Does your CCS Monitoring Plan Consider Patchiness?

What is patchiness? "Patchiness" defines how the effective rock behaves to passing seismic waves when there are two fluid phases present. When one of those phases is much more viscous and stiff than the other, then how the fluid is distributed in the pore space makes a big difference in how stiff the rock is. The stiffer the rock, the faster it is. I therefore like to think of "patchiness" as the name for the compressional velocity versus saturation function.

Supercritical CO2 has a viscosity in the range of 0.04-0.06 cp. Water has a viscosity of close to 1. The stiffnesses of the fluids are hugely different as well, with 2 GPa being the stiffness of water and 0.2 GPa being that of supercritical CO2. The large difference in stiffness and viscosity means one should consider patchiness if they plan to use time-lapse seismic methods to quantify CO2 saturation for CCS.

At the heart of patchiness is how slow the pore pressure will equilbrate in the pore space as an elastic wave (ultrasonic, sonic, or seismic) passes by. An important quantity to now consider is the "critical patch length scale". If all the pores have an equal amount of partially saturated fluid (green pores in figure), then we have "uniform mixing". If the rock has its partial saturation segmented in terms of pure water and pure CO2 pores, and the pores are smaller than the critical patch length scale, then we also have exactly the same uniform mixing. There is no difference in how the rock behaves in these two cases to a passing elastic wave; the stiffness felt is the same and it's uniform mixing.

The equation in the figure shows the critical patch length scale. This is a property of the rock and two fluids that will be mixing in the pore space. The variables that go into it are the rock permeability, bulk modulus of the most viscous fluid (water), viscosity of the most viscous fluid (water), and elastic wave frequency. Examples help illustrate the concept. Using a highly permeabile reservoir with 1000 mD (e.g., Quest BCS), I plotted the crtitical patch length scale in the graph on the lower right. The equation peaks at 0.5 meters at 10 Hz, and decays exponentially with increasing frequency. It does not get larger than 0.5 m (it only gets smaller for tigher rocks). For patch sizes less than the critical scale, the response is "uniform". For interconnected pore space with dominant patch sizes that are larger than this critical patch length scale, the response is "patchy". So for our 1000 mD rock example, if the interconnected pore space has patches of brine with a dominant size of 10 cm, then the response is "patchy" at sonic frequency ranges (red rectangle to the right), and "uniform" at seismic frequency ranges (red rectangle to the left). However, if one measures a uniform response with say sonic logging technology coupled with pulse neutron (for saturation measurements) from behind casing in the reservoir at a monitoring well, this guarantees a uniform response at seismic scales providing the rock at the well is representative of the rock throughout the reservoir.

How important is this for CCS? Stay tuned to the next article for the answer.