Predicting fluid flow through geothermal reservoirs in carbonates

The huge stores of heat far below the surface of the Earth are still a largely underused renewable energy source. In a few fortunate places, the right combination of geological conditions allows hot water to bubble naturally to the surface, meaning it can be readily used as a source of geothermal energy.

Elsewhere, however, it is necessary to drill deep into the Earth’s crust to tap into this constant, renewable supply of heat energy. Such activities can be expensive and fraught with complications. The local geology and hydrology at each location determines how deep boreholes need to be drilled, how easy it will be to circulate water through the rock, and how long a geothermal plant may remain operational for.

Among the most promising, but least understood geothermal systems are those contained within carbonate rocks – namely limestones and dolostones. The way these rocks deform can fundamentally determine the viability of these geothermal reservoirs.

The University of Leeds’ Carbonate Fault Research Group is using its expertise to fill crucial knowledge gaps about how fluids flow in carbonate rocks, and how fault systems affect this.

Its results are helping industry partners to better understand how to exploit both potential hydrocarbon and geothermal reservoirs in carbonate rocks.

Nigüelas fault in Spain

Nigüelas fault in Granada Basin, south of Spain. Note a flat fault wall in a creamy colour and a damage zone below it cross-cut by white veins.

Rock porosity

The flow of fluid through carbonates is highly variable, depending upon the rock’s porosity and how it deforms. This is ultimately determined by the size of the grains and how these are organised within the carbonate, but also on their depth.

“Shallow carbonates can be very porous,” says Ieva Kaminskaite, who helped to lead the work as part of the Carbonate Fault Research Group. “Once you start to get below one kilometre in depth, majority of carbonates become recrystallised or cemented. In very deep carbonates, all of the porosity comes from fractures and faults in the rock.”

In collaboration with Badley Geoscience Ltd, Kaminskaite and her colleagues have spent the past few years visiting carbonate outcrops around the world – including Italy, Germany, Malta, Greece, Oman, UAE, USA and the UK – to gather samples for the CARBFAULT project so they can analyse the properties of faults in the rock. “The first time I went to Italy, I was walking around the San Vito lo Capo peninsula to find faults, as not many people have studied them,” says Kaminskaite.

Using state-of-the-art laboratory facilities, the team were then able to examine the microscopic structure of the rock, how it deforms under strain and measure its flow properties.

Predictive algorithm

The results provide invaluable insights into how different fault structures within carbonate deposits allow fluid to flow through them. They have shown that in porous rock such as chalk, deformation caused by faulting can reduce permeability by causing macro-pores within foraminifera – the shells of single celled plankton that make up the rock – to collapse. Faults in porous limestones or dolostones can also act as a barrier to fluid flow through grain-scale breakage (cataclasis) and cementation. Tight limestones and dolostones with low permeability, however, behave differently.

“Deep carbonates are very brittle,” says Kaminskaite. “So when the rock breaks due to seismic movement or hydrofracturing, for example, it dilates rather than compacts, increasing the permeability. The faults then act as pathways for fluid.”

They also found that while limestone tends to deform along a single fault core, dolostone deformation spreads out over a large area to create a much wider fault zone with several fault cores.

This kind of information is crucial for potential geothermal energy projects, where understanding how deep carbonates will deform when water is injected and the effects this will then have on the ability to circulate fluid through the rock. To assist with this, the Carbonate Fault Research Group have been using their results to create a large, detailed database of faults within different lithofacies. This has allowed their collaborators at Badley Geoscience Ltd to create an algorithm that can be used to predict the fault rock permeability in carbonates, helping industry to model fluid flow in carbonate reservoirs.
“Previously most modelling was done using algorithms for sandstones, which isn’t great if you are trying to make predictions about carbonates as it deforms differently,” says Kaminskaite.

Drone image and transmissibility multipliers carbfault project

Drone image of the fault in San Vito lo Capo, Sicily, Italy (left); and a cellular fault surface showing fault throw and a fault rock thickness (right).

Cementing in faults

Fractures are only one important aspect of fluid flow in carbonate reservoirs. Mineral rich water flowing through faults can quickly lead to precipitation that seals them, otherwise known as cementation. The second phase of the CARBFAULT project is not only looking to extend the database of low and medium porosity carbonates, but it will use isotope analysis to understand the timescales of this cementation process. It will build up data about the depth and temperatures at which different levels of cementation takes place.

The project is also hoping to look at how different geochemical properties affect fault strength. “If you have another seismic event or you inject water to fracture the rock, we want to know which would break first,” explains Kaminskaite. “Will it be the host rock, which has not deformed previously, or would it be the fault that has already deformed.”

Knowing the answer to these questions could help with important decisions about where to place a geothermal facility. “If you know the fault is very hard to break because of the cementation or cataclasis processes, you might want to drill the injector and producer wells on one side of the fault to use the damage zone alone,” says Kaminskaite.

Until recently, carbonate rock formations have been largely studied for oil and gas exploration, but understanding fluid flow in deep rocks is also opening up their potential as a source of geothermal energy. Research at the University of Leeds is making this shift to renewable energy possible.

Video about the Carbfault project