US3526132A - Method of logging wells - Google Patents

Description

Sept- 1970 u J. wElNTRlT-r 3,525,132

METHOD OF LOGGING WELLS Filed May 14, 1968 2 Sheets-Sheet 2 Eli 210 2,/ l2 I3 I4 if l Y I 1 y 5A 5/6 @V6 @M5/NW6 APAC/W Q 45 o 2,5 a 55 4Q wd I""`` s* INVENTOR DOA/M0 J WE//VTF/i BYBAWW'V Q4-K United States Patent O 3,526,132 METHOD OF LOGGING WELLS Donald J. Weintritt, Houston, Tex., assignor to National Lead Company, New York, N.Y., a corporation of New Jersey Filed May 14, 1968, Ser. No. 729,109 Int. Cl. E21b 49/02 U.S. Cl. 73-153 4 Claims ABSTRACT OF THE DISCLOSURE The invention provides an improvement inwell logging, in which the capacity of formation shale samples to combine with a basic dye such as methylene blue is determined, and the capacities are then plotted as a function of well depth. The results are highly useful in judging the nature of the formations penetrated and are helpful in predicting the nature of the underlyingn strata.

This invention relates to formation-sampling well logging, and more particularly to shale logging, as used in the drilling of oil wells. Y

In drilling wells in search of oil and gas, the costs have increased steadily over the years, and at the same' time the ratio of productive wells to dry holesmhas decreased, since the most likely areas for productive horizons have long since been drilled. It has accordinglybecome essential, and indeed indispensible from an economic pay-out standpoint, to derive as much information' as possible from the drilling of a well, and more particularly, during the actual drilling of the well, so that intelligent decisions may be made concerning the desirability of coring, formation testing, continued drilling, adjusting the properties, especially the physico-chemical properties, f the drilling fluid, setting casing, and the like, all of which are expensive and time-consuming operations, and the proper use of which may be determinative for the success of any given well-drilling operation. i

In well-logging operations of various kinds, attentionl may well be given to shale formations, since in many ways they are almost uniquely diagnostic for several different kinds of information. For example, their density often has stratigraphic significance, since density may indicate overburden pressure, aswell as reflecting in some measure the ionic content of the water which gave rise to the shale sediment. Shale density alone, however, is an insufficient criterion on which to base many ofthe decisions which must be made concerning the further drilling of the well; and the same is true' for they readiness with which the shale has been drilled, as reflected in the drilling rate. l -f An object of the present invention is to provide a novel method of well logging wherein shale formation samples are tested with an organic dye and the results plotted quantitatively as a function of depth to give a meaningful result. l v

Other objects of the invention will appear as the description thereof proceeds.

In the drawings, FIGS. 1 and 2 show a number of well logging parameters plotted as a function of depth, for two different wells, exemplifying the invention.

' Generally speaking, and in accordance with illustrative embodiments of my invention, I collect samples of shale formations obtained from known depths, from a bore hole as is drilled in search of oil and gas. Such samples are most readily obtained as cuttings brought to the surface during the ordinary course of rotary drilling using circulating drilling fluid. Methods for obtaining such cutting samples and correlating them with the depth of origin are well known and need not be described here. A typical ICC presentation of the methods used is given in the article by W. H. Russell entitled Mud and Cuttings Logging which appears as chapter 17 of the book Subsurface Geology in Petroleum Exploration, edited by J. D. Haun and L. W. Le Roy, Golden, Colo. 1958.

Having obtained shale formation samples from known depths in the bore hole, I then determine the capacity of the shale sample to combine with a basic dye, such as methylene blue.

While I do not wish to be limited to any theory of operation, it may be mentioned that in general, shales comprise a substantial content of clay minerals which have appreciable cation exchange capacity, and which therefore combine by cation exchange with relatively large, water-soluble organic cations such as are furnished by the basic dyes. While the cation 'exchange capacity of the clay minerals in the shale sample is very likely a principal factor in the result obtained, nevertheless, the overall texture of the shale, and the medium molecular weight of the basic dyes, as compared with small cations such as tetramethyl ammonium on the one hand and very large cations such as the cationic polymers used in water clarication, are important factors which contribute to the unique diagnostic value of this particular type of assay.

The simplest method of determining the capacity of the shale sample to combine with the basic dye is t0 disperse or slurry the shaleA in water, add enough acid such as sulfuric or hydrochloric so that the pH is slightly on the acid side, and then add a solution of the selected basic dye in small increments to the shale suspension, with stirring after each addition. The initial increments of basic dye added will completely be taken up by the shale, so that while the solution of the basic dye itself is strongly colored, no color is imparted to the water in which the shale is suspended. When in this fashion sutlicient basic dye has been added to the shale sample suspension so that the capacity of the shale to combine with the dye has been exactly matched by the amount of dyeadded, then additional increments of dye will bring about a coloration of the water of the shale suspension.

The attainment of this equivalence is readily checked by placing a drop of the shale suspension on a piece of white lter paper, and noting if the liquid is colored. Generally when this is done, the solids of the droplet remain in a localized spot on the lter paper, while the water spreads away from the central spot, so that any coloration in the latter may be readily detected even though the mass of suspension, when stirred, .may be dark gray or even black. The drops so removed forthis test are so small in comparison with the usual suspension volume that the accuracy of the determination is not affected. Thus, a

sample of the dried shale, add it to from 15 to 25 ml. of

water and agitate. Motor-driven equipment for carrying this out is available from laboratory supply houses. Onehalf to 1 ml. of 5 normal sulfuric acid suffices to acidify the shale suspension, and an aqueous solution of the basic dye, preferably containing 1/100 equivalent per liter, is added in 1/2 ml. increments. The suspension is hand-stirred with a glass or plastic stirring rod for about one-half minute, whereupon a drop of liquid is removed with the rod and placed on lter paper. When equivalence has beenreached, the dye will then appear as a colored ring surrounding the solids on the paper.

As mentioned, water-soluble basic dyes generally are useful, such as methylene blue (922), malachite green (657), auramine O (656), safranine O (841), crystal violet (681), and rhodamine 6G (752). (The numbers in parenthesis following the dye name are the color index number.) Rhodamine 6G gives a striking fluorescence under ultraviolet light, which under some field con- 3 ditions may be advantageous. However, generally speaking, I prefer and fi'nd methylene blue to be best. Its color is intense and unmistakable, and it is readily available at known purity.

The basic-dye-combining capacity of the shale sample is conveniently expressed in milliequivalents of combined dye (dry basis) per 100 grams of shale sample.

As examples of the usefulness of my invention, reference may ybe made to the drawings. FIG. l shows welllogging results for a well drilled in Nueces County, Tex., for the depth interval of 8,000 feet to 10,500 feet. The column designated as 12 shows depth in feet, The first graph 10 to the left shows the normalized drilling exponent d as defined in the article by Jorden and Shirley which appeared in Journal of Petroleum Technology, November 1966, pp. 1387-1394. (See also U.S. Pat. No. 3,368,400.) The next graph 11 is a plot of the drilling rate in feet per hour as a function of depth. The abscissa scales for these graphs, as well as graphs 14, 16, 17 are given in the headings at the top of the chart. The next graph, 14, shows the percentage hydrocarbon of gas in the air drawn from the drilling mud exiting from the well, as determined with a hot wire gas detector. The readings set forth in the column marked 15 are the readings obtained for samples drawn other than during normal drilling heading. Thus, TG indicates trip gas while CO indicates that returns were being circulated out.

Graph 17 shows the density of the cuttings obtained from the respective depths.

Finally, graph 18, directly connected with the present invention, shows the basic dye combined capacity of the samples. As may be seen from the heading, these are plotted on a scale from -25 equivalents per 100 grams, and the actual graph 18 has an excursion between the values of approximately 8 and approximately 25 milliequivalents per 100 grams.

The great usefulness of the invention may be seen in this typical plot. For example, at a depth of 9,175 feet, no unusual values were exhibited by the drilling rate, or by the drilling rate taken together with the d exponent, as appears from graphs and 11 at this depth. However, the basic dye combining capacity of the shale sample taken at this depth (curve 18) shows an exceedingly high value of 25, much higher than the samples taken at 9,075 feet, above this depth, and 9,225 feet, below this depth, for which samples the basic dye combining capacities were approximately l1. At the same time, the shale density at 9,175 feet showed a minimum, dropping to the low value of 2.36. This in all likelihood indicates a clay cap layer lof tight, impermeable shale, below which one could expeet to find abnonmal pressure unless it had leaked olf through faulting or other stratigraphic anomaly. Another combination of high basic dye combining capacity and relatively -low shale density, as compared with samples immediately above and below, occurs at 9,400 feet. Here again, the drilling rate curve is uninformative, even when taken together with the d exponent. The low excursions of the basic dye combining capacity curve 18 are also most informative. When they occur together with low shale densities, as for example at 8,475 feet and 8,675 feet, zones of relatively high porosity are indicated.

Further information may be gained for the section from 9,750 to 10,250 feet, where the decreasing trend of the shale density combined with the increasing trend in the basic dye combining capacity indicates that transition to a zone of abnormally pressured shale is highly probable. In FIG. 2, there is shown a plot of shale density 20 as well as basic dye combining capacity 21 of the shale encountered in the interval 4,000 feet to 7,000 feet in a well drilled off the Coast of Louisiana, An additional usefulness of the invention may be seen from this figure. The very low formation densities at 5,100 and 5,45 O'feet might, without other information, be expected to indicate porous sands. The concomitant high values of the basic dye combining capacity of about 30 suggest, to the contrary, that these are water-wet shales which could be expected to give trouble with sloughing when drilled into. This gives the driller advance warning that attention should be paid to the mud composition to counteract this condition, when and if reached.

In the preceding discussions of the curves for shale density and for `basic dye combining capacity, the valuable information which may be obtained by comparing their trends synoptically with depth have been pointed out. A corollary thereto is that the trend of the differential between the two curves as a function of depth is likewise highly diagnostic. It is evident that one may feed the two curves into any commercially available curve plotter and/ or computing device, so as to provide a plot of the two curves expressed as a differential, either arithmetic or geometric, as a function of depth, or in any other way which will be evident to those skilled in the art of observing trends in well logs. Indeed, simply plotting the two curves in adjacent columns, both as a function of depth, as shown in the figures, is a particularly excellent method of enabling differential trends t0 be perceived and/ or registered in any instru-mental fashion desired.

. It may be mentioned that procedures for determining shale density are simple and well known. The simplest way is to use standard mineralogical techniques, by determining the tendency of the sample to sink or to float in a series of test liquids of known densities.

While my invention has been described with the aid Ot specific examples, and using specific procedures, it will be apparent that the invention is fundamentally a broad one and that many variations in detail are possible within the broad scope of the invention as defined in the claims which follow. i

Having described -the invention I claim:

1. In a process of logging wells wherein selected properties of the formations penetrated are determined and the results plotted in terms of quantitative values of the said selected properties as a function of well depth, the

' improvement which consists in the following:

collecting samples of shale formation from known depths; v

determining the capacities of said samples to react with a basic dye;

determining the densities of said shale samples;

plotting said capacities so determined as a function of well depth;

and plotting said densities of said shale samples on the same graph 'with said basic dye reaction capacities.

2. The process in accordance with claim 1 wherein said basic dye is methylene blue.

3. The process in accordance with claim 1 wherein said plotted density and said plotted combining capacities are registered so as to display a' differential value derived from said two plottings as a function of depth, whereby trends in formation parameters with increasing depth may be manifested.

4. The process in accordance with claim 3 wherein said Y basic dye is methylene blue.

2,995,027 8/1961 Bernard et al. 73-153 X JERRY W. MYRACLE, Primary Examiner

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