Бурение грунтовых зондов, установка энергетических колодцев

An influx of gas into the wellbore can have a significant effect on the annulus pressure.

Since there is such a large difference in density between the gas and the mud a gas bubble entering the well will be subjected to a large buoyancy effect. The gas bubble will therefore rise up the annulus. As the gas rises it will expand and, if the well is open, displace mud from the annulus. If, however, the well is shut in mud cannot be displaced and so the gas cannot expand. The gas influx will rise, due to buoyancy, but will maintain its high pressure since it cannot expand. As a result of this Pann will increase and higher pressures will be exerted all down the wellbore (note the increase in bottom hole pressure). the situation is as shown in Figure 14.

This increase in annulus, and therefore bottom hole, pressure will be reflected in the drillpipe pressure (Pph = bhp — pmd). This situation can, therefore, be identified by a simultaneous rise in drillpipe and annulus pressure.

It is evident that this situation cannot be allowed to develop as it may lead to the problems mentioned earlier (casing bursting or underground blow-out). From the point at which the well is shut in the drillpipe and annulus pressures should be continuously monitored. If Pann and Pdp continue to rise simultaneously it must be assumed that a high pressure gas bubble is rising in the annulus. In this case, the pressure must be bled off from the annulus by opening the choke. only small volumes (1/4 — 1/2 bbl) should be bled off at a time. By opening and closing the choke the gas is allowed to expand, and the pressure should gradually fall. The process should be continued until Pdp returns to its original shut in value (again P

is

dp

being used as a bottom hole pressure gauge). this procedure can be carried out until preparations to kill the well are complete. During this procedure no further influx of fluids will occur, provided Pdp remains above its original value.

Figure 14 Migration of gas bubble which is not allowed to expand

4.8 MAASP

Another important parameter which must be calculated is the maximum allowable annular surface pressure (MAASP). The MAASP is the maximum pressure that can be allowed to develop at surface before the fracture pressure of the formation

just below the casing shoe is exceeded. Remember that an increase in the annulus pressure at surface will mean that the pressures along the entire wellbore are increasing also. Normally the weakest point in a drilled well is the highest point in the open hole section (i. e. at the previous casing shoe). During the well control operation it is important that the pressure is not allowed to exceed the fracture gradient at this weakest point. The fracture pressure of the formation just below the casing shoe will be available from leak off tests carried out after the casing was set. If no leak-off test was carried out an estimate can be made by taking a percentage of the minimum geostatic gradient for that depth.

If an influx occurs and the well is killed with a kill mud this calculation should be repeated to determine the new MAASP. The MAASP should not exceed 70% of the burst resistance of the casing.

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