特低渗透砂岩油层钻井液侵入程度及影响

根据地质录井、实验分析、压裂试采等资料,基于动态分析、渗流理论及物质平衡原理,动、静态分析相结合,对特低渗透砂岩油层钻井液（泥浆滤液）侵入程度及其影响的评价方法进行了研究。利用压裂试采曲线特征将特低渗透油层的生产动态划分为初产与稳产2个阶段：1初产阶段主要表现为大量外来流体沿人工及天然裂缝系统的快速返排,产水量及含水率变化迅猛;2稳产阶段主要表现为地层原始流体在饱和度控制下的相对渗流,产水量及含水率变化处于低稳状态。正常油层具孔隙型单孔介质,通常无明显的钻井液侵入,其初产阶段的地层吞吐水量大致相当。低阻油层具微裂缝-孔隙型双孔介质,具有显著的钻井液侵入特征,其初产阶段的地层吞吐水量差异大致等于钻井液的侵入体积。利用地层初始吞吐液量差异确定了特低渗透砂岩油层的钻井液侵入程度,并以地层吞吐液量、钻井液侵入深度、结合压裂曲线特征,量化判断油层属性。建立了油层电阻率与钻井液侵入深度关系图版,定量评价了钻井液侵入的影响,分析了低阻油层成因。结果表明,钻井液侵入深度越大,则油层电阻率越小,低阻油层是钻井液超深侵入的结果;由于双孔介质及其微裂缝的发育,低阻油层的钻井液侵入深度通常超过了感应系列的径向探测半径,因此导致了"低阻"的发生。

Drilling fluid invasion degree and its impact evaluation in ultra-low permeability sandstone reservoir

Based on geological logging, experimental analysis, fracturing and test mining, etc., in combination with dynamic analysis, seepage theory and material balance principle, as well as dynamic and static analyses, this study explores the invasion degree of drilling fluid (mud filtrate) in ultra-low permeability sandstone reservoirs and the evaluation method of its impact. The production dynamics of ultra-low permeability reservoirs are divided into two stages, i.e., initial production and stable production, by using the characteristics of fracturing pilot production curves. The initial production stage is mainly characterized with the rapid return flow of massive external fluids along artificial and natural fracture systems, and a rapid change in water production and water content. The stable production stage is mainly characterized by the relative seepage of the original layer fluid under saturation control, and the change of water production and water content in a low-stability state; the normal oil layer has a single-porous medium, there is usually no obvious drilling fluid invasion; the amount of water invaded is roughly equal to the amount produced by the formation in the initial production stage. The low-resistance oil layer has a micro-crack-porosity dual-porous medium, showing significant drilling fluid invasion, and the difference in formation water throughput at the initial production stage is approximately equal to the invasion volume of drilling fluid. The invasion degree of drilling fluid in the ultra-low permeability sandstone reservoir is determined by the difference in the initial formation fluid throughput. Meanwhile, the properties of the reservoir are quantified by formation fluid throughput, the invasion depth of drilling fluid and the characteristics of fracturing curve. A map of oil layer resistivity and drilling fluid invasion depth is established to quantitatively evaluate the influence of drilling fluid invasion and analyze the genesis of low-resistivity oil layer. The results show that the deeper the invasion depth of drilling fluid is, the smaller the resistivity of oil layer will be. The low-resistance oil layer is the result of the ultra-deep invasion of drilling fluid. Due to the development of the dual-porous medium and its micro-cracks, the drilling fluid invasion depth of the low-resistance oil layer often exceeds the radial detection radius of the sensing series, thus leading to the occurrence of low resistance.