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Rock Fractures and Fluid Flow

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Fractured reservoirs

Although the historic focus of hydrocarbon production has been on conventional reservoirs, such as porous sandstones, it has always been known that the presence of fractures is highly important for fluid migration and, for some reservoirs, porosity and storage.

Even in a primarily porous reservoir, where fluid flow generally follows ‘Darcy’s law’, the presence of just a few fractures can largely control the permeability of the reservoir. Where fractures are present the volumetric flow rate, Q, through the fractures is calculated using the so-called ‘cubic law’

 rf32resn1imgeq

Here P1-P2 is the pressure difference over length L, b is the aperture or opening of the fracture, L is the length of the fracture (parallel to the flow direction), μf is the dynamic viscosity of the fluid, and W is the width of the fracture (perpendicular to the flow direction). Here it is assumed that the flow is horizontal, such as in a classical hydraulic fracturing, so that buoyancy plays no role. From the cubic law it follows that the parameter with the largest influence on the volumetric flow rate, and therefore permeability, is the fracture aperture, which is in the third power – cubed, hence the name of the law. Reliable estimates of fracture apertures are at depth are thus crucial for determining the permeability of subsurface reservoirs.

More recently, there has been focus on fractured reservoirs where the matrix porosity and permeability are low, but where the fractures themselves host the hydrocarbons instead of just acting as a fluid pathways, that is, where there is significant fracture porosity. It is known that highly brittle rocks at depth (or those that have been at depth in the case of outcrop analogues) are often extensively fractured. These fractures can be the result of processes during rock formation (e.g., cooling joints in igneous rocks), tectonic stresses (e.g. faulting and folding), fluid flow (be it geothermally derived or from the dewatering of sediments during burial), and weathering before burial (many basement reservoirs). In each of these cases the fracture network can be extensive and able to act as an effective reservoir, as has been demonstrated in Vietnam’s Bach Ho Field, which is a major oil field located in highly fractured granitic basement.

Crucially, fractures can also affect seal integrity for petroleum systems, in which case their existence is detrimental. Therefore, fractures play an import role in all parts of the petroleum system, namely as providers of fluid/migration pathways to and from reservoirs, seal integrity, and in hosting hydrocarbons and providing the porosity of the reservoirs.

The group works extensively on characterising fracture properties from outcrop data, using areas such as the Lower Jurassic shale/limestone alternations along the Bristol Channel, the Upper Carboniferous coalfields along South Wales, sandstones of the Bristol Channel Coal Measures, and sills from both the UK and Iceland as analogues for fractured reservoirs at depth. While the emphasis here has been on fractured hydrocarbon reservoirs, fractured geothermal reservoirs and groundwater aquifers are common and also form part of the research of the group.

rf32resn1img2r300 Click to enlarge rf32resn1img1r300

Fracture network within a limestone layer from the Jurassic limestone/shale alternations at Nash Point, South Wales. The sequence at Nash Point, and the fracture network within, is laterally extensive for several kilometres

The Whin Sill, North England. The sill is emplaced in sediments and is approximately 30m thick.


Contact: rf3@es.rhul.ac.uk

 
 
 

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