Numerical simulation of fault zone permeability enhancement by chemical stimulation operation at the Eden geothermal projects, UK

To increase fluid flow and thus obtain higher heat production from a geothermal well, chemical stimulation can be performed in their open-hole sections. Through chemical stimulation, fracture zone permeability in the vicinity of the borehole can be enhanced which results in higher fluid connectivity from the well to the reservoir fracture network. To understand the chemical stimulation effect on fault zone permeability enhancement and pressure development, the Eden geothermal project acted as a demonstration site in the framework of the MEET project. The open-hole section of the Eden geothermal project was characterized based on drill cutting analysis, several borehole geophysical logs including (Spectral Gamma Ray, Caliper, Borehole Image, etc.) as well as hydraulic testing. These hydraulic injection tests were carried out from January to March 2022 in the form of several low injection tests, where fluid was injected in a series of flow rates ranging from 5 – 15 l/s. Furthermore, a chemical stimulation test was performed in August-September 2022 followed by another series of hydraulic injection and production tests until late November 2022 at flow rates of 5 – 12.5 l/s. Rock cuttings from the target zone (3940 m TVD) indicated up to 3% of calcite which is easily dissolvable by acids. Therefore, the chemical stimulation was primarily aimed to treat possible near-wellbore damage resulting from an LCM cementation carried out during the drilling of the well in 2021, prior to installing the production casing.

Especially at high temperatures, commonly employed HCl tends to treat only near wellbore regions and show high corrosivity against metal tubular. To counteract such drawbacks, the strong organic acid system SSB-007 was used for chemical stimulation [1]. The two hydraulic injection test series of spring and autumn 2022 are compared against a coupled hydro-thermal numerical simulation solved using a finite element discretization approach in COMSOL Multiphysics. To numerically model the Eden geothermal system, a cubical reservoir including the well trajectory and casing design, and four individual fault zones are considered at a depth between 3830 m and 4280 m TVD. Due to similarity in the geological context on the regional level, permeability, porosity, and other petrophysical parameters of the faulted and matrix properties of the Cornubian granites are considered based on the United Downs geothermal reservoir [2,3]. Initially, the permeability and porosity of the fault zone and matrix are calibrated using the spring hydraulic testing data. Later, the downhole pressure evolution of the spring and autumn injection tests is compared to qualitatively understand any permeability enhancement. Initial results indicate that up to 20% of fault zone permeability enhancement was achieved by the chemical stimulation operation. Future reservoir characterization would be helpful for developing more accurate geochemical models predicting permeability enhancement.

1. Lummer, N.R., Rauf, O. and Gerdes, S., 2015, June. https://doi.org/10.2118/174242-MS.

2. Mahmoodpour, S., Singh, M., Obaje, C., Tangirala, S.K., Reinecker, J., Bär, K. and Sass, I., 2022. https://doi.org/10.3390/geosciences12080296.

3. Reinecker, J., Gutmanis, J., Foxford, A., Cotton, L., Dalby, C. and Law, R., 2021. https://doi.org/10.1016/j.geothermics.2021.102226.