Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/183

Environmental Impact of the Oil Industry

Although the Nigerian oil industry does not use chemical (oil based) drilling fluids, the discharge of the muddy drilling fluids (see section 3.4 of the previous chapter) into the neighbouring drainage systems is a serious problem, increasing turbidity and organic matter content often substantially increasing biological oxygen demand. These problems are particularly acute in mangrove environments and during the dry season when flushing rates are low. The 1995 World Bank report estimated that approximately 7 million m³ of drilling waste has been produced in the Niger Delta since oil prospecting began, an average bore hole giving rise to 2,500 m³.

If the well proves to be dry, some well linings and cement plugs are left in the ground: see section 3.7 of the previous chapter. 181