By Joanne Liou, associate editor
In the Brookeland Field in East Texas, conventional drilling methods failed in six attempts to drill a “straightforward” horizontal wellbore. The original well had surface casing installed at approximately 3,500 ft. Intermediate casing with a downhole deployment valve was run to begin drilling the lateral at 11,185 ft. “The original plan was to do underbalanced drilling (UBD) with this well. They drilled to 14,840 ft, and at that point, they ran a survey and found out they were crooked,” George Medley, Executive Vice President, SIGNA Engineering Corp, explained. The operator then decided to drill a sidetrack, but six attempts to do so failed using conventional drilling techniques. SIGNA was then brought into the project to drill the sidetrack using managed pressure drilling (MPD) and UBD, Mr Medley said at the 2014 SPE/IADC MPD and Underbalanced Operations Conference in Madrid on 8 April.
The MPD/UBD plan for the seventh attempt to drill a sidetrack first involved evaluating the prospect. “We really thought initially that MPD might be a good application due to the high-pressure regime,” Mr Medley said. The well had approximately 13,900-psi reservoir pressure and a bottomhole temperature of 334°F. “Our primary goal, as with all wells, is to reach geologic objectives in a safe manner with the tools that we have available.” The secondary goal is to determine downhole pressure environment limits, he added.
The MPD equipment setup included a rotating control device, a flow meter, an automatic hydraulic choke and a manual hydraulic choke monifold. Based on well history, a contingency mud-gas separator was also rigged up. “In this case, one of our contingencies was UBD,” Mr Medley noted.
While drilling the sidetrack with MPD, fractures were encountered, which resulted in multiple influxes. The mud weight was increased from 17.9 ppg to 18.7 ppg, “at which point there were some lost-circulation events. As you increase mud weight, you’re going to increase circulation system pressure.” Because of differences in pressure throughout the reservoir, “we found that you couldn’t do conventional constant bottomhole pressure MPD, which made the underbalanced contingency a lot more attractive.” SIGNA then switched to UBD.
A pressure-while-drilling tool was used to determine what was happening downhole. Some of the fracture zones had as much as 1 ppg higher pore pressure than other fractures, Mr Medley explained. “If you have a ppg higher pore pressure, you’re going to be seeing some lost circulation if you try to squelch all that flow.” Using UBD, mud weight was kept at 17.8-18 ppg using backpressure on the choke to try to maintain approximately 18.5-ppg bottomhole pressure (BHP). “We were not looking to be .5 ppg underbalanced. We still want to limit influxes as much as possible without breaking down the hole. Both of the MPD and the UBD that we were going to employ uses the same kind of choke manifold, so we didn’t have to change any of that equipment.”
Standard MPD was used on connections, and all trips were made in a balanced condition. “We would rate down the pumps and increase backpressure. Then, after we finish the connection, decrease backpressure and ramp up the pumps,” Mr Medley explained. “We replace circulating friction with backpressure just like we do with MPD. Whatever BHP we had while we’re drilling, we try to maintain for that connection.”
Although MPD was not 100% effective for the operation, it helped determine BHP limits and where to set pressure to prevent excess influxes. UBD increased penetration rate. Being able to use a combination of MPD and UBD and allowed “us to either circulate gas out, or if that circulation was going to result in too high of backpressure and too high of a surface pressure, we could just bullhead the gas back in,” Mr Medley said. “That’s one good thing about fracture formation; it can be very forgiving,”
SIGNA was able to drill the sidetrack on the seventh attempt with the MPD/UBD combination and reach the operator’s target. “It was really helpful to us to have one choke that we could move to a closed position and know it’s going to be closed,” Mr Medley stated. A thorough engineering plan and complete contingency plan included a lot of things we didn’t believe would happen but did anyway. They were good to have.”
For more information, please see SPE/IADC 168944, “Successfully Drilling Sidetrack #7 With MPD/UBD Combination After Six Failed Conventional Drilling Attempts.”