Managing Reservoir and Operational Decline without a Rig: An Okan Case Study
Abstract: The objective of this paper is to share learnings from the Okan field, highlighting successful strategies adopted to mitigate reservoir and operational decline almost 8 years without producer drilling or major rig workovers. Value gained is quantified to show that over a third of the current Okan production is tied to strategies adopted during the period of interest. Details of the different wellwork methodologies are provided to communicate how value was maximized using minimal cost. Key strate… Show more
Search citation statements
Paper Sections
Citation Types
Year Published
Publication Types
Relationship
Authors
Journals
Cited by 3 publications
References 5 publications
Acid stimulation has been used extensively to improve well productivity, as early as the late-19th century. The method was first applied to carbonate reservoirs. Over the years, it has seen application in sandstone reservoirs. However, it has been more of a hit-or-miss in terms of result. Success and failure of acid stimulation has often been attributed to candidate selection (Nnanna et al, 2009)1. The NNPC/CNL Joint Venture is made up of a total of 26 producing fields in the Niger-Delta basin of Nigeria with over 380 completions currently on production. Focus on production optimization efforts has helped to get the best from the wells, some of which have been on production since 1965. Over the years, production has declined in these matured fields for several reasons; reservoir pressure decline, reduced reserves, increased water and gas production, and near-wellbore damage among others. In cases where well productivity has declined over time in reservoirs with minimal drop in pressure and significant remaining reserves, acid stimulation has been identified as a method to boost productivity. Efforts were therefore made to identify acid stimulation opportunities in a cost-effective manner. With the paucity of cases when acid stimulation has been successfully used to improve oil production in these fields, it becomes pertinent to develop processes to select the best candidates that can yield incremental production with properly designed acid treatment. Also given the large number of completions available to be assessed, a set of technical selection criteria become important to identify potential underperforming wells with near-wellbore damage prior to carrying more detailed analyses like the classic Pressure Transient Analysis. This becomes more important in the current oil price regime with the limitations on spending. With over 180 well-head jackets spread over a total of 2.2 million acres in the JV, key challenges had to be addressed for optimal execution of the stimulation campaign. The mode of acid placement, selection of equipment and order of job execution were decisions that had to be made to arrive at the most economic but operationally successful execution strategy for the campaign. From a shortlist of 19 candidates, a 3-well pilot campaign was successfully executed using this approach with a total production gain of over 1900 BOPD. This paper discusses the detailed selection criteria used, the results of the pre- and post-stimulation characterization of the near wellbore damage using Pressure Transient Analyses, which supported the selection criteria and the holistic technical assessment carried out to determine the best treatment methodology for the candidates. The importance of a full team collaboration will also be highlighted. The results of the campaign, the lessons learned and the best practices based on the outcomes of the execution are documented alongside the operational challenges encountered and how these were managed. Background The NNPC/CNL Joint Venture is operated by Chevron Nigeria Limited (a Chevron affiliate company) with acreage covering both the onshore and offshore areas of the Niger-Delta (Figure 1). First oil discovery was in the Okan Field in December 1963. The JV commenced production in 1965. Currently, 26 oil and gas fields are producing from the Niger-Delta basin. The targeted reservoirs in these Niger-Delta fields are located within the NW-SE oriented Pliocene to recent depocenter, in wave and tidal dominated delta. The Basal Akata marine shale serves as the source rock for the overlaying Agbada paralic sandstone reservoirs. The system is overlain by continental to shallow marine sandstones of the Benin Formation (Figure 2).
Acid stimulation has been used extensively to improve well productivity, as early as the late-19th century. The method was first applied to carbonate reservoirs. Over the years, it has seen application in sandstone reservoirs. However, it has been more of a hit-or-miss in terms of result. Success and failure of acid stimulation has often been attributed to candidate selection (Nnanna et al, 2009)1. The NNPC/CNL Joint Venture is made up of a total of 26 producing fields in the Niger-Delta basin of Nigeria with over 380 completions currently on production. Focus on production optimization efforts has helped to get the best from the wells, some of which have been on production since 1965. Over the years, production has declined in these matured fields for several reasons; reservoir pressure decline, reduced reserves, increased water and gas production, and near-wellbore damage among others. In cases where well productivity has declined over time in reservoirs with minimal drop in pressure and significant remaining reserves, acid stimulation has been identified as a method to boost productivity. Efforts were therefore made to identify acid stimulation opportunities in a cost-effective manner. With the paucity of cases when acid stimulation has been successfully used to improve oil production in these fields, it becomes pertinent to develop processes to select the best candidates that can yield incremental production with properly designed acid treatment. Also given the large number of completions available to be assessed, a set of technical selection criteria become important to identify potential underperforming wells with near-wellbore damage prior to carrying more detailed analyses like the classic Pressure Transient Analysis. This becomes more important in the current oil price regime with the limitations on spending. With over 180 well-head jackets spread over a total of 2.2 million acres in the JV, key challenges had to be addressed for optimal execution of the stimulation campaign. The mode of acid placement, selection of equipment and order of job execution were decisions that had to be made to arrive at the most economic but operationally successful execution strategy for the campaign. From a shortlist of 19 candidates, a 3-well pilot campaign was successfully executed using this approach with a total production gain of over 1900 BOPD. This paper discusses the detailed selection criteria used, the results of the pre- and post-stimulation characterization of the near wellbore damage using Pressure Transient Analyses, which supported the selection criteria and the holistic technical assessment carried out to determine the best treatment methodology for the candidates. The importance of a full team collaboration will also be highlighted. The results of the campaign, the lessons learned and the best practices based on the outcomes of the execution are documented alongside the operational challenges encountered and how these were managed. Background The NNPC/CNL Joint Venture is operated by Chevron Nigeria Limited (a Chevron affiliate company) with acreage covering both the onshore and offshore areas of the Niger-Delta (Figure 1). First oil discovery was in the Okan Field in December 1963. The JV commenced production in 1965. Currently, 26 oil and gas fields are producing from the Niger-Delta basin. The targeted reservoirs in these Niger-Delta fields are located within the NW-SE oriented Pliocene to recent depocenter, in wave and tidal dominated delta. The Basal Akata marine shale serves as the source rock for the overlaying Agbada paralic sandstone reservoirs. The system is overlain by continental to shallow marine sandstones of the Benin Formation (Figure 2).
The Cement Packer completion has found great use and applicability in Chevron Nigeria assets by creating a cost-effective way of accessing ‘behind-pipe’ production opportunities. These isolated hydrocarbon pools could not have been otherwise developed due to the un-favourable cost of a Major Rig Workover. This has helped to maximize value from oil and gas assets by returning previously inactive wellbores to production. The typical through-tubing technique of deploying cement packers in candidate wells has been to set a plug in the tubing to isolate the deeper reservoir, punch a hole in the tubing, displace cement through the hole in the tubing and place it across the new target sand so that the displaced cement would serve as an artificial packer in the tubing-casing annulus. The tubing would then be perforated across the cement packer and into the target reservoir. Much success has been recorded with the use of this technique and though it has provided proper tubing-casing annulus isolation, there have also been a few challenges. Some of the challenges include significant skin (caused by the extra pressure drop during fluid flow through the cement tunnel), limited perforation efficiency in dual-string wells (caused by gun-size limitations due to the tubing size) and lack of radial flow since the perforations are oriented at zero-degree phasing to avoid perforating into the second string in dual-string wells. Eliminating these challenges would significantly improve the well production rate and project economics. This paper presents three case studies where these challenges have been effectively addressed and the attendant results. In the first case study, we show how over-displacing the cement ensured that the column of cement was placed above (rather than across) the proposed completion perforations while still retaining annular isolation. This significantly improved the expected initial production rate of the well by a factor of more than three since there was no extra skin due to fluid flow through the cement tunnel. In the second case study, we show how we improved the perforation efficiency in a dual-string well despite being constrained by the gun size by perforating twice at zero-degree phasing. In the third case study, we show how we overcame the challenge of always perforating at zero-degree phasing in dual-string wells by performing dummy simulations (at the surface) using pipe-in-pipe configurations to better understand the perforating gun orientation downhole relative to the tubings and casing. Based on the results of the surface simulations, we achieved additional phasing in two perforation runs and this significantly increased the productivity of the well. A major lesson learnt was the importance of performing dummy simulations at the surface using pipe-in-pipe configurations to mimic the tubings-casing configuration. This was crucial to the success of the job where additional phased perforations were added.
Sustaining Production in Brownfield Assets by Adapting Existing Rigless Workover Technology to Create Well-Specific Solutions
The changing landscape of global energy and fluctuations in commodity prices have driven companies to get more efficient means to sustain their businesses. With companies seeking more from their assets via rigless interventions, the trend has been that the low-hanging opportunities have been executed. The opportunities left are either too complex for rigless interventions or the expected production is too small to make major-rig workovers economic. We discuss such complex opportunities that were revisited to ensure sustained production from mature fields. The first case study discusses a gas-lift opportunity that was suspended when tubing leaks were discovered. The three holes posed a challenge since they were quite shallow and not suitable to be used as the injection-point of the gas. The work-around was to use a tubing patch to cover the holes without significantly reducing tubing ID as much as a pack-off would to allow for future well re-entry. The second case study discusses a well with a tubing restriction that prevented perforation addition. The job stalled when a gauge cutter (similar OD as the available gun) barely passed through the restriction during a dummy run. Even if the gun passed through and was detonated, the expended gun would be swollen and irretrievable due to the restriction. A modified perforating gun that disintegrated upon detonation was used, eliminating the need to retrieve the expended gun. The third case study was a well completed with mud five decades ago. The mud had caked; preventing any chance of executing a poor boy gas lift. The solution was to adapt the cement-bond-log technique to identify regions of moveable mud in the annulus with the mud subsequently displaced out by pumping. The lesson learned is that opportunities that have stalled due to their complexity can still be executed via rigless workovers by leveraging on existing technologies with slight adaptations to create well-specific solutions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
Contact Info
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Product
Resources
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.
