An Abu Dhabi operator faced a drilling challenge in the Bu-Hasa field. It wanted to drill trouble-free through the difficult Simsima formation, which primarily consists of badly fractured and vuggy limestones. Previous drilling attempts suffered from excessive mud losses, even with aerated mud systems.

The operator was concerned with drilling performance when attempting to penetrate severe loss zones due to the impact on wellbore integrity. It needed a special drilling fluid that could produce optimum hydraulics on the bit and help prevent mud losses, and would result in a well bore they could case and cement without problems. The operator invited Halliburton’s Baroid Fluid Services to engineer a fluid solution designed to meet these challenges in order to maximize its wellbore value.

Non-damaging fluid Baroid’s MAXDRIL-N fluid is a non-damaging drilling fluid designed to provide low plastic viscocity and a high yield point with a low funnel viscosity. These properties and its high low-end yield point generate hole cleaning efficiency at a low flow rate, resulting in a reduction of equivalent circulating density. It quickly gels up as fluid is lost to a fractured, vugular formation, which can reduce or halt fluid losses as fluid velocity declines and viscous drag increases.

In addition, increased effective viscosity and decreased shear rate adjacent to the wellbore help reduce seepage losses. And, with its suspension characteristics and thixotropic properties, it requires low pressure to break the gel. Its laminar flow, flatter velocity profile generates telescopic layers with high velocity at the central layers and reduces to zero toward the borehole, resulting in decreased friction-pressure losses.

The fluid mixed metal silicate system drilling fluid was tested in the Bu-Hasa field by drilling a 121?4-in. hole through the Simsima formation to the Shalaif formation. The results of the first hole drilled were more than satisfactory. The hole was drilled with an average rate of penetration of 113 ft/hour (34 m/hour), and the 95?8-in. casing was run and cemented to the surface without incident. Subsequently two additional wells were drilled with the same fluid with similar results.

The operator estimates cost savings on each well averaged US $65,000 from a combination of lower fluid losses and faster drilling. In addition, there are lower health, safety and environmental risks compared with using an air drilling package.