Abstract
Carbon sequestration in deep saline aquifers was recently developed at the industrial scale. CO2 injection experiences in carbonates are quite limited, most of them coming from projects carried out in porous mediums in the USA and Canada. Hontomín (Spain) is the actual on-shore injection pilot in Europe, being a naturally fractured carbonate reservoir where innovative CO2 injection strategies are being performed within the ENOS Project. CO2 migration through the fracture network existing on site produces hydrodynamic, mechanical and geochemical effectsdifferent from those caused by the injection in mediums with a high matrix permeability. The interpretation of these effects is required to design safe and efficient injection strategies in these formations. For this, it is necessary to determine the evolution of pressure, temperature and flow rate during the injection, as well as the period of pressure recovery during the fall-off phase. The first results from the not-continuous injections (8–24 h) conducted at Hontomín reveal the injection of liquid CO2 (density value of 0.828 t/m3) and the fluid transmissivity through the fractures. Taking into account the evolution of the pressure and flow rate showed variations of up to 23% and 30% respectively, which means that the relevant changes of injectivity took place. The results were modeled with a compositional dual media model which accounts for both temperature effects and multiphase flow hysteresis because alternative brine and CO2 injections were conducted. Advanced modeling shows the lateral extension of CO2 and the temperature disturbance away from the well.
Highlights
Most of the experiences on CO2 geological storage worldwide were conducted in rock formations with high permeability in the pore matrix, mainly in sandstones [1,2,3], and in some cases in carbonates such as the AEP Mountaineer Project [4,5], Michigan and Williston Basin CO2 -Enhanced Oil Recovery (EOR) projects [6,7] in USA, IEAGHG Weyburn-Midale CO2 project [8] in Canada and the UthmaniyahCO2 -EOR demonstration project [9] in Saudi Arabia
The results from the first not-continuous injection tests conducted in the ENOS project confirm that the fluid migration is through the carbonate fractures
When the pressure remains constant at the well-head with a value equal or higher than 75 bar for assuring the liquid CO2 injection, the flow rate decreases considerably as brine and CO2 expand within the fracture network, producing multiphase flow hysteresis and reaching values up to 23% lower than the initial
Summary
Most of the experiences on CO2 geological storage worldwide were conducted in rock formations with high permeability in the pore matrix, mainly in sandstones [1,2,3], and in some cases in carbonates such as the AEP Mountaineer Project [4,5], Michigan and Williston Basin CO2 -Enhanced Oil Recovery (EOR) projects [6,7] in USA, IEAGHG Weyburn-Midale CO2 project [8] in Canada and the UthmaniyahCO2 -EOR demonstration project [9] in Saudi Arabia. The pilot is located close to Burgos in Northern Spain, consisting of a naturally fractured carbonate reservoir with a poor matrix porosity [14]. It is at the early injection phase, and all the long-term effects that condition the geological storage of CO2 must be identified and analyzed. Hontomín is the operational storage field site in the ENOS project, where planned research activities are focused to demonstrate innovative injection strategies to increase the confidence of operators in the safe management of these sites [15]. The expected outcomes from the project are the increase in knowledge about the behavior of the injected CO2 in fractured carbonates and the development of safe and efficient operational procedures [16]
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