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specimen was sectioned at the conclusion of the experiment, concentric slip rings indicating plastic deformation could be seen around the spalled zone.

In another test, a specimen was subjected to stress well in excess of the yield stress, and a hole then drilled. The rate of radial deformation decreased with time, as shown in Figure 8-25. Other tests showed that creep continued indefinitely.

As previously discussed, in a tectonically stressed region the shear stress on the wall of the hole will be maximum parallel to the least principal horizontal stress; one would therefore expect spalling to be at those points. This hypothesis was confirmed by Hottman et al,n who reported that a high resolution, four arm caliper survey in the previously mentioned well in the Gulf of Alaska showed that, at the depth where spalling occurred, the diameter of the hole was I8V2 in. (47 cm) parallel to the least horizontal stress and to gauge (12!/4 in., 31 cm) 90° to it.

Stress concentrations may also be caused by out-of-round holes, e.g., key seats, doglegs, and anisotropic rock properties, such as bedding planes and oriented fracture systems. Spalling at specific points creates out-of-round holes, thereby increasing the stress concentrations. This self-perpetuating mechanism was demonstrated in the model borehole with specimens cut from a core of Mitchell shale, which has anisotropic rock properties because of old parallel fracture lines. The specimens were left in the tester under the same stress conditions, but for increasing lengths of time. Spalling increased in severity with time of exposure, and specimens left for several days eventually collapsed (see Figure 8-26). The points of spalling correlated with the old fracture lines.

The obvious way to cure spalling is to raise the density of the mud. The problem is that there is seldom enough data available to determine whether the spalling is due to excessive stress, to weakening of the formation by the mud filtrate, or to a combination of both. Under these circumstances it is usually preferable to

Hole Enlargement

The most common form of borehole instability is the hole enlargement that occurs when the yield stress is not exceeded until the formation is weakened by interaction with the mud filtrate. The filtrate invades a layer around the well bore, which then spalls and caves, exposing fresh surfaces to invasion, and the hole gradually enlarges. The only remedy is to use a shale stabilizing mud, as discussed later in this chapter.

Formations With No Cohesive Strength

Figure 8-9 shows that when a formation (such as unconsolidated sand) has no cohesive strength, the Mohr failure envelope passes through the origin. Therefore, when drilled with air or a clear fluid that exerts no confining stress on the walls of the hole, the sand sloughs into the hole. However, when drilled with a mud that has good filtration properties, the pressure drop across the filter cake imparts cohesive strength, and a gauge or near-gauge hole is obtained. The mud must contain enough bridging solids (see the section on the bridging process, in Chapter 6) to enable a cake for form quickly. Otherwise, the hole will be washed out to considerable distance by the turbulent flow conditions prevailing around the bit.

A similar but more difficult situation is often encountered when drilling through active fault zones. In this case, the grinding action of the fault has shattered the formation into loose rubble. Sometimes these zones consist of highly polished fragments of shale, in which case the shale is called slickensided.

Because of the lack of cohesive strength, sloughing of rubble zones is difficult to prevent. Use of a mud with good filtration properties is essential because rubble zones have fracture permeability. Sealing the fracture openings with a low permeability cake enables the mud pressure overbalance to be applied at the face of the formation. Special sealing agents are often added to the mud.

Good drilling practice is important when drilling through rubble zones. Keep annular velocities low to avoid fluid erosion and do not hang up and circulate with bit in a rubble zone. Reaming down fast with pumps on will push mud into the formation, and pulling up fast will pull it out again, bringing the shale with it.

Coal Seams

Coal is a very brittle material with a low compressive strength, often containing many natural fractures. In regions of high tectonic stress, such as the Rocky Mountain foothills in Canada, coal seams almost explode into the hole when the horizontal stress is relieved by the bit, often causing stuck pipe.35 Large chunks and slivers of coal are recovered at the surface. Caliper logs often show under-gauge hole in a coal section, indicating creep.

The best technique for drilling coal seams appears to be to drill very slowly through them, with a good hole-cleaning mud. YP/PV ratios as high as 7:1 have been used. High density muds cannot be used because of low fracture gradients.

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