Pore Structure Changes Occur During CO2 Injection into Carbonate Reservoirs

Mojtaba Seyyedi,Hisham Khaled Ben Mahmud,Michael Verrall,Lionel Esteban,Ausama Giwelli,Mohsen Ghasemiziarani,Ben Clennell

doi:10.1038/s41598-020-60247-4

Abstract

Observations and modeling studies have shown that during CO2 injection into underground carbonate reservoirs, the dissolution of CO2 into formation water forms acidic brine, leading to fluid-rock interactions that can significantly impact the hydraulic properties of the host formation. However, the impacts of these interactions on the pore structure and macroscopic flow properties of host rock are poorly characterized both for the near-wellbore region and deeper into the reservoir. Little attention has been given to the influence of pressure drop from the near-wellbore region to reservoir body on disturbing the ionic equilibrium in the CO2-saturated brine and consequent mineral precipitation. In this paper, we present the results of a novel experimental procedure designed to address these issues in carbonate reservoirs. We injected CO2-saturated brine into a composite core made of two matching grainstone carbonate core plugs with a tight disk placed between them to create a pressure profile of around 250 psi resembling that prevailing in reservoirs during CO2 injection. We investigated the impacts of fluid-rock interactions at pore and continuum scale using medical X-ray CT, nuclear magnetic resonance, and scanning electron microscopy. We found that strong calcite dissolution occurs near to the injection point, which leads to an increase in primary intergranular porosity and permeability of the near injection region, and ultimately to wormhole formation. The strong heterogeneous dissolution of calcite grains leads to the formation of intra-granular micro-pores. At later stages of the dissolution, the internal regions of ooids become accessible to the carbonated brine, leading to the formation of moldic porosity. At distances far from the injection point, we observed minimal or no change in pore structure, pore roughness, pore populations, and rock hydraulic properties. The pressure drop of 250 psi slightly disturbed the chemical equilibrium of the system, which led to minor precipitation of sub-micron sized calcite crystals but due to the large pore throats of the rock, these deposits had no measurable impact on rock permeability. The trial illustrates that the new procedure is valuable for investigating fluid-rock interactions by reproducing the geochemical consequences of relatively steep pore pressure gradients during CO2 injection.

Highlights

Capturing CO2 from large industrial sources and storing it in geological formations, such as saline aquifers, and depleted oil and gas fields, has become widely accepted as a viable solution for reducing high CO2 levels in the atmosphere[1,2,3]
As soon as the carbonated brine front meets the inlet face of the composite core (i.e., Core A), the pressure gradient across the composite core decreased slightly
One possible reason for such a sharp increase in the pressure gradient across the composite core is the exsolution of CO2 from the carbonated brine front due to the local pressure drop of 250 psi after passing the tight disk

Summary

Introduction

Capturing CO2 from large industrial sources and storing it in geological formations, such as saline aquifers, and depleted oil and gas fields, has become widely accepted as a viable solution for reducing high CO2 levels in the atmosphere[1,2,3]. At regions far from the injection well or when injection is stopped[21,22,23], the pressure of the carbonated brine front decreases This pressure drop alters the chemical equilibrium between the dissolved species leading to calcite precipitation. The potential impacts of mineral precipitation and consequent changes on rock porosity, permeability, pore structure, pore size, pore size distributions, and pore body roughness have likely been underestimated hitherto. We address these gaps in knowledge using a novel core flooding approach that examines effects from pore to continuum scales. The composite core was flushed by carbonated brine for a specified number of pore volumes and its hydraulic properties and pore structure were investigated prior to and following the carbonated brine injection step where the fluid-rock interactions occurred

Methods

Results

Discussion

Conclusion