Tophole Turbodrilling Offshore Qatar

1. ABSTRACT A technique has been developed for drilling the top hole of every development well offshore Qatar by means of a downhole turbine with tricone bits. This unusual technique was initiated as a means to combat drill string failures resulting from attack by hydrogen Sulphide contained in top hole formation waters. Turbo drilling has successfully eliminated all H2S related failures, has significantly enhanced drilling speeds and costs, has minimised hole enlargement and has proved to be easy, reliable and popular to apply. Many different parameters have been tested in an effort to optimise the technique, and these efforts are continuing. 2. HYDROCARBON OPERATIONS OFFSHORE QATAR Qatar is a small independent Emirate of some 13,000 square kilometres, jutting out as a peninsular from the Arabian land mass, on the southern side of the Arabian Gulf. To the West and South lies Saudi Arabia, to the East the United Arab Emirates, while to the North is Iran and North West Bahrain (see Figure 1). The population of Qatar is estimated as some 210,000 of which a large percentage are non Qataris. Up to 1977, offshore exploration, development and production operations were conducted by or in conjunction with international oil companies under concessionary arrangements. In phases, the Government acquired portions of the principle offshore regions or interest culminating with the acquisition in March 1977 of 100% of the offshore Contract Area shown in Figure 2. This Contract Area offshore comprises a Southern region where three oilfields, Idd el Shargi, Maydan Mahzam and Bul Hanine produce oil and associated gas, a Central region where several so far unproduced oil and gas accumulations lie, and a North-West region which accommodates a major gas field presently in the appraisal stage. The offshore Contract Area occupies 12,325 square kilometers and water depths vary between 65 and 200 ft. Ownership and operation of the offshore Contract Area and its related assets rest with the Offshore Operations Division of the Qatar Petroleum Producing Authority. A second oil and gas producing Contract Area on land to the West of the Qatar Peninsular is operated by QPPA's Onshore Operations Division. QPPA is itself owned by the Government's Qatar General Petroleum Corporation (QGPC). QPPA (Offshore) emp10ys three modern. Marathon Le Tourneau built jack-up rigs: QGPC's Rig Dana (250 ft water depth capability), Santa Rig 103 (250 ft) and Sea and Land Drilling Contractor's Rig Trident II (300 ft). The first two are staffed by QPPA (Offshore) personnel, the third by Contractor personnel. The vigorous work programme undertaken by these three rigs includes development drilling, exploration/appraisal activities and workovers. 3. PRINCIPAL DRILLING PROBLEMS ENCOUNTERED IN TOPHOLE The casing scheme and lithostratigraphic column of a typical development oil well in the Southern region of the Contract Area is shown in Figure 3. After setting 20" casing at ca. 600 ft bdf, 17½" hole is drilled to a maximum of ca. 4200 ft bdf where l33/8" casing is cemented. 95/8" casing and a 7" liner are set across upper and lower Jurassic reservoirs which overlie one another. This paper is concerned with the problems associated with drilling the l7½" tophole interval of some 3500 ft.