It is a challenge to drill in highly deviated or horizontal holes across the highly depleted formations. Wellbore instability, differential sticking and mud loss are frequently encountered problems while drilling a depleted reservoir in deep HTHP wells in Kuwait. Long-term production caused formation pressures to be highly depleted, and drilling became more challenging with considerable non-productive time. Therefore, it is necessary to identify a fluid’s solution when other options with casing zone isolation are not viable. Traditionally, oil-based mud (OBM) was used while drilling these formations with limited success. A customized fluid system was designed to overcome the issue of high overburden pressure in depleted formations targeting effective bridging, minimizing pore pressure transmission and strengthening the wellbore. A nano-size deformable synthetic polymer, along with sized calcium carbonate and graphite, was identified to effectively plug the pore throats and minimized the fluid invasion, which was confirmed by particle plugging tests. A well section was identified to comingle the highly depleted and pressurized formations. This was the first attempt on a high-angle well with deep drilling operations in Kuwait and was performed to facilitate the successful drilling of the reservoir. Traditional OBM was converted to a customized fluid system using a nano-size polymer and sized bridging additives based on proprietary software selection and series of laboratory tests. Drilling, coring and logging were successfully performed for the first time in the commingle section without incident. There was no wellbore instability or differential sticking tendencies, less torque and drag, as well as enhanced wellbore cleaning in the high-angle sections. This paper also presents the some of the successful applications of the nano-size deformable polymer in OBM to drill highly depleted formations in HTHP wells managing upto 3500 psi overbalalnceacross highly permeable formations.
Kuwait Oil Company (KOC) launched North Kuwait Gas Development Project in 2007 to fast track production of gas/light oil from deep HPHT Jurassic reservoirs in six fields. The exploratory wells require a large-hole casing design to isolate problematic formations and allow the operator to reach the deep HPHT target zones with maximum hole size. One of the most difficult/problematic hole sections is Zubair to Hith, traditionally drilled with 16-in bits. The formations are interbedded and consist of abrasive/pyritic sandstone, reactive shale, limestone and anhydrites with UCS that varies between 5-30kpsi. Historically, to complete the 3900ft section required more than three PDC bits or a combination of 4-5 PDC/TCI. To reduce costs, the drilling team developed the first slim hole well plan and casing string design to explore Cretaceous formations in Northern Kuwait. The objective would be to downsize the hole section from Zubair to Hith to 9¼-in and drill to section TD with three PDC bits. The production casing would then be run from surface to TD instead of the conventional liner tie-back and cementing process. The section was completed using the latest motor/PDC technologies in two bit runs saving 55% drilling time and the expense of a third bit. The operation set a new Kuwaiti record for the single longest 7¾-in production casing string (13,625ft) and cementing back to surface. Cement log indicated a good bond over all three zones of interest. The downsizing strategy and change in casing string design saved the operator over $1 million USD compared to a large hole and liner-string design. Production testing in Zubair, Ratawi and Makhul was positive, indicating a significant new oil discovery. The entire operation was completed with zero HSE related complications.
Drilling the 16" hole section through mostly carbonates in southern Kuwait presents several distinctive challenges. Interbedded formations with vastly different compressive strengths that vary between 5-30 Kpsi and a significant amount of blind drilling can result in twist-off and lost-in-hole situations. The most common tungsten carbide insert (TCI) rollercone bit failures mechanisms are impact induced cutting structure damage, hydraulic inefficiency issues and bearing failure caused by high weight-on-bit (WOB) through the hard Dammam to Ahmadi carbonate sections.To solve the challenges the operator's drilling team, working in conjunction with the bit supplier, launched a performance improvement initiative. The carbonate application was analyzed using finite element analysis (FEA) based dynamic simulation program to fully understand how to stabilize the bit in different downhole conditions and improve hydraulic efficiency. Based on the computer analysis and field run data, engineers conceived a new stable bit design. The technology platform includes improved hydraulics and a specialized cutter structure configuration with tougher insert carbide grades to mitigate impact damage. A high-aspect ratio seal employs two dynamic sealing surfaces (ID, OD) that reduces frictional heat and minimizes wear to improve bearing reliability.The new rollercone bit was run with an optimized drilling plan with outstanding results establishing a performance step change in southern Kuwait's carbonate applications. The bit recently set a new 16" world cumulative TCI footage record of 12,656ft drilling three wells shoe to total depth (TD) and 531ft of a fourth well. The bit amassed a total 446 hours of onbottom drilling time turning 2.4 million revolutions without a seal failure. The bit saved the operator 4.5 days of rig time plus bit spend. The new technology has enabled the operator to drill long footage intervals or multiple wells using a single bit with good penetration rates (ROP). The authors will discuss the formation challenges, application analysis and resulting performance improvement.
Operators in the current volatile oil and gas industry must optimise drilling performance and minimize well cost. Various drilling tools and methods have been implemented over the years to improve drilling efficiency and to reduce the non-productive time (NPT) while drilling through challenging formations. Many North Kuwait onshore wells are drilled with conventional drill bit technologies, such as tungsten carbide insert (TCI) and polycrystalline diamond compact (PDC) bits on either rotary or performance motor bottom hole assemblies (BHA). This paper discusses the breakthrough drilling performance of 16-in. hybrid drill bit technology on a performance motor BHA, delivering outstanding results and savings for the operator. The typical 16-in. vertical section ranges from 4000 ft. to 5000 ft., and contains limestone and sandstone with interbedded layers of shale and streaks of anhydrite with high variance in the unconfined compressive strength (UCS) across these formations. The downhole environment is unsuitable for PDC bits due to impact damage from the variable UCS, and TCI bits provide unsatisfactory rates of penetration (ROP) requiring a new drilling approach for the application. Poor dull conditions combined with the long section led to low overall ROP, further indicating the need for a new class of bit. The engineering process involved reviewing: –Current TCI and PDC cutting structure designs–Surface drilling parameters applied–Post-run dull analysis from current bit designs–Previous TCI and PDC bit design performance on rotary and motor BHA–Hybrid design performance on rotary BHA A collaborative technical analysis identified key performance objectives to ensure a step change in section drilling. Hybrid drill bit technology was deployed on a performance motor BHA to enhance the ROP. The hybrid bit enhanced the ROP by 153% with a 52% reduction in cost per foot compared to the average TCI performance in this field, saving the operator 2.2 days and over $ 0.4 Million operational cost. The hybrid drill bit design demonstrated its ability to serve as an alternative to conventional technologies and showed the potential for efficient drilling and reduction of NPT over multiple wells, assisting operators in the current cost-sensitive industry environment.
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