Inwell has recently drilled three-directional wells for a client in the Four Corners area of New Mexico from a small drilling pad located in the southwest corner of the lease, a natural area of significance to the local population as well as hikers, campers and other visitors. The acreage is under the protection of the US Department of the Interior, Bureau of Land Management, who has decreed that all surface facilities, including access roads, be located within a very small area that is hidden by the terrain. As a result, the client designed a small footprint drilling pad from which all wells will be drilled. To drain the reservoir effectively, the wells will step out as much as 2,500 ft (762.5 m) from the surface location. Wells will be drilled in an "S" profile with a vertical section, followed by a long ramp that leads from directly beneath the surface location to directly above the target. Lastly, another vertical section is drilled through the target reservoir. For the first seven wells the target is the Fruitland Coal. Three wells have been drilled to date and the program has shut down for the winter. Drilling will resume in April 2005. A typical well is 3,500-ft (1,067-m) true vertical depth (TVD). It steps out 2,500 ft (762 m) and has a total measured depth of around 4,500 ft (1,372 m). The ramp section builds from 5° to 45°. The Fruitland Coal overlies another target, the Dakota Sandstone formation, lying about 8,000 ft (2,439 m) below the surface, which will be a secondary objective from the drilling pad once the coalbed methane wells are drilled and completed. Both formations are gas plays.

What makes this different?

With the exception of the environmental restrictions, the project sounds like hundreds of others. In fact others drilling in the area have pushed down their vertical wells in as little as 31/2 days. The requirement to drill deviated outreach wells from the pad added about 5 days to the drilling curve, but still the specification was not unusually onerous.

"The technology we used is not new per se, but it's new to the San Juan Basin," said the company's drilling engineer. The objective was to use measurement while drilling (MWD) to geometrically steer the wells to target, and drill using an adjustable bent sub and mud motors. "Typically, we drill with an 83/4-in. rock bit to the top of the coal," she said. "Then we set our 7-in. casing there and drill the target formation using a 61/4-in. bit to make a pilot hole, followed by an underreaming tool that opens the hole to 91/2-in." She explained that the wells are completed barefoot in the coal and most do not require dewatering before production can commence.

Two possible problem areas existed. First, the operator was concerned that drilling the Fruitland Coal on anything other than a vertical trajectory might cause wellbore stability trouble or subsequent completion/production problems if coal sloughed off the walls of the well bore. Accordingly, it was deemed essential to penetrate the target formation vertically. Second, there were several unstable zones above the target where severe loss of circulation could occur. The occurrence of these zones was known, but their locations were somewhat unpredictable, so drillers had to be able to react quickly in case circulation was lost. Based on experience from more than 30 wells previously drilled in the area, engineers knew they might have to mix and pump up to 80 lb/bbl lost circulation material (LCM) to plug severe loss zones.
Responsible for directionally drilling to the target zone, the company recommended an instrumented drilling assembly from NQL Energy Services called Black Max. "The principle reason for using this equipment is that it uses electromagnetic (EM) telemetry instead of traditional mud pulse telemetry," explained Steve Allums, Inwell's President. Downhole data are immediately transmitted to surface using the electromagnetic telemetry link, and the high bandwidth signal allows all data to be transmitted in real time. The EM receiver is located at the rig, but in cases of very deep or extended reach wells a remote antenna can be placed to detect the signal from the downhole tools.

Mud pulse telemetry rapidly opens and closes a valve in the mud column to create a series of coded pulses that propagate drilling and logging data up the mud column to a surface sensor. It has two main disadvantages: First, mud pulse telemetry does not work in air, mist or nitrified muds, nor does it work when circulation is broken to make a connection. Second, few mud pulse telemetry subs can tolerate high volumes of LCM. Inwell didn't want to take the chance that they would lose vital information just at the time they needed it most.

"Mostly fiber and some drilling paper is used for LCM, because it's easy to mix and pump given the limited facilities onsite," said Allums. "The last thing we want is for the LCM to ball-up and plug off in the MWD tools."

Fortunately, once the well has been drilled and the 7-in. casing set, they can drill out and underream using an air mist system because there is no danger of a pressure influx. Even though the coal is inherently a "lossy" zone, drilling with mist instead of mud precludes loss of circulation at that point. The main purpose of the chosen technique is avoidance of unexpected loss of circulation during drilling and casing of the main well bore above the casing point.

Hole quality is a factor

"We have been very pleased in two specific areas of well construction," emphasized the drilling engineer. "These are accuracy in hitting the target and quality of the directional borehole." These are key factors in cost control and production performance. Production from the three wells drilled to date ranges from 2.0 MMcfg/d to 0.8 MMcfg/d. Rig personnel, along with Inwell, have learned quickly how to deal with the trouble spots. After experiencing some problems getting the first well cased all the way to total depth, they were able to quickly analyze the situation and correct it for the subsequent wells by adding gel to the mud. Because they had plenty of good data from the first wells, the learning curve was very steep. The company was able to take an analytical approach to solving problems that came up as well as improve operational efficiency. "I feel confident that I have all the time-relevant data I need to make the onsite drilling decisions that result in a smooth hole that is on-target," she said.

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