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135 140 145 150 155 160 165 Degrees, F.

135 140 145 150 155 160 165 Degrees, F.

Figure 9-39. Lost circulation zone located by temperature survey. (From Goins and Dawson.se Courtes/ API.)

and possibly damaging—trial and error approach. The symptoms and suggested treatment of the various types of lost circulation are given hereunder.

Loss into Structural Voids

When mud is lost into cavities, highly porous formations, propped fractures or other spaces, the mud level in the annulus falls until the hydrostatic head is equal to the formation pore pressure. An idea of the size of openings may be gained from measuring the rate at which the level falls. This rate may be very fast in cavernous limestones, or so slow into small openings that partial returns are obtained when circulating. Other indications are the depth at which the loss occurs: the greater the depth, the smaller the openings are likely to be.

Losses into large caverns occur only at very shallow depths, and usually the bit drops several feet when they are encountered. Losses into such caverns are very difficult to cure; enormous volumes of LCM slurries may be pumped in without appreciable increase in pumping pressure. Unconventional means, such as dropping sacks of cement (without removing the sacks) have occasionally been successful, but often it is necessary to reduce the pressure of the mud column, pm, below the formation pressure Pf, by drilling with aerated mud or foam until protective casing can be set. This method is tricky: If the mud density is reduced too much, formations fluids will backflow into the hole, and must be controlled by applying back pressure. Another method sometimes used is to drill blind with water and allow the returns, mixed with drill cuttings, to flow into the zone of loss.

Losses into smaller cavities, too large to be bridged by granular materials, may be plugged by pumping in viscous LCM slurries or time-set slurries that develop high shear strength. High squeeze pressures are unnecessary and undesirable because they introduce the risk of inducing a fracture.

Losses into vugular limestones, gravel beds, and propped fractures are best stopped by circulating granular materials while drilling, until the formation is fully penetrated. The required size and concentration can be established only by experience. It is generally best to start with a low concentration, say 5 lb/bbl (15 kg/m'), of a fine grade and add larger amounts and coarser grades if necessary. Ground nutshells are preferable to ground carbonates when drilling in non-productive formations, because they are less abrasive and about two-and-a-half tunes less dense. Therefore, about two-and-a-half times less by weight are required to provide the same volume of bridging solids, and lower gel strengths are required to suspend the particles. Carbonates must, however, be used in potentially productive formations because their acid solubility permits the removal of productivity impairment (see Chapter 10).

Materials and procedures for shutting off losses into structural voids have been discussed in detail by Canson.50a Problem analysis, treatment materials, and emplacement techniques are discussed.

Losses Induced by Marginal Pressures

When circulation is lost while drilling, but the hold stands full when the pumps are shut down (or will do so after a short waiting period), the loss is caused by the marginal increase in bottom hole pressure due to the hydraulic pressure drop in the annulus. In other words, pw exceeds p^, but pm does not. When the pumps are shut down, the fracture closes and mud solids bridge over the opening. Similarly, when pm is close to pfrac, circulation may be temporarily lost because of pressure surges when running pipe, or because of peak bottom hole pressures when breaking circulation after a trip.

Temporary circulation losses of this sort are best remedied by adjusting mud properties and operating conditions, rather than by the application of lost circulation materials. The following procedures are recommended:

1. Carry the lowest mud density consistent with well safety.

2. Use the lowest circulation rate that will clean the hole adequately.

3. Adjust the rheological properties to give maximum hole cleaning with minimum pressure drop in the annulus (see Chapter 5).

4. Do not drill with a balled bit and drill collars (see section on bit balling, earlier in this Chapter).

5. Run pipe in slowly, and above all, do not ream down rapidly with the pumps on (see Chapter 5).

6. Break circulation several times on the way into the hole. When on the bottom, break circulation slowly, and raise pipe while doing so.

7. Minimize gel strengths.

If mud losses occur even under optimum operating conditions, then it will be necessary to use some type of lost circulation material. Continuous circulation of ground nutshells while drilling appears to be the simplest and best method of control. Howard and Scott44 showed that these materials sealed fractures as they occurred, and prevented their extension. An alternative would be to squeeze soft DOB plugs, which, as already mentioned, adjust to seal transient pressures. Repeated applications might, however, be necessary to seal new fractures as they occur.

Losses Caused by the Hydrostatic Pressure of the Mud Column Exceeding the Fracture Pressure

In this casepm exceedsPiwc and the fluid level falls when the pumps are shutdown, until the hydrostatic head of fluid remaining in the hole equals Pfmc This kind of loss typically occurs when the density of the mud is raised to control a kick in (he lower part of the hole, and the mud pressure gradient thus increased exceeds the fracture pressure gradient somewhere higher up in the hole. Usually, a fracture is induced just below the casing shoe where the difference between pm and p^ is greatest (see Figure 9-40). Short of setting another string of casing, the only cure is to pump LCM into the fracture until the squeeze pressure exceeds the maximum transient pressure expected when drilling operations are resumed. Final squeeze pressures up to 1,000 psi (70 kg/cm2) have been reported.48 High squeeze pressures strengthen the borehole by widening the fracture, (hereby increasing (he hoop stresses around the walls (see Figure 9-41). Note that the squeeze pressure induces and seals new fractures below the initial fracture. The occurrence of such fractures is shown by fluctuations in the injection pressure.48

Fibrous materials, mixtures of fibrous and granular materials, DOBC, and high-filter-loss slurries may be used for high pressure squeezes, the principal requirement being that they develop sufficient shear strength, and do not flow back into the hole when the squeeze pressure is released.51

casing shoe

Figure 9-40. Diagram illustrating induced fracturing when mud density is raised to control kick.

u3 Least principal horizontal stress

Figure 9-40. Diagram illustrating induced fracturing when mud density is raised to control kick.

u3 Least principal horizontal stress

Loss of filtrate through sides of fracture

Loss of filtrate through sides of fracture

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