This paper focuses on the potential problems facing the operator in extended reach exploration and development drilling. A practical review is presented of world-wide extended reach drilling fluid experience with examples of successful technology application from specific projects. Critical drilling fluid success factors in the planning and construction of an extended reach well include borehole stabilisation, hole cleaning, lubricity and equivalent circulating density (ECD) management. The latest techniques in monitoring drilling fluid and wellbore behaviour while drilling an extended reach well are also introduced.
Drilling extended reach wells involves some critical issues that can pose significant challenges for the operator. From the drilling fluids' perspective, these include:
Narrow Mud Weight/Fracture Gradient Window
Torque and Drag
The drilling fluid is a key success factor in extended reach drilling (ERD) and, historically, oil or synthetic base muds have tended to be the fluids of choice. However, in the current era of increasingly stringent environmental constraints, the industry is striving to expand the fluids technology envelope with the development of more inhibitive water-based mud systems, in conjunction with suitable lubricants, to replace invert emulsion muds.
Drilling fluids for ERD wells are engineered to provide a flatter rheological profile in order to minimise the effect of the fluid rheology on ECD. Performance-enhancing products are used to boost the low shear rate viscosity, a critical factor in achieving good hole cleaning and avoiding barite sag.
Experience on some of these ERD operations has led to the development of many torque and drag reducing products and techniques. For example, a novel fibrous lost circulation material was found to dramatically reduce torque and played a major part in the successful drilling of world-record ERD wells at Wytch Farm and Tierra del Fuego.
Unique downhole hydraulics and hole cleaning modelling software has been used in conjunction with down-hole pressure while drilling (PWD) tools to accurately plan and predict fluid hydraulics and cuttings transport to great effect on the world's longest wells. Now incorporating the effect of cuttings loading in the annulus, together with the effect of pipe rotation and pipe eccentricity, this software has become an invaluable operational tool.
Invert emulsion muds have been the key ingredient in successful long reach drilling developments in many areas of the world, e.g. Wytch Farm, Argentina and the Gulf of Mexico. This is the direct result of their ability to provide high lubricity, stabilise reactive clays, preserve hole stability, resist contamination and produce firm, dry cuttings. Torque and drag readings and the potential for differential sticking are substantially lower, using an invert system, with an enhanced ability to slide and less tendency for cuttings bed compaction. All these factors combine to make invert emulsion muds the fluids of choice for extended reach wells. Invert emulsion muds, based on mineral or synthetic base fluids with low kinematic viscosity, are well proven in the field and provide a low ECD, excellent hole cleaning and cuttings suspension and extremely stable mud properties.
The most suitable water-base fluids currently available for ERD drilling, when shale inhibition is required, are potassium-based, non-dispersed, polymer muds containing glycol or silicates. When inhibition is not required, low solids polymer formulations or mixed metal silicates may be used. These systems will provide the required hole cleaning and their use, with a suitable lubricant, can be highly effective.