Abstract
The dissolution of CO2 into formation brines and the subsequent reactions of the CO2-charged brines with reservoir minerals are two key processes likely to increase the security of geological carbon-dioxide storage. These processes will be dependent on the permeability structure and mineral compositions of the reservoirs, but there is limited observational data on their rates. In this paper we report the cation and anion concentrations and Sr, oxygen and carbon isotopic compositions of formation waters from four extraction wells sampled at surface, over ~6months after commencement of CO2 injection in a five spot pattern for enhanced oil recovery at the Salt Creek field, Wyoming. Sampled fluids, separated from the minor oil component, exhibit near-monotonic increases in alkalinity and concentrations of cations but little change in Cl and Br concentrations and oxygen and deuterium isotope ratios. The increases in alkalinity are modelled in terms of reaction with reservoir calcite and silicate minerals as the changes in fluid chemistry and Sr-isotopic compositions are inconsistent with simple addition of injected fluids sampled over the course of the experiment. The reservoir mineral chemical and isotopic compositions are characterised by sampling core as well as surface exposures of the Frontier Formation elsewhere in Wyoming. The evolution of the fluid chemistries reflects extensive dissolution of both carbonate and silicate minerals over the course of the six months sampling implying rapid dissolution of CO2 in the formation waters and reaction of CO2-bearing brines with formation minerals. Rates of CO2 diffusion into the brines and advection of CO2 charged brines in the reservoir are sufficiently slow that, if present, calcite should react to be close to equilibrium with the fluids. This allows estimation of the CO2 partial pressures in the sampled fluids and comparison with the thermodynamic driving force for the relatively rapid average plagioclase dissolution rates of ~10−12mol·m−2·s−1.
Highlights
Dissolution of CO2 in formation brines and the consequent fluid-mineral reactions in reservoirs are potentially important processes which lead to the long-term security of geological carbon storage (IPCC 2005; Benson and Cole, 2008; Bickle, 2009)
The 2nd Wall Creek Sandstone at Salt Creek correlates with sandstone unit III of the upper part of the Belle Fourche Member of the Frontier Formation (Merewether et al, 1979) in eastern Wyoming
Sampling of fluids from sites where CO2 is injected for enhanced oil recovery enables evaluation of fluid-rock interactions, rates of CO2 dissolution in the sampled brines and gives a measurement of the heterogeneity of the formations sampled
Summary
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Rapid reactions between CO2, brine and silicate minerals during Geological Carbon Storage: Modelling based on a Field CO2 Injection Experiment. Mike Bickle1*, Niko Kampman, Hazel Chapman, Chris Ballentine, Benoit Dubacq, Albert Galy, Tule Sirikitputtisak, Oliver Warr, Max Wigley, Zheng Zhou
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