Carbon Capture & Storage: Rock Experimental Characterization & Modelling Prospective

Abstract

Geological storage of carbon dioxide is one of the options of greenhouse gas mitigation as it reduces the release of CO2 to the atmosphere. Eni is a major energy company in which decarbonization plays an essential role in its vision for a future with zero net emissions. H field carbon storage project comes in line with Eni strategy of carbon neutrality by 2050. Depleted gas fields are among the most suitable candidates for CO2 sequestration, with well-known high quality reservoir characteristics and good sealing. H is a depleted offshore gas field located in the North Sea; it has two main reservoirs: H1 and H2. Due to the massive size of both reservoirs, well-known reservoirs’ characterization, good reservoirs’ petrophysics, and the existence of thick and continuous cap layers (C1 & C2), H1 and H2 reservoirs have been nominated to be good candidates for a carbon sequestration project. The study objective is to build comprehensive mineralogical and geochemical models able to capture the physical and the chemical phenomena occurring in the short – medium – long terms after the CO2 injection in H1 and H2 formations. The proposed workflow relies on the following procedure: 1. laboratory experimental analyses (SEM-EDX, XRD, XRF) on core samples 2. data elaboration and integration 3. conceptual geological-geochemical models’ definition 4. 0D static batch models a. equilibrium approach b. kinetic approach Step 1, 2 and 3 are aimed at obtaining the mineralogical model, with attention devoted to the presence of the Fe-bearing minerals. In step 4, the rock-formation water geochemical system consistence is preliminary checked by performing a sequence of 0D equilibrium models with PHREEQC-V3 code, (USGS, 1999), allowing primary evaluations on mineral phases chemical stability (dissolution/precipitation). This step is also needed for obtaining a synthetic formation water composition at the equilibrium with the mineralogical model used in the subsequent reactive transport models (RTM), and to validate new-estimated site-specific minerals thermochemical parameters. Then, the CO2 injection is preliminary simulated by means of 0D kinetic models to assess the main acting geochemical processes triggered by rock-brine-CO2 interaction. The results show that, at the conditions considered in the project, the rock petrophysical properties (e.g. porosity) alteration does not seem to be an issue. However, the main criticalities/uncertainties might be related to the Fe-bearing minerals (carbonates, micas and clays) chemical behavior in presence of CO2. As a final remark, the simulation results obtained in this work cannot be regarded as values of general validity because they depend on the characteristics assigned to each reservoir model. Nevertheless, the study provides a novel geochemical modelling procedure, which implies building a detailed geochemical conceptual model modelled by means of a complex kinetic reaction network to simulate the mineralogical alterations. The presented procedure can be followed to preliminary assess the geochemical short- and long-term risks for a given carbon storage project.