Geological Model of a Storage Complex for a CO2 Storage Operation in a Naturally-Fractured Carbonate Formation

Abstract

Investigation into geological storage of CO2 is underway at Hontomín (Spain). The storage reservoir is a deep saline aquifer formed by naturally fractured carbonates with low matrix permeability. Understanding the processes that are involved in CO2 migration within these formations is key to ensure safe operation and reliable plume prediction. A geological model encompassing the whole storage complex was established based upon newly-drilled and legacy wells. The matrix characteristics were mainly obtained from the newly drilled wells with a complete suite of log acquisitions, laboratory works and hydraulic tests. The model major improvement is the integration of the natural fractures. Following a methodology that was developed for naturally fractured hydrocarbon reservoirs, the advanced characterization workflow identified the main sets of fractures and their main characteristics, such as apertures, orientations, and dips. Two main sets of fracture are identified based upon their mean orientation: North-South and East-West with different fracture density for each the facies. The flow capacity of the fracture sets are calibrated on interpreted injection tests by matching their permeability and aperture at the Discrete Fracture Network scale and are subsequently upscaled to the geological model scale. A key new feature of the model is estimated permeability anisotropy induced by the fracture sets.

An active tectonic field for CO<sub>2</sub> storage management: the Hontomín onshore case study (Spain)

“…The target CO 2 injection point is a saline aquifer formed by a dolostone unit, known as "Carniolas", and an oolitic limestone of the Sopeña Formation, both corresponding to Lias in time (Early Jurassic). The estimated porosity of the Carniolas reaches over 12 % (Ogaya et al, 2013;Le Gallo and de Dios, 2018), and it is slightly lower at the Carbonate Lias level (8.5 % in average). The reservoir levels contain saline water with more than 20 g L −1 of NaCl and very low oil content.…”

“…Furthermore, the presence of master faults could increase the occurrence of micro-earthquakes, due to the presence of secondary faults prone to triggering earthquakes by their normal seismic cycle (Scholz, 2018). It must be borne in mind that GSC onshore reservoirs used to be deep saline aquifers (e.g., Bentham and Kirby, 2005) as in the case of Hontomín (Gastine et al, 2017;Le Gallo and de Dios, 2018), which is confined in folded and fractured deep geological structures, in which local tectonics plays a key role in micro-seismicity and the possibility of CO 2 leakage.…”

“…Le Gallo and de Dios (2018) described two main fault sets affecting the reservoir with N-S and E-W trends, respectively. According to the present-day stress tensor described by Herraiz et al (2000) and Tavani et al (2011), E-W fault sets accommodate horizontal shortening, which means that the permeability could be low.…”

“…The tectonic characterization of the GSC of Hontomín was implemented in the geological model described by Le Gallo and de Dios (2018). Beyond the use of induced seismicity and potentially active faults, the scope of this method is to propose an initial analysis to manage underground storage operations.…”

“…). The main rifting phase took place during Late Jurassic and Early Cretaceous times, due to the opening of the North Atlantic and the Bay of Biscay-Pyrenean rift system (García-Mondéjar et al, 1986;Le Pichon and Sibuet, 1971;Lepvrier and Martínez-García, 1990;García-Mondéjar et al, 1996;Roca et al, 2011;Tugend et al, 2014). The convergence between Iberia and Eurasia from Late Cretaceous to Miocene times triggered the inversion of previous Mesozoic extensional faults and the development of an E-W orogenic belt (Cantabrian domain to the west and Pyrenean domain to the east) formed along the northern Iberian plate margin (Muñoz, 1992;Gómez et al, 2002;Vergés et al, 2002).…”