Hole Instability in Upper Zakum Field Offshore Abu Dhabi; Need Integrated Solution

Abstract

Abstract Hole instability is one of the serious problems adversely affecting drilling efficiency and costs. Generally, it occurred in different forms as: shale instability, unconsolidated formation eroded by mud flow...etc. The shale destabilization is the worst type of hole instability. In Upper Zakum drilling operations, shale instability is the dominant while drilling troublesome shale sections. The major shale sections are through Nahr Umr formations and thin shale streaks in Laffan, Halul, and top part of Thamma-I formations. This paper is primarily focused on shale instability encountered while drilling ZADCO wells in Upper Zakum field offshore Abu Dhabi. It presents an investigation and engineering analysis of the main causes and different mechanisms of shale instability; chemical interaction, mechanical deformation. The chemical shale destabilization occurs due to the chemical reaction between the reactive clays and incompatible drilling fluid formulations. This reaction leads to the dispersion of reactive shale minerals into the drilling mud. On the other hand, the mechanical destabilization of shale may result from the unbalanced formation pore pressure, hole deviation, drilling practices, even in the absence of reactive shale. The related troubles encountered while drilling the problematic shale section are; excessive torque and drag on connection and while tripping, mud solids build-up and consequently unsteady drilling fluid rheology, stuck pipe, and hole enlargement that results in poor cement jobs. Stuck pipe has costed ZADCO millions of dollars due to non-productive operational time involved in stuck pipe, fishing jobs, and side-tracking resulting from stuck pipe. Detailed analysis of drilled wells and observations along with the result of laboratory tests show that hole instability due to chemical destabilization can be improved by utilising compatible drilling fluid formulation and good quality mud chemicals, and optimising mud rehology. To minimize hole troubles due to mechanical destabilization of shale; it requires proper pre-planning of the wells and careful implementation of the best practice are the key issues. Hole instability needs an integrated solution including; well trajectory, mud weight and formulation, design of bottom hole assemblies, mud solids removal etc. The main objective is to stay out of the trouble better than get out of trouble or prevention is always better than cure.