An oil well with two perforated zones had an initial production rate of ~2,400 BOPD with water traces. Within 3 months, production decreased to ~1,000 BOPD and water cut increased to >25%. It was critical to identify the cause of decreasing productivity and increasing water cut to plan for remedial action.
The reservoir was evaluated by integrating several answers obtained from reservoir saturation characterization through use of pulsed neutron capture and water-flow log data, as well as from conventional and advanced production logging.
Production logging showed that water was sourced from the lower perforated interval, while only a small proportion of the flow came from the upper reservoir layer. Pulsed neutron logging confirmed a high amount of depletion in the zone below the lower perforation, with oil remaining in the upper part of the lower perforated zone and in the upper perforated zone. Bypassed oil was also found below the water-producing zone, confined between the oil/water contact and a thin shale break. This shale break is a major permeability barrier and allows lateral water movement through the high-permeability lower zone.
A multilayer test carried out with production logging tools showed that the productivity index of the upper zone was only one-tenth that in the lower zone, due to either intrinsically low permeability or high skin factor. The multilayer testing also showed that of the two comingled layers, the upper layer had approximately one hundredth the permeability of the lower layer. Reperforation of the upper zone resulted in only minor improvement in well production, thus confirming that the low productivity was not caused by high skin factor, but was due to low permeability.
From the testing, it is clear that to produce the oil from the upper zone, it will be necessary to produce the two zones separately, with different drawdown. Since similar conclusions were obtained in other wells in the field, these results provide a fieldwide strategy for improving field productivity.