Types of Perforations

In most wells, additional restrictions are imposed which can cause additional pressure drop between the average reservoir pressure and the well flowing pressure. One of such restrictions is perforations.

Perforations allow selective exposure of the rservoir rock to the well bore. After the well is drilled, and cased, using a perforation gun, the casing and the  cement behind the pipe is perforated to expose the reservoir to the wellbore.

The primary purpose of perforating the well is:

        To prevent water or gas coning

        To selectively produce from different zones within a well bore

The first purpose is accomplished by perforating the wells away from either water-hydrbocarbon contact, or gas-hydrocarbon contact. The second purpose is accomplished by perforating the well selectively such that only certain hydrocarbon-bearing zones will produce into the wellbore.

Three types of perforating techniques are used to perforate the wells: underbalanced perforations, overbalanced perforations, or extreme overbalanced perforations.

Underbalanced Perforations

Underbalanced perforations involve using less pressure within the well bore than the reservoir. In this case, as soon as the well is perforated, the reservoir fluid surges into the wellbore. This allows the clean-up of fines which are c reated as a result of the perforations. However, underbalanced perforations, in many instances, require the use of smaller guns which means smaller holes and shallower perforations. Underbalanced perforation is alos riskier if the reservoir pressure is not well understood.

Overbalanced Perforations

Overbalanced perforations involve maintaing a higher pressure in the wellbore than in the formation. As soon as the casing and cement is penetrated, the fluid surges outside. The advantage of overbalanced perforations is that larger guns can be used to perforate the well. It is also safer. The disadvantage is that the fines are lodged into the reservoir, which may adversely affect the pressure drop across perforations.

Extreme Overbalanced Perforations

Extreme overbalanced perforations has emerged in a last decade. In this method, a well with fluid column is pressurized with nitrogen to above fracturing pressure of the formation before gun is fired. Gas is injected, with or without acid or proppants, to create short fractures that extend from perforation tunnels. This procedure is found to be effective for low pressure and/or low permeability formations. The extreme pressure force acid deeper benefiting the wells with deep damage.