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A dynamic model for underbalanced drilling has been developed in a joint industry project. The model is of use for planning an operation, and especially evaluations of dynamic pressure and flow effects. Use of the model can help prevent excessive pressure transients during operations, which will have negative effects on well productivity. Experiments have been performed in a vertical, extensively instrumented, test well. The experiments focused on dynamic operations; such as kick-off or unloading, changes in liquid and/or gas injection rates and pipe connections as well as "steady state" periods. The tests were performed with either"parasite string" gas injection, or drillstring gas injection. Simulations with the dynamic model has been performed, using experimental quantities as input (geometries, liquids, rates etc.) Results show that the model performs very well for "parasite string"gas injection. For drilling gas injection system, the model
This paper was prepared for presentation at the 1999 SPE Asia Pacific Oil and Gas Conference and Exhibition held in Jakarta, Indonesia, 20–22 April 1999.
Abstract Underbalanced drilling is quickly becoming an important technology in the Canadian oil and gas sector. While drilling impairment has always been a concern, a significant increase in horizontal completions has brought this issue to the forefront. A number of stimulation techniques are available for overcoming impairment in vertical wells or shallow damage in horizontal wells, however, deeper matrix damage is often difficult to remove in long horizontal sections. With this in mind, Canadian producers have looked to underbalanced drilling to prevent damage caused by fluid leak-off and fines migration. Many of the oil and gas fields in Canada are subhydrostatic, therefore underbalanced drilling operations often require the entrainment of a gas phase in the drilling fluid to generate the appropriate bottomhole pressures. In most cases, because of safety concerns, nitrogen gas is used. Two basic styles of nitrified underbalanced drilling are currently used with conventional rig drilling operations. One involves injecting nitrogen at the standpipe to co-mingle with the drilling fluid. The other, referred to as parasite injection, utilizes an external gas injection conduit that allows gasification of the drilling fluid in the vertical annulus. Either approach has specific advantages which must be considered for each application. An essential component of underbalanced drilling (UBD) success is effective two-phase flow modeling. Initial simulation is required to investigate and optimize various design options in the planning stage. P. 507
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.
Dynamic UBD Simulator; DYNAFLODRILL.
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.