STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

656 Drilling and Well Completions
Resistivity. Control of the resistivity of the mud and mud filtrate while drilling
may be desirable to permit better evaluation of formation characteristics from
electric logs. The determination of resistivity is essentially the measurement of
the resistance to electrical current flow through a known sample configuration.
Measured resistance is converted to resistivity by use of a cell constant. The cell
constant is fixed by the configuration of the sample in the cell and is determined
by calibration with standard solutions of known resistivity. The resistivity is
expressed in ohm-meters.
Chemical Analysis. Standard chemical analyses have been developed for
determining the concentration of various ions present in the mud [23]. Test for
concentration of chloride, hydroxide and calcium ions are required to fill out
the API drilling mud report. The tests are based on filtration, i.e., reaction of
a known volume of mud filtrate sample with a standard solution of known
volume and concentration. The end of chemical reaction is usually indicated
by the change of color. The concentration of the ion being tested then can be
determined from a knowledge of the chemical reaction taking place [7].
Chloride. The chloride concentration is determined by titration with silver
nitrate solution. This causes the chloride to be removed from the solution as
AgCl, a white precipitate. The endpoint of the titration is detected using a
potassium chromate indicator. The excess Ag' present after all C1- has been
removed from solution reacts with the chromate to form Ag,CrO,, an orangered
precipitate.
The mud contamination with chlorides results from salt intrusion. Salt can
enter and contaminate the mud system when salt formations are drilled and
when saline formation water enters the wellbore.
Alkalinity and Lime Content. Alkalinity is the ability of a solution or mixture
to react with an acid. The phenolphthalein alkalinity refers to the amount of acid
required to reduce the pH to 8.3, the phenolphthalein endpoint. The phenolphthalein
alkalinity of the mud and mud filtrate is called the Pm and P, respectively.
The P, test includes the effect of only dissolved bases and salts while the Pm
test includes the effect of both dissolved and suspended bases and salts. The
methyl orange alkalinity refers to the amount of acid required to reduce the pH
to 4.3, the methyl orange endpoint. The methyl orange alkalinity of the mud
and mud filtrate is called the Mm and M,, respectively. The API diagnostic tests
include the determination of Pm, P,, and M,. All values are reported in cubic centimeters
of 0.02 N (normality = 0.02) sulfuric acid per cubic centimeter of sample.
The P, and M, tests are designed to establish the concentration of hydroxyl,
bicarbonate, and carbonate ions in the aqueous phase of the mud. At a pH of 8.3,
the conversion of hydroxides to water and carbonates to bicarbonates is essentially
complete. The bicarbonates originally present in solution do not enter the reactions.
As the pH is further reduced to 4.3, the acid then reacts with the bicarbonate
ions to form carbon dioxide and water.
The P, and Pm test results indicate the reserve alkalinity of the suspended solids.
As the [OH-] in solution is reduced, the lime and limestone suspended in the
mud will go into solution and tend to stabilize the pH. This reserve alkalinity
generally is expressed as an equivalent lime concentration, in lb/bbl of mud.
Total Hardness. A combined concentration of calcium and magnesium in the
mud water phase is defined as total hardness. These contaminants are often
present in the water available for use in the drilling fluid. In addition, calcium