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From Simpson.10 Copyright 1962 by SPE-AIME.

freed the pipe by penetrating between the pipe and the cake, thereby reducing friction. The work of Annis and Monaghan8 casts doubt on this theory: They found spotting oil was ineffective. Obviously the oil did not penetrate between the cake and plate, and if it had done so, it would not have reduced the area of contact because of the geometry of their apparatus. However, it is possible that under downhole conditions, the oil penetrates along the fillet as the cake is compressed, particularly if the pipe is being worked, and oil thus possibly assists in the freeing process. Note that excess of oil-wetting surfactants should not be used in spotting fluids, since they reduce capillary pressure.

Before spotting oil, the depth at which the pipe is stuck must be determined. The usual method is to measure the amount of pipe stretch produced by a given amount of pull. The length of free pipe can then be calculated from charts which list stretch vs. pull for the various pipe sizes and weights. Several logging devices are also available for locating the sticking point. The best of these is the drill pipe recovery log, which can define free and stuck sections by means of sound attenuation, even when there are multiple stuck sections.7

The oil slug must be weighted to the same density as the mud, if it is to stay in place. Based on field experience, Adams7 recommends using an excess volume of spotting fluid, and waiting at least 12 hours for the cake to be compressed.

Laboratory Drilling Tests

It has long been obvious that the properties of the drilling mud have a profound effect on the rate of penetration of the bit. Changing from drilling with air to drilling with water always results in a marked drop in penetration rate; changing from water to mud produces another sharp drop. Furthermore, there is a correlation between the penetration rate decrease and the depth, even when allowance is made for the older, harder rocks that are encountered as the depth of the hole increases. To understand why drilling fluids have such a profound effect on drilling rate, a review of basic drilling mechanisms is necessary. The principal factors involved were established by a number of investigators

using micro- or full-scale laboratory drilling machines. In these tests jacketed rock specimens were subjected to simulated subsurface vertical and horizontal stresses. Pbre pres-

sures were sometimes applied but usually set at zero, which was permissible since, as we have seen, the effective stress is the load less the pore pressure (see the section on the behavior of rocks under stress, in Chapter 8). These tests established beyond doubt that the critical factor governing penetration rate was a function of mud column pressure, not the stresses to which the rocks were subjected. The pressure of the mud column affected the penetration rate by holding the chips (created by the bit) on the bottom of the hole, as discussed below.

Static Chip Hold-Down Pressure

The differential pressure between the mud column and the formation pore pressure (pm — pj) causes the mud to filter into the formation beneath the bit, but the filtration rate bears no relation to the API filter loss nor to the filtration rate into the sides of the hold (see the section on filtration below the bit, in Chapter 6). Under the bit, the filter cake is continuously being removed by the bit teeth. If a tri-cone bit is rotating at 100 rpm, a tooth strikes the same spot about every 0.2 second. In time intervals of this magnitude, filtration is still in the mud spurt stage, and the amount of fluid that invades the formation depends on the concentration of bridging particles and their size relative to the size of the rock pores (see the section on the bridging process in Chapter 6) rather than the colloidal characteristics of the mud.20

The bridging particles establish an internal filter cake in the pores of the rock immediately below the bottom of the hole, and the finer particles invade somewhat further. This process is repeated over and over as successive layers of rock are removed. The magnitude of the resulting pressure gradients ahead of the bit was measured by Young and Gray19 in a micro drilling machine. In their apparatus, the invading fluid was constrained to flow downwards, and the change in pore pressure as the bottom of the hole approached a pressure tap was observed. Also, the distribution of permeability in the rock was calculated from Darcy's law. Figure 9-11 shows that the pressure gradient was very high in the first millimeter or so below the bottom of the hole, corresponding to the internal mud cake, and then decreased to almost zero in the invaded zone. Since a bit tooth will penetrate well into the invaded zone, practically the full pressure differential (pm - pf) will act across a chip (Figure 9-12).

That the pressure drop across the internal mud cake is one of the major factors affecting the penetration rate was confirmed experimentally by Black etalz>u in a full-scale model borehole. They measured the filtration rate while drilling Berea sandstone and while circulating only. The difference between the two rates gave the rate of filtration through the bottom of the hole while drilling. The pressure drop in the uncontaminated sandstone was calculated from this rate and the permeability of the sandstone by means of Darcy's law. This pressure drop subtracted from the total pressure drop across the sandstone gave the pressure drop across the internal mud cake. Tests were conducted on four water base muds at

5 x 10s

4 x 10s

1 x 10s

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0123456789 Core Length, cm io6 r

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0123456789 Core Length, cm

Figure 9-11. Permeability function and pressure gradient versus core length for high fluid loss mud (vertical Berea sandstone, pb = 1,000 psig). (From Young and Gray.19 Copyright 1967 by SPE-AiME.)

Formation Pressure Total invaded p, zone

Figure 9-12. Pressure distribution ahead of the bit (Schematic). Chip hold down pressure (CHDP) is virtually equal to pm - pf.

Formation Pressure Total invaded p, zone

Figure 9-12. Pressure distribution ahead of the bit (Schematic). Chip hold down pressure (CHDP) is virtually equal to pm - pf.

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