Boosting the Energy Efficiency of Water Processes

The handling of produced fluids, a mixture of oil, water, and gas, is essentially a huge waste of energy.

Water production is primarily caused by channeling and uneven drainage along the wellbore resulting from mobility contrasts within the reservoir. The water is a worthless byproduct of production, and its removal from reservoirs causes a loss of pressure that lowers production if it is not countered by reinjecting more water back in. This in turn increases energy consumption, as reinjection pumps are usually the second-largest energy consumers in offshore oil and gas production after gas compressors.

One of the goals of the LowEmission Centre, an eight-year research program managed by SINTEF, is to develop solutions that reduce the energy consumed by these processes. While focused on the Norwegian Continental Shelf, the solutions are anticipated to bring benefits globally.

Source: SINTEF

There is a balance between maintaining sufficient reservoir pressure and reducing selective inflow"

- Handita Reksi Dwitantra Sutoyo

Senior Research Scientist, Dr. Heiner Schümann, says energy efficiency is not only a climate measure but also an economic one. He is co-author in a recently published study, along with PhD candidate Handita Reksi Dwitantra Sutoyo and Prof. Carl Fredrik Berg, that analyzed the balance between high hydrocarbon demand and the imperative to reduce CO2 emissions by estimating how drainage strategies change under varying economic parameters.

Using model simulations that included both net present value (NPV), yield, energy consumption, and CO2 emissions, they concluded that increasing gas prices favor reducing reservoir pressure below the bubble point pressure, thereby releasing solution gas and leaving oil in the reservoir. Although lower reservoir pressure reduces the energy required for injection, the resulting increase in gas production necessitates more energy for gas compression, ultimately leading to higher emissions when gas prices rise relative to oil prices.

Source: SINTEF

In the absolute best-case scenario for processing, produced fluids would be transferred directly from the underwater wells to shore."

- Dr. Heiner Schümann,
Senior Research Scientist

For greenfield scenarios where well design matches production volumes, the impact of a CO2 tax is limited, leading to a single optimal drainage strategy. In this case, a non-linear relationship was evident between the reduced production and emissions. For increasing tax levels there were diminishing returns on lower emissions, reflecting reduced opportunities for emission reduction by changes in the drainage strategy. Some increments in tax rate will therefore have negligible impacts on drainage strategy, reducing profitability with negligible emission reduction.

In contrast, drainage strategies for less energy-efficient facilities are more dependent on variations in CO2 tax and oil and gas prices. Under-saturated oil reservoirs where water injection is used to maintain pressure may shift towards favoring gas production over oil when gas prices rise, prompting the adoption of a pressure depletion strategy. This transition can lower reservoir pressure and reduce energy consumption, but the increase in gas production necessitates more energy-intensive gas compression.

Source: SINTEF

Norway’s continental shelf accounts for about a quarter of our national CO₂ emissions, and 80 percent of those come from gas turbines powering offshore platforms.”

- Dr. Stefania Gardarsdottir,
Center Director

The impact of CO2 emission costs is substantial, particularly affecting project revenue in inefficient processing plants due to improper sizing, which is common in brownfield projects, says Schümann. Therefore, increasing CO2 tax levels is mainly a measure impacting emissions from fields with inefficient production facilities.

Further research from the group, published in October this year, includes analysis of the energy efficiency of inflow control systems under various drainage strategies and well completion configurations. Inflow control devices increase oil production and can reduce the volume of water lifted to the platform and then reinjected.

Autonomous inflow control devices can choke flow in intervals affected by water and gas breakthroughs by differentiating fluid properties based on viscosity. This eliminates the need for well interventions.

The devices increase back pressure, and therefore increase energy dissipation when choking the fluid flow. This also limits production. Consequently, maintaining reservoir pressure becomes increasingly important, requiring higher energy levels within the reservoir and resulting in increased injection rates, therefore higher energy consumption. However, they can still decrease overall energy consumption if they reduce water production.

“There is a balance between maintaining sufficient reservoir pressure and reducing selective inflow,” says Sutoyo. “Although it is crucial to maintain reservoir pressure, excessive increase in reservoir pressure appears to be disadvantageous to energy efficiency while concurrently giving limited increase in hydrocarbon recovery.”

Most recently, researchers from LowEmission are working on ultralong transport lines. Schümann explains: In the absolute best-case scenario for processing, produced fluids would be transferred directly from the underwater wells to shore – with no offshore platforms or logistics needed, eliminating their emissions by a 100%.

This isn’t possible today, because the mixture does not flow smoothly over long distances. Problems like the loss of pressure, separation into fluctuating gas and liquid pockets, and wax formation – which in worst-case-scenarios can block the pipeline – all increase with distance.

Existing measures involve injection of large amounts of chemicals for hydrate prevention, intensive pipeline insulation with active heating schemes, or scheduled pigging for wax removal. However, these measures are costly, energy intensive, and partly pollutive in case of chemicals.

The new approach targets the controlled cooling of the production fluids allowing the formation of wax and hydrate as particles in a transportable, stable, non-sticky (inert) slurry. This may be reached in dedicated subsea cooling units involving seeding techniques or the introduction of anti-agglomerants. The flow of a stabilized slurry will then allow transport over much longer distances (100+ km), potentially combined with a subsea boosting / multiphase pump system.

The potential is a considerable reduction in energy consumption and footprint, says Schümann. “And we don’t even need to make advancements all the way to shore to be effective in saving energy, costs, and emissions. Being able to have processing further away from wells could lead to more centralization, creating more efficient hub platforms supplied with production fluids from many different satellite fields.”

Researchers Dr. Per Eirik Bergmo and Dr. Ruben Mocholi Montanes are also involved in the range of research underway at LowEmission, and Dr. Stefania Gardarsdottir, Centre Director, says: “Norway’s continental shelf accounts for about a quarter of our national CO₂ emissions, and 80 percent of those come from gas turbines powering offshore platforms.”

Improving gas turbine efficiency and recovering waste heat through combined or bottoming cycles, could reduce emissions by up to 25%. A new demonstration facility is currently being built, showcasing compact and robust solutions for offshore use. Additionally, improving energy use in separation, compression, and other platform processes, reductions of 5-30% are possible, and optimizing operations, speed, and weather routing of supply vessels could deliver emission cuts of 5-35%.