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.

Horizontal wells enable drainage from a longer wellbore which helps to allow lower drawdown rate compared to vertical wells, minimizing gas or water coning. However productivity can be seriously affected unless mud cake damage is efficiently removed from all producing intervals along the horizontal wellbore. In recent years eco-friendly and non-corrosive bioenzymes (α/β-amylase) have shown great potential in cleaning wellbores uniformly and achieving higher well productivity. However in a low pressure fractured reservoir, there is always a possibility of localized reaction and loss of the clean-up fluid, unless the reactivity of the fluid is engineered based on the given well parameters. In this study α-amylase enzyme is modified to withstand higher thermal shock by structurally reinforcing the β-Helix layer to strengthen the catalytic centre by preferential protein hydration technique. Buffering was done to maintain different system pH and kinetic rate constant is derived through reducing sugar release measurement by DNS method using starch-xanthan gum-CaCO3 based drill-in-fluid as substrate. Though the overall reaction is extremely complex, a good correlation could be drawn between system pH and the rate of breaking mud cake into simple sugar. The kinetic rate constant index is used in final formulation of enzymatic clean up fluid for application in high temperature (110 °C) long horizontal wells drilled in carbonate formation, which allowed different soaking time due to operational constraints. The results show that there is excellent correlation between laboratory prediction and clean up efficiency in terms of well productivity.The study showed that each individual well demands a specific formula for clean-up fluid and higher than prognosed production could be achieved through custom formulation, based on well condition and operational requirement.

Achieving long term well integrity in deep gas wells offshore Abu Dhabi has presented some unique challenges. Historically, cases of poor zonal isolations has led to pressures in annuli and even sea-bed seepages. In one case this had caused well-head movement, causing concerns about the integrity of well-head platforms and surface equipment. Corrective actions to remedy failures has cost the operator millions of dollars in addition to taking up valuable resources and rig-time. The challenges were to ensure proper zonal isolations in these deep gas wells, where several stacked reservoirs have to be drilled prior to the target Khuff reservoir. Well-bore instability in shale formations and lost circulations into weaker, fractured carbonates presented significant complications, which seriously impacted the efforts of achieving zonal isolations in the multiple reservoirs. Thus, an integrated approach was undertaken to overcome these challenges. This encompassed well and casing design, drilling fluids formulations, cement designs, fit-for-purpose equipment, in addition to maintaining a stable well-bore and addressing the loss circulation issues that effectively raised the fracture gradients of weaker formations. The paper provides an overview of the optimization processes involved in the successful planning and execution of the wells. The paper describes the development and application of drilling fluids for different hole sections to achieve well-bore stability as well as mitigation of lost circulation incidents. The particular fluids technique used to strengthen the weaker formations resulted in the ability to place the cement slurries in the well-bore as designed. As described in the case histories, the adaptation and application of various fluid technologies to arrive at fit-for-purpose solutions and the engineering of the total integrated drilling and cementing fluids solution was crucial in achieving the necessary zonal isolations to ensure long-term well integrity of the gas producer and injector wells.