Seyyedi Mojtaba, Mahmud Hisham Khaled Ben, Verrall Michael, Giwelli Ausama, Esteban Lionel, Ghasemiziarani Mohsen, Clennell Ben
Australian Resources Research Centre, CSIRO, Kensington, Australia.
Curtin University Malaysia, CDT 250, 98009, Miri Sarawak, Malaysia.
Sci Rep. 2020 Feb 27;10(1):3624. doi: 10.1038/s41598-020-60247-4.
Observations and modeling studies have shown that during CO injection into underground carbonate reservoirs, the dissolution of CO into formation water forms acidic brine, leading to fluid-rock interactions that can significantly impact the hydraulic properties of the host formation. However, the impacts of these interactions on the pore structure and macroscopic flow properties of host rock are poorly characterized both for the near-wellbore region and deeper into the reservoir. Little attention has been given to the influence of pressure drop from the near-wellbore region to reservoir body on disturbing the ionic equilibrium in the CO-saturated brine and consequent mineral precipitation. In this paper, we present the results of a novel experimental procedure designed to address these issues in carbonate reservoirs. We injected CO-saturated brine into a composite core made of two matching grainstone carbonate core plugs with a tight disk placed between them to create a pressure profile of around 250 psi resembling that prevailing in reservoirs during CO injection. We investigated the impacts of fluid-rock interactions at pore and continuum scale using medical X-ray CT, nuclear magnetic resonance, and scanning electron microscopy. We found that strong calcite dissolution occurs near to the injection point, which leads to an increase in primary intergranular porosity and permeability of the near injection region, and ultimately to wormhole formation. The strong heterogeneous dissolution of calcite grains leads to the formation of intra-granular micro-pores. At later stages of the dissolution, the internal regions of ooids become accessible to the carbonated brine, leading to the formation of moldic porosity. At distances far from the injection point, we observed minimal or no change in pore structure, pore roughness, pore populations, and rock hydraulic properties. The pressure drop of 250 psi slightly disturbed the chemical equilibrium of the system, which led to minor precipitation of sub-micron sized calcite crystals but due to the large pore throats of the rock, these deposits had no measurable impact on rock permeability. The trial illustrates that the new procedure is valuable for investigating fluid-rock interactions by reproducing the geochemical consequences of relatively steep pore pressure gradients during CO injection.
观测和模型研究表明，在将二氧化碳注入地下碳酸盐岩储层的过程中，二氧化碳溶解于地层水形成酸性卤水，引发流体-岩石相互作用，这会显著影响储层的水力性质。然而，对于近井区域以及储层更深处，这些相互作用对主岩孔隙结构和宏观流动特性的影响仍缺乏充分的表征。从近井区域到储层主体的压力降对二氧化碳饱和卤水中离子平衡的扰动以及由此导致的矿物沉淀的影响，几乎未受到关注。在本文中，我们展示了一种新颖实验方法的结果，该方法旨在解决碳酸盐岩储层中的这些问题。我们将二氧化碳饱和卤水注入由两个匹配的颗粒灰岩碳酸盐岩岩心塞组成的复合岩心中，在它们之间放置一个致密圆盘以形成约250 psi的压力分布，类似于注入二氧化碳期间储层中的压力分布。我们使用医用X射线计算机断层扫描、核磁共振和扫描电子显微镜研究了孔隙和连续尺度上流体-岩石相互作用的影响。我们发现，在注入点附近发生强烈的方解石溶解，这导致近注入区域的原生粒间孔隙度和渗透率增加，最终形成虫孔。方解石颗粒的强烈非均相溶解导致粒内微孔的形成。在溶解后期，鲕粒的内部区域可被碳酸化卤水侵入，导致铸模孔隙的形成。在远离注入点的位置，我们观察到孔隙结构孔粗糙度、孔隙数量和岩石水力性质几乎没有变化或没有变化。250 psi的压力降轻微扰动了系统的化学平衡，导致亚微米级方解石晶体的少量沉淀，但由于岩石的大孔隙喉道，这些沉淀物对岩石渗透率没有可测量的影响。该试验表明，新方法对于通过再现注入二氧化碳期间相对陡峭的孔隙压力梯度的地球化学后果来研究流体-岩石相互作用具有重要价值。
