Acid Gas Modelling

Why is acid gas topical?

The term acid gas implies the presence of CO2 and/or H2S where there is no more than 2% hydrocarbon.  H2S is a concern because it is toxic and, in critical quantities, lethal.  CO2 is a concern because it is a greenhouse gas and, as such, has centre stage as a global environmental issue.  These concerns have spurred industry interest in understanding acid gas and have led to a variety of acid gas projects including disposal, sequestration, cycling and enhanced oil recovery in order to handle and process these fluids.

How widespread are acid gas operations?

There are currently over 50 acid-gas injection projects in Alberta (Pembina, Hansman Lake, Mirage, and Zama) and North East British Columbia, making Canada the location of the largest activities in acid gas operations worldwide.  In Alberta, the Energy Resources Conservation Board (ERCB) has implemented regulatory requirements to ensure standards are met to maintain public safety in areas of acid gas operation.

What makes modelling acid gas so different?

Acid gas behaviour is not intuitive because of its particular fluid behaviour which involves flashing between phases with significant changes in fluid density and viscosity.  These fluid property changes can significantly impact the flow regimes in the operation of wellbores and pipelines and in reservoir management.  Because of its complex behaviour, when modelling acid gas it is necessary to account for phase and property changes which affect the pressure and temperature profiles in the whole system.  Modelling allows for better development planning of the facilities required to process acid gas and understanding of the associated operational challenges.

What are Neotec's solutions for modelling acid gas?

Neotec's suite of software—WELLFLO, PIPEFLO, and FORGAS—are the best tools with which to model acid gas in the wellbore, pipeline and integrated reservoir-to-surface system.  With Neotec's exceptional and proven multiphase flow modelling technology in combination with the equation of state models provided by Virtual Material's Group (VMG), and optional coupling with the Computer Modelling Group's (CMG) reservoir simulator GEM, the best modelling solutions can be achieved.  Neotec's software allows engineers to examine the following:

Wellbore Behaviour
Neotec's program WELLFLO can be used to examine all aspects of wellbore hydraulics including:

• Phase behaviour
• Flow pattern determination
• Hydrate prediction
• Injection and production conditions

In the figure below, CO2 gas is being injected and the wellbore model can be seen with the depth, pressure, temperature, and liquid volume fraction profiles. The phase envelope is also plotted along with the pressure-temperature curve (black line) of the fluid in the wellbore.

At the start of injection at the wellhead the fluid is just within the phase envelope, close to the equilibrium line where flashing occurs.  The colour codes on the wellbore model show this with light blue being gas and black being liquid.  There is a multiphase mixture of gas and liquid at the top of the wellbore until the fluid enters the dense phase above the critical point.  At this point, the wellbore pressure losses become hydrostatic which give enough pressure to overcome formation pressure.   With the difficult balance between phases and the associated alternation between frictional and hydrostatic pressure losses, it is necessary to achieve a high enough wellhead injection pressure to get and keep the fluid in the dense phase to achieve a sandface pressure that can sustain injectivity.  WELLFLO allows the superior modelling of these complex behaviours.

Pipeline Behaviour
Neotec's program PIPEFLO can be used to examine all aspects of pipeline hydraulics including:

• Phase behaviour
• Flow pattern determination
• Hydrate prediction
• Facilities
• Elevation effects
• Liquid holdup

The figure below shows a pressure versus temperature plot with the phase envelope along with the pressure profile of the high H2S acid gas as it goes from the central facility, through the pipeline and down the wellbore.

The following can be noted in the figure:

• Fluid is in the liquid phase as it goes from the plant to the wellhead choke. The loss in pressure and temperature are evident once it reaches the choke at the wellhead;
• The fluid flashes across the choke;
• There is an increase in temperature with a slight increase in pressure up to the phase equilibrium line where the fluid enters the dense phase and the pressure losses become hydrostatically dominated.

PIPEFLO offers superior solutions in modelling the relation of acid gas fluid behaviour to hydraulic behaviour in pipelines along with many other complex flows in all sorts of systems.

 

Integrated Reservoir to Sales Point

FORGAS is an integrated forecasting tool that has a built-in material balance tank reservoir along with wellbore, pipeline, and facility modelling capabilities.  Depending on the nature of the reservoir, using FORGAS standalone provides a quick and reliable answer.  FORGAS' primary use is in modelling field developments including drilling wells and making alterations to the facilities of the gathering system.

For more complicated reservoirs (i.e. gas-condensate, fractured, faulted, or reservoirs with an aquifer) a reservoir simulator provides the most accurate modelling solution.  In this case, FORGAS has a dynamic link with CMG's GEM compositional simulator.  The link is particularly useful in forecasting gas systems since bottomhole pressure constraints change and it is difficult to make assumptions about these changes through time.  With GEM capturing the complexities of the reservoir and FORGAS accounting for the surface system constraints, the best forecasting solutions for acid gas can be achieved.

FORGAS coupled with GEM is proven technology and is used actively by industry to model commercial acid gas disposal operations in Alberta.

 

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