Experimental Limestone Dissolution and Changes in Multiscale Structure Using Small- and Ultrasmall-Angle Neutron Scattering

Small-angle neutron scattering (SANS), ultrasmall-angle neutron scattering (USANS), backscatter electron (BSE) imaging, and neutron computed tomography (NCT) were applied to the study of the pore size, pore distribution, and pore connectivity developed during the experimental dissolution of limestone. Eight cores of Indiana limestone having initial permeabilities of 2–4 and 70 mD were reacted with HCl solutions having a pH of 2 or 4 at flow rates of 0.1 or 10 cm3/min. NCT was used to image the structures developed during dissolution. Nine cross sections of each core from the inlet to the outlet were analyzed with SANS and USANS and with BSE imaging to characterize changes in the pore structure throughout the length of the core after reaction. The scattering curves obtained from SANS and USANS were combined with autocorrelation analysis of the BSE images to characterize porosity over length scales from ∼5 mm to 1 nm. Surface-fractal dimensions were ∼2.3 in the nanopore region, and mass-fractal dimensions were ∼2.75 in the micropore region. The transition from surface- to mass-dominated fractal geometry is at a pore size of ∼100 nm. There was no change in fractal behavior with dissolution, pH, permeability, or flow rate. Porosity was generally greater at the inlet, where most of the dissolution occurred, than at the outlet, where there was little or no reaction. There was also some evidence for porosity reduction near the inlet. The distribution of pore sizes peaked in terms of pore numbers in the nano, micro, and meso range, but there was little change in that distribution with dissolution. There was also little change in porosity in samples that developed preferential flow paths (wormholes), which formed in solutions of low pH and, in particular, at high flow rate. The initial permeability of each sample controlled the penetration and degree of branching of each wormhole into the cores. Samples with wormholes had little additional reaction. The composition of the solutions having a starting pH of 4.0 approached the equilibrium value of 9.5 at the outlet, with little regard to flow rate or permeability. In our experiments, the formation of wormholes and the change in porosity were most strongly influenced by the pH of the infiltrating solution, followed by the flow rate and initial permeability.本研究采用小角中子散射（Small-angle neutron scattering, SANS）、超小角中子散射（ultrasmall-angle neutron scattering, USANS）、背散射电子成像（backscatter electron, BSE）以及中子计算机断层扫描（neutron computed tomography, NCT），对石灰岩实验溶蚀过程中形成的孔隙尺寸、孔隙分布及孔隙连通性展开系统研究。实验选取8块初始渗透率分别为2~4 mD与70 mD的印第安纳石灰岩岩心，以pH为2或4的盐酸溶液为溶蚀介质，以0.1或10 cm³/min的流速开展溶蚀反应。其中，采用NCT对溶蚀过程中形成的岩心内部结构进行成像；对每块岩心沿入口至出口方向截取的9个横截面分别开展SANS、USANS及BSE测试分析，以表征溶蚀后岩心全长范围内的孔隙结构变化。将SANS与USANS获取的散射曲线结合BSE图像的自相关分析结果，可实现~5 mm至1 nm尺度区间内孔隙率的表征。研究发现，纳米孔区间的表面分形维数约为2.3，微孔区间的质量分形维数约为2.75；表面分形主导向质量分形主导的结构转变临界孔隙尺寸约为100 nm。分形特征未随溶蚀程度、溶液pH、初始渗透率及流体流速发生明显改变。岩心入口区域为主要溶蚀发生区，其孔隙率普遍高于几乎未发生反应的出口区域；同时存在入口附近孔隙率降低的实验现象。孔隙尺寸分布在纳米、微米及介孔区间均呈现孔隙数量峰值，但该分布未随溶蚀过程发生显著变化。在低pH溶液（尤其高流速条件下）形成优先流通道（虫孔）的岩心样品，其孔隙率同样未发生明显变化；单块岩心的初始渗透率决定了虫孔在岩心内部的渗透深度与分支发育程度，而发育虫孔的岩心样品几乎未发生额外溶蚀反应。初始pH为4.0的溶蚀溶液在出口处的组分趋近于9.5的平衡值，且该结果几乎不受流体流速与初始渗透率的影响。本实验结果显示，虫孔形成与孔隙率变化受侵入溶液pH的影响最为显著，其次为流体流速与初始渗透率。