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Operators targeting the highly-depleted reservoir sections in the United Kingdom Continental Shelf (UKCS) typically experience mechanical instability. The types of instability may include: induced differential sticking, mud losses and wellbore collapse. Throughout this mature field, equivalent circulating densities (ECD) are often close to leak-off test (LOT) results lead to seepage losses through the Jurassic sandstone/claystone sequence. Operators fight the ability to run liners to target depth, often requiring re-drills to access production targets. In this particular program, fracture sizes were predicted to be larger than 500 microns (μm), requiring a specialized drilling fluid technology to prevent and avoid lost circulation and wellbore instability.
In the 6-in. section, a sidetrack was drilled from 15,350-18,423-ft. measured depth (MD) using a 10.1 lb/gal oil-based mud (OBM) with residual FLC 2000®, an ultra-low invasion fluid additive in the drilling fluid. The FLC 2000 concentration (5 lb/bbl) was still in the mud system from the previous well, and the sand-bed-test (SBT) result was 6.5-cm, demonstrating the drilling fluid was able to create a seal against the fluid-rock interface. To protect against the larger fractures predicted, the program required an additional 2-3 lb/bbl FLC SUPREME in the active system, prior to drilling to the Upper Jurassic sandstone at 17,595-ft. MD.
FLC SUPREME was added to the OBM at 17,248-ft. with a SBT result of 3.5-cm. The result of the SBT provided confirmation to the operations team that enough product was in the active system to create a seal against the depleted formation. The FLC SUPREME concentration was maintained throughout the section until total depth (TD). Mud loss to the surface reduced to 0.09 bbl/ft. compared to 0.22 bbl/ft. on average and saved a total of 400 bbl of OBM. Hole cleaning was also effective, as several trips were made with no problems in and out of hole. The equivalent circulating density (ECD) was also consistent throughout the problematic formation, with no spikes or increasing trends.
Solids control was aligned to maintaining residual FLC 2000 in the system, with shale shakers equipped with API 60-mesh screens. Low gravity solids (LGS) ranged from 6-10%, slightly higher from previous wells drilled without FLC SUPREME, but within the acceptable range from the operator. Coarser screens had no effect on rheology or dilution.