致密砂岩储层成岩过程中水岩作用对岩石孔隙度影响的定量研究

Tight sandstone gas, one kind of important unconventional resources, is hosted in tight sandstone reservoir with extremely low porosity and low permeability. However, the formation mechanisms of these tight sandstone reservoirs remain unclear and the conditions leading to them are not well-understood. Focusing on the challenging problems of tight sandstone reservoir, this study aims to investigate water-rock interaction during continuous diagenesis and predict porosity evolution quantitatively by combining petrographic observations, laboratory experiments and numerical simulation methods. The Shihezi 8th formation, one of the Upper Palaeozoic reservoirs in the northern Ordos Basin, is investigated with reference to its reservoir property evolution during diagenesis. First, a total of eight 2-D models with various diagenetic conditions are constructed to find out main controlling factors of porosity and ascertain formation conditions of tight reservoir. Then, water-rock reaction experiments are conducted under six successive diagenetic stages. A total of six models corresponding to the experiment are simulated. An extended model at geological time scale is constructed after the parameters being validated. The combination of simulation and experiment could re-appear diagenetic evolution process quantificationally and ascertain the formation time of tight reservoir. At last, a 3-D model with porosity, permeability and mineral heterogenity is simulated to investigate the heterogeneous distribution of reservoir porosity. Overall, this study could not only solve theoretical problems such as diagenetic process of tight reservoir, but could also provide a quantitative evaluation and prediction of reservoir petrophysical properties for tight sandstone gas exploration.

致密砂岩气是一种重要的非常规资源，赋存于低孔低渗的致密砂岩储层孔隙中，储量巨大，但因储层的致密化机理尚未明确而开采受阻。在储层演化过程中，以水岩作用为主的成岩过程是影响储层的最直接因素，决定着孔隙度的分布特征和发展趋势。本项目拟选取鄂尔多斯北部盒8段致密砂岩储层，运用岩石学测试、室内实验、数值模拟和天然类比相结合的方法，定量评价成岩过程中水岩作用对孔隙度的影响。首先进行多因素的敏感性分析模拟，查明孔隙度的主控因素，确定储层致密化的条件；然后将成岩作用概化为6个连续的成岩阶段，分别进行不同阶段水岩作用的室内实验和数值模拟，再现整个成岩演化过程，定量恢复孔隙度随时间的演变，确定储层致密化的时间；最后在区域范围非均质条件下，评价并预测孔隙度的空间分布。本项目的研究不仅能实现孔隙度在时间和空间上的定量评价和预测，而且能明确致密砂岩储层的形成机制和空间展布，为致密砂岩气的勘探指明方向，提高开采效率。

致密砂岩气作为一种重要的非常规资源，赋存于低孔低渗的致密砂岩储层孔隙中，因储层的致密化机理尚未明确而开采受阻。在储层演化过程中，以水岩作用为主的成岩过程是影响储层的最直接因素，决定着孔隙度的分布特征和发展趋势。本项目选取鄂尔多斯北部盒8段致密砂岩储层，运用岩石学测试、室内实验、数值模拟和天然类比相结合的方法，定量评价了成岩过程中水岩作用对孔隙度的影响。首先进行了多因素的敏感性分析模拟，查明了孔隙度的主控因素为温度、压力、矿物和流体，其中矿物成分起着先天性作用，影响成岩作用的类型和储层演化方向，而成岩过程中的温度、压力、外界流体对孔隙度的演化起决定性作用，确定了碳酸盐岩胶结物为储层致密的主要因素；然后将成岩作用概化为成岩早期到后期共6个连续的成岩阶段，分别进行不同阶段水岩作用的室内实验和数值模拟，室内实验能够恢复成岩过程，而数值模拟能够通过反复正演和反演的方法，确定成岩条件，两者相结合，再现了储层整个成岩演化过程，定量恢复了储层孔隙度随时间的演变；最后在区域范围矿物非均质条件下，评价并预测孔隙度的空间分布，揭示了矿物相互转化的机理及其对孔隙度的控制作用。本项目的研究不仅能实现孔隙度在时间和空间上的定量评价和预测，而且能明确致密砂岩储层的形成机制和空间展布，建立的方法和模拟系统同样能解决碳酸盐、页岩等储层中的孔隙演化问题，从而为油气勘探指明方向，提高开采效率。