Numerous operators are using advanced slimhole drilling technologies in development programs throughout the Middle East. These programs involve new and re-entry wells with long horizontal sections, typically 6" or 6–1/8" in diameter. Rollercone bits are often used in these applications because of their overall robustness, superior directional characteristics and predictable sidecutting ability. Slimhole PDC bits are run when possible, but problems with high torque, toolface control and drillstring dynamics often prevent their use. Traditionally, operators have been reluctant to attempt long runs with slimhole rollercone bits due to the potential for lost cones after undetected seal failures. An innovative new metal sealed slimhole rollercone bit was recently developed. The downsized primary metal seal has proven its ability to significantly improve bearing life and reliability. The longevity of the new bearing has allowed operators to gain considerable confidence, which has translated into more on-bottom drilling hours and a high TD rate. The metal seal is especially useful on high speed (200 - 450 rpm) steerable motor assemblies, where elastomer seals wear at an accelerated rate. The new bits are achieving over 1100 krevs at 80% reliability throughout the Middle East, while conventional elastomer sealed bits achieve only 600 krevs at 80% reliability in head to head comparisons. In Abu Dhabi, the new bits are used to drill out from 7" casing, then geosteer horizontally for 4000ft+ of limestone with a UCS between 5,000 psi and 15,000 psi. The authors will document significant cost reductions through improved bit performance from several years of drilling in active fields in UAE (2003 - Aug 2004). The primary advantage of utilizing the new slimhole bit is they can drill the horizontal section in one fast run. Historically, it takes two elastomer sealed bits to complete the hole section. Reduction in drilling time due to higher ROP and elimination of tripping for a new bit has led to significant savings in rig time while minimizing formation damage by reducing the exposure of reservoir formations to drilling fluids. Introduction Historically, TCI bits with elastomer bearing seals (ES) were used to drill the slimhole sections in 6" size in Abu Dhabi. In other Middle East countries, 6–1/8" is the most commonly used bit size. Abu Dhabi Typically, the 6" hole section is drilled horizontally through the limestone reservoir (Shuaiba or Thammama group), sometimes crossing from zone to zone through the dense limestone. The reservoir tolerance set by operator is +/− 3 ft, but can allow +/− 5 ft; this tight tolerance requires the directional driller to geosteer in the reservoir section to keep the well on target. The reservoir section is porous and fractured limestone, with unconfined compressive strength (UCS) below 5 kpsi. The cap rock (dense limestone) ranges between 12 and 15 kpsi. The section length typically varies between 3000 ft to 4000 ft depending on the well, and sometimes extends to 6000 ft. Historically, two elastomer sealed IADC 4–3-7 type bits were required to drill the 4000 ft sections due to hours on the bit. Instantaneous ROP's are above 150 ft/hr and are at times limited by data capture speed of the MWD system. The additional trips for bit change-out not only cost rig time but also increase the exposure time of the reservoir to the drilling fluids, which can result in greater formation damage.
An analysis of recent run data from two fields in Eastern Venezuela documents large metal seal bearing roller cone bits (16" & 17–1/2"), with a new generation steel tooth cutting structure, can be used for a much greater number of total bit revolutions than prior generation bits. The new generation metal seal has proven its ability to significantly extend bearing life and reliability and thereby provide significant reductions in drilling costs. Penetration rates have more than doubled with total footage up 89% in some instances, with less overall cutting structure wear. Cost savings of up to $121,000 US per well have been documented. The authors will describe the application of these new bits and present case studies that document significant performance improvement in the Santa Barbara and Mulata fields of Eastern Venezuela. They will also describe the development of the latest generation metal sealed bearing package and the metallurgical and design improvements of the new steel tooth cutting structures. Application Background Since 1990, over 400 wells have been drilled in the oilfields surrounding Punta de Mata in Monagas County of Eastern Venezuela. Since 1994, a turnkey operator has drilled over 55 wells, under contract with Petroleos de Venezuela S.A. (PDVSA). These wells were mainly in the Santa Barbara, Mulata and Pirital Fields (Figure 1). The hole sizes are generally 26", 17–1/2", 12–1/4" and 8–3/8". The general casing program is depicted in Figure 2. The 17–1/2" hole section is typically vertical and generally drilled from 1,000 ft to 7,000 ft, measured depth (MD). However, several 17–1/2" sections were drilled directionally when high vertical displacement was required. By the end of the operator's turnkey contract with PDVSA, a much higher number of 17–1/2" directional sections were required. The 12–1/4" interval is where the majority of the directional work takes place in these "S" type wells. The subject wells are typically drilled to the Naricual (Triassic) or KP (Jurassic) sandstones. Formation penetrated during the drilling of the 17–1/2" hole section includes Mesa-Las Piedras, La Pica and Carapita Shale formations. In some instances the Sigmolina and Textularia formations are penetrated. All of these formations are Pleistocene to Miocene in age (Figure 3). The Mesa-Las Piedras formation is comprised of soluble claystones with sandstone stringers and coal seams. Additionally, some limestone stringers and siltstones can be encountered. The La Pica formation is comprised mainly of claystones and hydrateable shales. The Sigmolina and Textularia are also shales, with high amounts of claystone and siltstone. Rate of penetration generally slows when these formations are encountered. The 17–1/2" hole section usually bottoms in the Carapita Shale where 13–3/8" casing is set. Open hole sonic logs are not run in the 17–1/2" section, thus unconfined compressive strengths are not available. Attempts to drill the entire section with a single PDC bit have resulted in inconsistent performance. This is due mainly to durability problems with light set PDC bits while drilling the hard sandstone stringers, and balling problems with heavy set PDC bits while drilling the reactive claystones.
Thi s paper was sel ect ed f or pr esent at i on by an SPE/ I ADC Pr ogr am Commi t t ee f ol l owi ng r evi ew of i nf or mat i on cont ai ned i n an abst r act submi t t ed by t he aut hor ( s) . Cont ent s of t he paper , as pr esent ed, have not been r evi ewed by t he Soci et y of Pet r ol eum Engi neer s or t he I nt er nat i onal Associ at i on of Dr i l l i ng Cont r act or s and ar e subj ect t o cor r ect i on by t he aut hor ( s) . The mat er i al , as pr esent ed, does not necessar i l y r ef l ect any posi t i on of t he SPE, I ADC, t hei r of f i cer s, or member s. El ect r oni c r epr oduct i on, di st r i but i on, or st or age of any par t of t hi s paper f or commer ci al pur poses wi t hout t he wr i t t en consent of t he Soci et y of Pet r ol eum Engi neer s or t he I nt er nat i onal Associ at i on of Dr i l l i ng Cont r act or s i s pr ohi bi t ed. Per mi ssi on t o r epr oduce i n pr i nt i s r est r i ct ed t o an abst r act of not mor e t han 300 wor ds; i l l ust r at i ons may not be copi ed. The abst r act must cont ai n conspi cuous acknowl edgment of wher e and by whom t he paper was pr esent ed. W r i t e Li br ar i an, SPE, P. O. Box 833836, Ri char dson, TX 75083-3836, U. S. A. , f ax 01-972-952-9435.
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