An important first question when attacking a water production problem is, do significant volumes of mobile oil remain in the pattern or in the vicinity of the well of interest? Three types of observations are commonly used to make this assessment. First, a pumper may notice that certain well(s) exhibit a sudden increase in water cut. Second, a well or pattern of wells may be noted as producing at significantly higher water/oil ratios (WORs) than other similar patterns. Third, plots of fluid production versus time may show an abrupt increase in WOR at a certain point. Results from reservoir simulation studies constitute a fourth, less common method sometimes used by large oil producers to analyze water production problems in large reservoirs.

The oilfield operator should recognize that two distinct types of water production exist. The first type, usually occurring later in the life of a waterflood, is water that is co-produced with oil as part of the oil’s fractional flow characteristics in reservoir porous rock. If production of this water is reduced, oil production will be reduced correspondingly. The second type of water production directly competes with oil production. This water usually flows to the wellbore by a path separate from that for oil (e.g., water coning or a high permeability water channel through the oil strata). In these latter cases, reduced water production can often lead to greater pressure drawdowns and increased oil production rates. Obviously, the second type of water production should be the target of water shutoff treatments.

Understanding and conceptualizing the reservoir “plumbing” is a key to

  1. distinguishing between the above two types of water production,
  2. successfully diagnosing the water production problem, and
  3. successfully implementing and designing water shutoff treatments.