Remember PTTC's effort in organizing a series of workshops to highlight progress in DOE's microhole drilling program (http://
microtech.thepttc.org)? Most presentations from those workshops are available online. There were a couple of areas with direct significance to exploitation of mature reservoirs—one being refinement of coiled tubing drilling (CTD) (http://microtech.thepttc.
org/gas_technology/gas_tech
nology_fd_03222006.pdf) and the second being a program by BP for horizontal re-entry drilling in the Cleveland Sand in western Oklahoma and the Texas Panhandle (http://microtech.thepttc.org/
current_industry_effort/bp_
08162006.pdf).

There has been marked progress in coiled tubing drilling in the lower 48. Xtreme Coil Drilling Corp., a Canadian supplier, has had a strong focus on bringing their hybrid Coil Over Top Drive® rigs south. Their third quarter 2007 operations update (http://xtreme
coildrilling.com/news/releases/
071108xdc3Q.pdf) notes that they now have six rigs working in the U.S. The hybrid design mitigates the depth limitation of coil-only rigs. Continued technology advancements (www.ogj.com/
articles/save_screen.cfm?ART
ICLE_ID=296189) potentially open up even deeper horizons. Active operator drilling programs with Xtreme's rigs have proven their utility in the Piceance and Denver-Julesberg basins. Others become targets as more rigs become available and technology advances.

Back to BP's Cleveland Sand horizontal reentry drilling project. That technology thrust is just one element of realizing potential in a mature play. An SPE paper (109948, available through SPE's e-library at www.spe.org) by BP and Jones Energy, presented during SPE's recent annual meeting, provides an excellent overview of the "process over time, with technology improvements" of increasing recovery from the Cleveland. Since the concepts apply to many mature reservoirs, this article summarizes key points presented in the paper. Readers are encouraged to read the paper in its entirety.

initial rates about 0.4 Mmscfd. The initial horizontal well was drilled in 1997, but it was not until 2003 that a BP/EOG partnership for a horizontal well pilot caused horizontal drilling to take off. Now operators have virtually abandoned vertical drilling.

Evolution of Horizontal Completion Practices. Early horizontal well design was a 1,500–1,600 foot cased and perforated horizontal section with four sets of perforations spaced 400–500 feet apart. Laterals had a build angle of 9–12 degrees per hundred feet. Ball sealers were used as a diversion technique between stages. Production performance was widely variable. Subsequent micro seismic surveys indicated that the ball sealers were not a reliable diversion technique—one or more intervals were often not broken down. A four-well pilot by BP employed an open hole multiple packer (OHMP) system that causes independent fracture stimulations between packers along the lateral. A series of open hole packers and sliding sleeve fracture ports allow selective stimulation of one interval at a time, starting at the toe. Initial results were promising and, as more data became available, the improved performance with the OHMP completion became more distinguishable. Nearly all operators now employ OHMP systems to achieve independent stimulations along the full length of the lateral.

Image Logs and Fracture Mapping. While rarely observed in cores, open-hole image logs revealed they were more common, oriented roughly east to west. Image logs and core data indicate a link between lithology and the presence of natural fractures. Several micro seismic surveys during hydraulic fracturing were made. Taken together, the data indicate that hydraulic fractures tend to initiate in relatively clean sands, which should be considered in placing fracture ports or perforations. This could explain why the OHMP completion design works better—artificially restricting where one tries to initiate a fracture with the cased and perforated design (if its not the right lithology) would not work as well as open hole where the frac would move to the point of weakness (natural fracture) between the packers.

CT Reentry Drilling Program. As part of BP's effort to utilize the large number of existing vertical wellbores, BP executed a 10-well CTD re-entry field trial. This program employed an OHMP completion system. Five of the 10 wells were successfully drilled, but due to shale instability problems above the reservoir only three were successfully completed. Consistent with modeling, the wells confirmed only a small lower pressure region near the wellbores. Initial potential of the three wells averaged 1.3 Mmscfd, compared to the expected 1.4 Mmscfd for a transverse OHMP completion design.

To better understand shale strength and sensitivity, BP subsequently cut and analyzed a core of the Cleveland sand and overlying shale. Although the shale is not particularly reactive, water-based drilling fluids increase the shale's tendency to part. By incorporating core data into a geomechanical model, BP has developed an action plan for drilling through the shales, stabilizing them, then drilling the lateral.