Induced fractures often form during the coring of the rock. These need to be captured during a core description as should not be confused with those natural fractures that can actually contribute to the permeability of the reservoir. Have a look at this blog for some criteria you can use to distinguish between the 2…
http://www.ogilviegeoscience.co.uk/blog/2022/12/4/natural-vs-induced-fractures-in-rock-core
Induced fractures are often irregular and sinuoidal..
Some examples are provided here from the BGS core photograph database - from 2 wells in the Britannia Field (Outer Moray Firth) and 1 well from Shell’s Goldeneye Field in the same Lower Cretaceous Sand fareway.
The large aperture fractures in Fig. 1. are particularly striking. Their interaction with the rock fabric is interesting - note how the clay partings that run sub-parallel to the bedding appears to have affected the development of a fracture (red arrow on Fig. 1). These fractures are also quite sinuous in appearance (just above “DTI cut” text in Fig 2a and in Fig. 2b).
Given that these are observations from BGS photographs, we would need to visit the core store to carry out a more detailed analysis on the core itself.
Fig. 1. Induced fractures ? in sandstone in Britannia well 15/30-13
British Geological Survey © UKRI
https://largeimages.bgs.ac.uk/iip/index.html?id=20120104/S00156619
Fig 2. Induced fractures ? in sandstone in Britannia well 15/30-13 from 14516 - 14522 ft. Upper: red box shows location of Fig 1. Arrows show way up.
British Geological Survey © UKRI
https://largeimages.bgs.ac.uk/iip/index.html?id=20120104/S00156619
https://largeimages.bgs.ac.uk/iip/index.html?id=20120104/S00156620
Some induced fractures have distinctive geometries (e.g., petal, disc)
There is quite a regularity to the pattern in Fig. 3 which is from sandstone core in another well from the Britannia Field. It does look like sections through common petal fractures which are induced by the weight of the bit on the rock formation during coring. There are some useful diagrams on P85 of Lorenz and Cooper [1] to help with this. A discussion thread on the original post on LK mentionned that these are often (locally) called “pringle” fractures given their distinctive shape and that they may have resulted from hammering of the core barrel to free the core.
Alternatively, these could be fault-related (i.e. natural) which I think is unlikely given that Britannia has few natural (open) fractures and I wouldn’t expect this type of fracture pattern in that case. Note also how quickly the fracture zone transitions into an undeformed host sandstone (Fig. 3) which is a further line of evidence.
The examples (Fig.4) from the Goldeneye Field do not have the “pringle” geometry (in Fig. 3). Instead they are similar to those from Britannia in Fig 2. They are also found in correlatable Lower Cretaceous Sandstones across these fields, suggesting the induced fractures are common at this level. The turbiditic Captain sandstone is the target storage reservoir for C02 injection in the Acorn CCS project.2
Fig. 3. Petal fractures ? in sandstone in Britannia well 16/26-B4 from 13768 - 13780 ft.
British Geological Survey © UKRI
https://largeimages.bgs.ac.uk/iip/index.html?id=20111111/S00113007
https://largeimages.bgs.ac.uk/iip/index.html?id=20111111/S00113008
Fig 4. Induced Fractures in Lower Cretaceous (Captain Sst) sandstones, Goldeneye Field, Outer Moray Firth. Well = 14/29A-5.
In Conclusion..
It is important to separate out induced fractures from a natural fracture description as they do not contribute to reservoir permeability [1]. However, this is not always easy although there are certain types such as petal fractures, “pringle” fractures and disc fractures that are fairly common.
References
[1] Lorenz, J.C, Cooper, S.P. 2020. Applied Concepts in Fractured Reservoirs. Wiley, Blackwell