A new coring technique allows operators to obtain conventional size cores from target zones after a well has been drilled to total depth. Conventional cores are normally taken as the bit drills ahead, before the well is logged. Sidewall cores are taken after a well has been drilled and logged, but core recovery may be low.
One drawback of sidewall cores is their small size (1 1/2-2 in. diameter x several inches long). They may not extend beyond the skin of the well bore. In contrast, radial coring can drill the cores at an angle away from the well bore where part of the core may not be infiltrated with drilling fluid.
The radial coring system essentially consists of a conventional coring assembly run on small diameter pipe inside another string of drill pipe (the carrier string). After an open hole log has been run in a well, an up-stock is picked up and run in the hole on the carrier string. The upstock is a 12 ft long tool with a slight bend and a modified whipstock. The upstock is thrust into the side of the well bore, becoming wedged in place.
The carrier string is typically large diameter drill pipe, but almost any size drill pipe or casing can be used providing it is within practical limits of the well bore diameter.
For most coring operations, the orientation of the tool is not a concern. However, a wire line steering tool can be used first to position the upstock into a particular direction if the orientation is critical.
BLACK WARRIOR BASIN
Gardes Directional Drilling used radial drilling technology to core two coal bed methane wells for Meridian Oil Inc. in the West Cedar Grove field in the Black Warrior basin near Tuscaloosa, Ala. Radial drilling technology is typically used to drill a number of lateral extensions from the main vertical or horizontal bore of a well.
The first well, Findlay 17-13, was drilled conventionally to about 3,570 ft total depth. The second well, Morrison 15 10, was drilled conventionally to about 3,743 ft total depth. The 12 1/4 in. surface holes were first drilled to about 300 ft, and then 8 5/8 in. casing was run and set. The wells were drilled vertically with a 7 7/8 in. bit down to total depth.
On previous wells in the area, Meridian rigged up a core rig after surface casing was set. The core rig would core from the surface casing point to total depth. The elimination of these steps helps save thousands of dollars.
After a well was logged with electric line, the upstock was picked up and run in the hole on 4 1/2 in. X-hole drill pipe. The drill pipe was first drifted with a 3.14 in. rabbit to ensure that the 3.032 in. core bit and coring assembly could pass through. The upstock was then anchored into the borehole wall at the appropriate depth.
The coring assembly was picked up and run in the hole on 1 1/2 in. coiled tubing. The bottom hole coring assembly consisted of a 3 in. core bit, a 2 7/8 in. x 1 7/8 in. x 10 ft core barrel, a 2 3/8 in. x 12 ft positive displacement mud motor, and a series of crossovers to connect to the 1 1/2 in. coiled tubing.
The coiled tubing and coring assembly were run inside the carrier drill pipe. After a 10 ft core was cut, the coring assembly was tripped out of the hole in about 30 min. The use of coiled tubing as the drilling string reduced tripping time considerably. The upstock was then moved to a new target and set in the wall. Another coring assembly was made up and run in on the coiled tubing to repeat the process.
Three cores were retrieved from each well.
The cored holes did not interfere with the casing cement jobs during completion. The cement bond logs indicated good bond across the cored zones.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
