Bottom-Water and Edge-Water Effects on Low Gravity Oil Production

Abstract

Abstract Water encroachment into low gravity oil reservoirs tends to be irregular because of three mechanisms -selective encroachment,water-leg tonguing andindividual well coning. An investigation was made of 31 wells that were completed in low gravity oil sands with bottom water present in the base of the well-sand interval. It was found that initial water cuts occurred rapidly. Current average recovery for each of the 31 original completions is 10,184 bbl (31 per cent) clean oil and 22,542 (69 per cent) wet oil. Fourteen (45 per cent) of the 31 original completions have either been recompleted to other sands or are not feasible to produce because of high water-cut production. Average cumulative recovery for each original completion is now only 32,726 bbl of oil. Four low gravity oil reservoirs with well defined fault boundaries and original oil-water contacts were studied to ascertain the effects of irregular edge-water encroachment. This study showed that initial water cuts have rapidly occurred in all 12 (including the most up-dip) wells completed in the four reservoirs; only two of these wells were completed with an original water-leg extension in the base of the well-sand interval. Comparative clean and wet oil recovery from these four reservoirs has been 419,100 bbl (34 per cent) clean oil vs 810,500 bbl (66 per cent) wet oil. Ultimate recovery from the abandoned MG–30 reservoir is 1.9 per cent of the 8 million STB of oil initially in place. It is concluded that selective encroachment of edge water along thin, highly permeable, stratum has acted to reduce ultimate recoveries in the four reservoirs from 7 per cent - the recovery that would be expected if solution gas drive were the depletion mechanism. Two basic remedial techniques offer a possible solution for eliminating adverse (irregular) edge-water encroachment. One of these is gas pressure maintenance. The second technique embodies the production of water from the more down-dip areas of the reservoir to balance water influx from the associated aquifer.