Three productivity loss mechanisms - skin damage, water invasion, and reservoir compaction - are identified in one of the concessions co-owned by BG Group in the Nile Delta of Egypt. To inform the decision if and how to intervene to reverse the loss in productivity, it was important to understand the relative level of impact of each of these mechanisms on the observed drop in productivity.

Pressure Transient Analyses (PTA) of permanent downhole gauge (PDHG) build-up data, obtained during well shut-ins, allows identification of the contribution of skin and effective permeability thickness (kh) to a given well productivity loss. By monitoring build-ups over time, any loss in observed productivity can be attributed either to increased skin or reduced effective kh.

Knowing that effective kh reduction can be as a result of reservoir compaction, a reduction in net reservoir thickness due to water movement, or a reduction in relative permeability due to saturation changes, a number of control wells were selected and categorised, based on their levels of water production - negligible to low water, and moderate to high water producers. The moderate to high water producers already give indication of water breakthrough and possible gas-water-contact (GWC) movements.

Time lapse PTA from these control wells enabled the tracking of changes in skin and effective kh over their producing life.

The results of the investigation showed that although reservoir compaction may be occurring, the effective kh reduction for the low liquid producers is minimal. Hence increasing skin is believed to be the primary cause of the observed productivity losses. For the wells with high water cut, the productivity loss is attributed to both an increase in skin and a reduction in effective kh due to reduced net reservoir thickness and two phase flow in the reservoir.

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