Normal drilling operation is carried out with the hydrostatic head of fluids at bottomhole above the reservoir pressure.

The main reason is to maintain safety of the operation and to prevent production or blow- out during the various stages of the drilling operation. For some fields, this mode of operation may lead to lost circulation and damage to formation.

Under the underbalanced drilling (UBD) operation, the pressure of the fluids at bottomhole is slightly below the reservoir pressure. As a result, some production from the formation may also occur. The drilling and the associated process system should therefore be able to cope with all expected flow conditions such as gas kick and variations in the wellhead pressure.

A few years ago Caltec were approached by a drilling company interested in using the patented compact separation system, known as I-SEP/HI-SEP as part of their UBD system. The main duty of the compact separator was to operate close to wellhead, upstream of the choke package, and separate bulk of the produced gas at full wellhead pressure. The separated liquid phase and drilling cuttings would then pass through the choke system for further processing and treatment at a lower pressure, using the general UBD process and separation system. The unit was also required to be rated to full highest expected wellhead pressure of 5,000 psig.This unit was given the nick name of "THE GAS BUSTER". Flowrates and the operating conditions which the unit was designed for is shown in Table1 in the paper.

The initial tests and trials of the Gas Buster were carried out in three stages which included half scale laboratory tests, full scale testing of the unit with drilling mud, solids and compressed air at 10 barg in the UK and at up to 1,000 psig pressure in Canada.The unit was then transported offshore for its maiden trials as part of the UBD system on a platform in the Southern North Sea.

This paper describes the key features of the "Gas buster" and the tests carried out. The system was exposed to unexpected high gas flow rates, almost double its design capacity during the offshore trials, but still operated satisfactorily. However, as a result, some erosion of up to 6mm took place within some parts of the internal components of I-SEP.

The control of the system is the most important part of the operation as in addition to controlling the drilling operation and setting of the choke valves, satisfactory operation of the gas buster and maintaining the UBD mode was also required.The trials helped to improve the control of the system and the design of HI-SEP to cope better with gas cick conditions.

The benefits of using the Gas Buster are highlighted and include; ability to utilize the produced gas and prevent venting or flaring it, ability to continue drilling uninterrupted even under unexpected gas kick or similar surge conditions, Preventing the chokes to be eroded quickly, and protecting the downstream UBD process system to be exposed to extreme operating conditions which may lead to interruption to continuous drilling operation.Saving in drilling time is also expected.

You can access this article if you purchase or spend a download.