A Second Life for a Giant: Cased-Hole Pulsed Neutron Logging in Complex Completions and Challenging Fluid Scenarios
- Gabriele Duci (Eni S.p.A.) | Roberto Zarauti (Eni S.p.A.) | Alessandro Fasto (Eni S.p.A.) | Marco Pirrone (Eni S.p.A.) | Giuseppe Galli (Eni S.p.A.)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA 60th Annual Logging Symposium, 15-19 June, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
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Years and years of huge hydrocarbon exploitation from a giant field results in a not efficient production optimization strategy due to the high uncertainty in current reservoir fluid distribution. This scenario can be even more challenging in case of old and complex well completions and areal field compartmentalization.
This paper discusses design, interpretation workflow and results of a massive cased-hole pulsed neutron campaign performed in such conditions. The outcomes have driven several targeted well interventions for additional hydrocarbon production.
The presented case study deals with extensive pulsed neutron use in a reservoir characterized by more than three thousand meters of gas, oil and water bearing terrigenous sequence. An integrated capture (sigma mode) and inelastic (carbon/oxygen mode) approach overcomes the criticality of a strong changing in formation water salinity (one order of magnitude from hundreds to tens ppk). Small tubing in large casing environments, long perforated sections, different fluids in completion make the interpretation even more complicated. The available open-hole formation evaluation represents the input for the pulsed neutron modeling while a standalone cased-hole formation evaluation has been deployed in the oldest wells characterized by a limited open-hole log dataset. Actual water saturation and hydrocarbon type from the described approach have been used for water shut-off interventions and new perforations in front of bypassed oil bearing levels avoiding undesired gas production.
The aforementioned production optimization activities for all the analyzed and treated wells resulted in an overall increase in oil rate of about 650% and a watercut reduction of about 40% respect to the previous performances.
The onshore field described in this paper (from now on called Alpha) is characterized by a Plio-Pleistocene clastic sequence and by a complex structural setting. The main reservoir is represented by a terrigenous series, including more than three thousand meters of gas, oil and water bearing of sandstones.
Alpha is densely populated with many explorative, appraisal and development wells. It was put on production almost four decades ago, and it is still producing from several reservoir layers. Faulting plays a key role in the trapping mechanism, with many of the faults sealing thick sand units. It is also known that there is an element of stratigraphic trapping to some reservoirs. Hence, the presence of an intricate fault framework (see Figure 1) has strongly influenced the reservoir performance in terms of oil, gas and water production.
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