GAS MIGRATION MODELING IMPROVES VOLUMETRIC METHOD OF WELL CONTROL

Colin P. Leach, Kate M. Quentin
Well Control & Systems Design,
Houston

The use of a realistic, deterministic well control simulator helps accurately describe gas migration effects after a kick.

Gas migration effects cannot be generalized for all wells, but it is possible to describe migration for a particular well geometry and formation characteristics. If this simulation work is performed in advance during well planning, then the rig crews will be much better prepared to handle well control.

In the volumetric method, gas expansion during gas migration is allowed for by bleeding small quantities of fluid through the choke. When gas first reaches the choke, the influx is distributed near the surface in the annulus. Rapid gas migration then occurs, and mud and gas may need to be bled to maintain constant bottom hole pressure.

The volumetric method is a technique for controlling gas kicks when circulation is not possible.

The industry-recognized method is based on simple calculations which assume a single bubble of gas, the "classic kick." This technique can now be evaluated by using more realistic, deterministic kick models. 1-2 The results from such models cast doubt on some of the conventional procedures taught and used in the industry.

This article details the analysis of influx behavior following a typical volumetric method. Numerical modeling of fluid losses as the surface pressure rises, gas migration, and dispersion are included to correspond accurately with field observations of kicks. Revised procedures are suggested to deal with these events better, such that the goals of the volumetric method are still attained.

The goal of the volumetric method is to maintain constant bottom hole pressure while a gas bubble is allowed to migrate and rise through the annulus. Industry-accepted calculations are based on the results from single gas bubble calculations. Fig. I shows the start and end point of such a procedure addressed by this method.

As the gas rises, it is allowed to expand, Mud is bled off through the choke to accommodate this gas expansion. The volumetric method relies on the calculation of the hydrostatic pressure of the bled mud and the addition of this equivalent pressure to the choke presSure. In this way, the bottom hole pressure is maintained and further influxes are prevented while well bore pressure is kept to a minimum to avoid breakdown at the casing shoe.

The use of a more realistic deterministic model allows for the effects of gas migration and distribution to be considered fully.

GAS MIGRATION

The volumetric method was examined for a 20-bbl kick introduced at 12,000 ft in a 121/4-in. borehole. The influx was assumed to be gas. Two initial approaches were assumed for this influx:

Maintain constant bottom hole pressure
Keep the well shut in with no fluids being bled off.

Fig. 2 shows the results from these two approaches. The procedures were stopped when gas reached the choke.

For the constant bottom hole pressure case, small quantities of fluid were bled off. The gas reached the choke in 232 min. At that time 15 bbl had been bled off, at a typical rate of 2-5 gpm. The choke pressure increased 100 psi. Note the small amount of fluid to be bled off.

For the completely shut-in case, the surface pressure and casing shoe pressure rise to much higher levels. The choke pressure increases by 1,300 psi, reaching 1,800 psi. The casing shoe pressure rises by about 1,200 psi. In addition for this case, as the gas is distributed through the well, gas migration slows until the leading edge of gas is just short of the choke, when gas migration stops altogether.

This process is symptomatic of the general rate of gas migration, which varies from 0 for low gas concentrations to 5-6,000 ft/hr for high gas concentrations. Note that it is not possible to generalize on gas migration rates because the concentration of gas depends on hole size, and to an extent, on initial gas distribution.

Fig. 3 shows the gas distribution for the constant bottom hole pressure case as the gas reaches the choke. Note that the gas is distributed over most of the length of the well bore. The gas void at surface is about 50%, but below 150 ft the gas void is 5% or less. This means that for most of the well, 5% (or less) is gas and 95% of the annular volume is mud.

If no fluid is lost to the formation, it is only necessary to bleed off a small amount of fluid through the choke to maintain bottom hole pressure. In reality, it is likely that a small amount of fluid will be lost to exposed permeable formations through the mud filter cake. This loss may be enough such that no mud needs to be bled off at surface.

There is a risk that if mud is bled through the choke, the addition of this mud quantity and that lost to an exposed formation may be enough to induce a further (but small) influx. For the example shown in Fig. 2, bottom hole pressure will be maintained if the choke pressure is maintained at a level established by the dashed curve of the lowest track. In this case the required rise in choke pressure is very small. For a larger influx the required rise in choke pressure as the influx is allowed to expand will be greater (the dashed curve on Fig. 6).

It is difficult to see how these choke pressure profiles can be established in the field-the only practical procedure would rely on drill pipe pressure being available. It is therefore likely that the well bore will be overpressured or that further influxes will be introduced. The well control strategy should focus on which of these two is preferred.

As an aside, it can also be seen that the pressures observed at surface are very sensitive to small downhole fluid losses. This effect has very important implications for observation of shut-in pressures and estimation of gas migration rates. In this latter case, it is almost impossible to predict actual gas migration rates based on surface pressure observation.

In Fig. 2, the observed surface pressures will be somewhere between the shut-in and constant bottom hole pressure cases, depending on fluid loss to the formation. If much fluid is lost, there will be little choke pressure rise, and gas migration will be assumed (incorrectly) to be low. If little fluid is lost to the formation, surface pressures will rise and fluid must be bled off through the choke to maintain bottom hole pressure. However, the amount required to be bled off is likely to be very small.

GAS AT SURFACE

With gas at the choke, the conventional volumetric method implies that the well will be shut-in and no further action taken. Unfortunately, it is at this time that the bulk of the gas is near the surface, and expansion of this bulk of gas is really starting. Fig. 4 shows two alternative actions following the first arrival of gas at the choke:

Shut-in and bleed nothing
Maintain constant bottom hole pressure by bleeding.

If the well is shut-in and nothing is bled off, choke pressure will rise significantly. In the case shown, the choke pressure will rise from 600 to 1,300 psi in 100 min. As a result, the casing shoe pressure will also rise, perhaps to the point of shoe breakdown.

The second case where constant bottom hole pressure is maintained shows the amount of fluid that must be bled off. This volume is equivalent to 5 bbl over the 100-min period. This bleed-off rate is 10 gpm to start with, tailing off to 2 gpm over a period of 1 hr and is actually higher than during the earlier part of the procedure because the gas is near surface where pressures are lowest and gas expansion rates highest. This bled off mud is accompanied by large volumes of gas. The combined fluid must therefore be bled through the choke line up to the mud gas separator.

KICK SIZE

Fig. 5 is a snapshot of the gas distribution following migration of a 50-bbl kick to surface given the same well geometry as in the previous example. The influx is far more concentrated upstream of the choke. In this case, gas void fractions are about 30% less than the surface gas peak. The influx acts more like a single gas bubble than the smaller kick, but there are still some important differences.

Fig. 6 compares the migration of the 20 and 50-bbl kicks between initial entry and arrival at the shoe. The 50-bbl kick has a far greater gas concentration and as a result migrates with a much earlier arrival time at the choke. In fact, this large kick has the characteristics of the classic volumetric method.

Fig. 7 shows the two opposing courses of action once gas reaches the choke after 96 min:

Shut-in and bleed nothing
Maintain constant bottom hole pressure by bleeding.

It is necessary to bleed mud and gas to maintain the required well bore pressure while the influx tail migrates towards the choke. A gas flow rate of 3 MMscfd accompanies the bleeding of mud in this case. This high gas rate is a function of gas migration and distribution in the well bore and cannot in any way be explained by single gas bubble model theory.

VOLUMETRIC METHOD

The volumetric method can work. The application of the method is very different from that described in the literature and based on simple, single gas bubble theory.

The basic premise of the volumetric method, whereby gas expansion during gas migration can be allowed for by bleeding small quantities of fluid through the choke, is valid.
The quantity of fluid that must be bled until gas arrives at the choke will be very small, especially for small influxes. This is particularly true if a long open section of hole is exposed. In this case, a sufficient quantity of mud may be lost as filtrate such that no bleeding off at surface is required.
When gas first arrives at the choke, the influx is distributed, but near the surface. Rapid and significant gas migration now occurs. It is probably necessary to bleed mud and gas at this stage to maintain constant bottom hole pressure.
To be able to bleed a mixture of gas and mud, procedures must be established for the mixture to be routed to the mud/gas separator and for the mud returns from the bottom of the mud/gas separator to be measured.
Small gas kicks (even in water-based mud) may not even reach surface; they can become so dispersed that migration will stop.
It is possible to construct a perfect choke pressure profile that wis associated with a given influx size; however, it is very difficult to establish this profile in the field, and it is likely that the well bore will be overpressured or a further kick induced. Before a kick occurs, a field strategy should be established to determine which is preferable.
It is not possible to generalize gas migration effects for all wells; however, it is very easy to describe what will happen for a particular well geometry and formation characteristics. The use of a realistic deterministic well control simulator is required. If the work is conducted during well planning, rig site crews will be much better prepared to face difficult well control situations.

SURFACE GAS RATES

Results from recent simulations of gas kicks are presented here to alert the industry to a potential problem. Up until now, the industry has thought of the single gas bubble model as being the most conservative case. This model is the most conservative in calculating pressures in the well bore and in particular the surface pressures. However, the single gas bubble model can significantly underpredict surface gas rates because the effects of gas distribution and migration (slip) are ignored.

Fig. 8 shows the gas rates that could be obtained for a 100-bbl gas kick circulated from a 121/4-in. well bore at 12,000 ft. The results show gas rates at kick circulation rates of 0, 1, and 4 bbl/min. Note that there is a significant gas flow, even with zero pump rate. The classic single gas bubble calculation (Equation 3 in Reference 3) predicts a zero gas flow rate. 3

For the 4-bbl/min case, single gas bubble model (allowing for gas dispersion and slip) calculations predict a gas rate of 8 MMscfd (9 Mcf/min). These significant discrepancies were reviewed by the authors and by the team behind the more complex model and are considered to be explainable. The single gas bubble model underpredicted gas flow rates by 60% in this case.

Not all gas rate calculations based on the single gas bubble are as poor. In many cases, gas rates based on the single gas bubble model will be higher than those of the more complex model, because of the effects of gas dispersion. In other cases, such as more concentrated kicks that arrive at the surface as a segregated bubble, the single gas bubble model will only underpredict rates by 10-15%.

The single gas bubble model can significantly underpredict surface gas rates-this model is not necessarily conservative.
Procedures based on gas rates calculated from a single gas bubble model should be reviewed in the light of this analysis.
Cutting the pump rate in half during a well kill will not necessarily cut the surface gas rate in half.

ACKNOWLEDGEMENTS

The authors would like to thank Well Control & Systems Design for permission to publish this article. The authors would also like to thank Anadrill-Schlumberger, in particular the team in Cambridge, England, for its input.

REFERENCES

1. White, D. B., and Walton, 1. C., "A Computer Model for Kicks in Water-and Oil-Based Muds," Society of Petroleum Engineers paper 19975, 1990.

2. Rommetveit, R., et al., "An Advanced Kick Simulator Operating in a User-Friendly X-Window System Environment," Society of Petroleum Engineers paper 22314, 1991.

3. Butchko, D., et al., "Design of Atmospheric, Open Bottom Mud/Gas Separators," paper 13485, presented at the Society of Petroleum Engineers/International Association of Drilling Contractors Annual Drilling Conference, 1985.

Issue date: 12/26/94