No abstract

Efficiently drilling the abrasive Zubair sandstone is one of the Middle East's most daunting challenges. Adding to application complexity, the pyritic formation is also interbedded with hard shale streaks and has a compressive strength that ranges between 3-10kpsi. In Kuwait, the formation is first encountered at a depth of approximately 9000ft and been drilled with mixed performance results based on bit diameter. Generally, the large diameter PDC bits are still struggling to achieve the durability objective with some wells requiring more than two PDCs to complete the short 1400ft hole section. In the smaller hole sections, technological advances have overcome the cutter/bit durability issue but with no significant improvement in ROP. The objective of an intensive bit optimization effort has focused on increasing penetration rates while striving to improve overall bit life/cutter durability. To accomplish the operator driven objectives without time-consuming field trails, the drilling team used a software system to calibrate rock strength. This data was used in conjunction with an advanced FEA-based modeling system to analyze different PDC cutting structures to select a PDC bit with the blade count and shearing configuration that would produce dynamically stable drilling. The bit body would be equipped with a new O2 cutter to increase abrasion resistance and maintain temperature at the cutter tip by using: 1) enhanced HTHP sintering process; 2) refined post-pressing process to improve thermal stability 3) optimized hydraulics to maximizing cutter cooling.

Drilling the deep vertical 16" hole (starting at 9,300ft) in northern Kuwait is challenging due to the complex stratigraphic section that contains abrasive Zubair sand (with pyrite), reactive Ratawi shale, hard Ratawi, Minagish, Makhul limestones and Hith anhydrite (UCS 10-30kpsi). Completing the 16" section typically requires around 45 days of rig-time and four polycrystalline diamond compact (PDC) bits on a positive displace motor (PDM) driven bottom hole assembly (BHA). However, inconsistent PDC bit performance/durability and lack of downhole stability was causing premature bit/motor failure. The operator required new PDC technology that could efficiently drill the abrasive, Zubair and continue as far as possible into the Ratawi without premature cutter failure at acceptable rates of penetration (ROP). The PDC bit would also need to possess improved stability characteristics to reduce PDM failures/trip hours.An engineering team was assembled to analyze the latest drilling/PDC technologies, optimize BHA design, and determine the best practices to enable reaching section TD with two PDC bits/runs. A finite element analysis (FEA) based engineering system was utilized to predict the dynamic behavior of each BHA component. The resulting information led to greater bit stability when drilling in lithologies comparable to the specific field application. The development of a new abrasion resistant cutter was considered important to improving durability and maintaining cutting efficiency.The in-depth study and PDC design changes/advanced cutter technology produced a new nine-bladed PDC bit. The new bit was run on a performance motor in Raudhatain field and set new PDC footage and ROP records for drilling through Zubair (1,200ft) in northern Kuwait (Figure 1). Historically, the Zubair alone required three/four TCIs or two/three PDCs to complete the formation interval. Next an eight-bladed PDC, fitted with the new cutter technology, drilled 2,508ft from Ratawi to section TD for the first time in Kuwait. The improved performance reduced rig hours by an average of 13 days for a total savings of approximately $390,000USD compared to the closest two PDC offset runs. SPE 139873Conclusions KOC's Deep Drilling Team 1, working in conjunction with the service provider, set a new performance benchmark drilling the deep 16" vertical hole section utilizing teamwork and new PDC bit/cutter technology. The new HPHT PDC cutters have reduced accelerated cutter wear and diamond table loss enhancing penetration rates and bit durability. Using the advanced dynamic FEA engineering software systems helped prevent BHA and formation induced vibrations, associated failures and trips by providing dynamically stable BHA and PDC bit combinations. The application specific bit design was tested and certified according to the operator's requirements through the use of dynamic simulations and rock laboratory tests. The result saved the operator rig time and reduced drilling costs by eliminating the traditional trial and error improvement/optimization...

Efficiently drilling the 16" hole section in high temperature/high pressure (HTHP) conditions through hard/abrasive Jurassic formations in Kuwait's oil fields has been a distinctive challenge for more than a decade. The difficult sequence of lithologies starts with abrasive Zubair sand (with pyrite) followed by reactive Ratawi shale and then hard carbonates (UCS 10–30kpsi). The Zubair is approximately 1300ft thick and has historically been drilled with four/six rollercone TCIs or a combination of two TCIs and two PDC bits or two/three PDCs. The objective has been to reduce the total number of bits/trips and ultimately drill the Zubair and as much Ratawi as possible with one PDC bit. Initial attempts with PDC were encouraging, but not economically successful compared to rollercone cost/ft. To improve performance, an FEA-based modeling system was used to predict downhole behavior. Based on these knowledge gains, a new PDC bit design was produced with optimized blade orientation and reconfigured cutting structure. Although the new design did improve performance, the drilling team concluded the PDC shearing elements were unable to withstand the HTHP environment (thermal degradation) and vibration induced impact damage. The operator required new technology to enhance cutter longevity. To deliver a suitable solution, new cutter technology was developed using a two-step manufacturing process high temperature/high pressure pressing technique. Laboratory tests indicated the new style cutter has significantly increased resistance to abrasive wear and thermal fatigue compared to standard cutters without compromising impact resistance. The HTHP cutters were run in a nine-bladed PDC with outstanding results setting a new Kuwaiti single run Zubair footage record (1302ft) and Zubair/Ratawi field footage record (1347ft). The authors will discuss the application challenges and resulting performance improvement. Offset analysis will document a performance step-change that saved the operator an average of 10 days of rig-time (﹩500,000USD).

It is well documented carbonate formations cause a multitude of drilling problems. Because the majority of reservoirs in Kuwait are carbonates, the operator is consistently searching for technology to improve drilling efficiency. To reach the hard Jurassic carbonate reservoir, the operator must drill through a series of demanding Tertiary and Cretaceous formations. The fractured Dammam and Tayarat formations are notorious thief zones and can aggravate drilling problems. The strategy to get maximum hole size to the deep HPHT Jurassic reservoir starts by constructing a 28" borehole through Dammam to TD in Mutriba. The section is interbedded with anhydrite/shale and has compressive strengths between 4- 30Kpsi. At TD, 24" casing is run to isolate the problematic formations. The hole section is approximately 5000ft long but varies depending on the field. These interbedded formations and thief zones create a major challenge for rollercone bits including, impact damage from hard stringers, high WOB and on-bottom time in excess of 250 hours can reduce average ROP to an unacceptable level. Additionally, heat-induced bearing stress caused by low circulation rate in loss-prone formations adds to the applications complexity. To solve the challenge, the drilling team produced an optimized 28" TCI rollercone bit with custom insert geometry and updated carbide grades to increase cutting structure longevity. Over the last several years the new-style bit, along with optimized operating parameters has consistently drilled the entire section in one run and set a multitude of new footage and ROP records. The authors will discuss the application challenges and TCI bit development. They will present an in-depth engineering analysis that documents footage increases and several new ROP benchmarks in Kuwait. The improved performance has resulted in an average per application cost savings of 3.5 rig days/$175,000 for the operator.