The Source of the Steam in SAGD Operation: Flashing in the Reservoir or along the vertical section (Part 1)

The complexities associated with determining the source and mechanisms of steam generation in Steam-Assisted Gravity Drainage (SAGD) operations. This includes evaluating whether steam originates from the sandface, the possibility of flashing along the vertical section, and the impact of steam coning on production.

Steam generation and its behavior within the reservoir pose significant challenges for production engineers in SAGD operations. There's a misconception among many engineers regarding where steam originates and how it affects production. Some believe that steam production only occurs when it breakthroughs into the well, but in reality, steam is generated at the sandface before steam coning takes place. However, excessive steam production due to steam coning can impede liquid production and mitigate oil production in affected zones. And such failure cannot be resolved by ICD, and for most cases drilling a lateral well or in some cases patching may resolve the problem

This article aims to explain the mechanisms of flashing and steam coning in straightforward terms, providing examples to illustrate their occurrence during SAGD operations. It also addresses the exaggerated notion of steam flashing along the vertical section, highlighting the common miscalculation of pressure reduction without considering enthalpy change. Neglecting enthalpy conservation creates gaps in analysis, leading to reduced trust in modeling software among thermal operation managers.

This part delves into modeling steam flashing at the sandface, and how steam flashing is occurring at the edge of steam interface. The steam flashing starts from the edge of the sandface, and it grows into the reservoir as the pressure at the edge is reducing. In other word

Variations of saturation pressure and pressure within the liquid pool is presented in the following figure. The steam flashing zone is shaded in light-blue.

In SAGD operation, the pre-flashing temperature within the liquid pool decreases at a constant gradient. As shown, if operators produce the well at low subcools or high drawdown pressures, then the saturation pressure curve and pressure profile may intersect close to the wellbore. The intersection point is where the onset of steam flashing occurs, and the fluids in the zone close to the wellbore undergo flashing. Once flashing occurs in the near-wellbore region, the free steam vapor reduces the relative permeability and production of the liquid phases, i.e., the mixture of oil and water (emulsion). Eushaw dynamics consider the flashing based on the enthalpy conservation, and the pressure variation at the edge of the producer. But the amount of the steam can be produced at the sandface is controlled by the liquid-gas relative permeability. And the user has to understand the well, and change the rel-perm based on the historical vapor.

It should be noted that, in SAGD production wells, if this near-wellbore flashing does not result in steam coning, the reduction in local permeability and production rate will help the liquid pool to recover at a new higher liquid-pool level, mitigating flashing. Also, note that in Eushaw dynamics, horizontal flow masking temperature along the wellbore is neglected; this effect will be added to the future versions.

Knowing the volume of the vaporized water, we can calculate the water saturation change due to flashing. The relative permeability of the liquid portion can be estimated using the gas–liquid relative permeability curves. In Eushaw dynamics it is assumed that capillary pressure is minimal, and a modified version of Corey’s formula is used as:

The multiplier of ω is included to incorporate the effect of steam expansion on water volume reduction and its effect on water saturation reduction. If steam vaporizes locally and reduces the water volume without any leakage, ω should be considered as one. The parameter ω is calculated based on the previous data, and such calculation is the proprietary of the software.

If most of the vaporized steam is leaked without sweeping the water and reducing its volume, then ω should be zero. As shown in the following figure, the relative permeability at the flashed zone interface is one since there is no flashing at this point, and it is decreasing as we get closer to the well, and it reaches its minimum (kflash). In Eushaw dynamics, the following equation is suggested for steam flashing/steam coning in:

?In this formula the flashed zone mobility multiplier, which is a function of the geological properties and the heat capacity of the fluids.

As an example, a new feature within Eushaw dynamics that assists users in understanding and modeling steam flashing in SAGD operations. Before moving in the steam flashing section, let explain the new feature that helps the user to calculate the properties using formulas in a user-friendly manner, akin to capabilities found in software like Petrel.

The feature appears to enable users to define formulas for properties such as horizontal permeability, incorporating parameters like gamma ray (GR) in an example formula. in this example the formulations is give as: Horizontal Permeability = EXP(-3.5E-4 GR^2 – 3.5E-2 GR + 8).

?Users can also input the averaging model for grid permeability calculations,

?With such assumptions, it suggests that steam quality reaches 30% in one time step. Such amount of steam, it severely impacts liquid production in those areas.

?However, adjustments in liquid-gas relative permeability (such as altering the residual gas value) can potentially mitigate steam coning, leading to a reduction in steam production and a subsequent increase in liquid rates.

Eushaw dynamics calculates flashing based on relative permeability and enthalpy conservation, impacting liquid rates in the flashing zone. The presentation highlights how this affects liquid rates, particularly after a certain depth in the reservoir.

?Moreover, Eushaw dynamics offers insights into sandface subcool, aiding users in understanding its behavior around the wellbore. Additionally, the newer version enables the calculation and comparison of dynamic temperatures, which can be utilized to predict wellbore dynamics.

The next part of the discussion seems to focus on exploring steam flashing within the vertical section, presumably elaborating on the mechanisms and implications of steam generation in this context.

This analysis and modeling using Eushaw dynamics seem to offer a comprehensive understanding of steam flashing dynamics and their impact on production in SAGD operations.

?If you’d like to learn more about how Eushaw Dynamics Simulator model the reservoir/well coupling and FCD optimization reach out at?[email protected]?or give us a call at +1(403) 667-7293