Slimhole Technology Maximizes Middle East Horizontal-Drilling Programs
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 92376, “New Slimhole Technology Maximizes Productivity in Middle East Horizontal-Drilling Programs,” by Tariq Al-Wahedi, Mahmoud Abdel Hay, and Tamer Waheed, Abu Dhabi Co. for Onshore Oil Operations, and Bobby Grimes, SPE, and Sarvesh Tyagi, SPE, Hughes Christensen, prepared for the 2005 SPE/IADC Drilling Conference, Amsterdam, 23–25 February.
Many operators use advanced slimhole drilling technologies throughout the Middle East. These programs involve new and re-entry wells with long horizontal sections, typically 6 or 6⅛ in. in diameter. A metal-sealed slimhole roller-cone bit was developed for use in these slim wellbores. The longevity of the new bearing has enabled operators to gain considerable confidence, which has translated into more on-bottom drilling time and longer drilled sections.
Introduction
Historically, tungsten carbide insert (TCI) bits with elastomer bearing seals were used to drill the slimhole sections, 6 in. in diameter in Abu Dhabi. In other Middle East countries, 6⅛ in. is the most common bit size. Typically in Abu Dhabi, the 6-in.-diameter hole section is drilled horizontally through the limestone reservoir (Shuaiba or Thammama group), sometimes crossing from zone to zone through a dense limestone. A tight tolerance requires geosteering to keep the well on target. The reservoir section is a porous and fractured limestone, with unconfined compressive strength (UCS) less than 5,000 psi. The UCS of the dense limestone caprock ranges between 12,000 and 15,000 psi. The section length typically varies from 3,000 to 4,000 ft, depending on the well, and sometimes extends to 6,000 ft.
Two elastomer-sealed Intl. Assn. Drilling Contractors (IADC) 4-3-7-type bits were required to drill the 4,000-ft sections because of bit-use time limit. The instantaneous rate of penetration (ROP) is greater than 150 ft/hr, at times limited by data-capture speed of the measurement-while-drilling system. The additional trips for bit change-out cost rig time and increase the time that the reservoir is exposed to drilling fluid, which can result in greater formation damage.
Horizontal sections through limestone reservoirs are drilled in several other countries in the Middle East (e.g., Saudi Arabia, Oman, and Kuwait). The section length varies depending on country, operator, and well type, but in general, the length of bit run is limited by the risk of lost cones after undetected seal failures, not by the cutting-structure wear.
General Well Plan/Profile
There are two general Abu Dhabi well plans and profiles. In Case 1, the 12¼-in. upper vertical section is drilled, and 9⅝-in. casing is set at the top of a massive shale formation. The 8½-in. section is drilled through the shale and limestone reservoir section while building inclination from vertical to horizontal. The massive shale formation is open in this type of well. In Case 2, the 12¼-in. upper vertical hole is drilled to the top of the reservoir section, where the 9⅝-in. casing is set in a limestone member or below to cover the shale formation.