Well control by the volumetric method is a technique whereby bottom hole pressure is maintained slightly in excess of pore pressure whilst the gas influx is allowed to expand as it migrates to surface. The technique is a temporary well control method to be employed until other/better well control methods can be used to restore primary well control.

The volumetric method is applied initially when the well is closed-in and when conventional circulation is not possible (e.g. string plugged or partially/completely removed from the well, pump problems, etc.) and bullheading cannot be considered or is not recommended.

A worst case estimation of the pressures acting along the wellbore can be obtained using the standard well control formulae which are applicable to the "driller's method". It assumes that the original influx volume was on bottom at the time the well was closed in, after which the BHP is re-established at (D x r1).

The BHP becomes D x r1, as soon as the influx is above the bit. Well design calculations are therefore based on BHP = D x r1. The measured kick volume at the time the well was closed-in will be compressed due to the increased BHP to:

Actual pressures experienced during volumetric killing operations are usually lower than calculated Px values due to the fact that:

• The influx is fragmented and dispersed in the mud instead of remaining as a single slug.
• The influx is probably not on bottom at the time of closing in the well.

Gas migration rates depend on many variables, such as:

• hole size;
• difference in density of gas and well fluid;
• depth of the gas (the shallower the gas, the lower the gas density and the faster the migration rate);
• viscosity of well fluid;
• single bubble influx or separate smaller bubbles;
• type of well fluid (water based mud or oil based mud).

Because of these variables, gas migration rate estimates are not very reliable. As a rule of thumb, an estimated migration rate of between 150 and 300 m/hr (500 and 1000 ft/hr) is reasonable for gas in water based mud.

Migration rates of gas in brines are much higher.

The gas migration rate may be estimated in order to establish the approximate distance the influx has migrated during volumetric well control operations

Procedures concerning the volumetric method are discussed for the following conditions:

• Bit off bottom.
• String out of hole.

1 Bit off bottom

1.At first sign of flow, close in the well, monitor and record pressures and allow the closed-in annulus pressure to build up to Pchoke.

Pchoke = P_a + P_s + P_w

2.Maintain Pchoke constant until a volume of mud (DV1) has accumulated in the trip tank.

Since it is difficult to establish the exact position of the gas influx in relation to the drillstring, it is virtually impossible to apply the correct value of DV during the migration process of the gas influx along the hole. In order to simplify the volumetric method and to ensure that the bottom hole pressure is maintained above pore pressure, it is assumed that gas expands in the OH/DC annulus during the volumetric control exercise, until the gas reaches surface.

However, when the original influx volume is greater than the OH/DC annular volume, the DV and Ps values should be calculated using the average OH/DC and OH/DP capacity, not the OH/DC capacity.

In critical cases where a low MAASP does not allow for excess pressures to be applied to the well bore, a smaller working pressure increment (Pw) could be used until the influx is above the DC's. When the influx is considered to be above the DC's, a new Pw value can be calculated and applied assuming the influx is positioned around the drillpipe.

3.When the additional mud volume (DV1) has accumulated in the trip tank, the well is closed-in until Pchoke rises, under the influence of the migrating gas influx, by the value Pw and now becomes Pchoke 1.

4.By repeating steps 2 and 3 as often as necessary, gas is able to migrate upward and expand while a nearly constant BHP is maintained.

5.When gas is considered to be above the bit (calculated with migration rate), conventional circulation should, if possible, be initiated to remove the influx from the well.

6.If conventional circulation is still not possible, the method has to be continued until gas has reached surface. Gas must then be replaced with mud by pumping mud into the well intermittently through the kill line ("lubricating"). The casing pressure must be progressively decreased in steps of Pw for each DV2 mud volume pumped, to compensate for the gain in hydrostatic head of the fluid column.

When gas is bled off via the choke manifold, the lines downstream of the choke may be emptied completely, due to the sudden expansion of the gas. This erratic behaviour will result in less accurate volumetric steps.

1.1 Subsea BOP stack

In floating operations with a subsea stack, it is recommended to circulate the kill- and choke lines to mud immediately after the well is closed-in. This will simplify the operation, since only one mud weight is used in the system.

Basically, volumetric procedures used for a surface BOP stack also apply for a subsea BOP stack. However, the volumetric method is complicated when gas enters the choke line, because it causes the hydrostatic pressure in the well to drop sharply as a result of the small capacity of the choke line.

In order to compensate for this reduction in hydrostatic pressure, the choke pressure should be allowed to increase by DP before gas has entered the choke line. DP is calculated as follows:

DP = length of choke line x rmud.

After DP has been taken into account, the standard volumetric method is continued as before.

When gas arrives at surface, conventional circulation should be initiated, if possible, to remove the influx from the well.

If conventional circulation is not possible, gas can be replaced with mud using the following lubricating procedure:

1.Close the surface kill line valve and open the kill line valve at the BOP stack.

2.Wait until mud from the kill line is replaced with gas. This will be indicated when kill-and choke line pressures are equalised.

3.Pump a small amount of mud 0.3-0.6 m3 (2-4 bbl) into the kill line and let the kill- and choke line pressures equalise.

4.Bleed off Pchoke to the original pressure (i.e. the pressure prior to step 3) and repeat the cycle until a mud volume of

Delta V2 = P_w  x casing capacity / rho _mud pumped

has been pumped.

5.Bleed off gas until the choke pressure (i.e. the pressure prior to pumping DV2 mud) is reduced by Pw.

6.Repeat the lubricating exercise until all the gas has been replaced with mud.

Another method to replace gas from the well with mud is to start circulating across the kill-and choke lines at a reduced circulating speed via a small calibrated tank. The circulating pressure becomes the pressure indicator for continued volumetric well killing operations and should be increased or decreased according to the fluid level changes in the calibrated tank.

Circulation should start before gas is entering the choke line and the circulating pressure should have been compensated for the decrease in hydrostatic pressure due to gas entering the choke line (DP).

2 String out of hole

At first sign of flow, close in the well. Attempt to get pipe in the well immediately and follow the combined stripping and volumetric method.

If the string cannot be stripped in the hole, proceed as follows:

• Allow the closed-in annulus pressure to build up to Pchoke, where:

P_choke = P_a + P_w + safety margin

P_a = Initial closed-in annulus pressure before second build-up has taken place.

P_w = working pressure increment. Convenient values of P w are between 350-700 kPa (50-100 psi), bearing in mind the scale divisions of available pressure gauges.

A safety margin of 700-1400 kPa (100-200 psi) is recommended to reduce the chance of the well becoming underbalanced. The safety margin should be reduced for critical wells, where a low MAASP does not allow for excess pressures to be applied to the well bore.

• Maintain Pchoke constant until a volume of mud (DV1) has accumulated in the trip tank.
• When the additional mud volume DV1 has accumulated in the trip tank, the well is closed in until Pchoke rises under the influence of the rising gas influx by the value Pw and now becomes Pchoke 1, where:

Steps 2 and 3 are repeated as often as necessary, until the gas has reached surface.

• When the gas reaches surface, it must be replaced with mud. This mud is pumped intermittently into the kill line. From this moment, the casing pressure must be progressively decreased in steps by Pw for each DV2 mud volume pumped, to compensate for gain in hydrostatic head of the fluid column.

2.1 Subsea BOP stack

To avoid a rapid decrease in BHP due to gas entering the small choke line, extra choke pressure (DP) must be applied before the gas has entered the choke line (DP = choke line length x mud gradient).

The procedure for lubricating the well to mud is described in Section bit off bottom.

2.2 Operational considerations for the volumetric method

When there is a large difference in annular capacity such as in a casing/liner situation, more realistic DV mud volumes should be calculated taking the migration rate into account, in order to avoid excessive safety margins.

One should always remember that the volumetric method is only accurate when there are no losses.

Proper records should be kept during the well killing operation:

• Pchoke vs time: to identify an increase in the number of pressure build-ups which is an indication of gas entering smaller annular capacities.
• Pchoke vs DV: to maintain the correct bottom hole pressure.