Precipitation of Calcite in a Rock Fracture

Abstract

ABSTRACT A promising technology for permanently storing CO2 in the subsurface is through mineralization in mafic (e.g. basalt) and ultramafic (e.g. peridotite) rock, which are abundant in the earth's upper crust. Indeed, recent field experiments in Iceland and Washington State have demonstrated the ability of trapping carbon in mineral form by injecting CO2-charged water in the subsurface. One key question to be addressed in assessing this technology is whether the mineral replacement resulting from the combined dissolution / precipitation process will either clog the pore space or induce cracking that will expose fresh mineral surfaces to, and provide new pathways for, the reactive fluid. In other words, is the process self-limiting (clogging) or self-sustaining (cracking)? To explore this question, we designed a suite of experiments that involves injecting a solution supersaturated with respect to calcite in a rock fracture. The mixture was injected via a central hole drilled in a granite block that was fractured hydraulically. The paper reports on preliminary results of the precipitation of calcite on fracture surfaces, and resulting changes in fluid flow and fracture aperture. INTRODUCTION One option to reduce the release of anthropogenic carbon dioxide (CO2) to the atmosphere is the capture and storage of CO2 in geologic formations (Orr 2009). Although sequestration of CO2 via structural trapping in fluid-saturated porous sedimentary rock has gained much interest in the recent years, maintaining seal integrity remains a persistent issue (Benson and Cole 2008). To resolve problems associated with conventional methods for CO2 geologic storage (e.g. structural trapping; solubility trapping), permanent storage of CO2 as thermodynamically stable carbonate minerals has been suggested (Seifritz 1990; Kelemen and Matter 2008), where CO2-charged fluid is injected in Mg- and Ca-rich silicate rock. The carbon is fixed as MgCO3 or CaCO3 (Equation) Sustainable and large-scale storage of CO2 in geologic formations by mineralization (Equation 1), however, requires (i) continuous circulation of CO2-charged fluid in the rock mass, and (ii) access to fresh minerals enabling dissolution and precipitation to move forward. Potential clogging of the fracture network through vein formation, therefore, can pose significant limitations to carbon mineralization. Understanding the process of vein formation and whether a sufficiently permeable fracture network can be maintained over time by the pressure that growing crystals exert on the crack walls (Taber 1916; Rothrock 1925; Durney and Ramsay 1973; Wiltschco and Morse 2001) is an important question that should be addressed.