The pressure differences shown in Table 9-7 show that calculating bottom-hole pressures in deep hot holes from the density of the mud at the surface can result in bottom-hole underpressures quite sufficient to cause a blowout.

A computer program is available for predicting downhole mud temperatures from mud properties; surface operating data; and pipe, casing, and hole dimensions.5*- It is necessary to know the local geothermal gradient and the thermal properties of the downhole formations, at least approximately.

Downhole mud temperatures may be determined directly by measurement-while-drilling (MWD) techniques.584

High Temperature Drilling Fluids

In Chapter 5, it was shown that the degree of f locculation of bentonite suspensions starts to increase sharply with increase in temperature above about 250 °F (121 °C). The consequent increase in yield point can be controlled by the addition of thinning agents, but, unfortunately, thinning agents themselves degrade in the same temperature range. Degraded additives can be replaced, but as the rate of degradation increases, costs increase and eventually become excessive. For example, ferrochrome lignosulfonate is commonly used to maintain Theological properties and filtration control in clay muds at high temperatures. Kelly59 circulated such muds continuously at temperatures up to 350°F (177°C) in a laboratory unit, adding more ferrochrome lignosulfonate from time to time as required to keep the rheological properties constant. His results showed that the ferrochrome lignosulfonate started to degrade at 250°F (121 °C), but its thinning characteristics could be maintained up to 350°F (177°C) by the addition of small amounts of sodium chromate. The filtration properties did not start to degrade, with or without the sodium chromate, at temperatures below 350°F.

As mentioned in Chapter 4, the temperature at which degradation becomes excessive may be calculated from reaction rate mechanics. Usually it is established by experience, but it may be determined by laboratory testing.

Lignite is more temperature stable than is ferrochrome lignosulfonate. Muds containing lignite and DMS (see the section on surfactants, in Chapter 7) retained their rheological and filtration properties after heating under static conditions for 352 hours at 400°F (204°C).60-61 Bentonite-lignite-DMS muds are used extensively to drill geothermal wells with bottom hole shut-in temperatures around 450°F (234°C), but gel strengths and costs are high.62-63

Because of their low permeability, basement rocks may usually be drilled with air or water, both of which (especially air) have the advantage of temperature stability and fast drilling rates in hard rocks. Mitchell633 has developed an improved method for calculating air temperatures, pressures, and velocities. Water was used to drill two 15,000 ft wells at the Geothermal Test Site in New Mexico.*1* The wells were inclined at 35° to the vertical below 10,400 ft. 50 bbl sweeps of high viscosity bentonite pills helped clean the hole, and similar sweeps of pills containing triglycerides and alcohol reduced friction. Formation temperatures exceeded 600°F (316°C).

Oil muds are considerably more temperature stable than water muds. They have been used to drill wells with bottom hole logged temperatures up to 550° F (287 °C). Oil muds are best for drilling deep hot wells in the Gulf Coast and Mississippi. Because of high geopressures, mud densities of about 18 lb/gal (2.15 SG) are necessary to drill these wells. If a water mud is used, the combination of high temperatures and high solids content gives rise to high viscosities and gel strengths. If the mud becomes contaminated by salt water or other floe-culants, it becomes impossible to control the rheological properties.

One problem with oil muds is that at temperatures above 350°F (177°C) the organophilic clays used to provide structural viscosity degrade, and the cutting-carrying capacity of the mud deteriorates. To offset this problem Portnoy et al '" ' have recently introduced a lightly sulfonated polystyrene (i.e., a low ratio of sulfonated styrene units to styrene units) polymer (SPS). Laboratory tests have shown that SPS provides good rheological properties at temperatures up to 400°F (204°C), and it has performed satisfactorily in the field in wells with bottom-hole temperatures up to 432°F (222°C). Since SPS does not become activated until the temperature reaches 300°F (149°C), it is best used in combination with organophilic clay to provide rheological control at lower temperatures

The high temperature properties of some commercial muds have been evaluated by Remont et al.M To stimulate their behavior while circulating, the muds were rolled at 350°F (117°C) for periods up to 64 hours, and their rheological and filtration properties were then determined. Figures 9-47 and 9-48 show (he yield points and high temperature/high pressure (HTHP) filtration properties of 9 lb/gal (1.1 SG) water muds. These muds consisted primarily of bentonite and lignite, except Sample E, which contained sepiolite and an unspecified polymer.

Figure 9-48. HTHP filtrate of 9 lb/gal water muds after rolling at 350° F—300 psi. (From Remont, et a/.64 Courtesy of Sandia Laboratories.)
Figure 9-50. HTHP filtrate of 18 lb/gal oil muds after rolling at 350°F—300 psi. (From Remont, et a/.64 Courtesy of Sandia Laboratories.)
Figure 9-51. HTHP filtrate of 18 lb/gal water mud static aged 24 hours at the indicated temperatures and pressures. (From Remont, et a/.84 Courtesy of Sandia Laboratories.}

Note that Sample E had the lowest yield points but the highest filtration rates. Figures 9-49 and 9-50 show that the HTHP filtration rates of 18 lb/gal oil base muds are much lower than those of water base muds after rolling for extended periods at 350°F (note the difference in scale).

To simulate the behavior of muds left standing at the bottom of the hole for prolonged periods, the muds were aged statically at temperatures up to 500°F (260°C) and pressures up to 15,000 psi (1,055 kg/cm2). Figures 9-51 and 9-52 show that only three oil muds remained stable under these conditions.

Because of the interest in drilling for geothermal sources of energy, considerable research has been undertaken lately to develop muds that are more stable at high temperatures. When evaluating the results of such work, bear in mind that both high temperature and time of exposure to that temperature are pertinent factors. Claims based on exposure times of a few hours should be treated with caution.

Because of pollution problems, oil muds are not considered suitable for drilling geothermal wells, and research has therefore concentrated on water-base muds. As indicated by the work of Remont et al,M sepiolite appears to be a suitable viscosifier. Sepiolite is a fibrous clay mineral similar to attapulgite (see section on attapulgite, in Chapter 4). When slurries of sepiolite are subjected to high shear rates, the bundles of fibers separate to innumerable individual fibers. Mechanical interference between these fibers is primarily responsible for the

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