M´etodo del trabajo de deformaci´on homog´enea

In some cases, a formation is exposed to the wellbore that, when the drilling mud is weighted up to kill a kicking formation or zone, breaks down (fractures) and circulation is lost, either partially or completely. The formation into which wellbore fluids are being lost is called a thief zone. A thief zone can occur above or below a kick zone. Either situation complicates well control operations. Where (1) the thief zone is above the kick zone, (2) the losses are partial, and (3) crew members can mix enough mud to replace that being taken by the thief zone, one procedure is to continue normal kill procedures—that is, maintain the kill rate pump speed and SIDPP as determined earlier. This procedure maintains a constant bottomhole pressure above that of the kicking formation. Once the kick fluids go above the thief zone, the zone may heal. If, however, the loss rate is so high that not enough mud can be mixed to replace that going into the thief zone, another procedure is to add lost circulation material (LCM) to the mud and circulate it into the well. LCM includes such substances as mica flakes, walnut hulls, and other materials that can plug the permeability of the thief zone. Keep in mind that it may not be possible to pump LCM through small bit nozzles or other restrictions in the drill stem. In fact, many well-control specialists recommend installing a special circulating sub in the drill stem in areas where a kick and lost circulation may be expected.

If it is not possible to pump LCM, another way to handle lost circulation is to stop pumping and shut in the well completely. Sometimes, thief zones can heal themselves if given enough time in a static condition. Solids in the mud opposite the thief zone may plug the zone’s permeability.

One indication that the zone has cured itself is that SIDPP and SICP will stop decreasing. (Recall that one indication of lost circulation is that SIDPP and SICP drop and do not respond as they should to choke adjustments.) If the problem continues and circulation losses become severe or complete, it may be necessary to set a barite or gunk (cement) plug to heal the zone, as covered earlier in this chapter.

Sometimes, a thief zone can exist below a kick zone. While drilling into the thief zone, lost circulation occurs, which may cause the upper kick zone to flow. If the mud weight is lowered to avoid losses into the thief zone, the upper zone would kick, but control would be easy since the thief zone has not been penetrated.

Most contractors and operators agree that the primary concern is to control the well, so upon seeing the signs of the kick, the driller should shut in the well. If the pressure of the shut-in well is high enough to cause mud to flow into the lost circulation zone, SIDPP may be zero because the drill stem is full of mud, the kick zone is not at the bottom of the hole, and mud is being lost at the bottom of the hole. SICP may, however, be well above zero; the value for SICP depends on the size and fluid content of the kick

DAY-2 --- Session - IV Lost circulation

Loss of circulation is the uncontrolled flow of whole mud into a formation, sometimes referred to as a “thief zone.” This article discusses causes, prevention, and remedial measures for lost

circulation.

Lost-circulation zones

Fig. 1 shows partial and total lost-circulation zones. In partial lost circulation, mud continues to flow to surface with some loss to the formation. Total lost circulation, however, occurs when all the mud flows into a formation with no return to surface. If drilling continues during total lost circulation, it is referred to as blind drilling. This is not a common practice in the field, unless all of the following criteria are met:

 The formation above the thief zone is mechanically stable.

 There is no production.

 The fluid is clear water.

 It is economically feasible and safe.



Fig. 1—Lost-circulation zones.

Causes of lost-circulation zones

There are several situations that can result in lost circulation:

 Formations that are inherently fractured, cavernous, or have high permeability

 Improper drilling conditions

 Induced fractures caused by excessive downhole pressures and setting intermediate casing too high

Induced fractures

Induced or inherent fractures may be horizontal at shallow depth or vertical at depths greater than approximately 2,500 ft. Excessive wellbore pressures are caused by high flow rates (high

annular-friction pressure loss) or tripping in too fast (high surge pressure), which can lead to mud equivalent circulating density (ECD). Induced fractures can also be caused by:

 Improper annular hole cleaning

 Excessive mud weight

 Shutting in a well in high-pressure shallow gas

Eqs. 1 and 2 show the conditions that must be maintained to avoid fracturing the formation during drilling and tripping in, respectively.

... (1) ... (2)

Where λmh = static mud weight, Δλaf = additional mud weight caused by friction pressure loss in annulus, Δλs = additional mud caused by surge pressure, λfrac = formation-pressure fracture gradient in equivalent mud weight, and λeq = equivalent circulating density of mud.

Cavernous formations

Cavernous formations are often limestone with large caverns. This type of lost circulation is quick, total, and the most difficult to seal. High-permeability formations that are potential lost-circulation zones are those of shallow sand with permeability in excess of 10 Darcies. Generally, deep sand has low permeability and presents no loss-of-circulation problems. In non-cavernous thief zones, mud level in mud tanks decreases gradually and, if drilling continues, total loss of circulation may occur.

Prevention of lost circulation

The complete prevention of lost circulation is impossible, because some formations, such as inherently fractured, cavernous, or high-permeability zones, are not avoidable if the target zone is to be reached. However, limiting circulation loss is possible if certain precautions are taken, especially those related to induce fractures. These precautions include:

 Maintaining proper mud weight

 Minimizing annular-friction pressure losses during drilling and tripping in

 Adequate hole cleaning

 Avoiding restrictions in the annular space

 Setting casing to protect upper weaker formations within a transition zone

 Updating formation pore pressure and fracture gradients for better accuracy with log and drilling data

If lost-circulation zones are anticipated, preventive measures should be taken by treating the mud with loss of circulation materials (LCMs) and preventive tests such as the leakoff test and

formation integrity test should be performed to limit the possibility of loss of circulation.