US2759891A - Oil well casing wash composition - Google Patents

Description

Aug. 21, 1956 ROHRBACK ET AL 2,759,891

OIL WELL CASING WASH COMPOSITION Filed April 29, 1953 l l i 6 A A INVENTORS G/LSON H. ROHRBACK JOSEPH F CH/TTUM United States atent OIL WELL CASING WASH COMPOSITION Gilson H. Rohrback, Seattle, Wash., and Joseph F. Chittum, Whittier, Caliii, assignors to California Research Corporation, San Francisco, Calif., a corporation of Delaware Application April 29, 1953, Serial No. 351,833

2 Claims. (Cl. 252-855) This invention relates to a composition and method for inhibiting the corrosion of oil well casings.

Although relatively little attention has been given to corrosion failure of oil well casings in the literature, this problem is rather wide-spread. A number of theories have been proposed to account for casing corrosion and some attempts to control it have been made on the basis of the theories. Among the theories heretofore advanced to explain casing corrosion have been suggestions that the corrosion was due to bacterial action, due to electric currents flowing through the casing, due to localized corrosion at metal flaws, due to particular compositions of formation water, due to corrosion by selfpotential currents and due to interzonal migration of salt water. So far as is known, no successful method of controlling casing corrosion has been developed on the basis of any of these theories. Meantime, casing failure due to corrosion continues to be a serious problem in many oil fields. Repair of casing failure is difficult at best and if the failure escapes detection for an appreciable period of time, the well ceases to produce, and production may not be resumed, even though the casing be repaired, necessitating abandonment of the well.

A number of instances of casing failure due to corrsion in California oil fields have been carefully studied. From these studies it was concluded that corrosive attack upon ferrous metal casings is rapid and localized when different points of the exterior of the casing are in contact wit-h fluids differing markedly in pH or oxygen content.

Current practice in completing oil wells is such that the fluids resident in the annular space between the outer wall of the casing and the formation will commonly exhibit marked differences of pH at different points along the casing wall. When a well has been drilled to the desired depth and is ready for completion, the casing is full of drilling mud at the commencement of the cementing operation. It is rather common practice to introduce an aqueous solution of an alkali metal polyphosphate (commonly referred to as a pyro-wash liquid) into the casing before the cement is introduced. After the introduction of the aqueous polyphosphate solution, cement is introduced and forced down the casing. As the cement travels down the casing, there are three more or less distinct fluid phases in the casing, the bottom phase being drilling mud, the middle phase being an aqueous solution of a polyphosphate, and the upper phase being the cement slurry. When the necessary amount of cement has been introduced, drilling mud is introduced behind the cement phase and forced down the casing. The bottom mud layer, the pyro-wash layer, and the cement layer flow downwardly through the casing and around the lower circumference of the casing and upwardly through the annulus between the outer casing wall and the formation. When the cement has been forced into suitable position between the outer wall of the casing and the formation, it is allowed to set. The annular space between the outer wall of the casing and the formation when the well is completed will commonly contain a layer of drilling mud in the upper part of the annulus, below the drilling mud a layer of aqueous polyphosphate solution, below the polyphosphate solution a cemented zone, and below the cemented zone a layer of drilling mud extending to the lower extremity of the casing.

Drilling muds normally have pH values ranging from about 8.5 to about 12. The aqueous polyphosphate solution normally has a pH of about 12 in contact with cement. A difference in pH between the drilling mud layer and the aqueous polyphosphate layer present in the annular space between the outer casing wall and the formation is commonly so large as to cause rapid electrochemical corrosion of the casing which continues to the point of easing failure. In cases in which a high pH mud is used in combination with the polyphosphate wash, there will be little difference in pH between the mud and the wash solution, but formation water ordinarily having a pH of 7.5 very commonly breaks through the mud layer at one point or another andmakes contact with the casing so that large pH differences exist between the pyrowash solution and the formation water and between the mud, if it is a high pH mud, and the formation Water.

It has now been found that casing corrosioncan be very markedly reduced by incorporating a small amount of stannous chloride in the pyro-wash solution. Pyrowash solutions are normally wash solutions of polyphosphate such as tetrasodium pyrophosphate, sodium tripolyphosphate, sodium hexa-meta-phosphate, and the like, in which the polyphosphate content. is from, about 02% to 2% by weight. Large amounts of polyphosphate up to the point of saturation of the water may be employed. To inhibit casing corrosion from 0.05 to 5% by weight of stannous chloride is dissolved in the pyrowash solution.

While stannous chloride is preferred in compounding the wash solution of the. invention, it is the stannous ion which is the effective inhibitor. Accordingly, other stannous compounds which are soluble or dispersible in water, such as stannous sulfate, stannous nitrate, stannous hydroxide, etc. can be substituted for the stannous chloride and are found essentially equally effective.

Laboratory studies of the nature of casing corrosion and of compositions and methods of controlling it were made in the apparatus diagrammatically illustrated in the appended drawing. In the drawing, container 6 is a glass jar having about 4 gallons capacity. Electrode 1 was made up of 3 short sections of concentric pipe welded together. The weld areas were covered with a plastic paint to eliminate undesirable couples from these areas. The total area of this electrode was about 330 square inches. Electrode 4 is a small strip of iron having a surface area of approximately 2 square inches. Electrode 1 is welded to iron rod 2 which is attached to adjustable support 5 so that the position of electrode 1 in the jar can be adjusted at will. Electrode 4 is connected to rod 2 by wire 3. In making the experimental tests two fluids were introduced into jar 6. The fluids differed from each other in both pH and density. In each test, jar 6 was filled with the denser of the two fluids employed to level AA and then the less dense of the two fluids was introduced into jar 6, filling the jar to about level BB. Electrode 1 was positioned in the jar so that it was entirely surrounded by the fluid of higher pH. Electrode 4 was positioned in the jar so that it was entirely surrounded by the fluid of lower pH. The electrodes were then left in these relative positions for a period ordinarily of 14 days, at the end of which electrode 4 was removed and weighed to determine its loss of weight. In order to determine the weight loss of electrode 4 due to electrochemical action alone, a second small iron strip approxi:

mately identical with electrode 4 in size and shape was 3 suspended from the wall of the jar in the same fluid which surrounded electrode 4. At the end of the test period the weight loss of electrode 4 and the weight loss of the second metal strip were determined and the difference between these two losses was the loss of weight of elec- 5 trode 4 due to electrochemical action. Results of a series of experiments are set forth in the table below. The muds employed in the tests were typical commercial drilling muds. The formation water was a typical aqueous efliuent from a California well and the pyroflnid 10 was an aqueous solution of tetrasodium pyrophosphate such as is commonly employed for washing the casing prior to introduction of the cement and which is commonly left in the annulus between the outer casing wall and the formation when the well is completed. The etfect of 15 various additives on the weight loss due to electrochemical corrosion is shown in the table.

TABLE containing wash solution diffuses into the mud. Other natural tannin compounds such as chestnut extract, and the like, may be substituted for the quebracho.

A suitable pyro-wash solution can be prepared by dissolving 18 pounds of tetrasodium pyrophosphate and 5.4 pounds of stannous chloride in 10 barrels of water. This Wash solution can be improved in respect to its effect on mud viscosity by the addition of about 18 pounds of quebracho to the 10 barrels of solution.

We claim:

1. An oil well mud cake wash solution comprising an aqueous solution of an alkali metal polyphosphate containing a small amount from about 0.05 to about 0.15% by weight of stannous chloride.

2. An oil well casing wash solution suitable for use in filling at least a part of the annular space between the casing and the formation in a completed oil well comof treated and untreated fluids Electrode 1 Electrode 4 Measured Weight Loss from Elec- Test N 0. Fluid Additive pH Fluid Additive pH trade 4 Due to Current (Milligrams) Pyro N one 12. 3 Mud None 8. 5 968 Pyro Nonem. 12.3 Mud Sodium Ohromate- 8. 4 522 Mud None 8. 5 Pyro SnC 12.1 0 Formation Water. None... 7. 1 Pyro 12. 2 n Mud SnOhl2. 2 Pyro l2. 2 0

2Mud contained sodium chromate0.02 percent. 3-Pyro contained SnCl:-0.15 percent, and quebrach00.5 percent. 4-Pyro contained SI1C120.15 percent, and quebracho0.5 percent.

5-Both mud and pyro contained SnClz-OJS percent, and quebraeho0.05 percent. When pyro was treated with SnOh, the cell was set up with electrode 4 in pyro solution.

prising an aqueous solution of an alkali metal polyphosphate containing a small amount from about 0.05 to about 0.15% by weight of stannous chloride and from 40 about 0.1 to 3% by weight of quebracho.

References Cited in the file of this patent UNITED STATES PATENTS 2,322,484 Stuart June 22, 1943 2,490,291 Wrightsman Dec. 5, 1949 2,580,765 Hall et a1. Jan. 1, 1952

Claims (1)

1. AN OIL WELL MUD CAKE WASH SOLUTION COMPRISING AN AQUEOUS SOLUTION OF AN ALKALI METAL POLYPHOSPHATE CONTAINING A SMALL AMOUNT FROM ABOUT 0.05 TO ABOUT 0.15% BY WEIGHT OF STANNONS CHLORIDE.

Images