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Figure 8-21. Density of mud required to control plastic flow of salt formations. Mud weights given are the theoretical densities that will permit a creep rate no greater than 0.1 % per hour at the specified temperatures at the depth of interest. (From Leyendecker and Murray:31)

Figure 8-22b. Bulk density 2.22 g/cm3, 5% bentonite mud in hole, applied stress 1,000 psi (70 kg/cm2). (From Darley.6 Copyright 1969 by SPE-AIME.)

Figure 8-22a. Plastic yielding in model borehole, a. bulk density 2.0 g/cm3, air in the hole, stress at failure 1,700 psi (120 kg/cm2).

Figure 8-22b. Bulk density 2.22 g/cm3, 5% bentonite mud in hole, applied stress 1,000 psi (70 kg/cm2). (From Darley.6 Copyright 1969 by SPE-AIME.)

water from a fresh water mud causes the sides of the hole to swell and deform as shown in Figure 8-22b.

In a drilling well, plastic deformation of gumbo shales gives rise to large volumes of cuttings and spallings, sometimes in sufficient quantity to blank-out the shaker screen. Sometimes they agglomerate when rising in the annulus, forming mud rings large enough to block the flow line. Frequent reamings are necessary to avoid stuck pipe. The tendency of the cuttings and spallings to swell and disperse when rising in the annulus aggravates mud problems.

Gumbo shales can be drilled extremely fast, but hole cleaning considerations limit drilling rates. The maximum permissible rate is dependent on the circulation rate. Even with drilling rates thus limited, the time of exposure before protective casing is set is sufficiently short to enable the interval to be drilled with low density inhibited muds.33 The types of inhibited muds are discussed later in this chapter.

Lastly, plastic yielding may be experienced in geopressured shales. By definition, geopressured shales have a high water content relative to their depth of burial. For example, Figure 8-4 shows that the bulk density at 10,000 feet of a shale with a pore pressure gradient of 0.9 psi/ft is the same as that of a normally pressured shale at about 5,000 feet. The geopressured shale has to withstand an overburden load of 10,000 feet, and is therefore liable to deform plastically into the hole. Deformation is normally prevented because the mud density is raised to prevent the inflow of formation fluids from interbedded sands: the increase in density also prevents the inflow of shale. But when the shale contains no interbedded sands, the geopressure may not be recognized and the density of the mud not raised, in which case shale will be squeezed into the hole. Gill and Gregg34 state that in North Sea wells an underbalance of a few hundred psi— 1 ib/gal or less in mud density—may permit plastic flow.

We stated earlier that deformation of all rocks at depth was plastic because of the high confining pressure. However, penetration by the bit allows the radial stress at the wall of the hole to fall until it equals (pw — pf). Therefore, deformation may be brittle in the immediate vicinity of the hole and plastic further into the formation as the confining pressure increases. This type of yielding is most likely to occur in an air drilled hole because (pw — pf) is negative. This phenome non has been demonstrated in the model borehole (Figure 8-23). Specimens of consolidated shales, both natural and artificial, with air in a predrilled hole, were subjected to increasing triaxial stress. When the stress exceeded the yield stress, spalling developed around the hole, as shown in Figure 8-24. When the

Brittle-Plastic Yielding

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