Fastest Deep Marrat Well in North Kuwait: Case History of Raudhatain 206

Abstract

Abstract For the last 20 years, exploration in Kuwait has focused on finding commercial quantities of high quality oil in the deeper high pressure/high temperature (HTHP) Jurassic formations (13,000–17,000 ft), and for gas within the Triassic formations (17,000–21,000 ft). The HPHT conditions, the presence of H2S and CO2, the narrow pore pressure/fracture pressure (PP/FP) window to work with, and the high mud weights (18–20 ppg) required have made deep drilling conditions in Kuwait some of the most challenging in the world. This paper discusses the outstanding drilling performance achieved in drilling Ruadhatain 206 (RA-206) as the fastest deep Jurassic well to date in North Kuwait. New practices were incorporated and several firsts accomplished through optimum utilization of local built-in experience, combined with the latest technology and practices developed by International Operating Companies (IOC's) and service companies. Optimum bit selection resulted in a world record bit run in the 28" section, and record firsts for Kuwait in the 22" and 16" sections. Careful analysis of offset data and in selecting mud weights resulted in improved hole stability with minimum mud losses, and enabled the bottom two hole sections to be commingled. Extensive cement design and optimization resulted in an excellent CBL being achieved in the production casing. The excellent performance achieved in well RA-206, has now opened the possibility of efficiently drilling similar wells in 100 days (technical limit) instead of the 170–200 days normally taken. (Fig-1) The improved drilling practices significantly reduced the drilling time and well costs, and provided a step-change forward in KOC's understanding of drilling limits when applied to deep exploratory/development wells. Background on Kuwait Geology and Jurassic Well History Most Kuwait development and oil production comes from the shallow Cretaceous formations which extend down to 12,000+ ft, and which have normal pore pressures of less than 12 ppg. These formations consist primarily of sequences of sands, limestones and shales. The massive Burgan field of Kuwait is a good example, where multiple stacked oil zones exist in reservoirs ranging from the Wara formation down to the Minagish Oolite. These shallower formations contain a variety of challenging drilling conditions. The near surface formations are prone to loss circulation throughout Kuwait, with the highest severity of losses in the Tertiary Dammam limestones and the Cretaceous Shuaiba formation, both of which contain vuggy fractures. These formations often experience total losses while drilling, and are commonly penetrated using mudcap drilling techniques. Cretaceous shales in the Ahmadi and Ratawi formations are highly reactive, and require inhibitive muds to prevent hydration and hole sloughing. Hole stability is a concern in the Burgan sand/shales and in the depleted Mauddud formation. Hard and very abrasive sands exist in the Zubair formation, and dense carbonates in the Makhul formation result in slow rates of penetration (ROP's) at the bottom of the Cretaceous. Four casing strings are required to safely isolate the Tertiary and Cretaceous sequences prior to entering the overpressured Jurassic sequences below. With the shallower horizons extensively developed in Kuwait, exploration efforts have focused increasingly on drilling into the deeper Jurassic Najmah/Sargelu and Marrat formations. Penetrating into these Jurassic sediments poses significant additional drilling challenges.