DYNAMIC, IN-SITU IMAGING OF CAPILLARY IMBIBITION IN ROCK USING SIMULTANEOUS NEUTRON AND X-RAY

Dynamic, in-situ imaging of capillary imbibition in rock using simultaneous neutron and X-ray computed tomography

"The most common method of secondary oil recovery is waterflooding, whereby water is injected into a reservoir to displace the oil in the reservoir towards production wells. Under ideal conditions, the injected water (flood water) uniformly 'sweeps' the oil towards the wells. However, in fractured reservoirs, which contain around half of the world's oil reserves, the flood water preferentially flows through the network of fractures, leaving behind a large fraction of the original oil in the rock matrix. Then, more gradually, flood water in the fractures enters the rock matrix by buoyancy, diffusion, and spontaneous (capillary-driven) imbibition, displacing the oil that was left behind.

Salient features of imbibition under uniformly water-wetting (hydrophilic) conditions are well established. However, many fundamental questions remain unanswered for imbibition under mixed-wet conditions characteristic of oil reservoirs, under which grain surfaces display heterogeneity in wettability at the pore scale and sub-pore scale. To design efficient oil recovery schemes for fractured reservoirs, the relationship between rock wettability and the imbibition behaviour (i.e., rate and ultimate oil recovery) must be determined quantitatively.

Funds are requested for two members of the project team to travel to the National Institute of Standards and Technology (NIST) Center for Neutron Research in Gaithersburg, USA in a series of four trips to perform experiments at their X-ray/neutron imaging facility. We will dynamically image the displacement of oil within limestone samples by imbibing water for oils containing different wettability-altering constituents. Observed differences in imbibition behaviour will be correlated to the contact angle of the oil/brine interface on a calcite substrate measured independently; this macroscopic contact angle will be our measure of rock wettability.

The work will be undertaken by a multidisciplinary team comprising academic researchers with a combined expertise in experimental fluid mechanics, rock mechanics, and medical imaging at University of Aberdeen, together with physicists in the Neutron Physics Group at NIST. To our knowledge, NIST is the only high flux user facility with simultaneous X-ray and neutron imaging capabilities. Moreover, NIST will provide us with four highly specialized, constant pressure mode syringe pumps which we need for establishing uniform initial oil saturation.

Two key aims of the project are to consolidate the collaboration between the applicants and the named physicists at NIST and to evaluate the achievable specifications of the new combined X-ray/neutron imaging capability at NIST for our samples. This project will thus provide the basis for future work by the project team which will lead to a physically meaningful, predictive model for capillary-driven, two-phase flow in porous media under the full range of possible wettability and initial fluid distribution. Applications include petroleum engineering (oil recovery from fractured reservoirs), geological CO2 storage (migration of formation brine towards the injection well during well shut-in), geotechnical engineering (subsurface leakage from waste repositories), construction (water infiltration into concrete), and flood management, soil remediation, and irrigation (water infiltration in soil)."