Oil Recovery From Bottomwater Reservoirs

Abstract

Introduction Billions of barrels of oil remain unrecovered in "bottomwater" reservoirs with a highwater-saturation zone in communication with the oil zone. Typically, the water cut in such reservoirs increases rapidly during primary production, leading to low primary oil recovery. If a water zone were not present, many of these reservoirs would be good candidates for waterflooding. Instead, the performance is poor under conventional waterflood and worse if the oil is highly viscous. Several techniques and production strategies have been proposed recently to improve waterflood performance of such reservoirs. These are the subject of this paper. Problems In Producing Bottomwater Reservoirs Water coning is the most common problem in producing oil from a bottomwater reservoir. Once the water reaches the perforations, it is produced in preference to the more viscous oil, and the producing WOR becomes prohibitively high. In heavy-oil formations, a high water cut often causes excessive sand deposits in the wellbore. A specific minimum oil flow rate is required to mobilize sand in most heavy-oil reservoirs. In most cases, with an increase in the water cut, the sand-carrying capacity of the produced fluids drops considerably and the wellbore sands up. The detrimental effect of the bottomwater zone sometimes is alleviated by shale breaks, which reduce vertical permeability. A number of heavy-oil reservoirs with water legs have not been affected significantly by an underlying high-saturation water zone because of the presence of frequent horizontal shale breaks. Similar horizontal shale breaks probably will increase productivity in light-oil reservoirs with bottomwater as well. Field Experience The Bonnie Glenn field of Alberta is a light-oil bottomwater reservoir that has produced successfully under primary production. This light-oil (35 to 40 deg API) reservoir has a water zone, yet the oil production rate has been steady with little water influx from the bottomwater zone. One interesting feature of this reservoir is the presence of a gas cap. Also, the reservoir has a very high vertical permeability, resulting in an efficient gas cap drive with maximum pressure gradient from the top of the reservoir. As a result, oil can be produced with relatively small drawdown and minimum influx from the bottomwater zone. However, the reservoir is maturing after producing (with gas reinjection) some 80% of the original oil in place (OOIP). Currently reported WOR values are high.