Abstract:
A method is provided for selecting the optimum concentration of inhibitors such as soluble salts to effect maximum clay swelling and dispersion inhibition in an aqueous fluid of water-swellable, argillaceous materials including clays as exemplified by bentonite and argillaceous sediments, such as shales, encountered in drilling operations. In this method filtration measurements are first taken to develop a filtration parameter from which the optimum inhibitor concentration is readily determined.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for determining the optimum inhibitor concentration in an aqueous fluid having dispersed therein a water-swellable, argillaceous material. More particularly, this invention relates to a method for determining the optimum inhibitor concentration required in an aqueous fluid to effect maximum inhibition of clay swelling and dispersion of argillaceous materials including clays as well as argillaceous sediments encountered in drilling operations. In this method measurements which are a function of the swelling, e.g. hydration, are made on the drilling fluid and utilized to ascertain the optimum inhibitor concentration. 
     2. Prior Art 
     Frequently in oil and gas well operations argillaceous sediments such as shales are encountered that swell and disperse when contacted with drilling fluids that are of inadequate nature to prevent such clay swelling and dispersion. Previously, drilling programs have been designed to maintain a predetermined inhibitor (such as potassium chloride, sodium chloride, calcium chloride, selective flocculants and coagulants) concentration based on shale dispersion tests, mineralogical analysis and experience in the area. Drilling programs designed in this manner have not proven to be satisfactory since shale dispersion tests are empirical and mineralogical analysis do not yield data that is useful in determining optimum inhibitor concentration. Since clay content and reactivity of argillaceous sediments to water vary from one formation to another in the drilling operation, the inhibitor concentration required for maximum clay swelling and dispersion inhibition will vary from one formation to another. Since inadequate inhibitor concentration can cause clay hydration and dispersion resulting in wellbore sloughing, out-of-gage hole, stuck pipe and fill during trips, optimization of inhibitor concentration to more closely match the nature of the argillaceous sediment can achieve maximum economic and drilling performance when drilling through troublesome shale zones. 
     The use of filtration tests for determining the stability of colloidal systems in drilling fluids containing clay-swelling inhibitors and protective colloids has been disclosed by Diener et al in U.S. Pat. No. 3,296,126. Walter in U.S. Pat. No. 3,989,630 has disclosed the testing of aqueous low solids drilling fluids containing hydration inhibitors such as calcium sulfate, calcium chloride, etc., to determine the filtration rates. 
     Apparatus useful for filtering drilling fluids has been described in a number of patents including U.S. Pat. No. 3,172,286, Grubb et al, U.S. Pat. No. 3,286,510, Parker, and U.S. Pat. No. 3,289,467, Parker et al. 
     In view of the numerous and well-known problems as described above which are associated with drilling operations carried out in areas where formations containing substantial quantities of shale are found, it is apparent that there is a definite need in the art for a reliable, convenient method for determining the optimum hydration inhibitor concentration needed in aqueous fluids to secure maximum clay swelling and dispersion inhibition. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a new and improved method for determining optimum hydration inhibitor concentration of water-swellable clays or clay-containing materials of all types in aqueous fluids. 
     Another object of this invention is to provide a convenient, easily-conducted method for determining the optimum hydration inhibitor concentration for water-swellable clays or clay-containing materials in aqueous fluids. 
     The above and other objects of the present invention will become apparent from a reading of the specification, examples and claims which follow. 
     This invention provides a method for determining the optimum inhibitor concentration required in aqueous inhibited fluids for maximum clay-swelling and dispersion inhibition of water-swelling argillaceous materials including clays and argillaceous sediments such as shale and other clay-containing materials encountered in drilling operations comprising: 
     (a) determining a parameter for the aqueous inhibited fluid having dispersed therein a quantity of the water-swelling argillaceous material and the inhibitor concentration, which parameter approaches a substantially constant value with increasing concentration of said inhibitor, and 
     (b) determining from said parameter the minimum inhibitor concentration at which said substantially constant value is first obtained. 
     It has been found that filtration times as derived from the filtration parameter decrease with increasing concentration of the inhibitor to a point at which a substantially constant minimum filtration time is obtained. Surprisingly, it has also been discovered that the sorption index, i.e. the grams of fluid sorbed per gram of argillaceous material, e.g. clay, decreases with increasing concentration of the inhibitor to a point at which a substantially constant sorption index is obtained, the region of constant minimum filtration time and the region of minimum sorption index, e.g. constant maximum sorption or swelling inhibition of the clay components of the fluid occurring at the same general concentration range of the inhibitor. Thus, increases in the hydration inhibitor concentration beyond the concentration at which the constant minimum filtration time or constant minimum sorption index is reached do not result in increased sorption or swelling inhibition of the fluid. Therefore, the minimum concentration at which a substantially constant minimum filtration time or substantially constant minimum sorption index is first obtained, as ascertained by any suitable technique, is the optimum inhibitor concentration for that particular inhibited fluid. 
     In a preferred embodiment of this invention, the optimum inhibitor concentration is determined by first measuring the filtration times for a number of samples each containing the same amount of the argillaceous material in finely-divided form dispersed in water solutions containing incrementally increasing amounts of the inhibitor. A sufficient number of the samples are examined to determine the constant minimum filtration time from which the optimum inhibitor concentration is found. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The method of this invention in which the measurement of the hydration and/or dispersion properties of the inhibited aqueous fluid containing the argillaceous material dispersed therein is required is not limited to the use of any particular method and/or device. Any device that measures or indicates a function of the swelling, e.g. hydration of the clay such as a filtration rate, filtration time, sorption index or any other such fundamental property of a dispersed, partially or totally settled blend of wellbore cuttings (i.e. argillaceous sediments), with solids concentration of material such that a parameter can be obtained, in an aqueous fluid can be utilized in obtaining the data employed in this invention. Usually, when a filtration property is the parameter used, it is the filtration time. 
     This invention is applicable to the determination of optimum hydration inhibitor concentration in any aqueous fluid having dispersed therein a water-wettable argillaceous material and particularly to a formulation of the components designed to inhibit clay swelling and dispersion and to a filtrate of a drilling fluid or a synthetic filtrate of that fluid. 
     The optimum concentration of any of the wide variety of aqueous fluid inhibitors known in the art to be suitable for clay swelling and dispersion inhibition of argillaceous materials can be determined by the method of this invention. Inhibitors suitable for use in such inhibited fluids include, for example, alkaline earth metal salts such as calcium sulfate, calcium chloride, etc.; alkali metal salts such as potassium chloride, sodium chloride, etc.; organic flocculants and coagulants such as polyacrylamides, polydimethyldiallyl ammonium chlorides, polydimethylamino-ethylmethacrylates, etc. 
     This invention is best understood by reference to the following examples which are offered only as illustrative embodiments of this invention and are not intended to be limitative. 
     EXAMPLE 1 
     This example illustrates the relationship of the maximum clay swelling and dispersion inhibition capacity of aqueous inhibited fluids to the constant minimum filtration times of such fluids. 
     Wyoming Bentonite (15 grams) was blended with 250 ml of solutions of varying potassium chloride concentration in water. Filtration times of the resulting dispersed or inhibited systems (depending on potassium chloride concentration) were measured. Using a standard laboratory sorption meter, the Sorption Index, which is a measure of the grams of solution sorbed per gram of clay, also was measured. The following table summarizes the results of blending 15 grams of Wyoming Bentonite with 250 ml of various potassium chloride-water solutions and the effect of those solutions on the amount of water sorbed by the clay. 
     
                       TABLE I______________________________________Filtration TimeSeconds     1.8 cm    1.0 cm   Sorption IndexConc KCl, %     funnel    funnel   gm soln sorbed/gm clay______________________________________ 0        1558      --       10.712        21.6      65.4     1.914        19.9      61.0     1.618        19.0      61.3     1.6______________________________________ 
    
     The data in Table I indicate that the filtration times and sorption indices of the dispersed systems decreased with increasing potassium chloride concentration to a point at which a substantially constant minimum filtration time or sorption index was obtained. The concentration at which either of these values was first obtained is termed the critical inhibition concentration (c.i.c.). Finally, the data in this Table show that the optimum inhibitor concentration is about 14 weight percent potassium chloride. 
     EXAMPLE 2 
     This example illustrates the determination of the optimum inhibitor concentration for a water dispersion of an argillaceous shale. Such sediments or shales comprise clays, quartz, feldspar and other minerals in the colloidal size range. Clays, however, are the primary reactive component present in such argillaceous sediments. 
     Shaftesbury Shale cuttings from the lower cretaceous Shaftesbury formation, Alberta, Canada, were collected from a Shell Polymer Mud system from a well being drilled with 20,000-30,000 mg/l potassium ion at depth of 5700-5900 ft. Severe wellbore sloughing was observed two days later in this zone. 
     The washed shale cuttings were analyzed for potassium and chloride ion by x-ray dispersion technique. Chloride ion was not found, indicating that the shale cuttings were free of drilling fluid contamination. The following results were obtained by the filtration method of Example 1 with 15 grams of 200-325 mesh Shaftesbury Shale dispersed in 250 ml of solution containing various concentrations of potassium chloride (inhibitor). The data obtained are set out in Table II below. 
     
                       TABLE II______________________________________                    Filtration Time,Concentration        Filtration Time,                    Secondsof K.sup.+, mg/l        Seconds     two filter papers______________________________________  0          206.8** 1955        69.6*       175.7* 7820        62.1*       161.0*19550        58.1*       142.0*39100                    137.0*______________________________________ *average of 3 runs **average of 6 runs 
    
     These results show that a minimum filtration time is obtained between 19,550-39,100 mg/l potassium ion and that since filtration times of clay suspensions or suspensions containing clay have been shown to be dependent on their state of swelling and dispersion, the minimum filtration time corresponds to a state of minimum swelling and dispersion. Therefore, these data indicate the optimum inhibitor concentration occurs at a potassium ion concentration of about 19,550 mg/l . 
     While use of a filtration property such as filtration time is preferable, sorption index can be used with equally reliable results, the disadvantage being that the sorption technique requires longer times. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered, in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalence of the claims are therefore intended to be embraced therein.