Drilling and Early Testing of a Sidetrack to the Slant-Hole Completion Test Well: A Case Study of Gas Recovery Research in Colorado's Piceance Basin

ABSTRACT During the summer of 1990, a slant hole test well, funded by the U.S. Department of Energy, was drilled to 9,466 ft to evaluate the effectiveness of directional drilling in the tight, naturally fractured gas sands and coals of the Mesa-verde Group. Mechanical problems following cementing of a production liner resulted in loss of the completion interval, and operations were suspended. In early 1991, DOE decided to sidetrack the hole to permit production testing of the lost interval. The sidetrack was designed to parallel the original wellbore, but to be drilled 1,000 ft to the east to minimize the chances of encountering formation damage from the original hole. The sidetrack, like the original hole, was to intersect the paludal lenticular sands and coals at 60° and to penetrate the underlying Cozzette sand horizontally. This paper discusses the directional drilling operations and problems and the subsequent production testing of the horizontal portion of the sidetracked well. Sidetrack drilling was initiated at 4,162 ft in the original hole. All subsequent depths are given as measured depth. An 8-3/4-in. hole was drilled at 11 to 16° inclination, on a complex azimuth starting at S 71 °E at the first KOP at 6,550 ft, ana ending at Ν 13°E at the end of the first build at 7,309 ft. Drilling proceeded satisfactorily with 6-3/4-in. motors, until a bit and stabilizer were lost at 6,708 ft. Fishing operations created a 15°/100 ft dogleg, and subsequent efforts to ream out the hole resulted in sidetracking the sidetrack. The hole was plugged back to 6,450 ft and was then successfully redrilled with motors to the top of a long tangent section at a 57° inclination on an azimuth of Ν 14.6°E. A 1,230 ft tangent section was rotary drilled at 60° through the paludal Mesaverde, the Rollins sand, and the Mancos Tongue shale at 8,588 ft. The tangent was logged using drillpipe conveyed tools; the well was cased to 8,588 ft and cemented to isolate the tight target sands and preserve caving coals. A 3-1/2-in. drillpipe was picked up to drill a 6-in. hole to final depth. The second build to near-horizontal was drilled using 4-1/2-in. motors to 9,051 ft, at an angle of 86.5° and an azimuth of Ν 10°E. The Cozzette lateral was then rotaried to 9,308 ft, using an underbalanced mud weight of 14 ppg to prevent formation damage to natural fractures. A brief but quickly controlled blowout, resulting from inadvertently swabbing the well in during a trip, then forced the use of 16+ ppg mud over the rest of the hole. Significant mud losses during the last footage in the Cozzette lateral resulted in drilling being terminated at a total depth of 9,407 ft. A permanent packer with a landing nipple was set at 8,453 ft, just above the 7-in. casing shoe. The well was flowed through the drillpipe to remove as much mud as possible from the Cozzette fractures. It then was briefly tested at very high, but unmeasured, gas volumes from the Cozzette open hole before being shut in to await the formal testing program. After re-entering the well in late 1991, early production testing of the Cozzette showed that the 300 ft of in-pay horizontal hole can produce at rate 5 to 10 times higher than vertical wells in the same area. Well testing and results will be presented in the paper.