Time Lapse PTA to Determine the Impact of Skin, Reservoir Compaction, and Water Movement on Well Productivity Loss: a Field Example from WDDM, Egypt

Abstract

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.

PTA-metrics for time-lapse analysis of well performance

“…Permanent well and reservoir surveillance with PDG and time-lapse PTA is today employed in the oil and gas industry with focus on well performance and behavior with their changes during reservoir life-time, including fluid contact movements and recovery processes (Gringarten et al 2003), (Yaich et al 2012), (Ugoala et al 2013), (Rushatmanto et al 2017), (Suleen et al 2017), (Cassie et al 2018); well stimulations (e.g., acid and fracture jobs) and monitoring of well performance over time (Shchipanov et al 2014), (Shchipanov et al 2017a(Shchipanov et al , 2017b; and any kind of improved/ enhanced oil recovery pilots such as water shut-off (Aamodt et al 2018) and silicate injection (Skrettingland et al 2012), (Skrettingland et al 2014). Figure 1 illustrates typical timelapse pressure transient responses for sequential well shutins from (Shchipanov et al 2017a).…”