Cored slim holes drilled as part of Hawaiian project

No two holes are drilled exactly alike and careful planning is the key to any hole completion, say Noble Larsen and Percy Wicklund in a paper titled “Drilling the Hawaiian scientific observation holes” which was presented at the annual convention of the Canadian Diamond Drilling Association in Ottawa

The scientific observation holes (SOH) drilled at a project on the island of Hawaii have proven that cored slim holes can be drilled successfully in a hostile environment to depths in excess of 6,500 feet and have certain advantages over large size oilfield drills, say Larsen, president of Tonto Drilling Services in Utah, and Wicklund, a consultant from Colorado. Among the advantages listed:

— direct contractors fees are less than those charged by oilfield contractors. Equipment used for drilling and/or expended is smaller and costs less

— overall completion costs are reduced as location sizes are smaller, permitting costs, and the impact on the environment is greatly reduced — because of the nesting potential of core drilling, smaller diameter holes can be drilled with more possible reductions than on a rotary drill in the event of problems

— blow-out prevention equip-ment with the same ratings is available for a core drill as it is for large rotary drills

— unlike rotary drills, core drills are used to drilling blind (without circulation back to the surface)

— temperatures equilibrate faster in a smaller diameter hole because of the size and the smaller volume of fluids used for circulation while drilling. “Safety for the drill crew while drilling in a hostile environment is priority number one. There is not only the possibility of encountering harmful or deadly gases, but also the possibility of having the well blow out,” say Larsen and Wicklund.

“Three SOH holes have been drilled and completed with the Universal 5000 drill so far to date without any type of safety problems,” they say of the Hawaiian project. “In addition to drilling the SOH well, the crews with this drill entered the HGP-A well which was drilled in 1976 to conduct some tests and drilled down to the bottom of the well to make sure it was left in a similar condition as before the test starting.

“In the past six months, one contractor with an oilfield equipment rate at 16,000 ft. has experienced two blowouts in large diameter holes that were drilled adjacent to SOH-1.

“One of the disadvantages in drilling the SOH holes was that it was not possible to collect uncontaminated groundwater or reservoir fluids by bailing. If holes like the SOH holes are to be drilled in other areas, the holes must have the capability of being either pumped or flow tested. “The main thing to keep in mind when planning a drill

program is to minimize any possible trouble. Depending on the down-hole conditions, it doesn’t matter if you have a drill that can pull 50,000 lb. or one million lb. When you are stuck, you’re stuck.

“Depending on the drill pipe being used, there are limits as to how much abuse, either by stretching and twisting, the pipe can stand before deformation and breakage occurs and possibly the entire hole lost. “Anytime a dogleg occurs in a hole which exceeds three degrees over a short interval, a packed bottom hole assembly should be made up of stabilizers and reamers in order to straighten out that section of the hole or twistoffs and other problems will continue to haunt you.

“We cannot emphasize enough that knowledgeable drill personnel are the key to successful hole completions. Drilling is more of an art than a science and hole completions rely on the onsite drilling decisions made and completed by the drill crews.

“When an idea is generated, such as to core drill a hole in a hostile environment and a commitment made to pursue that idea, it is impossible to guarantee what the final costs will be of drilling a SOH type of hole.” Monitoring the daily and accumulated costs is mandatory. Meetings should be held frequently between the contractor and client to review expenditures and projected costs to determine if the drill cores obtained are worth the expenditures.