基于煤层气高效排采的煤层水响应机理及产出水动力模型研究

Coal bed is an important aquifer within the coal-bearing strata. The controlling effect of coal seam water involves in the whole processes of coalbed methane (CBM) generation, accumulation, and production. As the medium for CBM migration, the flow of coal seam water from the higher potential areas to the lower potential areas facilitates the accumulation of CBM. Moreover, the production of CBM from wells requires the dewatering of coal seam to reduce hydrostatic pressure which causes the desorption of gas from coal matrix. The study about the quantitative characterization of the heterogeneous distributed coal seam water and the controlling effects of coal seam water on CBM enrichment and production in Fanzhuang Block, which are closely related to the effective drainage of CBM wells, are still weak. Therefore, we will characterize the vertical and lateral heterogeneities of coal seam water distribution via geophysical logs inversion methods firstly. Then we will divide hydrogeological units based on the variations of chemical properties, and Sr, O, 18D isotopes for coalbed methane production water along the flow path of coal seam water. Associated with the dynamic changes of chemical properties, and Sr, O, 18D isotopes of produced water from wells with differentiated water production and gas production, we establish the preferential hydrological factors for CBM enrichment and high production. At last we will carry out hydraulic flushing simulation experiments under conditions with different hydrodynamics and water chemical properties. We will also conduct numerical simulations for coal seam water flow and construct the 3D geological model of water flow. Based on the dynamic changes of coal seam water during production, we will acquire the coal seam water dynamic models matching for the effective production, which will provide theoretical basis and technology support for the construction of effective production system.

煤层是重要的含水层，煤层水是煤层气运移的载体，伴随煤层水的运移路径由高势区向低势区富集。煤层气开采需要对煤层排水，以减少静水压力使得甲烷从煤中解吸。本申请项目针对樊庄区块煤层气高效排采的煤层水非均质性定量表征、控气效应机理研究薄弱等问题，以测井反演方法对煤层水在纵、横向上的分布规律进行定量表征；揭示煤层气井产出水的地球化学特征和锶、氢、氧同位素等沿煤层水运移路径的变化规律，精细划分水文地质单元。结合不同产水量、产气量区煤层水化学性质和同位素特征的动态变化，构建煤层气富集高产优选的水文地质指标体系。深入开展水型与水动力强度配置条件下煤层水冲洗煤层气物理模拟实验，并对煤层水的流动进行数值模拟，构建地下水流动的三维地质模型，刻画排采过程中地下水的动态变化，建立与煤层气高效排采相匹配的煤层产出水动力模型，为研究区煤层气高效排采制度的建立提供理论依据和技术支撑。

在定性评价煤层水控气效应的基础上，充分结合现场生产资料，采用测井解释、统计分析、岩（煤）心描述、测试化验、物理模拟及数值模拟等多种方法，探讨了煤层水控气效应的定量表征方法，并模拟煤层气实际生产过程中不同类型产出水的水文地质条件，开展煤层水产出模式，并构建了与煤层气合理排采制度相匹配的产出水动力模型。研究结果表明：樊庄区块内矿化度介于1385.75~2296.52 mg/L，PH值介于7.27~9.1，阳离子以Na+为主、Ca2+次之，阴离子以HCO3-为主、Cl-次之、SO42-含量最低，表明煤层产出水以Na-HCO3水型在区内占绝对优势，主体位于水动力滞流区，区块产气效果好；随着煤层气的不断产出，矿化度高值中心由中部向西部转移，Na++K+离子含量变化不大，Ca2+离子、Cl-离子及SO42-离子含量整体上升，HCO3-离子含量主要呈下降变化，说明中部优先形成面积降压区；排采过程中，Na++K+离子占比26.25~37.84%，随排采先升高后下降，变化幅度在5%以下；HCO3-离子占比55.03~71.68%，随排采整体上先下降后升高，变化幅度不大；Cl-离子占比1.37~13.66%，一般在4%上下，随排采整体为逐渐下降的变化趋势；研究区钠氯系数[rNa+/r Cl-]以上升趋势为主，部分井位为明显的递增变化；脱硫系数[(rSO42-×100）/r Cl-]普遍较低，整体变化较平稳；碳酸盐平衡系数[（rHCO3-＋rCO32-）/rCl-]随排采总体呈下降趋势；氢、氧同位素逐渐变轻。脱硫系数稳定或小幅度升高、碳酸盐平衡系数大幅度降低的叠加区域，煤层气井更易高产；构建了向斜翼部低产水高产气、岩溶水高产水中/低产气及构造导水高产水低/不产气等三种产出水模型，在低产水高产气高效排采模式下，煤层水中的氢、氧同位素表现为明显的沉积成因特点，钠氯系数小于6、脱硫系数小于1、碳酸盐平衡系数值高、87Sr/86Sr值偏向或高于壳源锶。