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FORMATION EVALUATION

In petroleum exploration and development, formation evaluation is used to determine whether a potential oil or gas field is commercially viable. Essentially, it is the process of "recognizing a commercial well when you drill one".

Lead –in

How can you determine a well to be a commercial one?

What are formation evaluation tools?

What is mud logging?

What is coring?

Terms and Vocabulary

1. Read the text “Coring” and do the exercises.

Coring

One way to get more samples of the formation at a certain depth in the well is coring. There are two techniques commonly used at present. The first is the "whole core", a cylinder of rock, usually about 3" to 4" in diameter and, with good luck, up to 50 - 60 feet long. It is cut with a core barrel, a hollow pipe tipped with a ring shaped, diamond chip studded bit that can cut a plug and retain it in a trip to the surface.

If no shales or fractures are encountered, the full 60 foot length of the core barrel can be filled. More often the plug breaks while drilling, usually at the aforementioned shales or fractures and the core barrel jams, very slowly grinding the rocks in front of it to powder. This signals the driller to give up on getting a full length core and to pull up the pipe.

Taking a full core is an expensive operation that usually stops or slows drilling for at least the better part of a day. A full core can be invaluable for later reservoir evaluation. One of the tragedies of the oil business is the huge amount of money that has been spent for cores that have been lost because of the high cost of storage. Once a section of well has been drilled, there is, of course, no way to core it without drilling another well.

The other, cheaper, technique for obtaining samples of the formation is Sidewall Coring. In this method, a steel cylinder — a coring gun — has hollow-point steel bullets mounted along its sides. These bullets are moored to the gun by short steel cables. The coring gun is lowered to the bottom of the well and the bullets are fired individually as the gun is pulled up the hole. The mooring cables ideally pull the hollow bullets and the enclosed plug of formation loose and the gun carries them to the surface.

Advantages of this technique are low cost and the ability to sample the formation after it has been drilled. Disadvantages are possible non recovery because of lost or misfired bullets and a slight uncertainty about the sample depth. Sidewall cores are often shot "on the run" without stopping at each core point because of the danger of differential sticking. Most service company personnel are skilled enough to minimize this problem, but it can be significant if depth accuracy is important.

Cores are cut where specific lithologic and rock parameter data are required. They are cut by a hollow core barrel which goes down around the rock core as drilling proceeds. Cores are preferable to well cuttings because they produce coherent rock. They are significantly more expensive to obtain, however.

A more serious problem with cores is the change they undergo as they are brought to the surface. It might seem that cuttings and cores are very direct samples but the problem is whether the formation at depth will produce oil or gas. Sidewall cores are deformed and compacted and fractured by the bullet impact. Most full cores that are taken from any significant depth expand and fracture as they are brought to the surface and removed from the core barrel.

Coring supplies intact specimens of the formation. It is the only method of making “direct” measurements of rock and fluid properties. This means that core samples are one of the most valuable sources of data for the study of subsurface rocks and reservoirs. Therefore, coring is a vitally important method of obtaining data for geologists, drilling engineers, petrophysicists, and reservoir engineers.