(a) Implementation of Kick Circulation Methods
The procedures for implementing one of the standard kick circulation methods are essentially the same for both the vertical and high angle or horizontal wells (as covered in the previous paragraphs). However, there are several points which should be considered before and during a well killing operation in a high angle or horizontal well.
• The advantages of the Wait & Weight Method over the Driller’s Method are less important in a high angle or horizontal well. This is because the weighted mud will not reduce the surface and casing shoe pressures until it has passed the horizontal or high angle section. By then the kick may have entered into the casing or been out of the well.
• The circulation should be started using the Driller’s Method once the well has been␣shut in and the stabilised shut-in pressures are established. In the mean time, the kill weight mud is prepared in the reserve mud pits. The earlier start of the circulation will reduce the risks of stuck pipe and other hole problems associated with the stagnant mud.
• Once the mud weight has been increased to the kill weight, the circulation should be switched to the kill weight mud, even if the influx is still in the annulus. The circulation continues until the kick is circulated out and the kill mud returns to surface. This will minimise the well pressures as well as the time of dealing with the kick.
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(b) Standpipe Pressure Schedule
When pumping down the kill mud through the drillpipe in a vertical well, the surface pump pressure should be reduced linearly from the initial circulating pressure (ICP) to the final circulating pressure (FCP), in order to maintain the bottom hole pressure constant. Thereafter, the pump pressure is kept constant at FCP until the kill mud returns to surface. Therefore, the pressure schedule during pumping the kill mud through the drillpipe can be obtained by simply joining a straight line between ICP and FCP. This has been covered in the previous paragraphs.
However, this is not the case in a high angle or horizontal well because the change in the hydrostatic pressure due to the kill mud is not linear. For example, when the front of the kill mud is going through a horizontal section of the drillpipe, the hydrostatic pressure at the hole bottom does not change at all. So in this case the pump pressure should be kept constant (or increased slightly due to friction pressure increase with kill mud). Therefore, the standpipe pressure schedule should be modified to take into account the effect of hole angle. To achieve this, the standpipe pressures when the kill mud reaches several critical depths in the drillpipe should be calculated. These include the depths at the kick-off, end-build, end-tangent, etc. The calculations can be performed as follows: i. Calculate the drillpipe size factor and the friction constant. This is necessary in
order to calculate the friction pressure increase due to the kill weight mud.
α1 = L1 / ID15
where: α1 = Size factor for drillpipe section 1, (m/in5) L1 = Length of drillpipe section 1, (m)
ID1 = ID of drillpipe section 1, (inch)
If there is more than one drillpipe section (tapered string), then the size factor should be calculated for each of the sections. BHA can be treated as part of the drillpipe section.
Pfc - Pscr
β =
α1 + α2
where: β = Drillpipe friction constant, (psi.in5/m)
α1α2 = Drillpipe size factors for section 1 and 2, (m/in5) Pfc = Final circulating pressure (psi)
Pscr = Slow circulating pressure with original mud MW1, (psi)
ii. Calculate the friction pressure increase when the kill mud reaches each of the critical depths in the drillpipe (kick-off, end-build, end-tangent, etc.).
• If the critical depth is above/at the drillpipe section cross-over point, then: MD
∆Pfriction = β x ID15
• If the critical depth is below the drillpipe section cross-over point, then: (MD - L1)
∆Pfriction = β x [α1 + ] ID25
where: ∆Pfriction = Friction pressure increase due to kill weight mud, (psi)
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MD = Measured depth at the critical depth, (m)
iii. Calculate the static drillpipe pressure when the kill weight mud reaches each of the critical depths:
TVD Pstatic = Pdp x (1.0 - )
TVDh
where: Pstatic = Static drillpipe pressure, (psi)
Pdp = Drillpipe pressure before the kill weight mud is circulated, (psi) TVD = Vertical depth at the critical depth, (m)
TVDh= Vertical depth at the open hole kick zone, (m)
iv. Calculate the standpipe pressure when the kill weight mud reaches each of the critical␣depths.
Pstand = Pscr + ∆Pfriction + Pstatic
where: Pstand = Standpipe pressure, (psi)
The results of the above calculations should be recorded in the Kick Sheet. These calculations should be carried out if the hole has a maximum angle greater than 30␣de grees.
Figures 6.1a shows an example of a completed kick sheet for a high angle well. Figure␣6.1b sho ws the standpipe pressure schedule for pumping down the kill weight mud. It shows that the standpipe pressures required to maintain a constant bottom hole pressure are lower for a high angle well (with build-hold profile) than if the well was vertical. So if the standpipe pressure schedule for a vertical well was used (dotted straight line in Figure 6.1b), excessive high well pressures would result, which would increase the risk of fracturing the formation at the casing shoe or openhole weak point.
(c) Trapped Gas in Inver ted or Horizontal Hole Section
If a kick containing free gas occurs in an inverted hole section (i.e. the hole angle is greater than 90 degrees), then the free gas will be trapped there unless the mud is circulated fast enough to flush the gas out of the inverted section. Similar scenarios also occur in washouts or undulations of a horizontal hole section.
A combination of the following is a possible indication that a kick has occurred in the inverted or horizontal hole section:
• There is an increased mud return flowrate • There is a positive pit gain
• When the well is shut in, the drillpipe pressure and the casing pressure are the same (under-balanced kick) or both are zeros (swabbed kick)
• The casing pressure is stable (no gas migration)
However the kick influx density/type (gas, water or oil) can not be determined based on the above data (as using the method described in Section 4.3). A gas kick is recognised when it is being circulated through the low angle or vertical hole section, where gas expansion causes a continuous increase in the casing pressure. So the first attempt to
kill the well should be using one of the standard techniques.
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If the kick can not be circulated to surface using the standard techniques, it indicates that the kick influx is free gas which has been trapped in the inverted or horizontal hole section. To remove the trapped gas, the mud must be circulated with an annular velocity above a critical value. This critical annular velocity is about 100 ft/min when the hole angle is between 90~95 degrees. In an 8-1/2" hole, this corresponds to a critical flow rate of 4.6 bbl/min, which is higher than the normal range of SCR during a well control operation. So prior to drilling an inverted or horizontal hole section, the pump pressure at a SCR corresponding to 100~150 ft/min should be recorded in the kick sheet. The following procedures may be attempted to remove the trapped gas from the inverted or horizontal hole section:
• Start circulation with the original mud at a flow rate corresponding to an annular mud velocity of 100~150 ft/min until the entire horizontal hole section has been displaced;
• Reduce the flow rate to a normal SCR and proceed using one of the standard well killing techniques.
• After one complete circulation, stop the pump and shut in the well to check the pit␣gain.
• If there is still a positive pit gain, that indicates that some gas is still trapped downhole. Repeat the previous procedures.
In cases where the high flow rate can not be achieved to remove the trapped gas, consider bullheading the gas back into the formation. As the trapped gas should stay near the kick formation, bullheading is more likely to succeed in an inverted hole section. The bullheading technique is described in Section 6.2.