Environmental effects of oil-based mud cuttings

Abstract

During both exploration and production drilling for oil, mud is used to lubricate and sometimes provide hydraulic power to the bit, bring cuttings to the surface and control reservoir pressure. Historically, most drilling in the North Sea has used water-based muds (WBM) however, drilling certain formations with WBM can prove difficult due to hole instability caused by swelling of water-absorbing rock. These problems can be greatly alleviated by using muds consisting partly of a mixture of hydrocarbons of which diesel is the most commonly used at present. The fluid part of oil-based mud (OBM) is an emulsion of water in hydrocarbons, typically with a water/oil ratio of 1 to 6 although this may range from 1:1 to 0:1. Oilbased mud has other advantages over WBM; notably it provides better lubrication, reducing the likelihood of differential wall-sticking. It has also been found that significant increases in drilling progress can be achieved by the use of oil-based muds. These advantages are brought to the fore in the North Sea, particularly in the deep-water Northern fields, where the very high investment required for construction of platforms dictates drilling as many wells from one platform as possible. This involves large horizontal well displacements bymore » deviated drilling to exploit effectively hydrocarbon accumulations or reach suitable locations for water injection.« less