Hydro-mechanical Simulation of CO2 Storage in a Depleted Chalk Reservoir: Impacts of Hydrocarbon Production, Injection Rate, and Rock-fluid Interactions on Fault Reactivation and Field Deformation

Abstract

Re-utilizing depleted hydrocarbon fields in the North Sea represents an attractive economic opportunity for the nascent CCS (Carbon Capture and Storage) industry, as the existing infrastructure and subsurface knowledge acquired over time drastically reduce costs and time. Additionally, the proof of containment of hydrocarbon fields at least at pre-production time limits the risk of leakage. In Denmark, a significant proportion of these fields consists of chalk, a highly deformable and reactive rock dominated by calcium carbonate. The present study aims at assessing fault stability and the elasto-viscoplastic response of a storage site in chalk during CO2 injection by performing field-scale coupled HM simulations. To this purpose, a sensitivity analysis is carried out by assuming different degrees of CO2-induced alteration of chalk’s mechanical properties, although experimental studies indicate an overall lack of mechanical weakening of chalk when flooded with supercritical CO2. The rate of injection also varies between scenarios and the hydrocarbon production phase is considered during simulations. This approach allows to analyse a broad range of potential deformation responses of depleted chalk fields being repurposed for CCS projects, a prerequisite for a comprehensive risk assessment, especially with regards to the technical challenges that the oil and gas industry have faced while exploiting chalk fields. It is important to note that the temperature of the injected CO2 is assumed to match that of the reservoir, allowing the study to focus on the effects of chalk-CO2 interactions. This study uses the Harald East gas field as a case study. Given the mechanical response of rocks and faults in the scenarios considered, the integrity of a storage site in chalk does not appear to be jeopardized by a potential rock strength reduction by supercritical CO2. Accounting for stress redistribution after decades of hydrocarbon production is, on the other hand, a prerequisite to design safe injection strategies using multiphysics simulation tools.

Keywords: CCS, Chalk, Fault reactivation, Deformation