ABSTRACT

During the past three years a new well logging method has come into general use in the sand sections of Southwest Texas. This new electrical resistivity logging instrument, which uses small electrode spacings of two inches or less and which keeps the electrodes in contact with the walls of the well bore, is being used in this area to assist in the evaluation of possible producing sands. Like any new technical aid for the geologist it has gone through a period of trial in all areas and in all producing formations in order to determine just where and to what extent it will be of benefit. In learning to use this new log, several problems of interpretation have arisen, but in many areas it has become a very useful aid in finding and in evaluating producing sands.

One of the major problems of interpretation is that of the producing oil sand which shows no or negative separation on the micro-contact log. As a general rule, micro-contact logs run through sand sections in the Southwest Texas area show no separation between the micro-resistance readings or the resistance of the longer spacing is smaller than the shorter spacing (negative separation) when the formation is not permeable. Permeable sands are usually indicated on the micro-contact logs by a larger resistance reading from the longer spacing (positive separation). However, some exceptions have been found in the producing oil sands of the Jackson group in Duval County. In the Jackson producing trend it has been found that micro-contact logs run through the producing sands may give no separation or negative separation in a part or all of the producing interval.

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Of a total 98 micro-contact logs run by Humble in this area, 66 have shown this exception to the usual results obtained with this log.

In the Hoffman Field in Duval County, producing from the Hockley sand in the Jackson, more than half the micro-contact logs run show negative separation or no separation in a part or all of the producing sand. In many of the wells only the top 4 or 5 feet of sand shows negative separation, while the remainder of the sand shows the expected positive separation. On all of these wells complete core analysis was made throughout the oil sand. A typical example from this field is the micro-contact log and electric log on the Humble Dougherty No. G-25, which is shown in Figure No. 1, showing negative separation in the producing sand. Also shown to the right on the 25-inch scale log is the permeability data given by core analysis. These permeabilities range from 132 to 517 millidarcys through the section on the micro-contact log having negative separation. This is the general range of permeabilities on the other wells in the field, both those having negative separation in the producing sand and those with the expected positive separation throughout the producing section. Upon completion, this well pumped 12 barrels of oil per day, but is one of the smaller producers in the field.

A second example from the Hoffman Field is shown on Figure No. 2 showing the micro-contact log and part of the electric log on the Humble Dougherty No. G-50. This log is shown on the 25-inch scale and the permeability data from core analysis is shown to the right. At a depth

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Figure No. 1. MICROLOGGING

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Figure No. 2. MICROLOGGING

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of 2208 feet the resistance from the 16-inch normal is 4.0 ohms, while the longer spaced micro-resistance curve reads 2.7 ohms and the shorter one 3.0 ohms. The mud resistance at this depth was 1.4 ohms. These micro-resistance readings are the opposite of what would be expected if there were a filter cake on the surface of the sand. It would be expected that the longer micro-spacing would read nearer the 4.0 ohms measured by the 16-inch normal, and the shorter micro-spacing being more affected by the low resistance filter cake, would read less than the longer micro-spacing. This well pumped 29 barrels of oil on the 24-hour potential test.

In the Colmena Field in Duval County, producing from the Cole sand in the Jackson, 9 micro-contact logs have been run, all of which show negative separation in a part of the producing sand and 6 of which have negative separation throughout the sand. Four of the logs run in this field were Halliburton Contact Logs and five were the Schlumberger Microlog. Figure No. 3 is a micro-contact log and electric survey run on Humble Duval County Ranch Company No. L-13 showing negative separation throughout the producing sand. This well pumped 37.5 barrels oil per day with 20 per cent salt water on the initial test. On this log at a point of negative separation at a depth of 1406 feet, the 16-inch normal reads 10 ohms, the mud 1.1 ohms, while the long micro-spacing reads 4.2 ohms and the short one 5 ohms. Again, this negative separation is the opposite of what would be expected if there were a filter cake on the surface of the sand.

In addition to these micro-contact logs in the Hoffman and Colmena

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Figure No. 3. MICROLOGGING

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Fields, negative separation has been found in the South Seventy-Six Field producing from the Chernosky sand in the Jackson, and in the North Lopez Field producing from the Mirando sand in the Jackson. In general, it can be expected that micro-contact logs run in Jackson oil sands may show negative separation in all or a part of the producing interval.

A possible explanation for this exception to the general results obtained from the micro-contact log is that in some sands the permeability to gas or to oil may be very different from the permeability of the same sand to fresh water, the difference being accounted for by the difference in the state of hydration of any hydratable material in the sand. In hydratable clays the individual particles swell because of absorption of water around them. When a clay particle, partially hydrated in the interstitial water, is subjected to any change in that water, corresponding changes in its state of hydration may result. Permeability is a measure of the capacity of a porous medium to transmit fluids when there is no interaction between the solids and the fluid. The permeability measured by core analysis in the Hoffman field sand was the permeability of the sand to gas. The permeability indicated by positive separation on the micro-contact log is the permeability of a sand to the drilling mud filtrate, which in general is fresh water. It should not be unexpected, then, if the micro-contact log and conventional core analysis do not always check one another for they are measuring different properties.

In a paper titled "Water Permeability of Reservoir Sands" by Norris Johnston and Carrol M. Beeson published in Transactions of the A.I.M.M.E., Petroleum Division in 1945, which was three years before

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the first micro-contact log was run in 1948, the authors bring out the fact that in their core analysis work several sands were found to have a permeability of several hundred millidarcys to air but were found to have no measurable permeability to fresh water, the difference being accounted for by the difference in the condition of hydration of any hydratable material in the sand. Their paper presented the results of laboratory work on over 1200 samples from 94 wells in 23 fields, mostly in California. Table I gives examples that have been selected from their Table II, titled "Differences Between Salt Water and Fresh Water Permeability", to illustrate the extreme difference found in field core samples.

From this it may be seen that it is possible that the reason for negative separation on micro-contact logs in the Jackson sands is that there is no no mud cake formed on the sand face since it may be impermeable to the drilling mud filtrate because of the presence of the hydratable material. That this is the case is strongly indicated by the results obtained in attempting to determine fresh water permeabilities in the Hoffman Field in preparation for the water flood project now in progress in that field. It was reported that fresh water permeabilities could not be determined in that field because the permeability to fresh water was zero or that the core disintegrated in the process. This disintegration may have been due to the swelling of hydratable material. The water flood of the Hoffman Field is now being carried out using salt water from another sand.

Not all of the Hoffman Field micro-contact logs have shown negative separation and many have shown it only in the top 4 to 5 feet. The

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Table I. PERMEABILITY IN MILLIDARCYS TO:

Table II. PERMEABILITY IN MILLIDARCYS TO:

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permeability examples shown in Table II have been selected from a table, titled "Contact Between Clean and Hydratable Sand", in Johnston and Daeson's paper to illustrate that a sand can have hydratable material in only a portion of the sand and that the presence of the hydratable material will not be shown by a conventional core analysis measuring permeability to gas but will be brought out in sharp contrast by fresh water permeability measurements. In this table no two samples are more than two feet nor less than one foot apart.

In the interpretation of the micro-contact log in Jackson oil sands, it is necessary to refer to the S.P. curve on the conventional electric log. The micro-contact log alone is not enough, but once the general section of the sand is determined from the S.P. curve, the micro-contact log can be used to pick accurately the top and base of the effective sand and is also used to separate out the hard impermeable streaks.

It is important to remember that the micro-contact log indicates only those formations which are permeable to drilling mud filtrate and in some instances may give a negative indication through a sand that will produce oil. The micro-contact log cannot be used by itself but must be interpreted along with the conventional electric logging readings. It is possible that as more experience is gained by continued use of the micro-contact log that producing sands in other areas may also show negative separation. Nevertheless, the micro-contact lognhas many effective uses in sand sections. It is used to determine accurately net sand thickness, regardless of what direction of separation the micro-resistance curves may have, and it is very effective in logging the exact location and thickness of hard impervious

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streaks. This aids considerably in taking sidewall samples and correlating core records with electrical logs.

One of the most important uses that Humble has made of the micro-contact log in the Southwest Texas area is on wildcat wells where after reaching the depth of possible productive sands it is a practice to drill 600 to 700 feet of reduced size hole, run electric logs, and then straddle packer drill stem test any likely looking sands. This is a great deal faster and cheaper than coring long intervals, but frequently in the Frio and Vicksburg formation the electric log shows what appear to be gas or oil sands but which are actually hard impermeable sandstones. The micro-contact log is an aid in determining which sands are permeable and which are the hard impermeable sandstones. It is used to eliminate some of these sands that might otherwise be straddle packer tested in the rat hole and is used to select the best sands for drill stem testing first. If the best looking sand on the micro-contact log gives a dry test, several other less likely looking sands can be eliminated. Figures Nos. 4 and 5 show an example of the use of the micro-contact log to differentiate between a hard streak and productive sand and also an example of a proven gas sand which would not have been discovered had the micro-contact log not been run in this well. Figure No. 4 is a portion of an electric log run in the Frio formation in Kenedy County showing what appears to be an oil or gas sand at 9870 to 9890 feet. No micro-contact log was run at the time this electric log was run. A straddle packer drill stem test was made in the middle of the probable productive sand from 9874 to 9882 feet but gave a dry test indicating no permeability. Later, after the micro-contact

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Figure No. 4. ELECTRICAL LOG

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Figure No. 5. MICROLOGGING

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log shown in Figure No. 5 had been run through this sand indicating permeability in the top four feet only, which was just above the interval tested in the open hole, casing was set through this sand and the top four feet produced gas and distillate through perforations.

Figure No. 6 is a portion of an electrical log run on the Humble No. 1 Mary A. Reynolds, a successful wildcat drilled in Live Oak County. The section of the log shown here is in the Yegua. The upper sand from 5367 to 5380 feet produced salt water on a drill stem test, but the sand drom 5406 to 5415 feet was not tested in spite of the favorable indication of the conventional electric log because the micro-contact log shown in Figure No. 7 clearly indicated that this section is a hard streak with a maximum possible net sand thickness of one foot. This is an example of a sand that might have been tested had the micro-contact log not satisfactorily explained the favorable appearance of the sand on the conventional electric log.

Figures No. 8 and No. 9 are portions of the electrical log and micro-contact log run on the Humble No. 85 King Ranch-Borregas showing a sand at 5940 feet with all the characteristics of an oil or gas sand on the conventional electric log. By using the micro-contact log to place sidewall cores in the center four feet shown to be permeable, good recoveries were obtained. Since these cores were sand with no show and since the favorable looking characteristics of the electrical log could be explained by the presence of the two hard streaks indicated on the micro-contact log, this sand was not tested.

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Figure No. 6. ELECTRICAL LOG

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Figure No. 7. MICROLOGGING

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Figure No. 8. ELECTRICAL LOG

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Figure No. 9. MICROLOGGING

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Figure No. 10 is an electrical log run through the producing Yegua sand of an stratigraphic trap type field in Starr County. This producing section has been cored in several wells and in all cases two to four feet of lignite was found immediately below the sand section. As the sand pinches out updip the lignite bed maintains a uniform thickness giving an electrical resistance log with the characteristics of a thick oil sand. The micro-contact log shown in Figure No. 11, run in the same well clearly indicates the difference between sand and lignite except for a one-foot interval between the two where it is possible that the electrode pad lost contact with the well bore.

This differentiating between lignite or hard streaks and gas or oil sands by the use of the micro-contact log is probably the most important single use we have made of this new tool. The micro-contact log has also been used for the selective perforating of permeable sections which are isolated by hard impermeable streaks and if the presence of hydratable material is indicated by the negative separation through oil producing intervals, the micro-contact log could also be used as a warning to any plans for water flooding with fresh water in that sand.

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Figure No. 10. ELECTRICAL LOG

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Figure No. 11. MICROLOGGING

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