Water Based Mud Chemistry

•The science of the composition, structure, properties, and reactions of matter, especially of atomic and molecular systems


•Mud engineers deal with chemistry every day.
•Properties such as mass, volume, density are important in drilling fluids
•Other common properties include alkalinity, calcium, magnesium, chlorides

LABRATORY TESTS

Pilot tests:
•In drilling fluid engineering work, calculations are based upon the oilfield barrel.
•For the purposes of the lab, an oilfield barrel is 350ml
–A 42 gal barrel would 350 pounds of fresh water
–1 g of a material is added to this barrel equivalent, this would be the same as adding 1 lb of material to an oilfield barrel
•Pilot tests can be done in the field prior to the addition of any treatment in order to predict the outcome of the treatment and the fluids properties
•Pilot tests are routine in the lab Aging Tests:
•To more closely simulate bottom-hole conditions samples are frequently heat aged at bottom-hole temperatures and for time periods equivalent to anticipated periods of no circulation.
•Most ovens used for this procedure allow the cell to remain either static, roll or rotate during aging Hot Rolling:
•This is an oven specially designed to take API stainless steel cells and rotate them over a period of time (usually 12 hours
•When filling the cells an adequate space must be left to allow expansion of the test fluid.
•Upon the cells removal from the oven the cell should be allowed to cool to room temperature. Static Age Testing:
•Samples are heated in an oven and left for a period of time. •Can simulate downhole conditions during trips Corrosion Tests with Corrosion Coupons:
•Corrosion Ring Coupons that are placed in the drillstring is one technique for monitoring the drilling fluid corrosion rate.
•Typically they are exposed to the drilling environment for about 100 hours
–Can determine type and rate of corrosion
•examination of the scale, if any, will indicate the dissolved gas that is causing the damage.
•If the source is known, the type of chemical treatment may be more effectively selected for better control. •Corrosion ring monitoring procedure
–Rings are design for different types of connections (X-hole, full hole, IF etc)
–Rings are run in pairs The first in the Kelly Saver Sub, and the one downhole in the last joint of drill pipe above the collars
–Rings should be run for 40
–100 hrs, record the total time in the drillstring, not just rotating hours •Corrosion ring monitoring procedure
–Completely fill out the corrosion ring form or envelope (date, mud properties etc)
–Run in the hole for 40 –100hrs
–After testing period pull out and examine the rings for severe corrosion
–If severe corrosion exist, action will be required immediately
–Clean off the corrosion ring and place it in the plastic bag and original envelope for lab analysis
•Laboratory test for Corrosion Coupon
–Prior to cleaning and weighing, determine the nature of the scale, if any.
–A magnet will indicate an oxide, and possibly oxygen corrosion. –With the drop-wise addition of the Iron Sulfide Test Solution, effervescence could indicate the presence of carbonates from the dissolved acid gas, carbon dioxide.
•Laboratory test for Corrosion Coupon
–the ring should be rinsed with a suitable solvent. This will remove any oily residue that may have been added.
–Verify that the coupon number agrees with the number on the shipping package.
•Laboratory test for Corrosion Coupon
–The presence of iron sulfide is also noted during this same test.
–A yellow precipitate of arsenic sulfide would appear as “miniature scrambled eggs”in nature.
–The mere presence of a yellow color just may indicate some of the steel is dissolving from the coupon, and without the yellow/green precipitate, no sulfide shouldbe noted on the report.
•Laboratory test for Corrosion Coupon •Laboratory test for Corrosion Coupon
–The ring should be cleaned with a detergent solution and a stiff bristle brush or equivalent.
–It may be necessary to dip the ring in inhibited 15% hydrochloric acid one or more times to aid in removal of the corrosion products.
–The ring should be scrubbed with detergent after each acid dip. –When the ring is clean, rinse with clear water and then anhydrous acetone or methyl alcohol to allow the ring to dry.
•Laboratory test for Corrosion Coupon
–After the ring has been properly processed, then it should be reweighed at least to the nearest milligram.
–The weight loss can be calculated.
–If there is significant loss of metal due to mechanical damage, it is recommended that Mechanical Damage (MD) be noted, and all of the calculations be shown at the bottom of the ring report.
–The weight loss in g multiplied by the “K”-factor, divided by the total hours in the string yields the corrosion rate.
•Laboratory test for Corrosion Coupon
–Visual inspection •Note if corrosion is pitted or generalized attack
•Erosion may be a factor
•The normally accepted value for corrosion rates of freshwater muds is 2 lb/ft2-y or less with no pitting
•Calculations of Corrosion Rates
–The difference in the weight of the cleaned coupon before and after the test period is determined by weighing
–This weight loss multiplied by the “K”-factor divided by the total hours will yield the corrosion rate. •Calculations of Corrosion Rates
–If no K-factor is supplied then lb/ft2-y = wt loss (mg) x 144 x 365 453,600 x *area (in2.) x days exposed** Fann Model 70 •HP/HT Viscometer (500°F –20000 psi) to evaluate mud rheology, simulating bottom hole conditions

MUD FILTRATE ALKALINITY

Acidity is one measure of alkalinity that is indicated by the pH. However, the nature and amount of other ions such as Carbonates or Bicarbonates can also effect mud filtrates alkalinity.
For fresh water mud systems, these ions can be indicative of the rheological stability of such mud systems.
Concentrations of either ion can result in high, low shear rate viscosity (Yield Point) and high, progressive Gel Strengths.
•Three methods can be employed for the determination of Carbonate and Bicarbonate concentration. The very common Pf / Mf method is restricted to mud systems having a low organic content, whereas the P1 / P2 method or Garret Gas Train may be used for better, more quantitative method, especially in the systems with high organic content. Pf / Mf Method test procedures
•Using a 1 ml pipette, measure 1 ml of filtrate into a white titration dish. Dilute with distilled water.
•Add 2-3 drops of Phenolphthalein Indicator.
•if no color change occurs, then the Pf = 0; continue to step 4
•if a pink or red color develops, the Pf > 0; continue to step 3 •Using a pipette, add N/50 Sulfuric Acid continuously while swirling or stirring until the sample changes from pink to colorless (or original filtrate tint).
•The number of ml of N/50 Sulfuric Acid to reach this point is recorded as the Pf value.

•To the sample, which has been titrated to the Pf end point, add 2-3 drops of BromoCreosol indicator solution to obtain a light blue color. Continue titrating with swirling (or stirring) until the color changes from light blue to apple green (pH 4.0-4.5).
•This end point is recorded as the Mf end point.

ALKALINITY OF THE MUD (Pm
•This test measures the alkalinity of the whole mud. When used in conjunction with the filtrate alkalinity determination, the amount of excess Lime present in the mud can be determined.
Test Procedure
•Measure 1 ml of a freshly stirred sample of mud into a titration dish using a syringe.
•Dilute the mud in the dish with about 50 ml of distilled water.
•Add 4-5 drops of Phenolphthalein indicator solution.
•If the sample does not change color, record the Pm as 0.
•If the sample turns pink, titrate rapidly with N/50 Sulfuric Acid until the pink color disappears.
Calculations
•Report the alkalinity of the mud, Pm as the number of ml of N/50 Sulfuric Acid added until the pink color disappears.
•Note: If the mud sample is deeply colored and the color change is hard to see, use 0.5 ml of mud, and report the Pm as the volume of Sulfuric Acid added X 2.0. If N/10 (0.1N) Sulfuric Acid is used, the Pm is reported as the volume of acid added to 1 ml of mud X 5.0.

TOTAL HARDNESS (Calcium & Magnesium)

•Water containing large amounts of Calcium or Magnesium Salts is commonly referred to as “hard water”. Make up waters that are hard make it difficult to obtain the maximum yield from Bentonite, so it becomes necessary to treat out excess Calcium.
As a general rule, the total hardness as Calcium should be brought to less tan 40 mg/L.
The presence of Calcium in the mud filtrate may also indicate the presence of contaminants, such as anhydrite or cement.

Test Procedures Total Hardness (as Calcium)::
•Using a pipette, measure 1.0 ml of filtrate into a white titration dish, and dilute to a convenient volume with distilled water.
•Add 4-5 drops of strong Buffer solution, and 2-3 drops of Indicator. A red or wine color will develop if Calcium is present.
•While swirling or stirring continuously, add Total hardness titrating solution with a pipette until the color changes from red to blue. At this end point, record the number of milliliters of Total hardness titrating solution added. Calculations: Total Hardness (as Calcium): mg/L Hardness (as Calcium) = 400 X ml Total hardness titrating solution added Calcium Hardness:
•Using a pipette, measure 1.0 ml of filtrate into a white titration dish, and dilute with a small amount of distilled water.
•Add 2-3 drops of 8N KOH (Potassium Hydroxide) solution. •Add several grains of CalverII, and swirl or stir to mix.
•Using a pipette.
Titrate with Total hardness titrating solution solutionto a color change from red to blue Calculations: mg/L Calcium Ion = 400 X ml Total hardness titrating solution added Magnesium Hardness: –The Magnesium hardness is calculated as follows: mg/L Magnesium = mg/L Total Hardness (as Calcium) –mg/L Calcium

CHLORIDE DETERMINATION

•Chloride ions exist in a mud system as Salts of Sodium, Magnesium, Calcium, or Potassium.
The determination of the Chloride ion present in the mud filtrate may give an indication of a Salt water flow, or the presence of a Salt formation or stringer.
•In mud systems to which Salt has been added, the Chloride measurements show the amount of salinity present in the mud.

Test Procedure:
•Measure 1.0 ml of filtrate into a white titration dish and dilute to a convenient volume with distilled water.
•Add a few drops of Phenolphthalein indication solution. If a pink color develops, add N/50 Sulfuric Acid until the pink color completely disappears.
It is not necessary to record the volume of N/50 Sulfuric Acid added.
•Add 4-5 drops of Potassium Chromate indicator to obtain a yellow color.
•Add Silver Nitrate while swirling or stirring until the color changes from yellow to orange-red (brick red), and persists for 30 seconds. Calculations:
•If 0.0282 N Silver Nitrate is used: –mg/L Chlorides = 1000 X ml of Silver Nitrate added
•If 0.282 N Silver Nitrate is used: –mg/L Chlorides = 10000 X ml of Silver Nitrate added Remarks:
•mg/L Salt (NaCl) = 1.65 X mg/L Chlorides
•The Chloride test may be run on the same sample used in the Pf determination, if the Mf test was not performed.
•Avoid contact with the Silver Nitrate solution. Wash immediately with water if Silver Nitrate gets on the skin or clothing.
•The end point of the reaction is when the Silver Chromate is when the first detectable permanent color change from yellow to a light brick red occurs.
•When using the weak Silver Nitrate solution, the end point is approached very gradually. Therefore, the formation of the Silver Chromate can be seen by a color change for yellow to brick red.
•If the strong Silver Nitrate is used, the end point is approached much more rapidly. Hence the early formation of the Silver Chromate, and is brick red color may be missed due to the larger amounts of Silver Nitrate being added. The color change will go from yellow to red. As soon as the red color is seen, the titration is complete.
•White lumps of Silver Chloride form when titrating high concentrations of Salt. This should not be mistaken for the end point.
•A high pH will precipitate Silver Oxide.

pH DETERMINATION

•The acidity or alkalinity of a drilling mud is indicated by the Hydrogen ion concentration, which is commonly expressed in terms of pH.
A perfectly neutral solution has a pH of 7.0, whereas alkaline (basic) solutions have a pH range between 7.0-14.0, and acidic solutions have a pH less than 7.0.
•The pH measurement is used as well to indicate the presence of contaminants such as cement or anhydrite.
Test Procedure
•pHydionPaper:
–This method may be used on the mud filtrate, or whole mud directly. –Place a 25 mm (1 inch) strip of indicator paper on the surface of the mud to be tested and allow it to remain until the liquid has wet the surface and the color has stabilized.
This takes approximately 1 minute.
–Compare the color standards provided with the test paper (which was not in contact with the mud solids) to the color standards provided with the test paper, and estimate the pH of the mud accordingly.
•Color pH Strips: –This method applies onlyto the mud filtrate.
–After obtaining a sample of mud filtrate, totally immerse the colored portion of the color pH strip into the filtrate, and remove immediately.
–After a short period of color stabilization (10-15 seconds), compare the color of the wetted strip with the color standards provided in the color pH plastic container.
•An estimate may be necessary if a color does not exactly match a particular pH value.

SAND CONTENT

The API sand content is defined to be that portion of the drilling mud solids whose size is greater than 74 microns. The test can be used to give a qualitative, relative indication of the solids removal equipment effectiveness, the relative amount of coarse Barite present, and the abrasiveness of the mud.
Equipment:
•The sand content set consists of a 63.5 mm, 74 micron (200 mesh) sieve, a small funnel to fit snugly over the sieve, and a conical glass tube graduated in percent, and having two fluid level indicator lines





Test Procedure
•Fill the glass measuring tube to the indicated mark with the mud to be tested. Add water to the next mark. Close the mouth of the tube and shake vigorously.
•Pour the mixture onto the screen tapping it lightly to aid passing of the diluted mud through the screen. Add more clean water, and repeat this wet screening procedure until the wash water in the tube is clear. Wash the sand retained on the screen to free it of any remaining mud

Test Procedure
•With the sieve in an upright position, fit the funnel over the sieve. Invert slowly and fit the funnel tip into the mouth of the cleaned measuring tube.
•Back wash the sand from the sieve using a fine spray of clean water with the measuring tube positioned vertically upright, allowing the sand to settle in the tube for a few minutes.
•Report the sand content as the volume fraction of sand, (the volume percent divided by 100).
•For example, if the sand content is read as ¼%, the volume fraction is reported as 0.0025%

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