Wellhead combustion is an extraordinarily complex system involving an extremely large range both of scales and of physical hardware. Moreover, the diverse properties of crude oils and the different geographic settings of the wellheads (e.g., Arctic, off-shore) lead to significant challenges for developing predictive models of wellhead combustion. OSRR 1063: Bureau of Safety and Environmental Enforcement Report: Computational Fluid Dynamics Model for Predicting Wellhead Oil-Burning Efficiency at Bench and Intermediate Scales: Interim Report (July 30, 2020), produced by the U.S. Naval Research Laboratory (NRL) and funded by the Bureau of Safety and Environmental Enforcement (BSEE), includes summaries of computational modeling and experimental efforts to represent wellhead combustion toward the ambitious goal of predicting the combustion efficiency of wellhead flames. An ad hoc committee convened by the National Academies of Sciences, Engineering, and Medicine was charged with performing a peer review of this interim report encompassing the study methods, the quality of the data informing the study, and the strength of any inferences drawn by the NRL authors; accordingly, this final peer review report focuses on the technical nature of the interim NRL report (OSRR 1063).
The committee found that the authors performed foundational work for modeling and experimental research on some of the physicochemical mechanisms for physically downscaled wellbore processes. They identified some of the important aspects to be considered and developed some foundational understanding of physical and chemical processes relevant to wellbore ignition and combustion problems. They also summarized some relevant literature to provide context for their work. However, the consensus conclusion of the committee is that the model developed is not adequate for predicting the combustion efficiency of wellhead flames.
The major concerns identified by the committee through deliberation on the questions posed in its charge as provided by BSEE (Appendix A) can be divided into three general categories:
- Gaps in the study approach and the assumptions chosen to represent the physical system of wellhead combustion limit the utility and accuracy of the approach and the model.
- Several modeling approaches employed are not the state of the art.
- Other modeling methods employed are the state of the art, but their related uncertainties and known weaknesses are not considered.
Regarding the utility of the approach and the assumptions applied in developing the model and its components, the wellhead system is not well defined, and the level of accuracy required or desired for the model is never identified. These are fundamental concerns that dictate which approaches for the modeling and experimental work—e.g., Reynolds-averaged Navier-Stokes (RANS) or Large Eddy Simulation (LES) modeling methods—are appropriate, or how crude properties are to be considered. Another key concern is the lack of well-defined initial and boundary conditions in the context of wellhead combustion.
Designing appropriate experiments with which to validate the model or help scale the results to actual wellhead combustion conditions is difficult without a well-defined problem. Without a well-defined problem, moreover, it is not possible to evaluate the adequacy of the model.
Other high-level technical findings as to the completeness of the modeling results for predicting wellhead oil-burning efficiency identified by the committee are as follows:
- It is unclear whether the authors considered the correct flow system.
- Is the correct configuration for the multiphase flow considered? Specifically, is co-annular two-phase flow appropriate for representing wellhead oil flow?
- What are the thermophysical and chemical properties of the crude oil? How are those properties captured, or not captured, by the simpler fluids used in the study?
- Wellhead conditions were applied based on results from a worst-case discharge (WCD) model by Hilcorp; however, details of this model are either not adequate or not provided.
- Naturally imposed external flows were not considered.
- The verification and validation process was not rigorous.
The committee suggests that a broad-based research program may be appropriate to address the complex challenges of wellhead combustion. To this end, identifying better unit problems to frame such a research program will require more substantive understanding of the underlying conditions of wellhead combustion, as well as the goals for the stakeholders of such work.