Over the years, the drilling fluids industry has focused on solving problems related to non-productive time, primarily wellbore stability and lost circulation. Today, drilling fluids have evolved into high technology fluids that maximize drilling efficiencies, overcome anticipated drilling risks, and provide a stable wellbore within budget.

Drilling a single hole section to accommodate formations with different pore pressures, formations such as unstable shales and into a depleted sand reservoir under a high temperature environment can be very challenging. To promote wellbore stability, a higher fluid density is often required to balance the highest possible pore pressure and minimize the potential for any formation break-out due to prevailing in-situ stress conditions. Increased fluid density will lead to a higher differential pressure across the depleted reservoir layers, and hence, the risk of lost circulation will increase. The risks of formation stability and lost circulation therefore create greater challenges for the planning team in order to drill the depleted reservoir in a one-hole section.

A novel approach of using polymeric nanoparticles (NPs) in the design of a non-aqueous drilling fluid with an optimized and selective range of pre-determined, sized bridging materials based on software modelling has been applied successfully at the Saqqara field in the shallow waters of the Gulf of Suez to eliminate or minimize the risks associated with drilling the depleted reservoirs. A case study is included, which was the main motivation for writing this paper. The case study covers two wells that were sidetracked and drilled successfully as 6-inch hole sections, penetrating the abnormally pressured formations with a relatively lower fluid density compared to data from offset wells and the subnormally depleted reservoirs, with up to 2,600 PSI overbalance with a bottom hole temperature (BHT) of up to 350° F.

The effectiveness of the polymeric NPs can be attributed to unique characteristics such as a very high specific surface area of 33.7 m2/g, narrow particle size distribution (PSD) between 135 and 255 nm with a D50 value of 179 nm, and their deformable nature leading to effective sealing performance. The effect of these NPs on the properties of oil-based drilling fluids has been evaluated through a variety of experimental lab tests. Rheological properties at different temperatures and filtrate loss were measured to study the effect of the NPs on the base fluid. The study indicated that small concentrations of NPs could provide a better performance for the drilling fluid along with high temperature resistance.

This work focused on the role of NPs in designing a smart drilling fluid with tailor-made rheological and filtration properties. The paper also briefly describes the operations and discusses the challenges related to drilling in depleted reservoirs and how new drilling fluid technology makes difficult wells possible to drill

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