polluted by drilling fluids, macroscopically showing high damage to permeability of the coal reservoir. However, when the original permeability of coal reservoir was large, the pollution degrees of micron, and nano-sized fractures failed to be macroscopically observed, having insignificant influences on the permeability of the coal reservoir. The results showed that the combination of the Monte Carlo method and the Matlab numerical simulations can more distinctly reflect the pollution degrees of the drilling fluids on different scales of fracture compared to traditional methods that have been used to test the permeability of coal reservoirs before and after pollution.

中文翻译：

---

钻井泥浆对煤储层不同尺度裂缝渗透率破坏程度的定量评价

摘要 本研究旨在调查煤储层不同尺度裂缝上钻井泥浆​​的污染程度。在实验室制备了现场使用的钻井泥浆，在被钻井液污染前后的煤和岩石样品中产生了不同尺度的裂缝，并用扫描电子（SEM）和光学显微镜进行了观察。使用蒙特卡罗建模，分别确定了污染前后的毫米 (mm)、微米和纳米级裂缝的网络。编程后，利用Matlab软件分别构建了毫米、微米和纳米级裂缝的渗流模型。根据渗流原理计算出钻井液在不同尺度裂缝上的污染程度，并与测得的污染钻孔渗透率进行比较。与毫米级和更大的裂缝相比，微米级和纳米级的裂缝对钻井液更敏感。当煤储层原始渗透率较低时，不同尺度的裂缝均受到钻井液的严重污染，宏观上对煤储层渗透率损害较大。但当煤储层原始渗透率较大时，宏观上未能观察到微米、纳米级裂缝的污染程度，对煤储层渗透率影响不显着。