In overbalanced drilling conventional well control is affected by using a hydrostatic pressure greater than the pore pressure. In under balanced drilling this differential is removed and bottom hole pressure is maintained at a value less than the pore pressure. Therefore there is a continuous "well control situation" during the under balance section.
To do this requires some form of rotating control device, drilling chokes, specialised fluids and different operating techniques.
Under balanced drilling is still, despite its increase in use, a form of drilling which is still an unknown to many.
On average only 10% of all wells drilled have some section drilled under balanced.
Until the last few years under balance operations were seen as a form of drilling specifically undertaken to overcome unique problems. These were often localised which has led to pockets of expertise being formed. As with most problems, there are often multiple solutions. Similar problems have often been addressed by overbalanced techniques, which although costly, were often accepted as part of legitimate drilling expenditure.
With today's economic climate, all means of creating a better return on investment are being explored and, as a result, under balance technology is moving to the forefront of drilling.
The areas where under balanced technology has provided its best returns so far have been in dealing with
Reducing "skin" damage
Drilling rate enhancement
Of these categories it is only when produced hydrocarbons are brought to surface that the hazards increase noticeably.
There have been situations where UBD has been used to address some of the above areas and has failed to produce the desired returns.
The causes of these failures can be categorised under the following headings.
Poor candidate selection
Mishandling of hydrocarbons and drilling fluids on surface
From even a cursory examination of the causes, providing appropriate training could be identified as part of a tangible solution for each cause.
When constructing a training programme, an important step is to establish the existing level of knowledge of the students and recognise any preconceived points of resistance to the subject matter.
The following points were identified during the development work that was undertaken for R.S.T.S under balanced drilling and well control courses.
It must be recognized that preconceived ideas based upon supposition must be considered as possible barriers and should be dealt with as factual until proven otherwise. Providing training will in most cases dispel these.