Numerical study of asymmetric vertical fluid intrusion in deep reservoirs: Effects of stress, temperature and salinity

Abstract

Non-symmetric upward and downward pressure diffusion from the point of intrusion is a common phenomenon in fluid-triggered seismicity. In most studies, a stress and pressure dependent permeability is applied resulting in a depth-dependency. An alternative source of non-symmetric diffusion are variations in fluid properties with depth due to temperature or salinity gradients causing buoyancy and affecting hydraulic diffusivity. We evaluate these processes to identify to what extent each may result in non-symmetric diffusion. We find that over short time scales of a couple days, stress-dependent permeability has the strongest influence on non-symmetric diffusion processes, while temperature effects become more important over longer time scales of months. Salinity plays only a very minor role. We verify these findings by applying our model to the 2008 West-Bohemian swarm, which is most likely triggered by degassing CO2 from the mantle. As the first swarm phase lasts thirty days with high pressure intrusion for approximately 7 days, we verify that asymmetric diffusion is dominated by the mechanical and hydraulic state of the system. Although density and viscosity of supercritical CO2 vary strongly in the pressure and temperature range present, due to the small amount of intruding fluid, the temperature and buoyancy effect is limited. Hence, we require unrealistic permeability gradients with depth to explain the asymmetric vertical pressure diffusion observed. We introduce an increased permeability based on fracture (re-)activation as contributor to intensify asymmetric diffusion and reducing the necessity of exaggerated permeability gradients. We conclude, that for short-term events up to 50 days simulations of realistic seismic cloud behavior will require constraints with respect to depth-variations of the hydraulic parameters and should consider increased permeability due to fracture (re-)activation. Long-term simulations over months require a sophisticated coupling of heat and fluid flow in combination with an adequate equation of state for the fluid phase.