Continuous Circulation Systems (CCS) have been used in the past to help drill wells where interactions between the mud weight and theformation may increaserisks, thereby allowing the well to be drilledwithout encountering drilling problems or damaging the formation, whilst reaching total depth in the planned time. Previous systems have,however,relied on people working in the red zone. This paper discusses the process of making drill pipe connections, with continuous circulation,utilising a fully automatic deployment system to add value by removing the risk of people around the drill pipe during the connection, working in conjunction with other automated rig systems todeliver the advantages of continuously circulation over each connection.
The continuous circulation subs are installed in every stand of drill pipe to be used in the drilled interval and facilitate circulation while the next stand is picked up and made up to the stump. A valve manifold is utilised to divert flow from the pumps to the subs, instead of the top drive, during the connection. During each connection, circulation is maintained down the drill string, from bit to surface, at drilling rates. Once the connection has been made the continuous circulation surface equipment is disconnected from the drill string allowing drilling to resume.
After deploying the continuous circulation system on a number of geothermal projects, the results of using the system for top hole and intermediate sections suggest that while drilling with low rheology water-based mud systems,a high percentage of cuttings are returned to surface while the next stand of drill pipe is being picked up,limiting any hole loading and allowing the driller to increase the rate of penetration (ROP) through these sections. Fewer hole collapse issues were observed while drilling through volcanic tuff and ash, where wellbore stability is low due to poorly consolidated rock formations, thereby reducing non-productive time associated with stuck pipe and the costs of lost bottom hole assemblies (BHA's) and sidetracks. Most Geothermal projects work on very tight budgets and geothermal exploration costs need to be kept low. Improved drilling performance has improved the viability of these projects and increased the number of exploration wells that can be drilled in a campaign by reducing the days versus depth P90 estimate, that being the 90% probability of the rig matching the days v's depth curve predicted.