Well Control - Preliminary record of information

Well Control - Preliminary record of information

Pre-recording of information.Proper secondary well control follows a planned course of action. Pre-recording relevant information avoids unnecessary delay. The following aspects concerning the pre-recording of information are discussed:

• Kick control worksheet.
• Checking pump pressure at reduced pump speeds.
• Establishing the weighting material mixing capacity of the rig.
• Relationship between bottom hole pressure, formation strength at shoe and mud gradient.
• Determining the maximum allowable annular surface pressure (MAASP).

1 Kick control worksheet

A Kick Control Worksheet is available (CLICK LINK).

During drilling operations, the Worksheet must be updated regularly to correct depth-dependent data and additionally, whenever significant changes occur in parameters such as casing and hole sizes, mud gradient, drillstring configuration, nozzle sizes, formation characteristics, etc.

Kick control worksheets should be available at all times that BOPs are installed and in use.

For development they should be updated before entering the potential oil reservoirs. For exploration or appraisal wells, they should be updated at least daily or every 200 m, whichever is the sooner.

2 Checking pump pressures at reduced pump speeds

Well control operations are performed at reduced pump speeds in order to:

·allow weighting up and degassing of the mud;
·reduce loads on surface equipment;
·increase reaction time for correct choke adjustments.

Operation of the pump at a pressure too close to the setting of its relief valve is dangerous. If the relief valve opens, mud will flow back through the drillpipe and standpipe manifold.

Circulation pressures at the reduced pump speeds must be known. Before starting to drill with a new bit and at the beginning of each shift, the driller shall observe pump pressures at selected circulation rates and record the results in the daily drilling report and kick control worksheet.

Circulation pressures may differ considerably when using a different equipment hook-up (e.g. circulating head/chicksans vs kelly or top drive).

Slow circulation rates should be taken with the equipment hook-up which is planned, or most likely to be used during the well killing operation.

Slow circulation rates and pressures can be plotted as a straight line on a graph using log log paper. Appropriate pump pressures for any pump rate can be found using this graph, provided the mud properties and drillstring configuration do not change.

2.1 Friction losses in subsea BOP stack kill-and choke lines

Friction losses in subsea BOP stack kill-and choke lines represent a significant proportion of the back pressure developed in the annulus and, if not taken into account, could lead to a fracture of the formation around the casing shoe.

The friction losses in kill-and choke lines (both separate and combined) should be determined at various rates after landing and testing the BOP stack, but before drilling out the cement in the casing.

The pressure losses can be established by comparing pump pressures obtained whilst circulating:

·Down the drillpipe and up the marine riser.
·Down the drillpipe and up the kill-and choke lines separately and combined.

Alternatively, the value of the friction losses can be measured directly, and perhaps more accurately, by pumping down the lines, separately then combined, with returns up the marine riser.

In deep water operations, suitably low circulation rates should be selected.

Choke and/or kill line pressure losses for a particular subsea BOP stack set-up and waterdepth will only change when different mud properties are used.

3 Establishing the weighting material mixing capacity of the rig

The weighting up capacity depends on the maximum rate at which barytes can be added over prolonged periods. It is recommended that this rate is established for mud gradients in normal use.

An important advantage of the balanced mud method is that a well can be controlled in one circulation. This is done by employing a constant pump speed whilst circulating a mud which has been weighted up.

If mud mixing facilities cannot achieve the required increase in mud gradient, it will not be possible to control the well in one circulation.

Several circulations may be required, the gradient being raised in stages during each circulation, or on a gradual continuous basis while pumping.

One option to obtain the required mud weight quickly is to prepare kill mud prior to drilling ahead in possible overpressured formations. This practice will facilitate speedy well killing operations.

Preparing kill mud in advance is recommended for wells in which increased mud weights are expected to be used. The recommended kill mud gradient depends on the expected formation pressures to be encoutered, but as a general rule may be 10-20% higher than the actual mud weight in use. The correct mud weight can be obtained quickly by diluting the kill mud with the original mud.

4 Relationship between bottom hole pressure, formation strength at the casing shoe, mud density, and influx volume

There is a close relationship between the above parameters and the ability to safely handle a kick without causing formation failure in the open-hole section. Given certain fixed (or assumed) values of these parameters, combined with an acceptable kick tolerance for the rig drilling the well, a minimum required formation strength at the shoe may be specified.

The ability to safely handle a kick without causing formation failure in the open hole section needs to be reviewed as soon as the actual formation strength at the shoe is known and whenever any of the other parameters change.

Section 6.13 contains equations which show how the minimum required formation strength is calculated for specific influx volumes.

4.1 Kick tolerance

Influx volumes of 5-15 m3 (30-90 bbl) may be used as the maximum allowable kick tolerance. This depends on the following:

• knowledge about geological control, PVT characteristics, etc.;
• accuracy and reliability of the kick detection system;
• rig type (floater/bottom supported rigs);
• crew reaction time to close in the well.

Likely influx volumes for the drilling and reservoir conditions prevailing can be calculated/determined from equations given in the Casing design guide However, results largely depend on realistic values of kick detection threshold volumes for the rig used to drill the well. Experience has indicated that well-trained crews can perform BOP drills in three to four minutes. BOP drills shall be carried out to achieve and maintain this performance standard.

Periodic checks shall be made to establish the magnitude of the influx which can still be safely handled with the mud currently in the hole (kick tolerance), i.e. without causing formation failure. These checks will indicate the extent to which the density can be increased to control formation pressure or whether casing should be set prior to drilling deeper. New calculations shall be made whenever any of the parameters have, or will be changed.

5 Determining the maximum allowable annular surface pressure (MAASP)

In drilling operations the borehole fluid pressure must not exceed the formation strength at any point in the open hole. During well control operations the maximum allowable borehole fluid pressure at the casing shoe is normally considered to be the critical factor, based on the assumption that the weakest formation is at the shoe. For practical purposes, this pressure is referred to at the surface as the Maximum Allowable Annular Surface Pressure (MAASP).

The MAASP equals the "formation strength" at the casing shoe minus the hydrostatic head of the mud and/or influx in the casing. The "formation strength" is generally established by making a leak-off test during the course of the drilling operation.

In principle, MAASP should not be exceeded during well control operations, as long as the top influx has not yet reached the shoe, because of the danger of breaking the formation which may result in a (internal) blowout. However, there are situations which may justify exceeding the leak-off value during well control operations when the detrimental effects of having larger influx volumes entering the borehole are considered worse than the risk of encountering losses. There are exceptional cases where it would be better to take the risk of breaking the formation and to have an internal blowout than to have a flow to surface. Exceeding MAASP before top influx is at the shoe will not always have such detrimental effects, because of hidden safety factors in MAASP which are explained as follows:

• the formation intake pressure obtained during the leak-off test is usually lower than the formation breakdown pressure;
• the influx is dispersed in the mud and the actual position of top gas is much higher than the theoretical top of gas when well killing commences. This means that the influx will not reach the shoe as one bubble, but part of the influx is already inside the casing shoe when the theoretical top of influx should be at the shoe. The actual shoe pressure is therefore lower than the calculated shoe pressure using MAASP;
• leak-off tests are usually done in "virgin" hole. In principle, higher leak-off test results are obtained with time due to the plastering effect of mud.

The decision to exceed MAASP and the extent to which it could be exceeded depends very much on the circumstances under which the kick is experienced.

Drilling staff should be sufficiently conversant with well control principles that they can recognise and handle such situations. Main criteria to be considered are:

• severity of the kick (inflow performance low/high?);
• casing shoe depth (deep shoe, less chance of cratering);
• strength/depth of weakest formation;
• position of influx in open hole when MAASP is reached and estimation of how much MAASP will be exceeded when the theoretical top influx has reached the shoe.

5.1 Automatic MAASP control

Some control panels of remotely controlled chokes have an automatic MAASP control feature which keeps MAASP constant as soon as it is reached. When this feature is used in situations whereby MAASP should be exceeded, for instance when top influx is already inside the casing shoe, more influx is allowed to enter the hole unnecessarily which may lead to worsening situations. It is recommended not to use this feature, but to adjust the remotely controlled choke(s) manually from the control panel.

5.2 Calculation of the MAASP

To calculate MAASP, three parameters should be known:

• the hydrostatic head in the annulus above the weakest formation;
• the true vertical depth of the weakest formation;
• the strength of the weakest formation.

If any of these three parameters changes, the MAASP has to be re-calculated.