CCS research by the Transport properties research laboratory

Heavy duty shear rig: fluid flow along fractures and faults in caprock

Rock mechanics testing: changes in flow properties in caprock under complicated pressure conditions.

Supercritical CO2 rig: fluid flow through caprock at reservoir conditions (up to 150°C and 250 bar).

Led by Dr Caroline Graham as part of the Transport properties research laboratory.

A growing team of staff work in the transport properties research laboratory (TPRL) and collaborate with other fluid processes researchers such as Dr Robert Cuss in the fracture physics laboratory.

Research in the TPRL is focussed on the physical aspects of fluid flow underground (both water and gas) and the influence of deformation. Thirty years of development in this field has led to the development of high precision techniques, allowing exceptionally small amounts of flow to be measured where conventional techniques can be unsuccessful. As such, the TPRL is able to investigate the long-term properties of rocks that act as 'seals' to fluid on geological timescales. Staff working in the TPRL have a mixture of backgrounds in fluid flow testing, rock mechanics and engineering, which are used to design, construct and operate complex experimental apparatus tailored to a specific scientific problem.

The TPRL is involved in a number of CCS projects and topics of interest.

Caprock properties

To store carbon dioxide (CO2), the gas is converted to a high pressure, liquid-like form known as supercritical CO2, which is injected underground directly into sedimentary rocks. The CO2 is stored in a 'reservoir' rock. This is usually a porous (containing holes, or 'pores', much like a sponge) sandstone, which allows the CO2 to flow into these pores. The caprock is located above the reservoir rock and is a material that does not allow fluid to flow through easily, because its pores are very small (e.g. a siltstone or shale). This rock acts as a 'cap' and traps the CO2 in the reservoir.

This laboratory investigates the physical properties of caprocks and identifies the pressure conditions under which these materials do and don't act as barriers to flow. This is important in order to determine how suitable a caprock is for trapping CO2 in the reservoir. Because the caprock has to support the weight of the hundreds of metres of rock and water above, it will be under pressure. These pressure conditions will have changed over its geological history and will also be changed by injection of CO2 in the reservoir below. These changes in the stress conditions will affect the caprock properties and can cause it to deform or fracture. If a bar of toffee is warm it will stretch and deform when you try to bend it, but if it is cold it will snap under enough force. Rocks can behave in a similar way under different conditions, so it is important to understand how a caprock may behave in such situations.

Fractures and Faults

Caprock may already contain faults and fractures before CO2 is injected into the reservoir. It is therefore important to study these natural features to understand if injection will affect them (for example, causing them to start sliding again, or 'reactivate'). If a fault does move again it may or may not create a better seal than before and affect movement of CO2 along it. If a fault does move, it may create a small earthquake, or 'micro-tremor'. Even if a fracture does not begin to move, fluid flow along the fracture may change. This area of research is so important that BGS has now developed a new fracture physics laboratory.

Well-bore interfaces

Just as it is important to study the flow of fluid along a fracture, the potential for fluid to exploit the surface between the caprock and the borehole (the well-bore interface) is also relevant. We want to know what conditions might affect sealing here, as the borehole could form a direct route to the surface. As the borehole will generally be made of steel and surrounded by cement, there may be reaction front as CO2 interacts with the cement.

Changes in flow properties due to CO2

Precipitation and dissolution can change the permeability, and carbonation can affect the flow properties of the reservoir rock or caprock. We are investigating how these chemical changes affect different flow properties of the reservoir and caprock.

Projects that involve this type of work

The Ultimate CO2 (Understanding the Long-Term Fate of Geologically Stored CO2 — EC FP7) and BIGCCS (Norwegian Research Council) projects are looking at fluid flow along fractures in caprock and how the pressure in a reservoir and the minerals in a fault may affect flow. Also studying fault reactivation by changing pressure conditions around a fault.

For the Green River Project (Department of Energy and Climate Change), rock samples have been drilled from a 'natural analogue' site in Utah, where natural subsurface CO2 is finding its way back to the surface. The project is looking at the sealing properties and flow behaviour of the reservoir sandstones and the siltstones and shales that may have acted as a caprock in the past.

CONTAIN (Impact of Hydrocarbon Depletion on the Treatment of Caprocks within Performance Assessment for CO2 Injection Schemes — Engineering and Physical Sciences Research Council) uses laboratory testing to simulate the pressure conditions that a caprock may go through over time. Much like a balloon, the reservoir will 'deflate' (during oil/gas extraction) and 're-inflate' (after CO2 injections).

The Irish Basins Project (Geological Survey of Ireland) focusses on reservoir sealing in the Irish Sea and is looking out how CO2 flows through and interacts with intact caprock at a range of temperatures and pressures.

Contacts