Underground blowouts can be identified by several indicators. One or more can be observed, but also none at all may be noticed. Very often personnel fail to recognize the symptoms because they are focused on loss circulation. The following situations indicate underground flow (Tarr, Flak, 2017; Barnhill, Adams 1979):
1. An initial increase in drill pipe and casing pressure followed by a decrease. When the kick is detected, the preventers are closed and the surface pressures begin to increase to the value that is necessary to balance bottom hole pressure. In case of an underground blowout, fracture formed downhole will relieve wellbore pressures.
2. Fluctuating or unstable pressure readings. Pressure fluctuations can be caused by unsteady fluid flow from one or several formations, or from the fractured zones tending to open and close as the pressure changes on the interval.
3. Gas, oil or saltwater will surface via drill pipe. Mud inside drill pipe will be replaced with flowing fluids (no drill string check valve).
4. Unable to get mud returns. Mud flows to loss zone with blowout fluids.
5. Able to strip up or down with no change in annulus pressure. Controlling pressure is fracture pressure or pore pressure in the loss zone.
6. Thermal anomalies recorded on temperature logs. Other production logs may also provide indications.
7. No direct indication of pressure communication between drill pipe/tubing and annulus. The drill pipe pressure can change without affecting casing pressure and vice versa. This is due to loss of integrity of the borehole.
8. Lower than normal shut-in tubing and annulus pressures on a producing well.
9. A sudden change in GOR or WOR in a producing well with annulus pressure.
10. Christmas tree or BOP vibration of a shut-in well.
11. Sudden tubing or drill pipe vibration when the pipe is lowered past the point in the well where the flow is occurring.
1.1 Problems faced after underground blowouts occur
1. Environmental problems
· Fresh water aquifer contamination
· Shallow supercharging
· Mud losses
· Loss of equipment
· Lost holes, sidetracks, wells
· Wasted rig time
· Rig loss
· Loss of reserves
3. Life danger
4. Control Methods
Conventional control methods alone are not sufficient when the underground blowout is established. There are no unique control methods for such cases. Several items have to be known when we are dealing with underground blowouts: the cause of the underground blowout, the location of the thief zones and flowing zones, formation pressures and limitations of proposed procedures.
4.1 Location of fractured zone
Selection of particular control or kill method depends on the location of the fractured zone. The location of the fractured zone is important so that exact density and volume of the kill mud can be determined. The history of the field can provide necessary information in detecting location such as a structurally weak zone, unsealed fault plane, depleted sands. There are several tools that help in locating the fractured zone (Barnhill, Adams, 1979).
1. Temperature log. The most common used tool to locate fractured interval. It records differential temperature, not the absolute temperature. The tool records the abnormal fluid heat for the depth at which is encountered. In the case of gas expansion, cooling effect will be noticed. However, the important is to notice abnormal changes in temperature.
2. Radioactive tracer tool. Radioactive materials are pumped with drilling mud into the well and then gamma ray detector is used to detect an area where is the highest radioactive concentration. The depth at which are highly radioactive materials detected is the area where fluid enters the formation.
3. Noise log. The noise logging tool is a sonic detector that records the sound in and around the wellbore. The tool delineates static fluid column above fractured zone from moving fluid below the zone. It is often used with temperature logs to have better information about circulation problem.
4.2 Control methods
After the fractured zone is determined there are several blowout intervention methods that can be considered. Control methods can be divided into two groups depending on intervention location (Schubert et al., 2004):
1. Surface intervention – controls blowout by direct access to the wellhead or exit point of the wild well.
2. Relief well methods – used in situations when it is impossible or impractical to use surface intervention methods.
According to Schubert et al., blowout interventions can also be divided depending on certain method:
2. Momentum kill/bullheading
3. Relief well
4. Dynamic kill
5. Gunk plugs
Although capping and bullheading mostly are not performed in cases of underground blowouts, a brief explanation is given.
Capping as a surface intervention refers to stopping the uncontrolled flow by closing exit point on the surface so it is necessary to have access to an exit point of a wild well. The equipment in capping method consists of pipe fitted with a ball valve or blind rams and diverter line. The capping equipment is built into the wellhead or into the pipe components with valve or rams in open position. Once the equipment is stacked in place and connection to the wild well is sealed, valve or rams are closed and flow is stopped. Capping method has many limitations and disadvantages and can be very dangerous for the personnel. First, the pipe has to be guided to the exit point of the well which may be on fire. If we have a gas blowout that did not ignite, danger from an explosion is very high. For a shallow-water gas blowout, it can be difficult to access the wellhead because of the reduced buoyancy in the area. In deeper water, buoyancy may not be a problem because of currents and the length of the plume of gas. However, this method is not a good solution for the underground blowout. When the flow is abruptly stopped, the pressure will increase instantly and that can even cause underground blowout.
4.2.2. Momentum kill/bullheading
Momentum kill or sometimes called bullheading, also a surface intervention involves injecting kill fluid through drill string with greater momentum than the flow of formation fluid coming up the wellbore with annulus being closed. If drill string is not present in the wellbore, it will have to be snubbed down the wellbore using a snubbing unit. An advantage of this method is that the drill string does not have to be all the way to the bottom of the well. It is frequently used to prevent sour gas kicks reaching the surface as it can harm the personnel. Another version of bullheading is injecting fluid through the annulus with drill pipe being closed. In case of conventional well control, this method is not recommended because it requires that lost zone is established. However, in case of an underground blowout, this technique is applicable because the fracture zone already exists.
4.2.3. Relief well
Relief well is drilled parallel to the blowing well and used to relieve pressure by producing from the flowing formation. Later it was discovered that flooding reservoir with water is more effective method. The relief well would connect with wild well through fractures and vugs and further flood it with water until it was dead. The disadvantages and limitations of this methods are the inability to be applied in low permeability formations such as tight gas reservoirs and in cases when the relief well could not be drilled in close proximity to the blowing well. Nowadays with directional drilling, it is easier to drill relief well that could intersect wild well which is difficult to reach due to distance or unapproachable location. On figure 3, scheme of two relief wells is shown.