Abstract
Eighteen (18) water boreholes were studied for lost circulation. When locations of the boreholes associated with lost circulation were plotted on the map of Anambra State a lost circulation belt was observed around the River Niger – Onitsha – Oba – Nnewi axis. Lost circulation intervals range between 20-50m and 75-90m depth and were found to be associated with lignite and peaty clay members of the Ogwashi-Asaba Formation and/or the Ameki/Bende Formation. This suggests that natural rather than anthropogenic factors are responsible for the widespread occurrence of lost circulation in the State, although the latter could aggravate the former. Lignite and peaty clay of the Ogwashi-Asaba and Ameki/Bende Formation cause lost circulation because they are woody to earthy in nature and are easily eroded when they occur above the water table. These conditions favour the development of vugs, leading to increase in permeability and porosity, making the formation prone to lost circulation. Pre-drilling geophysical, geological and hydrogeological investigations are recommended, since these would go a long way to identify the presence or absence of lignite and thus help to avert possible lost circulation intervals. Journal of Applied Sciences and Environmental Management Vol. 10 (3) 2006: 31-35
Highlights
The study of the occurrence of lost circulation associated with lignites was carried out for some localities of Anambra State, southeastern Nigeria
Air drilling is very valuable in these areas and compressor-mounted rigs are recommended for combating lost circulation
Lost circulation could be very expensive to remedy if it is drastic as recorded in some parts of the Anambra Basin
Summary
The study of the occurrence of lost circulation associated with lignites was carried out for some localities of Anambra State, southeastern Nigeria (fig. 1). The study of the occurrence of lost circulation associated with lignites was carried out for some localities of Anambra State, southeastern Nigeria Lost circulation is loss of substantial quantities of drilling mud to an encountered formation during borehole drilling. This is evidenced by a total or drastic reduction of returning mud and a reduction in the volume of mud in the mud pits. The undesirable effects of lost circulation include; Loss of drilling energy, sudden undesirable speed increase of the rotary, deflection of the bit along joint planes or even breaking of the bit, drilling fluid may be totally lost, increased cost of operation, time wasted in pulling back and/or combating lost circulation, drop in annular level may cause blow out in over-pressured or gas-bearing formations, loss of information from the down-hole, the chances of stuck-pipe and fishing exercise are increased, if lost circulation occurs in an aquiferous zone, or slightly above it, completion and development of the borehole may be impaired. The source of the sediments into the basin is principally from the Cameroon massif and the Abakaliki synclinorium, (Nfor, 2003)
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