Surveillance Planning in A Mature Multi-Stacked Oil and Gas Field - Enabling Production and Lessons Learnt
Authors: Umoren Odudu , Olominu Oluwafemi , Idehen Festus , Ayuba Aminat , Bassey Anthony , Jaiyeola Jeffrey , Adelanke Bola , Ubendu Obisike , Avwunudiogba Andrew , Dimkpa Benson 
Published: October 2020

Abstract
This paper outlines the systematic approach adopted in carrying out post-production fluid content and contact analysis in a mature Niger Delta field with more than 50 years production of oil and gas in multi-stacked reservoirs with varying fluid properties and moderate to high rock properties. The surveillance data acquired revealed varying dynamic behavioral patterns in the reservoirs. The result was a vital guide in rigless production adding intervention activities, in addition to well placement decisions.
A sand penetration chart was developed from the initial analysis and stacking of the fluid distribution in all the reservoirs encountered by the wells defining the valuable wells to be logged. The volumetric and production information from the reservoirs were reviewed for reservoir ranking purposes. The structural location on the wells was also a screening consideration for the well selection. Having established the reservoir viability, the wellbore accessibility was investigated and wells for logging were ranked based on all the parameters earlier established. The logging activities were sequenced to minimize production deferment. Processing and interpretation of data acquired was an iterative process between the logging contractor and the asset operator.
Post-production Carbon/Oxygen saturation logging was carried out in 2018 with 8 out of 40 wells logged. The results of the data acquisition revealed different dynamic behavior patterns in the reservoirs logged.
Some reservoirs exhibited non uniform flushing, as crestal wells were observed to be more flushed than flank wells in the same reservoir. Reservoirs earlier interpreted to be oil-bearing were logged and interpreted to be gas-bearing, indicating that a secondary gas-cap had developed Some gas-bearing reservoir columns were observed to be re-saturated with oil, resulting in additional scope for oil completion in some wells Reservoirs with high viscous crude showed patchy drainage patterns with a lot of by-passed oil in wells close to the vicinity of high BSW producing wells. Few reservoirs also exhibited piston-like production pattern
The variability in the result indicates that the reservoir system is not as simple as previously interpreted. Though the reservoirs are continuous, and the rock properties are generally good, there is likelihood of the existence of baffles, which also have controlling influence on the dynamic behavior of the reservoir. However, the results were a key input in identifying intervention activities and optimal in-field well placement.
Subsurface engineers most times have the understanding that the dynamic behavior in the Niger Delta reservoirs is controlled mainly by the structural configuration due to the continuity and good rock properties of the reservoirs, however this is not necessarily the case in all situations as stratigraphic components can also have a controlling influence. Acquiring time-lapse saturation data for calibration and better understanding of reservoir dynamics is vital for optimal reservoir development.