Intelligent Drill-Stem Test for Well-Scale Flow Assurance Monitoring and Elemental Sulfur Deposition Prevention Enables Field Development in an Offshore Deep Gas Reservoir
Authors: Al Blooshi Ahmed , Al Yaqoubi Amna , Alloghani Jasim A. , Elmahdi A. , Abdelmawgod Ahmed , Bulekbay Aslan , Belova Natela , Ibrahim Mahmoud , Nuriakhmetov Ruslan , Hassan Ahmed , Sarac Sukru , Mugundan Mahidara , Radwan M. Sherif 
Published: November 2024

Flow assurance challenges pose significant risks for production. Lab-scale flow assurance experiments are common, but well-scale experiments are required during reservoir appraisal to understand risks, conditions, and solutions for field development decisions. This paper presents the first well-scale, real-time flow assurance monitoring and mitigation experiment with an instrumented DST string to understand production conditions for flow assurance problems caused by Elemental Sulphur deposition (ESD). The successful mitigation method that was experimented and proved during the DST unlocked the production potential from an offshore deep carbonate reservoir.

Elemental Sulphur deposition during production can quickly plug the tubing and cause the production to cease in a few hours. Previously, flow tests using a standard completion string were attempted in an offshore deep carbonate reservoir. These tests were not successful because the production tubing quickly got plugged due to ESD and the well ceased to flow. Critical information such as well productivity, formation properties and representative samples were not obtained successfully. A new well test approach with a DST was selected to test 2 zones in this formation. The DST string encompassed strategically distributed pressure sensors along the tubing and a surface controlled downhole chemical injection valve. During the DSTs, wireless downhole gauge data were analyzed in real-time with automatic data processing to monitor ESD in the wellbore at different depths. Downhole chemical injection rate was varied to change the dosage at different times to understand the minimum required chemical injection to gas production rate ratio to avoid ESD and tubing blockage.

Two DSTs were carried out to test 2 separate zones in this formation. These DSTs acted as a well-scale in-situ flow assurance experiment to understand the conditions for ESD deposition, tubing plugging as well as the required inhibitor injection dosage to sustain production from this reservoir.

In the design phase, transient wellbore and reservoir simulations were run to understand variations in the pressure and temperature distribution in the DST string during flow and build-up periods. The well test design was modified to maintain high temperature in the tubing by changing the well test method from modified-isochronal test to a flow-after-flow test, where a continuous flow is targeted, and well shut-in periods are minimized. The simulated pressure-temperature conditions showed that the flowing conditions would be outside the expected ESD phase envelope (no ESD expected); however, actual deposition was observed during the test, which highlighted the importance of high-quality downhole fluid sampling for elemental Sulphur analysis and the critical role of performing an in-situ well-scale flow assurance test under actual operating conditions of a well. During the test, downhole pressure sensor data were automatically converted to pressure gradients using a real-time data processing software and compared with modeled pressure gradients. Deviations from the modeled responses indicated the time and the depth interval of deposition. During the first DST, ESD was observed twice, at different pressure and temperature conditions but at similar chemical dosages. This information revealed the minimum chemical injection rate required to avoid ESD and safely produce the well.

During the second DST in the same well with a similar fluid, the minimum chemical dosage requirement was respected, and the test was completed without any flow assurance problems. The findings in this study showed production conditions that lead to downhole deposition, the chemical injection depth and rate requirements to avoid tubing blockage and production loss in future production wells. Deposition monitoring and inhibition techniques implemented in these DSTs act as a proof of concept for future production well design and operation. Successful DST operation enhanced with flow assurance monitoring and prevention system enabled testing the well to its full potential to obtain representative formation characteristics and well potential. Obtaining reservoir properties, well productivity and the flow assurance method unlocked the potential of this deep gas reservoir and enabled a confident field development plan.

This work is the first well-scale flow assurance study with a DST to monitor and prevent flow assurance problems. Findings of this study revealed an effective method to produce from an offshore gas field and unlocked the production potential after multiple attempts to flow and test previous wells. The methods presented in this paper can be universally applied to other wells with any type of flow assurance problems in early stages of reservoir appraisal to de-risk field development.