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(3) 1t the time for test, run the bailor to determine amount of
increase of fluid and depth of hole. It is assumed that care-
of formation. Depth of Hole.
Under the caption "Depth of Hole" in Tables I to V inclusive, is shown the total depth drilled and, where a bridge was reported, the depth to top of bridge. When no bridge was reported the depth shown is that which the hole measured at the time of test. Rules for measuring depths of wells and casing, as applied to work passed upon by the State Oil and Gas Supervisor, were published in the Second Annual Report'. The depth of well, in all tests, is determined by running a bailer or string of tools to the bottom of the well. It is necessary to check carefully the distance the derrick measures-over on the sand-line or drilling-line. This distance is the basis of all measurements at tests. In rotary holes there is often a considerable discrepancy between the depth of hole measured by drill pipe before cementing and the depth measured at the time of test. The first depth, especially in contract holes, is usually “long." Where there is a considerable discrepancy in these measurements the question arises as to whether or not the hole is actually open below the shoe of the water string. For example, report No. T 4-10122 states that the casing was first reported cemented at 2205 feet, and later reported cemented at 2188 feet, based on measurement of drill pipe after drilling out the cement. With this discrepancy of 17 feet, it would be difficult even to imagine the exact depth of hole and position of shoe.
For the ideal testing conditions, the hole should be open to formation in place, and no deeper. Ordinarily, five feet ahead of the shoe is sufficient.
When the hole has been bridged, aside from the condition discussed above, an entirely different set of factors enters the problem. These will be discussed later under Factors in Results of Test. The Method of Drilling.
Discussion of methods of drilling is here confined to the efficiency of operations with rotary or cable tools in landing casing to shut off' water. Cost and time sheets of the adherents of either method should, as far as economy of operations, up to the time of shutting off water, is concerned, present substantial reasons for the preference for which ever method is used. However, the question of economy in drilling an oil
Second Annual Report. Bull. 82, Cal. State Min. Bur. pp. 59-60.
well is not finally determined until water is excluded and the well is completed. If, in cementing the water string, or attempting a formation shut-off, the job is not successful, the operator is confronted with the alternatives of redrilling, or of recementing, or of landing another string of casing or a liner. The expense of such operations is a considerable addition to the originally estimated cost. In order to operate at a minimum cost, it is well to consider, therefore, in what manner, if any, the method of drilling influences the operation of shutting off water.
In Tables I to V, under the caption "Water String," the method of drilling, whether by rotary or cable tools, is given for most of the wells. Also, in these tables, under the heading “Result," the successes and failures in each instance are given. Table VI gives a recapitulation of these items, with respect to successes and failures, for both methods of drilling. It also gives the percentage of failures for each method and the average percentage for both. The first six fields listed are those in which there was sufficient use for both methods to warrant comparison. There are ten fields, including Ventura County, in Group 7, in which the majority of the work was done with cable tools. This group shows that the proportion of failures, in landing water string with cable tools, is 15.5 per cent. The proportion of failures with cable tools, for all fields in the state, as shown in Group 8, is 14.4 per cent; and failures with rotary tools, for all fields, 21.8 per cent. In other words, for every three failures with rotary tools, there are only two with cable tools.
A study of Table VI shows, however, that this apparent disadvantage in the rotary method is due principally to results of operations in the Coalinga and Sunset fields. The failures, with rotary and with cable, for these two fields, are in the ratio of four to one. The Coalinga field shows 3.6 per cent failures with cable tools, which is by far the lowest for the state. The failures with rotary tools are 16 per cent for the Coalinga field. This is 6 per cent lower than the average for the state, and 3 per cent higher than the Midway field. The latter has the lowest percentage of rotary failures of all fields. The Sunset field shows 11.8 per cent failures with cable tools, which is two per cent lower than the average for all fields, and ranks next below Coalinga and Midway. The failures with rotary tools are 50 per cent for the Sunset field. This is by far the highest for either kind of tools in any field in the state.
It should be noted, before passing to the probable causes of failures in Coalinga and Sunset fields, that the respective percentages of failures for the rotary and cable tool methods, in the Midway, Coyote Hills, Montebello, and Lost-Hills-Belridge-McKittrick fields, do not vary a great deal from the percentages of combined failures for the respective fields.
8 -- 41994
Comparison of Success and Failure of Water Shut-off as Between
Casings Landed With Rotary and With Cable Tools.
In the Montebello field the proportion of failures for both kinds of tools-rotary 35.7 per cent and cable 35.3 per cent—are double the average for both methods for all fields-17.2 per cent. This is probably due to the uncertainties and hazards of drilling in undeveloped territory. Until a number of wells have been drilled, formations carefully logged, and experience has indicated the stratigraphic position of suitable formations in which to land casing for water shut-off, the hazard is greater than that which should obtain under settled conditions for drilling. In the developed fields, the operator's engineer can forecast with considerable accuracy, by the use of well logs, cross-sections, pegmodels, subsurface contours, and other data, the depth at which a desirable stratum should be entered by the drill. The accuracy of the data is due to the additional fact that nearly all of the early development was done with cable tools.
The advantages that accrue from the use of such data is instantly reflected in a study of the percentages for Coalinga field operations. Coalinga operators are virtually the pioneers in the use of engineering methods for the study and direction of operations for shutting off water. The disadvantages of the use of rotary tools are, therefore, emphasized by the results in this field.
Inspection of Tables I to V, inclusive, shows that of 67 rotary failures for all fields, only two of the jobs were formation shut-offs. The Perkins cementing method explained on page 133 was used in 81 per cent of 65 cementing jobs.
Rotary Drilling in Coalinga and Sunset Fields.
Use of the rotary drill is confined principally to the East Side field in ('oalinga. Stratigraphic conditions are quite consistent there. The Red Rock and top of Brown Shale' are good formational markers, and engineers' estimates, based on these markers for depth of oil bearing formations give close results. For example, the following statement is taken from comments on report No. P 5-3182, in the list of Decisions, Coalinga field:
“The well was surveyed by this department, and from the correct data an estimate was made which agreed within 4' with the result as found by drilling.'
Rotary failures are undoubtedly due to (1) discrepancies in measurements of rotary holes, already mentioned under “Depth of Hole,” page 112; (2) the inability to log accurately the nature of formations entered; and (3) impossibility of bailing the hole to identify oil or water-bearing strata while drilling. It is not only possible, but it is quite necessary, to make close estimates for depths to land water string in the Coalinga East Side field. This point is illustrated by reference to recommendations on “Reports of proposed operations for the Coalinga field?. Report P 5-81 recommends shutting off water at 2370 feet instead of 2355 feet as proposed. Report No. P 5–100 recommends that water be shut off at 1980 feet instead of 1955 feet. Report No. P 5-3306 recommends cementing at 24955 feet, instead of 2515 feet, as proposed. It will be noted that these differences are 15, 25 and 20 feet, respectively.
1See First Annual Report, Bull. 73, Cal, State Min. Bur. East Side field-principal formations, pp. 62-65.
Decisions, Coalinga Field, Sec. 2, T. 19 S., R. 15 E., M. D. B. & M. Standard Oil CO.. Well No. 10, Report No. P 5-318. Chapter VII. post.
*See list of Decisions, Sec. 14, T. 19 S., R. 15 E., M. D. B. & M., Coalinga field.
The above differences in estimates indicate that it is necessary to make water shut-off in a stratum of no great thickness. It is also necessary for the rotary driller to recognize and accurately log the marker formation and water and top-oil, in order that the engineer's estimate may be checked and revised, if necessary. In a number of cases in the Coalinga field and elsewhere, the rotary has not fulfilled this necessity. The items listed under Sec. 29, T. 19 S., R. 15 E., VI. D. B. & M., Table V, give a case in point, with the exception that the red rock is not known to be present in this area. The various operations at this well are covered in list of Decisions for Coalinga field'. These operations exemplify the difficulties and additional expense mentioned in the openinz paragraph of this section. Brietly, the operations were these:
The company proposed to cement casing at 2675 feet, estimating top of oil at 2695 feet. The department recommended cementing at 2650 feet. After considerable disenssion relative to location of top oil sand (based on rotary Jog), the company cemented 10-inch casing at 2696 feet. Report No. T 5-68 shows that, after bailing for test, 760 feet of water entered the hole in 21 hours. It was later conclusively proved, by ripping the 10-inch casing at 2655 feet, that an oil sand existed at that depth. The record shows that not only had the rotary passed through an oil sand, but that the shoe of the casing was landed in a zone composed of the unrecognized oil sand and sandy shale-material entirely unsuitable for shutting off water. It may be noted, in passing, that the rotary log of this well catalogs a noninformative succession of strata, logged as clay and boulders, hard sand, shale and shell, for 2669 feet. Rotary "boulders” are usually the evidence of a condition, but not of a formation.
Careful operators in the Coalinga field are already questioning the practicability of present rotary programs. The writer is informed that at least one of the larger Coalinga operators is preparing to make a radical change in the present method of drilling. Such a change will involve landing a string of casing with rotary near the red-rock marker and carrying a smaller diameter casing with cable tools for water shutoff at the top of first productive oil.
The following evidence, taken from the transcript of a hearing” before the State Oil and Gas Supervisor, gives the opinions of rotary drillers and operators relative to the difficulties mentioned above. The evi
Decisions, Coalinga field, Sec. 29, T. 19 S., R. 15 E., M. D. B. & M., Shell Co. Well No. 7.
"Hearing relative to the drilling of Union Oil Company's "International" Well No. 7, Ser: 4, T. 11 N., R. 23 W., S. B. B. & M., Maricopa Flat, Sunset Field, held at Taft, California, April 9, 1918.