Physical property evolution of granite during experimental chemical stimulation

Abstract

The Soultz-sous-Forêts geothermal reservoir (France)—in particular, the GPK-4 well—has been proposed as a target for chemical stimulation under the DESTRESS Horizon-2020 framework. With a combination of batch reaction tests and acid flow-through experiments using hydrochloric acid (HCl) at different molarities and temperatures, we investigate the potential for acid-induced permeability enhancement of a granite analogous to the Soultz-sous-Forêts reservoir rock, by means of acid stimulation. In the batch reaction experiments, we find that the propensity for increase or decrease in porosity and permeability depends on the physico-chemical properties of the starting material: unaltered granite underwent a significant increase in both porosity and permeability relative to its initial state, altered granite exhibited a moderate increase in both porosity and permeability (modulated slightly by HCl molarity), whereas initially more porous and permeable thermally and naturally fractured granite exhibited an increase in porosity accompanied by a relative decrease in permeability. The extent to which permeability increased or decreased appears to be tied to the initial fluid-flow characteristics of the material. Using a new, custom-built acid permeameter, flow-through tests were performed on unaltered granite, while the acid was sampled at regular time intervals. Element release into solution recorded throughout the experiments, indicated dissolution of granite minerals. Despite this operative micromechanism, however, the absolute change in sample permeability is limited, both at room temperature and at 100∘C. Ultimately, these data suggest that the potential for geothermal reservoir enhancement using HCl is low at Soultz-sous-Forêts. Nevertheless, the possibility remains that a more targeted thermal or chemical stimulation approach—or hybrid thereof—could prove effective in the future.