Challenges in Completing Long Horizontal Wells Selectively

Abstract

Although drilling technology is now able to deliver ultra-long horizontal wells, completion technology has been slower to evolve. Running long liners, effective stimulation and completions are some of the areas that require more attention. The paper discusses some of the challenges in the development of the Al Shaheen reservoirs offshore Qatar and how they were overcome. Some of these wells have a step-out in excess of 30,000 feet with a TVD of 3,100–3,500 feet and thus they offer some unique challenges. Running and cementing liners to these depths is difficult and some of the methods to effectively achieve this are discussed. Perforating guidelines are presented that allow sand control without having to resort to conventional sand control practices. Stimulation of such long reservoir sections is a particular challenge and a fine balance must be struck between acid coverage and cost, both for barefoot and perforated intervals. The so-called Q-CAJ technique of acid distribution is presented, which has allowed acceptable stimulation despite these conditions. The technique also offers opportunities to stimulate long horizontal boreholes more effectively and at lower cost. Some of the wells are completed with multi-zone selective completions, which push the design limit of equipment. A number of such intelligent wells have been completed successfully and more aggressive wells are planned. In order to optimize the development cost, dual-lateral wells have also been drilled with complete control over each lateral. Discussed are some of the underlying completion techniques such as running completion in stages, use of mechanical latches, hydraulic control line wet connects, interval control valves and limitations imposed by control lines. The paper concludes with some of the challenges that have yet to be overcome. Introduction Maersk Oil Qatar is the operator of the Al Shaheen Field, which is located on the central axis of the Qatar Arch some 70 kilometers north-east of the Qatar peninsula (Figure 1). The main targets comprise of the Lower Cretaceous Kharaib B and Shuaiba carbonate formations and the Nahr Umr sandstone (Figure 2). The Kharaib Formation is a laterally uniform carbonate deposition. The thickness of the Kharaib is generally 80 feet with a reservoir target of some 25 feet. The Shuaiba Formation exhibits lateral facies changes and permeability contrasts. The Shuaiba thickness is generally 200 feet with a reservoir target of some 20 feet. The Nahr Umr Formation is a 20 foot sand sequence with reservoir targets of some 5 feet of permeable sand. The decision to develop with long horizontal wells was primarily driven by 1) the large areal extent of the accumulation, 2) poor vertical well productivity, and 3) number of platform locations required if a conventional approach with vertical or slant wells was pursued. Currently, extended reach wells are being drilled from 9 platform locations. The length of these horizontal wells has been extended during the course of the development and some of these wells are pushing the limit of what can be achieved with today's technology (Figure 3). Completion Philosophy With depletion and the need for water injection support, control of fluid offtake/intake is an important feature of the current development plan in both production and injection wells. It is also important that this control can be effected both quickly and cost effectively. Consequently, independent interval control valves were chosen for this purpose. These valves can be opened and closed from surface as required, depending upon the reservoir conditions pertinent to that particular well.