Acid Stimulation by Bull-heading and Back-production: The Bonga Experience
- Ehimhen Okoh (Shell Nigeria Exploration and Production Company) | Olugbenga Olatunbosun (Shell Nigeria Exploration and Production Company) | Oluseye Ogunsina (Shell Nigeria Exploration and Production Company) | Enor Uche (Shell Nigeria Exploration and Production Company) | Kingsley Eta (Shell Nigeria Exploration and Production Company) | Lukman Oduola (Shell Nigeria Exploration and Production Company) | Maye Beldongar (Schlumberger Nigeria Limited)
- Document ID
- Society of Petroleum Engineers
- Nigeria Annual International Conference and Exhibition, 6-8 August, Lagos, Nigeria
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 1.8 Formation Damage, 5.2 Reservoir Fluid Dynamics, 5.1.1 Exploration, Development, Structural Geology, 5.6.4 Drillstem/Well Testing, 3.3 Well & Reservoir Surveillance and Monitoring, 4.5.3 Floating Production Systems, 4.3.4 Scale, 2.4.3 Sand/Solids Control, 4.3.1 Hydrates, 4.1.2 Separation and Treating, 2.2.2 Perforating, 4.1.5 Processing Equipment, 3.2.4 Acidising, 1.10 Drilling Equipment, 3.2.5 Produced Sand / Solids Management and Control, 3 Production and Well Operations, 1.4.3 Fines Migration, 4.2 Pipelines, Flowlines and Risers, 4.2.3 Materials and Corrosion, 1.2.3 Rock properties, 4.5 Offshore Facilities and Subsea Systems, 2.5.2 Fracturing Materials (Fluids, Proppant), 1.6 Drilling Operations
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Bonga producer (Well-X) became severely impaired during a series of interventions (to address SCSSV failure), with oil potential dropping from 18 to 3 kbod. The Well and Reservoir Management team believed the initial impairment was most likely caused by fines migration and secondly by fluid (MEG and brine) losses to the formation during interventions. A two-fold treatment was recommended -solvent (surfactant) and half strength mud acid to target the two impairment mechanisms. Considering the relatively low productivity and remaining reserves, the downside risk was low from a subsurface perspective. The major concerns were related to HSSE and integrity risks pertaining to unspent acid flow-back to the Bonga FPSO. Lessons learnt from successful executions of similar jobs by other Operators enabled the Bonga team to demonstrate the necessary risk management and pursue the concept towards execution.
To minimize cost, the recommended deployment method was by bull-heading the treatment from the Field Support Vessel, via a flexible hose connected to the tree, and subsequently to back produce the well fluids to the FPSO with injection of Soda Ash to neutralize any unspent acid on the topsides. The key challenges that needed to be addressed were: Identifying an appropriate stimulation recipe, Ensuring adequate pump rate of stimulation fluid given limited pressure rating of the flexible hose, Managing flowback of unspent acid to topsides, Metallurgy compatibility with the stimulation fluid, Hydrate risk and Production of H2S from chemical reaction. Full integration of the various functional aspects was essential for effective planning and execution.
The Well-X stimulation led to an increase in production from 3 to 22 kbod, with PI improving from 1.4 to 86 bpd/psi; a PIF of 60. Industry experience shows that gains from mud acid stimulation to attack fines can typically be sustained for up to 12 months, while the partial gain from surfactant to remove MEG/brine impairment is expected to be sustained permanently. This success paves the way for further acid stimulation in the Bonga field where fines migration is typical among producers, and provides opportunities for production acceleration in the field and other upcoming developments.
Stimulating of oil producing wells is an established means to increase oil production potential without requiring extra facilities or drilling new wells. Well stimulation methods increase well productivity by removing (or bypassing) formation damage in the near-wellbore region or by superimposing a highly conductive structure onto the formation. The technical objectives of well stimulation are to remove, reduce or bypass the formation damage, reduce sand production and clean-up the perforations, while the economic objectives are to increase flow rate and optimize production from the reservoir.
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