ABSTRACT: Drilling, Testing, and Geologic Setting of a Slant Hole in the Mesaverde Group Tight Gas Reservoirs of Colorado's Piceance Basin

FROHNE, KARL-HEINZ, U. S. Department of Energy

In 1990 the U.S. Department of Energy drilled an experimental slant hole through the Mesaverde Group near the town of Rifle, Colorado. The objective of this test well was to evaluate the tight gas production potential of two types of directional wells in one hole. The Mesaverde sands are mostly very tight with rock matrix permeabilities in the nano- to microdarcy range, but are often naturally fractured. The slant hole penetrated, cored, and logged two completion targets: (1) a 60 degree inclination high angle hole through a 500 ft (152 m) thick interval of interbedded tight fluvial channel sands and gassy coal seams, and (2) a near-horizontal hole in the marine Upper Cozzette sand. Both intervals were cored, 266 ft (81.13 m) of core were cut at 60 degree inclination, and 115 ft (35.0 m) cores were cut in the lateral Cozzette hole. Multiple natural fractures were cored, some fully or partially mineralized and others not, including open fracture apertures of up to 0.1 in. (2.5 mm) wide. Many gas kicks were taken during drIlling and coring of the Mesaverde sands and coals. Limited wireline and drill pipe conveyed logs were run in the slant hole. After being drilled to total measured depth of 9466 ft (2887 m), and after many signs of high gas production potential, the Mesaverde part of the well was lost due to mechanical problems while attempting to cement a liner in the Cozzette lateral.

In 1991 the slant hole was successfully sidetracked and redrilled through both completion targets. The sidetrack was directionally drilled along a complex path to place the Cozzette lateral about 1100 ft (335.5 m) to the east and parallel to the original hole. The objective was to avoid formation damage caused by the loss of perhaps 500 bbl (79,485 L) of heavy drilling mud to the Cozzette while drilling the original slant hole. The paludal zone was cased and cemented, while the Cozzette lateral was completed open hole. After drilling underbalanced over 300 ft (91 m) of Cozzette without incident, the well was inadvertently swabbed in during a bit trip whIch resulted in a brief flash fire when a well control line failed. It took several days and the pumping of very heavy mud into the fo mation before drilling of the final 100 ft (30 m) was resumed. After being drilled to a total measured depth 9407 ft (2869 m), or 7948 ft (2424 m) true vertical depth, the Cozzette sand was very briefly flowed back to remove drilling mud from the open hole prior to temporarily shutting in the well. Over a 3 hour period about 150 bbl of mud were recovered, before the well flowed straight gas. The flowback was controlled through a choke manifold and, before being shut in, was flowing steadily at a pressure of 2500 psi (17.2 MPa) through a 1/2- to 3/4-in. (12.7 to 19 mm) variable choke. The slant hole is currently shut in, pending production testing of both completion zones scheduled to begin late this fall.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)