Underbalanced and lightweight drilling is very promising method to solve numerous drilling problems in depleted reservoirs, mature fields as well as deep-water regions. Also, maintaining underbalance during the whole drilling operation will prevent or reduce formation impairment in various formation types. In order to properly design an underbalanced operation, good control of the downhole pressures is necessary. A technology programme to develop a dynamic underbalanced drilling simulator, for proper design of operations has been undertaken. This work has included laboratory experiments, modelling achievements as well as full scale testing. Comparison of full scale data as well as field data with simulations show that the simulator predict very well the steady state downhole pressure conditions for most gas - liquid rate combinations within the realistic operational range. Also, transient pressures during rapid operational changes can be predicted well.
In parallel, an innovative separator system and methodology for utilising UBD techniques in offshore operations have been developed.
These technologies have been utilised in an offshore environment, for the purpose of developing lightweight drilling offshore.
This paper will present the various technology elements developed (dynamic simulator; separator system and methodology for lightweight drilling) and present verification tests and field applications of these in an offshore environment.
Underbalanced drilling has been increasingly used to address many field and operational problems during the last years. Some of the advantages of applying underbalance drilling are to reduce formation damage, avoid lost circulation, minimise differential sticking and increase the rate of penetration and bit life.
Successful UBD depends on a complete understanding of the reservoir as well as proper design and computer modelling of the complex multiphase flow system1. Dynamic pressures2 should be evaluated using an advanced transient model; and procedures to minimise such effects developed. However; such a model had to be developed and verified, in order to meet the needs of complex operational scenarios and fluid systems3.
Most UBD operations have so far been conducted onshore. However, a few operations have been performed offshore from barges (Lake Maracaibo, Venezuela) and jack-ups (Southern North Sea). Since rig heave is not pronounced, no special modifications were necessary to perform the operations.
Offshore wheather conditions however, are much more critical than in a lake, and therefore a number of challenges needed to be addressed and overcome to apply UBD technology from a floater4. Of special importance was the need to develop a more compact, efficient and cost-effective separator system.
These technology elements have been tested and implemented in a real operation in the Campos Basin offshore Brazil.
In the following sections the development of the technologies as well as field applications will be described.
The need of a dynamic design tool for any aspect of the hydraulic, multi-phase operation during underbalanced drilling, was identified early. A R&D programme to develop such a tool was defined in co-operation between Rogaland Research and several major operators, including Petrobras.