Simulator improves control of kicks taken during tripping operations

David J. Element
AEA Technology
Dorchester, U.K.

Nigel P. Brown
Offshore Safety Division, Health & Safety Executive
London

A recent analysis of well incident data reported to the Offshore Safety Division (OSD) of the Health & Safety Executive (HSE) for 1991-1992 has shown that around one third of kicks were taken with the bit off bottom.¹ However, the extensive studies of gas kicks in recent years have focused solely on on-bottom kicks.

The Kicks-While-Tripping simulator has been developed to provide a greater understanding of handling kicks under circumstances where an influx is detected with the bit off bottom. It has been based on an existing gas kick computer code which provided accurate descriptions of downhole mud and gas properties that include gas slip and solubility.

The development of the simulator to allow modeling of movement of the drillstring has led to a tool capable of examining the flow of mud or mud/gas mixtures during tripping and stripping operations. By linking calculations with a commercial swab/surge package to Kicks-While-Tripping simulations, it would be possible to estimate the likely size of a swabbed kick, and then to model subsequent well control actions.

This article presents preliminary results from using the Kicks-While-Tripping simulator, showing how a deep well responds to control actions, particularly stripping back to bottom. This simulator will find uses in well control training, well planning, and accurate post-event analysis studies for kicks with the bit off bottom.

Background

Several sophisticated gas kick simulators have been developed in the last decade.^{2 3} Some of these state-of-the-art simulators are available commercially. These simulators have been put to numerous uses, including the following:

• Scenario-focused crew preparedness training

• Optimization of well design

• Assessment of well control options

• Incident audit and post-event kick analysis.

The development of sophisticated simulators has been supported by research projects aimed at gaining a better understanding of the behavior of well bore fluids during a kick. A good example is the research into gas rise velocities, for which experiments have been conducted in experimental facilities and full-scale wells.⁴

These experiments have shown that during a shut-in period in a conventional well geometry, a gas kick may be expected to slip past drilling mud at a velocity of approximately 6,000 ft/hr. This velocity is considerably higher than the previously assumed value of 1,000 ft/hr which had been adopted as a "rule of thumb" for many years.

Significant though the advances in simulator development and gas kick understanding may have been, little attention has been focused on the control of kicks when the influx is detected with the bit off bottom.

Off-bottom control

The choices available for controlling a kick detected with the bit off bottom include the following:

• Shut in the well, strip the drill pipe back to bottom, then perform a conventional well kill operation.

• Perform an off-bottom kill.

• Attempt to bullhead the influx fluids back into the formation.

• Perform a volumetric kill-attempting to control downhole pressures by periodically bleeding-off fluid at surface.

None of these control methods is without difficulty. For instance, some of the potential problems associated with stripping back to bottom include the following:

• Stripping may not be possible if surface pressures are excessive.

• Excessive surge pressures may cause problems while pipe is being run.

• Slip or expansion of the gas influx may lead to difficulty in maintaining correct bottom hole pressure (BHP).

• As the drillstring is run into the influx, there is a risk of losing hydrostatic integrity.

• String weight may be insufficient to overcome well bore pressure.

• Reduction in hydrostatic pressure forces as the pipe enters the gas will cause annular pressures to rise (to prevent further entry of formation fluids), which may lead to a risk of fracturing.^7-9

Nonetheless, there are advantages to be gained by stripping back to bottom while taking pressure measurements and then using this information to perform a conventional kill. Once the bit is on bottom, well bore pressures may be determined and interpreted conventionally.

This is not the case when an off bottom kill is attempted. There may be uncertainty as to the distribution of the influx above and below the bit. Although a number of off-bottom kills have been carried out successfully, there have been instances in which off-bottom kills have revealed deficiencies in procedures and failure to understand the conditions in the well while circulating off bottom. Avoidable secondary influxes have occurred during off-bottom kills, complicating the situation through taking large kick volumes and high shut in pressures.

Simulator

The Kicks-While-Tripping simulator has been developed by AEA Technology for the Health & Safety Executive. It has been based on the HSE's gas kick model (R-model).¹⁰ Like R-model, at this time the Kicks-While-Tripping simulator is intended for use as a research tool.

The aim of developing the simulator was to provide a tool for furthering the understanding of the processes that occur in the well bore when a kick is detected with the bit off bottom. The simulator does not compute swab pressures; it would have required a much more ambitious project to couple a bit-off-bottom, gas-kick model, and a swab/surge capability.

Changes which have been introduced to the finite difference equations reflect the extension of the model to incorporate changing well bore geometry. In both R-model and Kicks-While-Tripping, the finite difference equations represent conservation of gas mass, conservation of mud mass, and conservation of momentum.

A well bore temperature profile is computed at the start of a simulation and is assumed to remain static throughout.

The physical laws and fluid property algorithms in the Kicks-While-Tripping simulator have been based on those in R-model, so the first version of the new simulator contains several approximations:

• The simulator is not designed to compute the transient swab/surge pressures while tripping.

• Below the bit, mud/gas slip is assumed to obey the same slip laws as in the annular space above the bit.

• The simulator does not model any region of mixing near the bit but simply combines the contributions from the different flow paths which converge there (three fluid flow paths converge: mud pumped down the drill pipe, mud/gas below the bit, and mud/gas in the annulus around the drill pipe). It should be noted that the mixing region will be very short compared to the length of the entire well.

• Annular friction does not account for the relative motion between the inner and outer walls of the well bore annulus.¹⁰

The preliminary results illustrate some of the features of the new model. The scenarios are based on the well geometry of a generic deep well of 14,000 ft total depth (Fig. 1 [18494 bytes]). Although the simulator can model solubility of kick gas in oil-based drilling muds, the examples presented here assume the use of a water-based mud, so solubility is limited.

Detection and shut in

In the simulations, a 10-bbl swabbed kick was induced while the drill collars were tripped past the casing shoe. A flow check followed, then the well was shut in. Well and formation pressures were chosen so that the swabbed influx was just sufficient to lead to underbalance at the start of shut in. In this simulation, however, the influx during shut in was significantly less than the swabbed kick.

During shut in, both drill pipe and casing pressures increased, chiefly because of free-gas migration (Fig. 2 [20332 bytes]). In all figures in this article, "zero time" corresponds to the start of the simulation. Tripping began 5 min into the simulation, and the swabbed influx entered the well at 10-11 min. All gas remained below the bit during the short shut-in period modeled here, so drill pipe and casing pressures were essentially identical (Fig. 2 [20332 bytes]).

Following detection of the influx and the subsequent shut-in period, two alternative courses of action were investigated:

• Case 1-Strip back to bottom and then kill

• Case 2-Circulate with the drill pipe off bottom

.

Case 1

The well bore snapshots in Fig. 3 [29364 bytes] illustrate the movement of the influx gas during the shut-in period and as the string was stripped back to bottom. The time labels below each snapshot image (Time A1, Time A2, etc.) are also marked on the line graphs.

Soon after the stripping operation began, the gas region was displaced around the bottom hole assembly (middle snapshot in Fig. 3 [29364 bytes], Time A2).

Changes in the shape of the gas region are caused by the combined effects of displacement of the gas as the drill collars were lowered into the gas region and gas migration due to buoyancy.

One of the aims of stripping is to maintain an approximately constant bottom hole pressure (BHP). For the simulation, an idealized stripping operation was modeled, maintaining BHP precisely at a constant value (equal to BHP at the end of shut in plus a 100-psi operating margin). It is recognized that in practice, a perfectly constant BHP could not be achieved while stripping.

Future modifications to the Kicks-While-Tripping simulator could readily enable it to model real stripping operations, which typically involve a combination of operations at constant well volume (well pressure builds up as the drill pipe is stripped without releasing fluids from the well) and constant well pressure (well fluids are allowed to flow from the surface, allowing the kick gas to expand and thus lower the BHP).

Fig. 4 [21679 bytes] shows the simulated surface pressures modeled for the idealized stripping operation (stripping began at a simulation time of about 26 min). As kick gas was displaced by the drill collars, the length of the gas region increased rapidly. This elongation gave a reduction in the annulus hydrostatic pressure drop, which (since BHP was constant) caused surface pressure to increase (around 2 min after stripping commenced, the point at 28 min in Fig. 4 [21679 bytes]).

After further stripping, the top of the gas was displaced above the collars where the annular cross sectional area is wider. The length of the gas region was reduced, causing a decrease in surface pressure. Once the bulk of the gas had been displaced above the collars (around 6 min into the stripping operation-the point at 32 min in Fig. 4 [21679 bytes]), annular pressures became effectively constant.

Once stripping was completed, a driller's circulation was simulated to circulate the kick out of the well. During the kill operation, the gas was circulated up the well, expanding as it neared the surface (Fig. 5 [29568 bytes]). Fig. 6 [20238 bytes] is a plot of choke pressure, which adopts the conventional shape expected for kick circulation with drill pipe on bottom.

These simulations include the effects of gas slip, whichever operation is being modeled (tripping, shut in, stripping, or killing). The gas slip model has been based on extensive gas migration experiments which indicate that the movement of free gas depends on well bore geometry and void (gas) fraction. At low void fractions, the experiments have shown that gas is entrained in the mud and there is no migration. This effect is incorporated in the Kicks-While-Tripping simulator and explains why the bulk of the gas region is followed by a long tail of gas at low void fraction as the kick was circulated out of the hole (Fig. 3 [29364 bytes] and 5 [29568 bytes]).

Case 2

In the second simulation example, the shut-in period was followed by a driller's circulation, with the bit off bottom. The pumping pressure was determined from the slow circulation pressure computed by the simulator (adjusted to account for the reduced length of drill pipe in the hole). During this period of circulation, gas migration below the bit raised some of the gas to the bit depth, where it was mixed with the circulating mud. Gas, once in the flowing mud stream, was carried farther up the annulus (Fig. 7 [32923 bytes]).

As the off-bottom circulation continued, the concentration of gas below the bit reduced, and (because of the relationship between gas concentration and gas slip) the migration rate gradually decreased. However, for as long as the migration below the bit continued, there was a steady supply of gas mixing with the flowing mud at the bit depth. This gave rise to a lengthened gas region, significantly longer than was observed when the string was stripped back to bottom before circulation commenced (Fig. 7 [32923 bytes]). Eventually, most of the gas would be circulated out of the well, but some could remain below the bit entrained in the mud at void fractions so low that there is no migration.

If an oil-based mud were used, gas dissolved in the mud would also remain below the bit. Such kick fluids below the bit could cause problems later (e.g., while the drill pipe is being tripped back to bottom), especially if its existence is overlooked.

Fig. 8 [20894 bytes] illustrates the changes in choke pressure for the off-bottom circulation. Because the pump pressure was kept constant, the bit pressure remained effectively constant. Changes in the choke pressure reflect the hydrostatic pressure changes in the well bore annulus as: