Advanced slim hole drilling technologies being introduced for creating cost-effective deep exploration and development wells require a totally new type of drilling fluid that minimises parasitic pressure losses while maintaining effective suspension properties at high temperatures.
A set of high-density organic brine systems based on formate salts has been designed to meet these, and other performance criteria visualised as necessary for an ideal deep slim hole drilling fluid. High-temperature-high-pressure rheology and fluid loss experiments, in both conventional laboratory equipment and IFP's fluid test loop, have confirmed that the new brine formulations have good thermal stability to at least 150 degrees C.
The results from many detailed rheological measurements on these formulations will be fed into advanced fluid hydraulic models to validate the original design concept for minimising parasitic pressure losses in deep slim hole wells by using low-viscosity brine-based drilling fluid systems.
Slim hole drilling technologies, utilising miniaturised conventional hardware, are being increasingly adopted by Shell companies to help reduce drilling costs and minimise waste production. Encouraging results have been obtained with slim hole drilling at depths down to 5,000 metres, but as Shell's search for new hydrocarbon reserves shifts to even deeper horizons it has become evident that more sophisticated slim hole technologies will have to be developed to deal with the increasingly difficult downhole conditions that will be encountered in the future.
FLUID DESIGN ISSUES
There is a particular need for an improved drilling fluid system that has high density and good thermal stability and can transmit hydraulic power downhole with minimal parasitic pressure losses, while maintaining borehole integrity and promoting effective hole cleaning. Traditional solids-weighted drilling muds may be unsuitable for this purpose because they can develop high frictional pressure losses during circulation in deep slim holes, resulting in low hydraulic power transmission to the downhole mud motors, and the occurrence of high equivalent circulating densities (ECD's) and high swab/surge pressures in the narrow annulus.
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