Mineral Spatial Distribution and Flow Velocity in Determining Calcite Dissolution Rates in Porous Media

Open Access
Author:
Chao, Tse-hua
Graduate Program:
Petroleum and Natural Gas Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 03, 2013
Committee Members:
  • Li Li, Thesis Advisor
Keywords:
  • calcite dissolution rate
  • spatial heterogeneity
  • mineral spatial distribution
Abstract:
This work investigates the effects of mineral spatial distribution in porous media on dissolution rates. We measured the calcite dissolution rates by using column experiments, which are packed with the same amount of calcite mass but different mineral distributed patterns in quartz-sand column (the mixed, 2-cylinder, 1-cylinmder column). The scale of each column is 2.65 cm in diameter and 10 cm in length. The mixed column has a homogeneously distributed calcite in the quartz column , and the 1-cylinder/2-cylinder column confines all the calcite particles within the middle cylindrical zone(s) that is parallel to the flow injection. We flushed the columns with acidic water at various flow velocities. Experimental data show the dissolution rate of calcite is approximately 1.6 to 2 times slower in the 1-cylinder column than in the mixed column, and the rates are about 1.6-2 orders of magnitude lower under the slowest flow velocity (0.31 m/day) compared to the fast flow velocity (m/day). Ratio (α) demonstrates the mineral spatial distributed effect playing an important role in column-scale rates. This is, the mineral spatial distributed effect is more significant under the conditions with fastest flow velocity and less homogeneously distributed reactive minerals (1-cylinder column). In the 1-cylinder column, the 2-D spatial profile modeling shows that the dissolution rates are larger by orders of magnitude at the calcite-quartz interface than the central region within the middle calcite-packed zone. Also, the range of dissolving calcite area at the calcite-quartz interface becomes wider with the increasing transverse dispersivity and under fast flow velocities. In contrast, transverse dispersivity does not affect local dissolution rates in the mixed column and all the calcite are dissolving. From those observations, we infer that transverse dispersivity caused by different mineral spatial distribution controls the mass transport in transverse direction. Two calculations in effective surface area are introduced to understand the above observations:(1) total surface area (AT), representing the total calcite surface area by assuming that all the calcite particles are dissolving; (2) effective surface area (Ae), which is the surface area of effectively dissolving calcite at the calcite-quartz interface and inlet. Column-scale bulk rates (mol/s) increase with the increasing Ae values while remain irrelevant to AT values. Also, the Ae values increases with the increasing flow velocities. This suggests that the different measures in the surface area and flow velocities can be the possibilities causing the discrepancies between field and laboratorial measure rates. This work provides the way to minimize the discrepancies in order to approach the calcite dissolution rate under natural subsurface conditions. The surface area of dissolving calcite at the interface of calcite and other non-reactive minerals needs be measured first, and then the rate constant (mol/m2/s) obtained in laboratory work can be used to infer the real rates. The results from this work also compare with the literature (L. Li, Salehikhoo et al., 2013), which applied the same experimental sets but used magnesite as their reactive mineral. The both cases have the similar conclusions about the effect of mineral spatial distribution. This work systematically quantifies the impact of mineral spatial heterogeneities on dissolutions using both experimental and modeling approach.