Famn Resistivity Meteb

Figure 3-20B. Fann Model 88 C, resistivity meter. (Courtesy Fann Dresser.}

termined by calibration with standard solutions of known resistivity to convert the measured value to ohm-meters. Most instruments, however, are direct reading in ohm-meters because the cell constant has been accounted for in the cir cuitry of the electrical meter. Details of operation of the resistivity meter are supplied by the manufacturer. The conductivity of the sample is the reciprocal of the measured resistivity.

Hydrogen Ion Concentration (pH)

Die significant influence of the hydrogen ion concentration on the properties of drilling fluids has long been recognized and has been the subject of numerous studies. Hydrogen ion concentration is more conveniently expressed as pH, w hich is the logarithm of the reciprocal of the hydrogen ion concentration in gram mols per liter. Thus, in a neutral solution the hydrogen ton (H. + ) and the hydroxyl ion (OH — ) concentrations are equal, jnd each is equal to 10 7. A pH of 7 is neutral. A decrease in pH below 7 shows an increase in acidity (hydrogen ions), while an increase in pH above 7 shows an increase in alkalinity (hydroxyl ions). Each pH unit represents a ten-fold change in concentration.

Two methods for the measurement of pH are in common use: (1) a co I or i metric method using paper test strips impregnated with indicators, and (2) an electrometric method using a glass electrode instrument.

Color ¡me trie Method. Paper test strips impregnated with organic dyes which develop colors characteristic of the pH of the liquid with which they come in contact afford a simple and convenient method of pH measurement. The rolls of indicator paper are dispensed from a dispenser which has the reference comparison colors mounted on its sides. Test papers are available in both a wide-range type, which permits estimation of pH to 0.5 unit, and a narrow-range type, which permits estimation to 0.2 unit of pH. The test is made by placing a strip of the paper on the surface of the mud (or filtrate), allowing it to remain until the color has stabilized (usually less than 30 seconds), and comparing the color of the paper with the color standards. High concentrations of salt in the sample may alter the color developed by the dyes and cause the estimate ot pH to be unreliable.

Glass Electrode pH Meter. When a thin membrane of glass separates two solutions of differing hydrogen-ion concentrations, an electrical potential difference develops that can be amplified and measured. The pH meter consists of: (1) a glass electrode made of a thin-walled bulb of special glass within which is sealed a suitable electrolyte and electrode; (2) the reference electrode, a saturated calomel cell; (3) means of amplifying the potential difference between the external liquid (mud) and the glass electrode; and (4) a

pressure is then increased gradually up to 1000 psi (70 kg/cm-) or until the seal fails. The volume of mud and the maximum pressure are recorded. The mud may be discharged either through a slot (simulating a fracture) or through a bed of marbles or BB shot (simulating a coarse granular formation).

The data obtained with a slot is of limited value because the seal obtained is external, whereas when LCM is pumped into a subsurface fracture, the seal is internal, and depends largely on the frictional resistance offered by ihe material. Moreover, if the fracture is induced, its width increases wish increase in injection pressure. For these reasons, it would be belter to evaluate fibrous or granular LCM with the method used by Howard and Scott."2 which consists of pumping the slurry into a simulated, expandable fracture (see Figure 3-22). For evaluation of time-setting lost circulation materials, such as diesel-oil-bentonite-cement slurries, the spring-loaded discs apparatus described by Messenger4-1 permits an observation of the slurry pumpability and the shear strength of the plug (see Chapter 9), Other than an API filtration test, apparently no test exists for evaluating LCM slurries which depend on loss of filtrate into the sides of a fracture foi forming a sea!.

Maintenance of Hole Stability

The four tests that should be made on local shales when formulating a mud for maximum hole stability are: X-ray diffraction to determine the clay

Pipe Section 5-Vz" Diameter m

Flow

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