Improving the Physical and Economic Performance of CO2 Capture, Utilization, and Storage in Saline Reservoirs by Producing Brine

Technical deployment barriers can be grouped in three categories for CO₂ capture, utilization, and storage (CCUS) in saline reservoirs: 1) net cost (after utilization benefits are considered); 2) water intensity of the CO₂ capture process, and 3) uncertainty about well injectivity and reservoir storage capacity. The third category is often considered to be the most challenging. Overpressure, which is fluid pressure that exceeds the original reservoir pressure due to CO₂ injection, is a limiting metric for injectivity and capacity because it drives key physical risks: caprock fracturing, fault reactivation, induced seismicity, and CO₂ leakage. Variables that control overpressure include: 1) the quantity of CO₂ and the rate at which it is injected, 2) the size of the reservoir storage compartment, and 3) reservoir permeability. Geologic surveys, geologic logs, and core data from exploration wells provide information that can be used to estimate the size and permeability of the reservoir compartment, but large uncertainties will be narrowed only when there is operational experience with moving large quantities of fluid into and/or out of the reservoir. Unlike CCUS applied to CO₂ Enhanced Oil Recovery (CO₂-EOR) in mature oil fields, CCUS in a saline reservoir will typically have less geologic information and little or no production and injection history with which to estimate how much CO₂ can be safely stored. Moreover, saline-reservoir CCUS does not typically share the advantage of depleted reservoir pressure prior to CO₂ injection with CO₂-EOR.

Reservoir pressure management by producing brine to minimize pressure buildup due to CO₂ injection has been evaluated in many studies. Most of these studies assume that separate injection and production wells will be used and that brine production will begin during the CO₂ injection phase. We present an approach where brine is produced prior to the CO₂ injection phase, using the wells that will be subsequently used for CO₂ injection. In this approach, all wells are initially used for exploration and monitoring and then to produce brine prior to injecting CO₂. This pre-injection, brine production approach has several advantages. First, pressure drawdown observed during brine production mirrors pressure buildup during CO₂ injection, providing necessary data to directly estimate reservoir storage capacity before CO₂ is injected. Second, pressure drawdown is greatest where CO₂ is about to be injected, which is more efficient on a per well basis and per mass of removed brine basis. Thus, pre-injection brine production in a saline reservoir shares two key advantages of CO₂-EOR: a) greater knowledge about reservoir properties and storage capacity prior to CO₂ injection and b) depleted reservoir pressure, which increases storage capacity. A third advantage of this approach may be applicable where the composition of the produced brine is amenable to treatment for beneficial uses, such as saline cooling water or for water generated through desalination, to reduce water intensity of CCUS. If net fluid injection is zero (net brine removal is volumetrically equal to CO₂ injection), there is the added benefit of minimizing interference with neighboring owners and users of subsurface pore space, and the other advantages will be maximized.

We investigated well strategies that involve pre-injection and co-injection brine production, as well as reinjecting residual brine in an overlying permeable formation. Achieving zero net injection may necessitate extracting a volume of brine that is greater than the injected CO₂ volume. We examine how this can be practically achieved and illustrate how pre-injection brine production can be used as a tool for site selection and characterization, including assessments of CO₂ storage capacity.

This work was sponsored by the USDOE Fossil Energy, National Energy Technology Laboratory, managed by Traci Rodosta and Andrea McNemar. This work was performed under the auspices of the USDOE by LLNL under contract DE-AC52-07NA27344.