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Rate of penetration (ROP) increases significantly if salt formations are drilled using undersaturated fluids. This is especially true when drilling riserless. Of primary concern is the amount of hole enlargement that will occur. However, if managed, then a step-change in drilling performance and costs can be achieved. This paper presents, in detail, the results of a comprehensive and large-scale laboratory testing program performed on outcrop salt samples that replicate drilling salt formations with undersaturated drilling fluids with flow rates of up to 1000 gpm. The laboratory-testing program includes tests performed in a CT-scanner to map hole enlargement in real-time, as well as variously sized borehole leaching tests with borehole diameters of up to 6". Flow rates are scaled to produce comparable levels of turbulence occurring in the field. An analytical hole enlargement prediction model is presented that incorporates the effects of ROP, pump rate, drill-fluid saturation, dissolving salt drill-cuttings and salt leaching from the borehole wall. This model accurately predicts, to within 10%, the measured hole enlargements produced in the scaled laboratory tests, for a wide range of flow rates and fluid saturations. Predictions of field performance are made, and the implications of the predicted hole geometry discussed. Provided high rates of penetration are realized, acceptable hole geometries will result, even when pumping seawater at flow rates of up to 1600 gpm. A field application is described, whereby the historical practice of drilling the last 200 ft of the 20" hole into salt using a saturated brine drilling fluid was discontinued in preference to continued use of seawater. The competing influences of increasing rate of penetration and avoiding the cost of a sacrificial mud system, offset by the increased cost of cement, resulted in cost savings of $250,000 per well. Implemented in a multi-well sub-salt development, cost savings of over $1.5 million will be realized in the drilling program. Further applications of this technology are now being sought. |
