Drilling fluids monitoring

How well the drilling fluid fulfills these functions is determined primarily by the response of the well. The whole drilling operation suffers if the fluid is not adequate. In extreme cases, drilling may be stopped or a hole may have to be redfilled. A variety of physical and chemical properties are monitored to ensure satisfactory performance and guide fluid formulation and treatment (21,22). 

Corrosion Theory 1259. Forms of Corrosion Attack 1268. Factors Influencing Corrosion Rate 1292. Corrodents in Drilling Fluids 1300. Corrosion Monitoring and Equipment Inspections 1312. Corrosion Control 1323. Recommended Practices 1340. 

Linear polarization instruments provide an instantaneous corrosion-rate data, by utilizing polarization phenomena. These instruments are commercially available as two-electrode Corrater and three electrode Pairmeter (Figure 4-472). The instruments are portable, with probes that can be utilized at several locations in the drilling fluid circulatory systems. In both Corrater and Pairmeter, the technique involves monitoring electrical potential of one of the electrodes with respect to one of the other electrodes as a small electrical current is applied. The amount of applied current necessary to change potential (no more than 10 to 20 mV) is proportional to corrosion intensity. The electronic meter converts the amount of current to read out a number that represents the corrosion rate in mpy. Before recording the data, sufficient time should be allowed for the electrodes to reach equilibrium with the environment. The corrosion-rate reading obtained by these instruments is due to corrosion of the probe element at that instant [184]. 

The galvanic probe continuously monitors the corrosion characteristics of the drilling fluid. The probe (Figure 4-473) consists of two dissimilar metal electrodes, usually brass and steel. The electrodes are mounted on, but insulated... 

As mentioned earlier, microorganisms can attack drilling fluid additives and introduce corrosive agents to the system. Therefore, it is very important to monitor their activity and detect any source of problem as early as possible. API RP 38 is probably the most widely used testing procedure in the industry [201]. The methods that can be used to monitor the microbial activity can include the following [201,208] ... 

Drilling fluid systems Improved design and operation of drilling fluid systems can also reduce the need for water. Waste minimization opportunities (such as solids control and detailed system monitoring) have been proven effective in reducing the amount of makeup water needed in a drilling operation. 

Murch et al. (183) have recently described the application of the on-line measurement of the physical properties of drilling fluids that may eliminate the need for laborious manual rig site testing. The fluids monitoring system (FMP) constructed by Murch et al. gave continuous... 

H2S presence in drilling mud is usually detected by electronic sensors and continuous monitoring equipment. The following are the drilling fluid related indicators of HjS contamination (Scott 1994) ... 

Garrett, R. L., Carlton, L. A., and Denekas, M. O. (1990) Methods for Field Monitoring of Oil-Based Drilling Fluids for Hydrogen Sulfide and Water Intmsions, SPE Journal, p. 93. 

Monitoring Techniques for the Control of Drill Pipe, Casing, and Other Steel Components in Contact with Drilling Fluids," NACE Publication 1D177, Materials Performance, Vol. 16, No. 2, Fehruaiy 1977, p. 9. 

The inspector should always bear in mind the possibility of inadvertent drilling fluid returns. The right of way should be examined regularly for inadvertent returns. Particular attention should be paid to locations of underground utilities and pile foundations. If inadvertent returns are found, they should be cleaned up inunediately and the location monitored for continuing problems (particularly during pull back). 

Sentinel holes are used as a simple form of thickness testing. A small hole of about I - 6 mm diameter is drilled from the outer wall of the piece of equipment to within a distance from the inner wall (in contact with the corrodent) equal to the corrosion allowance on the equipment (Fig. 9.11). The technique has been used even in cases where the corrodent spontaneously ignites on contact with the atmosphere. The philosophy is that it is better to have a little fire than a big one which would follow a major leak from corrosion through the wall. When the sentinel hole begins to weep fluid a tapered plug is hammered into the hole and remedial maintenance planned. Siting the sentinel holes is somewhat speculative although erosion at the outside of a pipe bend is often monitored in this way. 

Temperature measurements in boreholes through ice sheets are now available with millikelvin accuracy (Clow et al. 1997 Cuffey and Clow 1997). The heat from drilling dissipates rapidly, and can be monitored and corrected for by repeat profiling, as can effects of convection in the fluid used to prevent borehole collapse. The temperatures in an ice sheet, as measured in a borehole, in turn contain information on past surface temperatures of the ice sheet. The ice about halfway down through central Greenland has not finished warming from the last ice age, and is observed to be colder than ice above or below (Fig. 9). 

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