Micro-CT imaging of reservoir condition CO2 during multi-phase flow in natural rock

Abstract:

Micron-resolution X-ray microtomography has allowed researchers to examine the processes controlling fluid flow behaviour at the pore scale, offering the promise of a transformation in our understanding of flow and transport in porous media. Until recently wettability has only been directly accessible in extremely simplified systems. A new method is presented for the measurement of the contact angle and capillary pressure of multiple immiscible fluids at the pore scale at reservoir conditions in the scCO₂-brine-carbonate system. Contact angle is found by resampling the micro-CT data onto planes orthogonal to the contact lines, allowing for vectors to be traced along the grain surface and the scCO₂ – brine interface. A distribution of contact angles ranging from 35^o to 55^o is observed. This distribution can be understood as the result of contact angle hysteresis and surface heterogeneity on a range of length scales. Ganglion capillary pressure for each ganglion was found by measuring the curvature of the CO₂-brine interface, while the pore structure was parameterised using distance maps of the pore-space. The formation of the residual clusters by snap-off was examined by comparing the ganglion capillary pressure to local pore topography. The capillary pressure was found to be inversely proportional to the radius of the largest restriction (throat) surrounding the ganglion, which validates the imbibition mechanisms used in pore-network modelling. The potential mobilization of residual ganglia was assessed using a new formulation of both the capillary and Bond numbers, rigorously based on a balance of pore-scale forces, with the majority of ganglia remobilized at N_cmacro around 1. By the use of synchrotron tomography it is possible to create high quality 4D images of dynamic processes involving the flow of multiple fluid phases. We show how the drainage process take place as a series of discreet Haines jumps. Two different types of Haines jumps were seen, one where CO₂ moves into a pore and remains connected with the rest of the CO₂ and one where it immediately snaps off, forming a disconnected ganglion in the filled region. We also observe the change in capillary pressure associated with a Haines jump, showing that the capillary pressure reduction during the event can cause snap-off far away from the Haines jump.