[Paper Review] Numerical modeling of carbon dioxide sequestration on the rate of pressure solution creep in limestone: Preliminary results

François Renard, Elisabeth Gundersen+3 more|ArXiv.org|Jan 3, 2008

CO2 Sequestration and Geologic Interactions39 references4 citations

TL;DR

This study uses a 2D numerical model to investigate how CO2 sequestration affects pressure solution creep in limestone, showing that high CO2 partial pressures (up to 30 MPa) increase compaction rates by 50–75 times and reduce rock matrix viscosity, significantly altering long-term reservoir deformation and storage capacity.

ABSTRACT

When carbon dioxide (CO2) is injected into an aquifer or a depleted geological reservoir, its dissolution into solution results in acidification of the pore waters. As a consequence, the pore waters become more reactive, which leads to enhanced dissolution-precipitation processes and a modification of the mechanical and hydrological properties of the rock. This effect is especially important for limestones given that the solubility and reactivity of carbonates is strongly dependent on pH and the partial pressure of CO2. The main mechanism that couples dissolution, precipitation and rock matrix deformation is commonly referred to as intergranular pressure solution creep (IPS) or pervasive pressure solution creep (PSC). This process involves dissolution at intergranular grain contacts subject to elevated stress, diffusion of dissolved material in an intergranular fluid, and precipitation in pore spaces subject to lower stress. This leads to an overall and pervasive reduction in porosity due to both grain indentation and precipitation in pore spaces. The percolation of CO2-rich fluids may influence on-going compaction due to pressure solution and can therefore potentially affect the reservoir and its long-term CO2 storage capacity. We aim at quantifying this effect by using a 2D numerical model to study the coupling between dissolution-precipitation processes, local mass transfer, and deformation of the rock over long time scales. We show that high partial pressures of dissolved CO2 (up to 30 MPa) significantly increase the rates of compaction by a factor of ~ 50 to ~ 75, and also result in a concomitant decrease in the viscosity of the rock matrix.

Motivation & Objective

• To quantify the impact of CO2-rich fluid injection on pressure solution creep in limestone reservoirs.
• To understand how acidification from CO2 dissolution enhances dissolution-precipitation processes in carbonate rocks.
• To assess the influence of elevated CO2 partial pressure on rock deformation and porosity reduction over geological timescales.
• To model the coupling between mass transfer, fluid chemistry, and mechanical compaction in porous limestone.
• To evaluate the implications for long-term CO2 storage capacity and reservoir integrity in carbonate formations.

Proposed method

• A 2D numerical model is developed to simulate the interplay between fluid chemistry, mass transfer, and rock deformation.
• The model incorporates dissolution at high-stress grain contacts and precipitation in low-stress pore spaces, representing intergranular pressure solution creep (IPS).
• Chemical reactions are governed by pH and CO2 partial pressure, with dissolution rates dependent on carbonate solubility and fluid diffusivity.
• The model tracks changes in porosity, fluid chemistry, and stress distribution over time under varying CO2 partial pressures.
• Rock viscosity is recalculated dynamically based on evolving porosity and mineralogical changes due to precipitation and dissolution.
• Simulations are run over long timescales to assess cumulative compaction and permeability evolution under CO2-rich conditions.

Experimental results

Research questions

• RQ1 How does increased CO2 partial pressure affect the rate of pressure solution creep in limestone?
• RQ2 What is the impact of CO2-induced acidification on dissolution-precipitation processes in carbonate rocks?
• RQ3 How does the presence of CO2-rich fluids alter the mechanical properties of the rock matrix, particularly its viscosity?
• RQ4 To what extent does CO2 sequestration accelerate porosity reduction and compaction in limestone reservoirs?
• RQ5 What are the long-term implications for reservoir integrity and CO2 storage capacity under elevated CO2 partial pressures?

Key findings

• High partial pressures of dissolved CO2 (up to 30 MPa) increase the rate of pressure solution creep in limestone by a factor of 50 to 75.
• The compaction rate enhancement is directly linked to increased fluid reactivity due to CO2-induced acidification.
• The rock matrix viscosity decreases significantly under high CO2 conditions, indicating a softening effect on the mechanical behavior.
• Porosity reduction is accelerated due to enhanced dissolution at grain contacts and precipitation in pore spaces.
• The model predicts a strong coupling between chemical alteration and mechanical deformation, with implications for long-term reservoir stability.
• These effects suggest a substantial risk of accelerated compaction and potential loss of storage capacity in carbonate reservoirs during CO2 sequestration.