Drilling Fluids

To properly control the hole cleaning, suspension, and filtration properties of a drilling fluid, testing of the fluid properties is done on a daily basis. Most tests are conducted at the rig site, and procedures are set forth in the API RPB13B. Testing of water-based fluids and nonaqueous fluids can be similar, but variations of procedures occur due to the nature of the fluid being tested.

Water-Base Muds Testing
To accurately determine the physical properties of water-based drilling fluids, examination of the fluid is required in a field laboratory setting. In many cases, this consists of a few simple tests conducted by the derrickman or mud Engineer at the rigsite. The procedures for conducting all routine drilling fluid testing can be found in the American Petroleum Institute’s API RPB13B.

Density Often referred to as themudweight, densitymaybe expressed as pounds per gallon (lb/gal), pounds per cubic foot (lb/ft3), specific gravity (SG) or pressure gradient (psi/ft). Any instrument of sufficient accuracy within ±0.1 lb/gal or ±0.5 lb/ft3 may be used. The mud balance is the instrument most commonly used. The weight of a mud cup attached to one end of the beam is balanced on the other end by a fixed counterweight and a rider free to move along a graduated scale. The density of the fluid is a direct reading from the scales located on both sides of the mud balance .
Marsh Funnel Viscosity
Mud viscosity is a measure of the mud’s resistance to flow. The primary function of drilling fluid viscosity is a to transport cuttings to the surface and suspend weighing materials. Viscosity must be high enough that the weighting material will remain suspended but low enough to permit sand and cuttings to settle out and entrained gas to escape at the surface. Excessive viscosity can create high pump pressure, which magnifies the swab or surge effect during tripping operations. The control of equivalent circulating density (ECD) is always a prime concern when managing the viscosity of a drilling fluid. The Marsh funnel is a rig site instrument used to measure funnel viscosity. The funnel is dimensioned so that by following standard procedures, the outflow time of 1 qt (946 ml) of freshwater at a temperature of 70±5◦F is 26±0.5 seconds. A graduated cup is used as a receiver.

Direct Indicating Viscometer
This is a rotational type instrument powered by an electric motor or by a hand crank . Mud is contained in the annular space between two cylinders. The outer cylinder or rotor sleeve is driven at a constant rotational velocity; its rotation in the mud produces a torque on the inner cylinder or bob. A torsion spring restrains the movement of the bob. A dial attached to the bob indicates its displacement on a direct reading scale. Instrument constraints have been adjusted so that plastic viscosity, apparent viscosity, and yield point are obtained by using readings from rotor sleeve speeds of 300 and 600 rpm.
Plastic viscosity (PV) in centipoise is equal to the 600 rpm dial reading minus the 300 rpm dial reading. Yield point (YP), in pounds per 100 ft2, is equal to the 300-rpm dial reading minus the plastic viscosity. Apparent viscosity in centipoise is equal to the 600-rpm reading, divided by two.

Gel Strength
Gel strength is a measure of the inter-particle forces and indicates the gelling thatwill occur when circulation is stopped. This property prevents the cuttings from setting in the hole. High pump pressure is generally required to “break” circulation in a high-gel mud. Gel strength is measured in units of lbf/100 ft2. This reading is obtained by noting the maximum dial deflection when the rotational viscometer is turned at a low rotor speed (3 rpm) after the mud has remained static for some period of time (10 seconds, 10 minutes, or 30 minutes). If the mud is allowed
to remain static in the viscometer for a period of 10 seconds, the maximum dial deflection obtained when the viscometer is turned on is reported as the initial gel on the API mud report form. If the mud is allowed to remain static for 10 minutes, the maximumdial deflection is reported as the 10-min gel. The same device is used to determine gel strength that is used to determine the plastic viscosity and yield point, the Variable Speed
Rheometer/Viscometer.

API Filtration
A standard API filter press is used to determine the filter cake building characteristics and filtration of a drilling fluid
The API filter press consists of a cylindrical mud chamber made of materials resistant to strongly alkaline solutions. A filter paper is placed on the bottom of the chamber just above a suitable support. The total filtration area is 7.1
(±0.1) in.2. Below the support is a drain tube for discharging the filtrate into a graduated cylinder. The entire assembly is supported by a stand so 100-psi pressure can be applied to the mud sample in the chamber. At the end of the 30-minute filtration time, the volume of filtrate is reported as API filtration in milliliters. To obtain correlative results, one thickness of the proper 9-cm filter paper—Whatman No. 50, S&S No. 5765, or the equivalent—must be
used. Thickness of the filter cake is measured and reported in 32nd of an inch. The cake is visually examined, and its consistency is reported using such notations as “hard,” “soft,” tough,” ’‘rubbery,” or “firm.”

Sand Content
The sand content in drilling fluids is determined using a 200-mesh sand sieve screen 2 inches in diameter, a funnel to fit the screen, and a glass-sand graduated measuring tube . The measuring tube is marked to indicate the volume of “mud to be added,” water to be added and to directly read the volume of sand on the bottom of the tube.
Sand content of the mud is reported in percent by volume. Also reported is thepoint of sampling (e.g., flowline, shale shaker, suctionpit). Solids other than sand may be retained on the screen (e.g., lost circulation material), and the presence of such solids should be noted.

read also Sand Control

Liquids and Solids Content
A mud retort is used to determine the liquids and solids content of a drilling fluid. Mud is placed in a steel container and heated at high temperature until the liquid components have been distilled off and vaporized. The vapors are passed through a condenser and collected in a graduated cylinder. The volume of liquids
(water and oil) is then measured. Solids, both suspended and dissolved, are determined by volume as a difference between the mud in container and the distillate in graduated cylinder. Drilling fluid retorts are generally
designed to distill 10-, 20-, or 50-ml sample volumes.

For freshwater muds, a rough measure of the relative amounts of barite and clay in the solids can be made (Table 1.1). Because both suspended and dissolved solids are retained in the retort for muds containing substantial
quantities of salt, corrections must be made for the salt. Relative amounts of high- and low-gravity solids contained in drilling fluids can be found in Table 1.1.

pH
Two methods for measuring the pH of drilling fluid are commonly used: (1) a modified colorimetric method using pH paper or strips and (2) the electrometric method using a glass electrode . The paper strip test may not be reliable if the salt concentration of the sample is high.
The electrometric method is subject to error in solutions containing high concentrations of sodium ions unless a special glass electrode is used or unless suitable correction factors are applied if an ordinary electrode is used. In addition, a temperature correction is required for the electrometric method of measuring pH.
The paper strips used in the colorimetric method are impregnated with dyes so that the color of the test paper depends on the pH of the medium in which the paper is placed. A standard color chart is supplied for comparison
with the test strip. Test papers are available in a wide range, which permits estimating pH to 0.5 units, and in narrow range papers, with which the pH can be estimated to 0.2 units.
The glass electrode pH meter consists of a glass electrode, an electronic amplifier, and a meter calibrated in pH units. The electrode is composed of (1) the glass electrode, a thin-walled bulb made of special glass within
which is sealed a suitable electrolyte and an electrode, and (2) the reference electrode, which is a saturated calomel cell. Electrical connection with the mud is established through a saturated solution of potassium chloride
contained in a tube surrounding the calomel cell. The electrical potential generated in the glass electrode system by the hydrogen ions in the drilling mud is amplified and operates the calibrated pH meter.

Resistivity
Control of the resistivity of the mud and mud filtrate while drilling may be desirable to permit enhanced evaluation of the 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 id determined by calibration with standard solutions of known resistivity. The resistivity is expressed in ohm-meters.

Filtrate Chemical Analysis
Standard chemical analyses have been developed for determining the concentration of various ions present in the mud. Tests for the concentration of chloride, hydroxyl, 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 can be determined from a knowledge of the chemical reaction taking place.

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 Cl− has been removed fromsolution reactswith the chromate to formAg9CrO4, an orange-red precipitate. Contamination with chlorides generally results from drilling salt or from a saltwater flow. Salt can enter and contaminate themudsystem 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 of the filtrate to 8.3, the phenolphthalein end point. The phenolphthalein alkalinity of the mud and mud filtrate is called the Pm and Pf , respectively. The Pf test includes the effect of only dissolved bases and salts, whereas the Pm test includes the effect of both dissolved and suspended bases and salts. The m and f indicate if the test was conducted on the whole mud or mud filtrate. The Mf alkalinity refers to the amount of acid required to reduce the pH to 4.3, the methyl orange end point. The methyl orange alkalinity of the mud and mud filtrate is called the Mm and Mf , respectively. The API diagnostic tests include the determination of Pm, Pf , and Mf . All values are reported in cubic centimeters of 0.02N (normality= 0.02) sulfuric acid per cubic centimeter of sample. The lime content of the mud is calculated by subtracting the Pf from the Pm and dividing the result by 4.
The Pf and Mf 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 reacts with the bicarbonate ions to form carbon dioxide and water.
ml N/50H2SO4 to reach pH=8.3
CO 3(-2) +H2SO4→HCO3(-) +HSO4
carbonate+acid→bicarbonate+bisulfate
OH−+H2SO4→HOH+SO4=  hydroxyl+acid→water+sulfate salt
The Pf 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
(Table 1.2). This reserve alkalinity generally is expressed as an excess lime concentration, in lb/bbl of mud. The accurate testing of Pf, Mf , and Pm are needed to determine the quality and quantity of alkaline material present
in the drilling fluid. The chart below shows how to determine the hydroxyl, carbonate, and bicarbonate ion concentrations based on these titrations.

Total Hardness
The total 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 makeup. In addition, calcium can enter the mud when anhydrite (CaSO4) or gypsum (CaSO4 ·2H2O) formations are drilled. Cement also contains
calcium and can contaminate the mud. The total hardness is determined by titration with a standard (0.02 N) versenate hardness titrating solution (EDTA). The standard versenate solution contains sodium versenate, an
organic compound capable of forming a chelate when combined with Ca2 and Mg2.
The hardness test sometimes is performed on the whole mud as well as the mud filtrate. The mud hardness indicates the amount of calcium suspended in the mud and the amount of calcium in solution. This test usually is made on gypsum-treated muds to indicate the amount of excess CaSO4 present in suspension. To perform the hardness test on mud, a small sample of mud is first diluted to 50 times its original volume with distilled water so that any undissolved calcium or magnesium compounds can go into solution. The mixture then is filtered through hardened filter paper to obtain a clear filtrate. The total hardness of this filtrate then is obtained using the same procedure used for the filtrate from the low-temperature, low-pressure API filter press apparatus.

Methylene Blue Capacity (CEC or MBT)
It is desirable to know the cation exchange capacity (CEC) of the drilling fluid. To some extent, this value can be correlated to the bentonite content of the mud. The test is only qualitative because organic material and other clays present in the mud also absorb methylene blue dye. The mud sample is treated with hydrogen peroxide to oxidize most of the organic material. The cation exchange capacity is reported in milliequivalent weights (mEq) of methylene blue dye per 100 ml of mud. The methylene blue solution used for titration is usually 0.01 N, so that the cation exchange capacity is numerically equal to the cubic centimeters of methylene blue solution per cubic centimeter of sample required to reach an end point. If other adsorptive materials are not present in significant quantities, the montmorillonite content of the mud in pounds per barrel is calculated to be five times the cation exchange capacity.
The methylene blue test can also be used to determine cation exchange capacity of clays and shales. In the test, a weighed amount of clay is dispersed into water by a high-speed stirrer ormixer. Titration is carried out as
for drilling muds, except that hydrogen peroxide is not added. The cation exchange capacity of clays is expressed as milliequivalents of methylene blue per 100 g of clay.