Impact of Thermal Pressurization on Fluid Pressure Build up in Carbon Capture and Storage Projects

Over the past decade or so, induced seismicity due to high pressure fluid injection has been a point of controversy for emerging energy geo-technologies such as unconventional hydrocarbon reservoirs, enhanced geothermal systems, and carbon capture and storage projects (CCS).

When CO₂ is injected, pore pressure in the reservoir increases and effective stress decreases. The changes in effective stress and pore pressure cause expansion in the reservoir which leads to deformation of the overburden. This phenomena may threaten the storage integrity by creating new fractures and reactivating existing faults.  In the presence of an existing fault combined with favourable injection characteristics, fault slip occurs which may increase the leakage risk and can induce low-magnitude seismic events. 

Once slip is initiated, two mechanisms having opposite impacts compete to control slip and rate of slip (Segal and Bradly 2012). First there is shear-induced dilatancy of the fault core, which tends to increase fault permeability contributing to fluid pressure decrease. Countering this potentially stabilizing mechanism, fault heating promotes increased fluid pressure that can weaken the fault (Rice 2006, Segal and Bradly 2012). Dilatant strengthening is considered as a mechanism for slow slip events. The faster the fault slips, the harder it is for fluid flow within the fault zone to keep up with dilatancy (Segall & Bradley, 2012). An earthquake occurs if the thermal weakening process during the fault’s early slip takes over the shear-induced dilatency due to the release of tectonic stress driving the fault motion. Two thermal mechanisms, referred to as flash heating and thermal pressurization, can weaken the fault and decrease frictional resistance along the fault (Rice 2006). Flash heating occurs in rapid slips and mostly depends on the slip rate. This mechanism deals with highly stressed micro-scale contacts during slip and decreases the fault friction coefficient (Rice 2006).

In thermal pressurization (TP), frictional heating generated during slip may increase pore fluid pressure within the fault core, which consequently leads to a fault weakening mechanism. When shear sliding happens, the fault fluid expands in volume much more than the surrounding rock. This is due to the thermal expansion coefficient of the fluid being greater than the shear-induced dilatancy of the rock matrix. Consequently, frictional heating increases pore pressure which decreases the effective normal stress and the coefficient of friction and thus, reduces fault strength (Rice 2006).

In this study, we investigated the role of TP of a pre-existing fault on temperature and pore pressure changes in CCS projects. This paper is not dealt with nucleation phase but rather with the coseismic period after that the thermal weakening dominates rate and state dependent weakening at slip rate exceeds 10⁻⁴ to 10⁻² m/s. Similar approach has been used in several studies focusing on the TP impact (e.g., Rice 2006). We developed a 2D coupled Thermo-Hydro-Mechanical (THM) model using the finite element method in COMSOL Multiphysics 5.1 to investigate how pore pressure and temperature evolve due to TP. Thermal pressurization was simulated by implementing a simple constitutive model which couples pore pressure changes with temperature rise using the PDE interface of COMSOL. The problem was solved in one dimension in the fault-normal direction and the fault-normal stress (σ_n) was assumed to remain constant during the slip. Injection was modelled in the center of a fault zone which is already activated and the velocity was considered to be 0.1 m/s. In this study, the effects of hydraulic diffusivity (ω) and slipping zone thickness (w) on pore pressure and temperature changes were investigated. In order to illustrate the effect of TP, two sets of models were developed and compared with each other. In the first set of models, only poroelasticity was taken into account whereas in the second set, poroelasticity was coupled the TP model. Based on the results, it can be concluded that, in general, ignoring the TP role can lead to a significant underestimation of pore pressure built up during CO₂ injection. However, the difference between the models with and without TP becomes smaller for thinner shear zones.

  References

Rice, J.R., 2006. Heating and weakening of faults during earthquake slip. Journal of Geophysical Research, 111(B5), p.B05311.

Segall, P., & Bradley, A. M., 2012. The role of thermal pressurization and dilatancy in controlling the rate of fault slip. Journal of Applied Mechanics, 79(3), 031013.