What is a cemented fracture ?

A fracture that now contains a mineral fill or cement. When fluids travel along fractures, cements can precipitate out into the fractures. Calcite is a common fracture cement (Fig. 1) but many minerals form cements. For example, fluorite can be found within the deformation bands in Fig 2 (a) and anhydrite fills the lowermost fracture in Fig 2 (b).

http://www.ogilviegeoscience.co.uk/blog/2022/12/4/natural-vs-induced-fractures-in-rock-core

Fig. 1. Calcite cemented fracture in limestone, Kilve Pill, Somerset, UK. Looking at one of the fracture walls - the other side has been removed by erosion.

Fig 2. (a) Fluorite cemented fracture network in the Permian Hopeman Sandstone, UK. (b) Deformation bands in Rotliegende sandstone from the Southern North Sea. The lowermost (thickest) fracture has been reactivated and cemented by anhydrite.

What are the consequences of a completely “filled” fracture ?

Most likely it will impede flow and hinder the productivety of a fracture network, as the permeability and storage capacity (porosity) is significantly reduced.

What about a partially cemented fracture ?

The fractures (in sandstone) from the Clair Field (West of Shetland, UK) in Fig. 3 appear to have some cement i.e., they are partially filled. In areas of the field, this has been beneficial in preserving fracture aperture during hydrocarbon production. Otherwise, the fractures may close.

Fig. 3. Fractured sandstones from the Clair Field, West of Shetland, UK (From Barr et al. 2005). Note the partial fill on the right hand most fracture in (b).

Further examples of partially filled fractures are given in Fig. 4. Sometimes the aperture is obvious (Fig 4a) other times we can just pinpricks along the extent of the fracture (Fig 4b).

Fig. 4. (a) Partially open fracture in dolomite, Middle East (b) partially open fracture in sandstone core

This together with fracture surface roughness (where asperities can prop open the fracture even as the normal stress increases) may explain why pressure drawdown does not always result in closed fractures. If the fracture is reactivated, these asperities may collapse (depending upon the rock) and new points of contact taken up, creating new apertures. In strong rocks such as igneous rocks (e.g. granite) these asperities will be less likely to collapse.

Can we see partially filled fractures on image logs ?

Yes, but it is valuable to have a core calibration for any image log interpretation. On the image on Fig. 5, note how the aperture appears to be thickest at [1] but thins towards [2]. This could indicate a partially filled fracture. Rock core in this well would be most useful to calibrate this interpretation i.e., can we pick the same structure in core that is demonstrably a partially filled fracture or perhaps the fact that partially filled fractures dominate the core is sufficient calibration.

Fig 5. Electrical Resistivety image where a partially open fracture is interpreted.

In conclusion..

Cemented fractures will usually impede fluid flow in a hydrocarbon or geothermal reservoir and would likely negatively impact production. However, a small amount of cement can act as a natural propant, keeping fractures open during production and is therefore beneficial to the field development. These are most confidently identified on core and it is valuable to use core (if available) to calibrate image log interpretations.

References

Barr, D, Savory, K.E., Fowler, S.R, Arman, K, McGarrity, J.P. 2005. Pre-development fracure modelling in the Clair Field, west of Shetland. From Lonergan, L, Jolly, R.J.H, Rawnsley, K and Sanderson, D.J. (eds) Fractured Reservoirs. Geological Society Special Publications, 270, 205 - 225.