Open hole logs

Open hole logs

Fracture are important in hydrocarbon production as they are the source of high permeability in some reservoir rocks that have low matrix permeability to produce without fractures. Fractures may contribute only from 0.5% to 1.5% porosity towards the total porosity of the reservoir. This figure is too low to be detected with the help of porosity logs. It is difficult to detect fractures in shale beds.

There are two types of fractures:

1.    Natural - existed before drilling

2.    Induced – created by drilling of the borehole

The natural fractures are often short  in length (inches to feet) and may be vertical or horizontal. In this case the fractures are assumed to be open and capable of carrying fluids, sealed fractures may also be detected but they have no commercial importance.

The induced fractures due to drilling are generally vertical. These fractures may contribute to production if they join up with an existing fracture and jointing system in the rocks.

Fracture identification:

1.Direct Identification Techniques:

The fracture can be directly detected in the cores, by photographic systems or impression packers. In the photographic technique, a camera and a lighting system is lowered in the hole. It requires clean borehole liquid or no bore hole liquid.

Impression packers require significant amount of time to obtain data. The packer is run with the help of drill pipes and set opposite the zone where the impression is required. Each packer will cover only few feet and hence several packers will be required to be lowered to get complete impression of the thick reservoir rocks.

2.Identification with the help of well logs

Mostly acoustic method is employed for detection but resistivity measurements may also help to detect fractures.