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).

1. Career development
To our knowledge, combined neutron/X-ray imaging has not been attempted on geological materials to date. The project thus represents an opportunity for the applicants - two of whom are early career researchers - to become champions of this technology amongst geological fluid dynamists in the UK. Furthermore, by working closely with physicists at a large user facility, the applicants will have the opportunity to further expand their professional network and to develop awareness of the suite of neutron measurement tools. The international dimension of the project will benefit all staff involved by exposing them to different research cultures and perspectives.

2. Public
There is much public interest in fracking in the UK following several 'focusing events', e.g., the anti-fracking protests in Balcombe, Sussex in July 2013. This project will produce high quality videos of fluid flow through rock, which can serve as an effective tool for communicating the nature of fluid flow through geological materials - whether they be soil or shale - to the public. The videos will also be used in outreach activities, which will encourage interest in STEM and, in the long term, contribute towards addressing our chronic shortage of engineers - a need that has been highlighted, e.g., by the UK parliament [1] and by the Royal Academy of Engineering [2].

3. Government
The reactor at NIST Center for Neutron Research represents significant capital investment and operational cost by the US government. This project will accelerate its use to a relatively new application - geological materials - thereby increasing its user base and contribute towards the "advance[ment of] the use of neutrons by... academia and industrial scientists" (www.ncnr.nist.gov/20150203NCNRGrant.pdf) - a goal of the partner organization, which falls under the US Department of Commerce. In doing so, the project maximizes the return on this investment.

4. Industry
The project will contribute direct measurements of capillary-driven flow towards a comprehensive data set comprising bulk properties of fluids, capillary-hydraulic properties of rocks, and flow behaviour of the fluids, through the rocks, associated with waterflood (forced displacement) and spontaneous imbibition. The data set will enable validation of reservoir simulators with unprecedented rigour. There is thus potential for a wide range of commercial sectors to benefit. Groundwater hydrologists in environmental consultancies will be able to better design remediation schemes for NAPL-contaminated soils and aquifers. Better design and reduced uncertainties will translate to reduced costs for their clients, who in turn span many sectors including agriculture (e.g., chemical spills, runoff), defence, energy (e.g., abandoned refineries), mining, construction, as well as government agencies. Independent stakeholders (e.g., NGOs with interest in the environment and public health) will also benefit from improved confidence in proposed schemes. Similarly, reservoir engineers in the oil and gas sector will be able to better design waterflood/enhanced oil recovery and CCS schemes which, in the longer term, will promote their deployment, attract more players and investment, and create more jobs.

5. Regulators and government agencies
One of Ian Wood's recommendations for the Oil and Gas Authority, UK's new regulator for oil and gas operations, is that it sets "clear expectations on... stewardship factors such as production efficiency... and... ensure[s] that they are met." Data from this project can inform these expectations and standards.

Similarly, the Environment Agency, in partnership with the British Geological Survey, produces monthly forecasts of groundwater levels and river flows. Improved simulators will lead to better forecasts, and hence increased confidence in flood prevention, irrigation, and remediation schemes based on them.