Roller cone bits have dominated the 8–1/2" steerable application in Abu Dhabi due to their proficient steerability, consistently achieving the required build rates in the soft Nahr Umr shale. In the vertical section, PDC bits often achieve up to four times the penetration rates of TCI bits. However, directional performance when using PDC bits to build angle was compromised by fluctuations in reactive torque resulting in poor tool face control and inconsistent build up rates. The overall result was poor penetration rate and performance compared to roller cone bits. The operator and a service company utilized a new design process and cross functional team approach to aggressively seek new steerable PDC technology to drill the curved section with controllable torque response and consistent directional behavior while achieving the full penetration rate advantage of PDC bits. This process was complemented by the Well Delivery Limit process (WDL) that was already established by the operator that focused on delivering high value wells with significantly reduced costs. The team analyzed the drilling operations from virtually every perspective using numerical models, laboratory drilling tests, and field testing. The key to the PDC solution was the team process that identified the relevant drilling problems and performance requirements in order to develop the needed technology. After 30 bit runs from November 2001 to June 2002, the team reduced drilling costs by $44.26 per foot for a total savings of $3,792,367 over 85,682 ft and 2,386 drilling hours. The average ROP for the section doubled from 18 ft/hr with TCI bits to 36 ft/hr. The current ROP record stands at 60 ft/hr over a 3,169 ft interval. Introduction The operator and bit manufacturer have a history of joint optimization of roller cone technology in this 8–1/2" interval, improving cutting structure and metal face sealed bearing performance. The operator has documented these achievements, along with two failed attempts to drill wells with PDC bits.1 Despite advances in directional drilling technology and more powerful positive displacement motors (PDM), even as recently as late 2001, IADC code 517 roller cone bits were the only technology that could provide both tool-face control and build up rates required for the well plans. The lithology for this section consists of short section of limestone, followed by long section of soft shale to the landing point in a limestone reservoir. All PDC attempts failed to achieve any significant drilling improvements because of poor tool face control and inadequate build rates through the soft shale. Seeing an opportunity to reduce drilling costs, engineers from the operator, bit supplier and motor supplier developed a new technology for steerable PDC bits through an open and cooperative process. Well Plan The 8–1/2" interval starts at approximately 6,600 ft at the bottom of a limestone interval where unconfined compressive strength (UCS) ranges between 9 and 21 ksi. At approximately 7,400 ft, a 3 to 5 ksi UCS shale is encountered which extends to the top of the reservoir at 8,100 ft. Almost half of the directional work takes place in the soft shale before the reservoir is penetrated at around 40 degrees of inclination. The limestone reservoir features alternating soft and hard layers, with the softer rock around 9 ksi UCS and the firmer rock 15 to 26 ksi UCS. Figure 1 shows a plot of the rock type and compressive strength versus depth while Figure 2 shows two typical well plans. Directional work continues until the well is horizontal and measured depths often extend to 9,500'.

Drilling offshore Abu Dhabi presents some unique challenges primarily related to wellbore instability in directionally drilled wells through the unstable Nahr Umr and Laffan shale formations. This not only affected drilling performance and cost millions of dollars due to non-productive time (NPT), but had also placed constraints on the Company’s field development plans, as it was not possible to drill high deviation wells through these shales with a high degree of success. It was a major challenge to develop and engineer a water-based drilling fluid that could effectively eliminate shale instability encountered while drilling these high deviation wells. The stabilization of these highly laminated shales required an effective synergistic blend of products to both maintain the chemical stability of the shales and minimize the transmission of fluid pressure into the shales, while sealing both natural and induced micro-fractures. The development of the ideal types and blend of products, treatments and engineering techniques tailored to these specific rock requirements is crucial to drilling success. The net result of the correct application of this unique drilling fluid system has been to mitigate the incidences of cuttings accretion, wellbore instability, time-related failure and induced losses into permeable formations, thereby eliminating drilling non-productive time. This paper describes the development and field application of a uniquely engineered water-based drilling fluid that demonstrates enhanced shale stabilization properties, much like the invert emulsion systems, but without the associated waste management equipment and logistics and environmental exposure. Additionally, the development and use of a new class of sealing additives is outlined showing how improved stabilization of the wellbore can be achieved while drilling these older, highly laminated shale formations. Extensive laboratory work was carried out under simulated conditions, on both the fluid system and sealing additives to optimize the fluid formulation. Thereafter, successful field applications were carried out in high inclination wells in the area. Drilling performances were greatly enhanced compared to conventional fluid systems, which accrued significant cost savings to the operator. Further, there is additional flexibility to drill high deviation wells through these shales, as required to reach a reservoir target. This development approach and the use of the unique stabilization additives can be utilized in many other areas where issues of wellbore instability in older and laminated or fractured shales exist.

Cementing gas wells in Khuff reservoir where sustained annular pressure has been reported in many wells, presents a big challenge offshore in Umm Sheif field in Abu Dhabi. The main challenges are preventing gas migration and achieving long-term zonal isolation using a competent cement sealant system able to withstand downhole stresses and high temperatures during production cycles. An extensive study was carried out on two previously cemented wells, and an in-depth analysis was performed on the cement systems pumped in this field. The knowledge gained paved the way for designing, planning, and executing successful cementation on a recent well. A mathematical model simulating downhole stresses in terms of pressure and temperature was used in designing the cement sealant system. Unlike conventional cement systems, properties such as high Poisson's ratio and low Young's modulus value compared to that of the rock were optimized in the new system to achieve mechanical resistance and durability. This paper describes a case history whereby this cement sealant system was used to cement and secure the critical phases of this well. Cement evaluation logs across all the liners showed complete cement coverage and zonal isolation, especially across the Khuff gas reservoir. Introduction Any well containing a gas bearing formation is a potential candidate for annular gas migration. Cementing wells with potential gas migration is a critical operation. It requires from both service company and operator full and complete cooperation to gather the necessary data in order to optimize the design of the cement slurry, as well as its placement to mitigate the risk with the gas migration. The severity of gas channeling or migration can range from the most hazardous, blowouts, to less severe cases of residual gas pressure of a few psi at the wellhead. The main concern is the gas migration behind shallow casing strings or behind long liners For shallower casing strings cemented to surface the slurry volumes are usually large, holes are washed out, centralization is poor and frequently the fracture gradient will not allow the pumping of heavier fluids. For liners, more attention is usually paid to design and mud removal, however long liners pose problems of large temperature differentials between the top and bottom sections. This poses problems when the gas zone is near the top of the liner section.

ADCO's experience while drilling the sixth deep Khuff well with water base mud to total depth of 21682 ft and a bottom hole static temperature of 465 F has highlighted mud related problems. The mud has shown gelation, barite sagging, sensitivity to formation water influx and possible instability at densities higher than 122 pcf. These problems were considered in the selection criteria of the drilling fluid for the seventh deep Khuff well which required a drilling fluid density of 110–140 pcf with stable HTHP fluid loss at temperature up to 465 F, capable to tolerate influx of CO2, H2S, and high salinity formation water. Studies on various oil & water base muds were performed in various laboratories including ADCO's, the Shareholders' and the mud companies'. The development of stable water base mud was not possible. Oil base mud formulation was developed and has been used. The well has reached its total depth at 20085 ft & a bottom hole temperature of 440 F after facing operational challenges requiring mud weight of 155 pcf to control the well and during which, the OBM choice proved to be a success. Introduction ADCO's experience in drilling five deep Khuff wells, using water base mud encouraged the drilling of the sixth well utilizing water base mud. The well was drilled to total depth at 21682 ft. Loss of circulation occurred into Jilh formation at 14570 ft and was controlled by decreasing mud weight from 100 pcf to 97 pcf. Intermittent formation water influx from the Lower Khuff and Pre-Khuff formations from 19146 ft to 21682 ft occurred although 7" liner was set between the two zones at 20220 ft, The bottom hole temperature of 465 F caused mud degradation in the from of gelation, barite sagging and failure to tolerate higher densities. Differential sticking occurred across Upper Khuff at 17940 ft after increasing the mud weight from 96 pcf to 110 pcf in an attempt to stop formation water influx from Lower Khuff. Failure to shape the mud density and rheology, as required, probably was the cause of the problem which occurred while cementing the 4 1/2" liner and ended up in cementing the setting tool and drill string in hole and abandoning the well primary objective after excessive fishing operations. P. 257

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