Bridging the Gap between...GAP and OLGA
Bridging the Gap between...GAP and OLGA
Enabling better understanding between subsurface and subsea
Background
Working in the flow assurance space, there is a big emphasis on the accurate modeling of pipelines. Great care goes into ensuring that the simulated fluids, heat transfer factors (insulation, burial level, ambient temperatures, medium speeds, etc.) and pipe specifications (inner diameter, roughness, bathymetry, etc.) are all precise. Sometimes the smallest details can have BIG impacts on the thermal-hydraulic nature of a pipeline and the resulting impacts to operations.
To carry out this work, steady state and transient modeling software like PIPESIM, OLGA, and Ledaflow are used. And these tools are “industry standard” for good reason… decades of R&D efforts along with industry wide collaboration have gone into their development.
But, these tools are certainly not the only modeling tools that engineers in the oil and gas industry use. In fact, arguably, these are just the tools of flow assurance engineers. A likely even more popular piece of software is GAP, created by PE limited, which seeks to “bridge the gap” between reservoir and production.
GAP is rather ubiquitous throughout the industry as a means of modeling reservoir behavior while also including everything downstream of the reservoir (trees, pipelines, etc.). It sees use in both offshore and onshore projects, although our experience in the onshore space, where ‘type curves’ still dominate, would indicate there is potentially more value to be had improving flowing wellhead pressure constraints, particular in multiphase operation systems.
The true value in GAP is being able to run complex life of field assessments quickly to understand how a given field will perform both physically, and more importantly, economically. Generally this kind of analysis is referred to as IPM or integrated production modeling.
The Problem
The team at PE limited have made great advances in the underlying correlations they use to predict the thermal-hydraulics of pipelines and some even argue they have caught up to the likes of OLGA and Ledaflow.
But just like financial managers don’t get fired for buying Apple stock, flow assurance engineers don’t get in trouble for using OLGA and Ledaflow. In fact, most in the industry would still prefer to see them being used for key flow assurance modeling because they think they perform better at predicting complex thermal-hydraulic behavior of multiphase pipelines.
A few specific examples of where differences can emerge between the two softwares include:
Differences in liquid holdup seen in a multiphase operation scenario
Differences in heat transfer, and thus fluid properties, due to partial or fully buried pipelines
Differences in modeling of production chemistry issues such as emulsions
Differences in modeling ability of injection chemicals in substantial volumes such as mono-ethylene glycol, in the case of gas condensate systems
Inaccuracies associated with the above examples (and there can be many more) have the potential to leave you with a very different impression of the back-pressure your wells will see, and ultimately, what recovery you will get from the reservoir. This can have drastic effects on a project’s economic evaluations.
"I'd rather be lucky than good." - Lefty Gomez
Bridging the gap
To overcome some of the potential pitfalls discussed above, the truly ideal scenario would be if GAP had the same level of sophistication with respect to pipeline modeling that some of the flow assurance tools do. But, there are two issues with this:
Despite the capabilities they have put into GAP, it was not designed for the same intent as OLGA/Ledaflow and therefore it will always be playing catch up in terms of the modeling capabilities specifically for pipelines.
Even if GAP did reach that same level of sophistication (or potentially the softwares and companies merged), we would now be requiring that reservoir engineers, or field developments managers who use GAP, suddenly also be required to know the intricate details of flow assurance modeling too. There are multiple disciplines involved at every stage of oil and gas development for good reason.
Since these issues exist, at Pontem we have developed another approach…
Within GAP there is an ability to set the back pressure for a given pipeline (or pipelines) using what are referred to as VLP (vertical lift performance) tables. Traditionally, these have been used to supply the hydraulic performance of the tubing, thus immediately giving downhole pressure estimates for a reservoir. However, by including them for other pipelines, you can essentially allow the back pressures to your GAP model be given at the wellheads for example, where further downstream hydraulics have been “pre-calculated” by outside tools, in this case OLGA or Ledaflow.
A very simple example of one of the VLP tables we have produced is shown below. It indicates some of the variables we have included such as gas rate, tubing head pressure (which is really an outlet boundary pressure), water-gas ratio, and oil-gas ratio.
A real-world example
For many years, we have supported as asset that produces a gas-condensate through a long subsea tieback. Several flow assurance issues exist in the tieback, including a slow building restriction in the line that has lead to steadily increasing pressure drops at comparable rates.
As the project looks to undergo expansion, they have kept a close eye on the economics, which of course means that they have run long forecasts that couple reservoir and pipeline dynamics with hydrocarbon recovery estimates, all using GAP.
Because the conditions in the pipeline are changing steadily over time, and because the cause of those changes are rather complicated flow assurance phenomena that GAP has struggled with modeling, the choice of OLGA was made to supply the required back pressures for economic evaluation.
Using the approach outlined above, we routinely tune our OLGA model for the subsea tieback to ensure we are consistent with the hydraulic behavior in the field, and from that produce VLP curves that are feed into the GAP model.
Of course, we’d love to fix the building restriction too, but that’s a complex matter for another day… In the meantime, this workflow continues to ensure that project economics are evaluated using tools that focus on what they do best.
Final thoughts
GAP is a powerful tool used throughout the oil and gas industry daily. And while in many cases, the built-in functionality that it has is more than sufficient to meet the needs of a particular project, occasionally higher fidelity is required. For these scenarios, using predictions from specialized software designed for multiphase fluid dynamics and flow assurance challenges can provide that missing detail.
At Pontem we’ve developed complex workflows to automate this entire process setting up runs in software like OLGA and producing tables in their correct formats for use in GAP. We have a great deal of experience in helping clients find the best of both worlds, bridging the gap between GAP and OLGA.