The Effect of Water-based Mud on Borehole Stability of Deeper Formations in Field X

Abstract The migration from water-based drilling fluids in the 1990s to predominately synthetic based drilling fluids in the 2000s by a major operator in the Niger Delta was not predicted on any scientific research on the effects of water based drilling fluids on borehole stability. In spite of this migration, the operator continued to have borehole stability problems in field X. In addition the cost of drilling fluids continued to rise to about 10% of the well cost in field X. Also there has been increasing stringent requirements by the regulators on the use of synthetic based drilling fluids. This paper describes the results of the investigation on the possibility of using water based drilling fluids to drill the deeper sections of wells in field X. The overall intent of the investigation is to reduce the drilling fluid cost and improve the environmental and regulatory performance of the operator. The project methodology consists of two approaches. First is a study of borehole stability problems in the project area. The second method consists of experimental studies of the field X formation samples to determine the mineralogy of the formations. Based on the study, a water-based mud system that could be used to drill these formations was developed. The result of the borehole stability study showed that borehole problems occurred in wells drilled with water based mud and as well as those drilled with pseudo-oil based muds. The experimental studies showed that the highest percentage of the smectites/illite reactive clays in the samples examined was 12%. Use of 7% KCl solution caused a reduction in the shale reactivity by as much as 39.6%. Use of a 9% KCl with 8% base oil water-based fluid led to a very low clay dispersion of 6.5%