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Fobex 12: “The good, the bad and the ugly: Fractured Basement Reservoir”

Nowadays, the most energies needed in the world are still oil and gas. The demands that remain high with the reserves that declining time by time makes oil and gas becoming more and more difficult to be found. On the first phase of exploitation, many wells only produce shallow reservoir which usually contains huge volume of oil and gas. The declining of the production rates insist to drill deeper in the previous wells to gain new reservoirs that have bigger hydrocarbon reserves.

Generally, well production rely on thick reservoir that ideally found in the log data. However, those ideal reservoir that have already been produced for more than 30 years getting declined dramatically on pressure and production rates. When the reserves of hydrocarbon are becoming harder to find, a geologist induced to keep looking, and sometimes things that are impossible become possible.

Usually, oil and gas companies will stop the drilling when they already found granite or any other crystalline rocks as the basement. Although they found naturally fractures that contain hydrocarbon, those things are assumed as a non-economic reservoir to produce hydrocarbon. A few years ago, the interest of Fractured Basement is increasing when a filed in Yemen, offshore Vietnam and other places have been successfully producing oil in a significant number.

Until now, the potential of Fractured Basement Reservoir is looked as a new prospective thing to be more explored. It is also supported by some cases on the basement field that found actively produce hydrocarbon in recent decades.

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Picture 1. The location of fractured basement reservoir in the world.

A fracture can be detected by using log image. A fracture not always can be identified using core, because other than economic reasons, it is also possible a new fracture can be created while acquisitioning the core, so that the using of image log to identify in situ fracture is very useful. On the other hand, in some cases, it is still found a difficulty to identify the fracture. Those difficulties are depend on the sequences and litology factors. For example on the sequence sand-shale, the respond of sedimentation is more dominant rather than in carbonate rocks, when fracture is relatively easier to be identified.

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Picture 2. Graphic of relationship between fracture permeability and width of fracture and the calculation of permeability in fractured reservoir, when e = width of fracture, D = distance between two fractures that are parallel (Nelson, 2001)

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Picture 3. Graphic of relationship between fracture porosity and width of fracture and the calculation of porosity in fractured reservoir, when e = width of fracture, D = distance between two fractures that are parallel (Nelson, 2001)

 

It can be seen from the picture 2 and 3 that the graphics of relationship between permeability and porosity of fractures with fracture width mean that the width of fractures are important to find the number of permeability and porosity.

 

In conclusion, fractures that created by the process of drilling are often found in the cores and those fractures also have characteristics that differentiate with the fractures that naturally created, although it is really hard to differ them. Other than core, there are also other parameter that can identify fractures, but still, an accurate calculation of the effect of drilling is a must.

 

Created by: Putra Herianto

References:

Nelson, R.A. 2001. Geologic Analysis of Naturally Fractured Reservoirs Second Edition. Boston: Gulf Professional Publishing

 

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