Pressure Buildup During CO2 Injection into a Closed Brine Aquifer

Abstract

CO 2 injected into porous formations is accommodated by reduction in the volume of the formation fluid and enlargement of the pore space, through compression of the formation fluids and rock material, respectively. A critical issue is how the resulting pressure buildup will affect the mechanical integrity of the host formation and caprock. Building on an existing approximate solution for formations of infinite radial extent, this article presents an explicit approximate solution for estimating pressure buildup due to injection of CO 2 into closed brine aquifers of finite radial extent. The analysis is also applicable for injection into a formation containing multiple wells, in which each well acts as if it were in a quasi-circular closed region. The approximate solution is validated by comparison with vertically averaged results obtained using TOUGH2 with ECO2N (where many of the simplifying assumptions are relaxed), and is shown to be very accurate over wide ranges of the relevant parameter space. The resulting equations for the pressure distribution are explicit, and can be easily implemented within spreadsheet software for estimating CO 2 injection capacity. 123 384 S. A. Mathias et al. List of symbols A Formation plan area [L 2 ] b Forchheimer parameter [L −1 ] b r Relative Forchheimer parameter [-] c o Compressibility of CO 2 [M −1 LT 2 ] c r Compressibility of geological formation [M −1 LT 2 ] c w Compressibility of brine [M −1 LT 2 ] h CO 2 brine interface elevation [L] h D = h/H Dimensionless interface elevation [-] H Formation thickness [L] k Permeability [L 2 ] k r Relative permeability [-] M 0 Mass injection rate [MT −1 ] p Fluid pressure [ML −1 T −2 ] p D = 2π Hρ o k r kp/M 0 μ o Dimensionless pressure [-] q o CO 2 flux [LT −1 ] q oD = 2π Hr w ρ o q o /M 0 Dimensionless CO 2 flux [-] q w Brine flux [LT −1 ] q wD = 2π Hr w ρ o q w /M 0 Dimensionless brine flux [-] r Radial distance [L] r c Radial extent of reservoir [L] r cD = r c /r w Dimensionless radial extent of reservoir [-] r D = r/r w Dimensionless radius [-] r w Well radius [L] S r Residual brine saturation [-] t Time [T] t cD = αr 2 cD /2.246γ Dimensionless time at which the pressure disturbance meets the reservoir boundary [-] t D = M 0 t/2π(1 − S r )φ Hr 2 w ρ o Dimensionless time [-] α = M 0 μ o (c r + c w )/2π(1 − S r )Hρ o k r k Dimensionless compressibility [-] β = M 0 k r kb r b/2π Hr w μ o Dimensionless Forchheimer parameter [-] γ = μ o /k r μ w Viscosity ratio [-] = (1 − S r )(c o − c w )/(c r + c w ) Normalized fluid compressibility difference [-] μ o Viscosity of CO 2 [ML −1 T −1 ] μ w Viscosity of brine [ML −1 T −1 ] ρ o Density of CO 2 [ML −3 ] ρ w Density of brine [ML −3 ] σ = b r ρ o /ρ w Density ratio [-] φ Porosity [-]

Heat transport and pressure buildup during carbon dioxide injection into depleted gas reservoirs

“…Consequently, throughout the injection duration, the vast majority of the reservoir pore space continues to be occupied by brine. Therefore, in contrast to depleted gas reservoirs, the compressibility of the injection fluid is found to have very little impact on pressure buildup (Mathias et al 2011b). Furthermore, because of the much larger viscosity difference between the CO 2 and the brine, along with the IFT that develops between the CO 2 -rich and aqueous fluid phases, the mobility difference between the injection and reservoir fluids has a much more significant impact on the pressure buildup process (Mathias et al 2009(Mathias et al , 2013a.…”

Dynamic modelling of a UK North Sea saline formation for CO 2 sequestration

“…This is 532 because the pressure buildup in the storage site is unable to dissipate (see Mathias et al 2011). 533…”

“…To accurately model injection well pressure a very fine horizontal grid resolution 240 (~ 5 mm) is needed around the injection well (Mathias et al 2011). As the purpose of our model is to 241 look at the overall capacity of the storage site to store injected CO 2 it was not deemed necessary at 242 this stage to carry out detailed modelling of injection pressures.…”

Pressure Buildup During Supercritical Carbon Dioxide Injection From a Partially Penetrating Borehole into Gas Reservoirs

“…Analytical solutions have also been obtained for estimating risks of pressure buildup resulting from CO 2 injection (see Oruganti et al, 2011) and for pressure buildup in overlying formations (Zeidouni et al, 2011). Further, Mathias et al (2011) presented an explicit approximate solution for estimating pressure buildup due to injection of CO2 into closed brine aquifers of finite radial extent.…”