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.