US 2234790 A
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Patented 11, 1941 UNITED STATES PATENT OFFICE v I uunwatzfi izmmnsron Vernon 13.710110, venous, cam; assignm- Tidewater Associated Oil Company, New York,
N. Y., a corporation of Delaware No Drawing. Application l'ebruary 1c, 1939,
Serial No. zsazar a claim (Cl. 166-21) m suitable power source about its longitudinal axis.
The bit is provided with teeth of various shapes designed to bite the rockaway in small pieces, and is lubricated by a continuous stream of mud pumped down through the hollow'pipe and bit.
15 The stream returns'to a surface-sump, carrying the rock particles or cuttings with it, by pass! ing upwardly in the bore on the outside of the pipe. The mud is used to carry abrasive material in suspension and to cool the bit. 'The mud 001-.
2'0 umn also controls the oil and gas pressures encountered, and has other important functions.
As the drill is driven deeper into the earth, a
. wallof partially dried mud is gradually built up around the outside of the bore.
25 countered, especially in deep wells, may cause a filtering or "baking action, and it is frequently foundthat the mud wall becomes so hard and imp rvious that it. must be removed before the oil can iiow into the bore. This removal'is hence to very important in determining the size and output of the well.
Qne of the common methods of removal involves treating the wall with an acid which will cause it wholly or partially 'to decompose, permit- 3o ting the mud to slough off and be carried away in suspension by the fluid in the hole. Four of the factors governing the type of acid which is suitable for'such use are: the rate of disintegrationofithe wall; safety inhandling; chemical I 4'0 inertness toward steel casing: and cost. Hydrochloric (muriatic) acid has been widely used.
It is relatively inexpensive," and reacts rapidly with the mud, but is diiiicult to handle in the concentration necessary for treatment and tends to 48 attack the casing and drill pipe in the hole unless inhibited;
Inhibiting the acid consists in adding thereto a substance which will reduce as far as possible the reaction of the acidwith the casing, while at the '50 same time not interfering with the break-down of the mud. Such a substance could be a salt of any metal lying below=hydrosen in the'electromotive series. One of the best and cheapest inhibitors is cuprous chloride. Others are described as in United StatesPatent 1,877,504 to crane and The heat env -Sanford, which claims the use as inhibiting agents Q -of arsenic acid, arsenic tri-oxlde, and other soluble arsenates and arsenites, as well-as a variety of other substances. Such inhibitors, however, are at best a compromise. Even with their use, it is necessary to introduce the inhibited acid to 5 the bore in undesirable concentrations. As the reaction progresses, the acid becomes continually weaker; while a comparatively rapid reaction may occur at first, it tapers oif rapidly;- The time required for a treatment is usually from five to eighteen hours, but is dependent on a-number of factors, including the wall thickness, the depth of hole, and others.
My invention overcomes the diiiiculty esperil enced with even the most successful or the previously known methods by the use of a weak acid, together with a substance which will combine therewith in the bore in such a way as to produce continually a freshsupply of a strong acid. The" concentrations thus secured will break down the mud wall rapidly without deleterious eifects on the casing. In pursuance of this method, ,I preferably introduce in solution. acetic acid and magnesium chloride, which will react in the bore to form an inert salt and hydrochloric acid The fundamental reaction in the chemical breakdown of a mud sheath is that between the acid and calcium carbonate, which is an important constituent of nearly all native clays. s The calcium carbonate is dispersed throughout the mud; it combines with acid to form an unstable compound, carbonic acid, which again breaks down intowater and carbon dioxide gas. The mudbecomes honeycombed by the decomposition of the calcium carbonate, so:that the remainder of the wall is easily penetrated by and sloughs oil with the fluid circulating or standing in the hole.
It will be seen from the above description that M the primary object of my invention is to provide more eifective means for removing the mud sheath from'well bores. Other; objects are: to
. provide a continuously replenished supply of acid sheath dlsintegrator which may be handled without danger by operators; and to provide a disintegrating agent in more economical form.
My invention possesses numerous other objects and features of advantage, some of which. together with the foregoing, will be set forth in the following description. It is, therefore, to be understood that my invention is applicable to other substances, and that I do not limit myself, in any way, to the exact showing 0! the present application, as I may adopt various other apparatus embodiments, utilizing the method, within thescope of the appended claims.
Removal is accomplished by pumping the solution of my invention into the bore, either directly or through the drill stem. The solution is allowed to remain in the hole for about ten hours. This time is, of course, a variable, depending on the percentage of carbonatesin the wall, the permeability and the thickness, the temperature in the hole, and the pressure head due to the overlying fluid column. .Where ordinary acid treatment is used, the usual range of times is from five to eighteen hours, so it will be seen that my method compares very favorably with the prior art in this respect.
If the wall is to be removed along only a portion of the bore, the solution is spotted" in place by displacing the mud or liquid in that portion of the hole.
My departure from the previous art lies in the use of acetic acid instead of hydrochloric acid, and the addition of substances thereto which will increase the reactive rate with the mud. I prefer to use magnesium chloride for this pur-- pose, the most eflfectlve proportions being 4.0%
magnesium chloride and 25% of glacial acetic acid, by weight.
A milld reaction occurs when the solution is mixed, due to the small proportion of magnesium. chloride present. Some magnesium acetate and hydrochloric acid is formed, leaving surplus I acetic acid. Using the abbreviation Ac to represent the acetate radical (CHaCOO), this initial reaction may be written:
The mixed solution represented by the rig-ht half of the Equation 1 then begins to react with the mud in the formation. The hydrochloric acid reacts with the calcium carbonateto form calcium chloride and carbonic acid. The latter is unstable and breaks down into water and carbon dioxide;- the former provides a source of the chloride radical. for subsequent reaction with one excess acetic acid. At the same time, the acetic acid reacts with the calcium carbonate to form calcium acetate and carbonic acid. The latter breaks down into water and carbon dioxide, and the former remains inert as did the magnesium acetate previously iormed. These reactions may be written:
The surplus acetic acid remaining is still available for reaction with the mud directly, as in Equation 3, and through intermediate combination with the calcium chloride oi Equation 2:
The hydrochloric acid produced as in Equation 4 will in turn react with the mud as in: Equation 2, the process continuing until all the acetic acid is-spent.
The continual replenishment of the hydro-- \chloric acid as it is consumed makes possible a comparatively rapid breakdown of the mud wall without there being present at any time a corrosive concentration, while the reactive power is much greater than if dilute hydrochloric alone was used.
The proportions suggested above have been found to be the most economical for a positive reaction. An increase in the amounts of glacial acetic acid and magnesium chloride present has little effect on the rate of reaction, while a decrease causes a marked decrease in the rate of disintegration.
I have found that the breakdown may be speeded by the addition to the solvent of a wetting agent,.which will reduce the surface tension of the solution and permit it to penetrate the mud wall more freely. One such wetting agent which is satisfactory is known by the trade-mark "Aerosol OT", the composition of which is not known exactly to me, although it is believed to be an ester of a sulfonated bi-carboxylic acid. The proper proportion is 1 to 2 parts of the commercial 10% strength solution to 1,000 parts of solvent, by weight, or 1 to 2 parts of full strength Aerosol CT" to 10,000 parts of mud solvent, by weight.
Another means of increasing the completeness of the reaction is to add more calcium carbonate to the drilling mud. Experience in the field indicates, however, that the method is entirely satisfactory with Ventura New Mud, "Ventura Reweak alkali and a strong acid could be used, subject to the further limitations, that it be soluble in the initial solution, and that all the reaction products be similarly soluble. Chlorides are preferred. because the hydrochloric acid is the most active. The limitation as to solubility eliminates all the other alkalis but strontium. Strontium chloride could be substituted for the magnesium chloride, in the same proportions. Its cost, however, is greater, rendering it less economical to use at present.
It is also apparent that other weak acids might be substituted for acetic acid, provided that the same requirements as to solubility specified above, are met. Such a substitute would be readily ,reactive with the mud, inert toward the steel pipe and tools, be low in cost, and react with the magnesium chloride to liberate hydrochloric acid I through the reaction period.
The safety in handling the solution I have described may be judged from the fact that it may be placed in the mouth'for a while with no un-,
. changes makes it possible for the remaining mud.
to slough ed and be carried in suspension by the the usual manner.
It will be apparent to those skilled in the art that I have invented a disintegrating solution which meets the requirements as to speed, safety, inertness toward tools and casing, and low cost; It provides a non-corrosive solution which will react in the bore to provide a continuous supply of acid reacting rapidly with the bore, and a method of treatment which eliminates the industrial hazard always present in handling very strong acids.
What I claim is:
1. The method of disintegrating the mud sheath within a well bore which comprises treating the mud with a solution including, by weight, substantially 25% of glacial acetic acid, and 4.0% of magnesium chloride.
2. The method of disintegrating the mud sheath within a well bore which. comprises injecting a solution including, byweight, substanand combining in situ with said salt to release an acid strongly reacting with constituents of said sheath, permitting the'solution to remain therein for about ten hours, and removing the resultant solution from the bore;
4. A method or treating a mud wall in a bore having a casing therein, which comprises pumping therein a solution having 25% by weight. of an acid not appreciably reactive with said casing and 4% by weight of a soluble salt of a weak alkali and a strong acid, said non-reactive acid and said soluble salt being reactive to form an inert soluble salt and an acid strongly reactive with said mud wall, permitting the reactions to continue for about ten hours, and pumping out the end products resulting from said reactions.
5. The mud wall disintegrating treatment which comprises placing in contact therewith a solution comprising one to two parts in ten thousand of a wetting agent, four per cent of a soluble reactive with said mud, assisting said highly reactive acid to penetrate said mud thoroughly, maintaining contact between said mud and said solution for about ten hours, and pumping the resultant products out of said hole.
6. A mud wall disintegrating solution, comprising a soluble salt formed from a weak alkali and a strong acid, and an acid relatively inert with respect to steel but able to combine with said salt adjacent said wall to form a strong acidand other soluble salts.
7. A mud wall disintegrating solution, com