A two-year, nine-well MPD campaign in the Gulf of Thailand resulted in dramatic improvements in nonproductive time and overall drilling performance, reports a paper presented at the 2010 SPE/IADC Managed Pressure Drilling & Underbalanced Operations Conference, 24-25 February in Kuala Lumpur.
The paper was presented by Paul Fredericks, At Balance with Smith, who stressed that this was not a case of MPD drilling “undrillable” wells. “The operator had good drilling practices and kept the hole clean,” Mr Fredericks said. “It was the time that it was taking to keep the well clean with good equivalent circulating density (ECD) that was the issue.”
MPD was used to drill through a depleted zone and to reduce NPT, and therefore cost, compared with previous operations drilled conventionally.
“In the conventional scenario,” Mr Fredericks said, “the mud weight was 10.5 ppg.” MPD allowed the operator to decrease the mud weight to 9.0, a 1.5-ppg savings.
“Herein lies an important point for managed pressure drilling – that MPD is a true enabling technology,” he added.
As a consequence of the reduced mud weight, the ECD fell from 12.5 ppg to 10.59 ppg. The 2.0-ppg difference was critical. Any higher than 11.9 and the ECD would’ve exceeded the fracture gradient. Mr Fredericks said that drilling was possible without losses because ECDs were below 11.3 ppg. However, this choked performance, requiring cutbacks on ROP and flow rate, restrictions on string rotation speed, and use of high pV and solids concentrations.
And, importantly, each connection required two to three back-reaming operations.
Still, on the first MPD well, back-reaming was required. “In any long-term MPD project, there is a learning curve,” Mr Fredericks said.
By the fifth well, the operator decided to drill out of the 9 5/8-in. shoe with MPD rather than the conventional approach. This resulted in 800 m drilled in two days, or 400 m/day, compared with less than 300 m/day on a conventional offset.
Mr Fredericks noted that back-reaming was still required; however, this fell to twice per connection, rather than three times.
Due to a problem with a seal, the operator chose to switch to conventional operations in the 8-in. section. Resulting ROP fell dramatically to 87 m/day.
ECD profiles also changed for the better with the use of MPD. The ECD spiked significantly (approximately 1 ppg peak to peak) in conventional drilling, due to swab and surge caused by back-reaming. In the first MPD well, using 9-ppg mud and a higher flow rate, ECD was flat at 11 ppg. In the seventh well, ECD ranged between 10.5-11 ppg. Surge and swab was experienced, but less back-reaming was required.