In this Case Study, AFS examines the process of developing a hydrate management philosophy post-start up based around acceptable risk tolerance for the operator and capabilities with existing architecture.
Issue
A hydrate strategy for a field is typically included as part of early project design and system architecture is designed with mitigation and/or remediation methods in mind. For the project in question, the operator did not have a robust hydrate management plan in place despite the field being several years into production. The field consists of 3 drill centers with looped flowlines from the platform to each drill center. The client communicated that the field experiences frequent shutdowns (1-2 per month) where the field occasionally enters hydrate conditions. The field has the ability to blowdown the flowlines and limited injection points at the manifolds for chemical inhibition. The operator had expressed interest in avoiding continuous chemical injection or minimizing chemical requirements.
Process
Historical data for past shutdown and restart cycles were provided by the operator for analysis. Based on the data, the field has been able to depressurize outside of hydrate conditions in some cases, however it is not always possible based on well routing / production rates. Multiphase modeling of future production profiles was utilized to determine optimal well routing between looped flowlines based on total water cut and to minimize the flowline pressure following a depressurization.
To quantify the hydrate risk and demonstrate why the field has not experienced a hydrate event to-date, a series of rocking cell and autoclave tests were carried out with fluids from the field. Rocking cell tests successfully demonstrated the stochastic nature of hydrate formation with identical cells forming hydrates either immediately, within several hours, or even not at all. A series of autoclave tests were performed to determine hydrate risk over a range of pressures at ambient temperature and to optimize chemical dosage (LDHI) for a continuous injection strategy. Chemical dosage was further optimized by exploring LDHI + MeOH combinations.
Combining the results of multiphase modeling and the autoclave testing, hydrate risk was quantified based on achievable flowline pressure following depressurization. The operator was provided the risk matrix shown in Table 1 that can be referenced at any point to assess hydrate risk. If the flowlines cannot be depressurized below the desired threshold and the risk exceeds the operator’s tolerance, options for continuous chemical injection are included.
Figure 1: Hydrate Risk Matrix and Strategy for High/Severe Risk
Outcome
In summary, this study evaluated how a post-start up hydrate management strategy can be developed around operator risk tolerance and existing field architecture. The hydrate strategy was developed through a combination of laboratory testing with fluids from the field, analysis of historical field data, and multiphase modeling of current and future production. The operator was provided a risk matrix based on field conditions that can be referenced to determine the steps to protect the field, if necessary.
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