USE O F OIL-BASE FLUIDS I N DRILLING AND COMPLETING WELLS -

6 1

RESULTS AND USE OF OIL-BASE FLUIDS IN DRILLING AND COMPLETING WELLS -f

GLENN V. KERSTEN " ABSTRACT

Eviclence is shown indicating an improvement in initial well productivity with the use of oil-base fluids. There is inadequate evidence that the improvenlent is of last- ing benefit. More records are necessary for additional evidence.

Swabbing, scratching, or washing is seldom necessary for well co~npletion when using oil base.

The effect of oil base on drilling operations tends, in general, to increase rig time and expenses. Preeau-

Reasons for Use of Oil Base

The majority of oil personnel a re intensely interested in obtaining the best possible type of well coinpletion under operational condibons. Every effort is made to obtain maximum oil entry and nlaxiinum continued production of oil. Drilling fluids a re conditioned to the lowest possible water losses through the oil zones. Oil-base fluids are being used where conditions of sand- fluid content and low permeabilities indicate the neces- slty of the smallest possible water-loss fluid.

Shortly over a year ago the industry was shown the results of nunierous pernleability tests on core samples of differing types of sands.'" I t was demonstrated that sands containing hydratable n~sterlal or exceedingly tlght sands are seriously affected by contact with water-lowering the permeabllities, in many cases, to ail alarming extent. Fresh water affected the cores more than salt water. I t was indicated that 111 many instances no water a t all would be beneficial, and that wells coinpleted or repaired with a fluid having no water loss would perform more satisfactorily.

The only drllllng fluid a t hand which will fulfill the above conditions is an oil or oil-base fluid. Almost everyone is convinced from basic premises that this 1s the best type of fluid with which to penetrate or work on ail oil zone. Yet, to date, there 1s a lack of factual evidence that substantiates an initial and contlnulng improvement through a variety of conditions.

Results of Use of Oil-Base Fluids

Earlier ~nvestigatlons have shown z 3 that there is ail initial indicated improvement in well completion from the use of 011 or 011-base fluids. Fig. 1 to 7

* Western Gulf 011 Co , Bakersfield, C a l ~ f t Presented nt the spring lueetlng of the Pacific Coast District.

D~rlvlon of Prodoct~on, B~ltlilore Hotel Los Angeles, Ca l i f , Bpr IS, 1916, pres~dlng, Lot Bowen, ~ e g t e r n Gulf 011 Co , Los Angeles, Calif

t F ~ c # # ~ r . * rrsfer to REFERENI'ES nl i 13 li6

tlons are necessary to prevent fires, water contamination, and unnecessary fluid losses. Con~pletion problems are n~ultiplied if the program calls for more than a drilled hole with a liner. Cen~ent work on blank sections and squeeze work on gun perforations are not so easy as with water base.

The expenses of drilling and completion with oil base are increased over those of water base. Improvements

I in technique and handling should rednee future costs.

were developed to show that this indicated improve- ment is not confined to one field or one set of condi- tions. The figures shown are for five different fields- four froin the southern portion of the San Joaqum Valley, and one from the Los Angeles Basln Harbor area. Type of sands vary from low-permeable sands contaming hydratable material through high-permeable clean sands, and high-permeable silty sands. In every field for which data were available, i t 1s evldent that ail indicated initial improvement is made in the abllity of the oil zones to yield fluid per-pound-pressure drawdown.

The plot of the ability of the sand to yield fluld per-pound-pressure drawdown against the number of inillldarcy-feet of sand open was chosen a s the best yardstick w t h which to measure well capacity for any one set of conditions or field. In general, for the same

Initial Comparison-Oil-Base Vs. Water-Base-Field "A," Central Sat1 Joaquin Valley.

FIG. 1

m~llidarcy-feet of sand, there is an indicated improved difference between 011-base ~nit ials and water-base ~nitials, although there are a few cases where the 011 completions are not so g06d and where the water- base completion is exceptionally good I t is also indi- cated that wall scraping with oil base after peaetra- tlon with water base is better than water base.

I t is important that the possible improvement gained by 011-base conipletion be malntalned, for there are

Colnparison after Producing 200,000 Bbl of Oil-Oil- Base Vs. Water-Bas-Field "AT Central San Joaquin Valley.

FIG. 2

Initial Comparison-Oil-Base Vs. Water-Base--Field "B," West Central San Joaquin Valley.

FIG. 3

numerous cases of well-productiv~ty increase several months after water-base completion. I t IS possible that the inltlal Increase froin oil base could be caught up wlth, and even surpassed by, many water-base comple- tions. In the event this happened, the initial bother and expense from the use of oil base w ~ l l have been in vain.

Fig. 2 and 4 were assembled to glve an idea as to what happens after producing several thousands of barrels of oil. Only two fields were found wlth enough experience data to plot, although there are nunierous cases of oil and oil-base completions of long years' standing. Field "A" first had oil-base coinpletions during 1939 and 1940. Field "B" had oil-base com- pletions during 1942. Good records were kept as to productivity and volume. Both fields seen1 to indicate

Comparison after Producing 150,000 Bbl of Oil-Oil- Base Vs. Water-Base-Field "B," West Central San Joaquin Valley.

FIG. 4

Initial Comparison-Oil-Base Vs. Water- as-Field "C," South Central San Joaclnin Valley.

FIG. 5

USE OF OIL-BASE FLUIDS IN DRILLING AND COMPLETING WELLS 63

that the ~ n i t ~ a l increase has been sustained through the production of 200,000 bbl of oil from wells in field "A," and 150,000 bbl of oil from wells in field "B." Ua- fortunately some of the wells completed and wall scraped with oil base in field "B" had to be shut In due to high ratio.

If this indicated sustained spread could be substan- t ~ a t e d by more and better records, a true evaluation of the results of oil base on well capacity and recovery could be made. I t 1s suggested that, wherever pos- s~ble, permeability, net oil sand, and productivity-index data be gathered so that an evaluation of the l a s t~ng effects of oil-base completions can be made before deple- tion occurs. It 1s not enough to show the management that the well flowed more ~ni t ia l oil and seeins to per- form better.

Other benefits derived from the use of oil base is

Initial Comparison-Oil-Base Vs. Water-Base-Field "D," West Side, San Joaquin Valley.

FIG. 6

~ -

Oil-Base Vs. Water-Base--Initial Comparison-Field "E," Harbor Area, Los Angeles Basin

FIG. 7

In completion work and In the poss~b~hty of obtaining better interstltlal-water content. Coinplet~ons are, in general, made easy for flowing wells, as it 1s usually only necessary to d~splace the oil base w ~ t h 011, and the well will either flow immediately or after several hours Swabbing time is largely el~~tunated I ts use opposite low-pressure sands ehm~nates the espense and work involved In washing and scratching. Fluid is changed to 011, and the well is pun~ped in.

Use of Oil-Base Fluids

Oil or 011-base f lu~ds have been In use for a great many years and in numerous fields. At present oil base IS, or has been, used In nearly all of the fields In the state Depth of hole or temperature conditions now offer no problem, because suspending agents for weight and viscosity control have been developed to a high degree of effic~ency. Water has a detrnnental effect on all oil-base fluids, but can be controlled by the addition of unslaked lime. Most operators experl- ence no trouble with up to 10 per cent water content.

I t is often necessary to change casing prograins when uslng 011 base, a s ~t 1s dangerous to have such flulds opposite a long open hole. The usual procedure IS to obtain a primary water shutoff above the oil zone, then change to 011 base to drill Into the horizon. Open hole up to 2,000 f t has been n la~nta~ned satisfactor~ly A sacr~fice in hole size is necessary unless the expense and time for underreaming or wall scraping are deemed adv~sable.

Effect of Oil Base on Drilling Problenls

The use of 011 base multiplies the. problems of dr~l l - lng operations. Drilling crews have to have extra compensation-usually 75 cents per day per man. Fire hazard has to be guarded against !vith rigid rules A large amount of t ~ m e and material is spent on pump repalrs, although the newer type rubbers for pistons and valves w l l last longer than the older types up to 1,500 p s ~ circulat~ng pressure, above whlch the usual amount of trouble 1s to be expected. I t is often necessary to repair one pump wh~le the other is In use- one pump being down cont~nually.

As the cost of oil-base fluids is escessively high, it IS necessary to guard aga~ns t losses. A wiper must be used on the drill plpe or tubing. The standback should have two drams-one to the ditch for oil, and one outside for wash~ng. Water-cooled drawworks and steam englnes should have special guards to prevent water ge t t~ng into the cellar. Mud p ~ t s have to be protected from water drainage Storage tanks are needed, especially where the operation 1s isolated from a central mud-mixing pit.

Use of oil base results in smaller hole slzes than where the zone is normally penetrated before any casing IS

run. Footage per bit 1s lowered, due to the smaller sized dri l l~ng or corlng heads. Exper~ence also has shown that, for the same size bit, shales will d r ~ l l about the same as water-base muds, but sands will d r ~ l l

considerably harder, and will take more bits to make the same footage.

All of the above has a declded effect on r lg tlme, as all tend to Increase the amount of time spent on the same length of hole. Drlll-tlme chart, Flg. 8, shows the effects visibly, although considerable tlme was spent on 011-base holes "A" and "B" by under- realrung and speclal testing for required structural data, which should be deducted in maklng a compari- son to wells "C" and "D."

The.operator first uslng oil base is apt to experience considerable "growing pains" when handling 011 base and should, ~f possible, use the experience of h ~ s

Oil-base costs vary from $5 00 to $10.00 per barrel for 11ght-welght muds (68-75 lb), and from $10.00 to $15 00 per barrel for heavy-welght muds (75-85 lb). Losses per hole in use vary from 50 bbl to 150 bbl with an average of around 100 bbl. Precautions outlined above will cut losses still further. The fluid may be used a number of times in different holes. Assuming that there is a loss of 100 bbl per hole, and a depreciation of one-elghth per hole of 400-bbl-fluid content, 6 holes may be completed with 1,000 bbl of stock oil base. This spreads the cost considerably. Where operations permit, a central mud-storage and -mimng plant 1s very

neighbor to avold unnecessary costs 1 deslrable

DAYS

+ FOR COMPARISON DEDUCT TIME FOR UNDER REAMING & TESTING

Drill-Time Coniparison Chart-Water-Base and Oil-Base Completions, Central San Joaqnin Valley Field.

FIG. 8

USE OF OIL-BASE FLUIDS IN DRILLING AND COMPLETING WELLS 65 I

The Effect of Oil Base on Con~pletion Problems I aniount needed therefrom, and by preced~ng the cement

Where the completion program can be made simple, as ~n drilling or coring the hole and se t t~ng a perfor- ated liner, running tubing and bringing the well in, the coinpletion problem is not ser~ously affected except as to rlg tinie and extra mud expense. If the program for completion lnvolves intermediate waters and the necessity of cementing blank liners or blank sections, an amount of extra time will be found necessary to complete.

Electric-logging equipment for oil base has been improved, and i t IS now possible to make fair correla- t ~ o n s on major breaks in tlie oil zone. I t is not satis- factory for third-curve work or for minor markers or for sand counts. Correlation section, Fig. 9, shows 3 oil-base and 3 water-base electric logs for a field in which normal water-base electric-log correlation IS

sometimes difficult. The base and tops of the major sand bodies appear to correlate satisfactorily, although the ininor breaks are difficult to ~nterpolate. There are instances of cores being badly flushed by oil base, 0

making known analysis procedure and results uncertain. New experience factors and diffe~ent methods may pro- duce better results for this type of core. It is often advisable to test drllled or cored intervals to determine the proper fluid content.

Open-hole packer-test work seems to be more posi- tive with 011 base than with water base. One company which makes open-hole interval tests as a part of com- pletion practlce has not had a stuck packer which could not be jarred loose easily.

Obtaining satisfactory cement jobs in 011 base is d~fficult without certain precautions. A quicker job IS effected by running a hole caliper and figuring the

w ~ t h d~stillate. As the 011 base was Installed to pre- vent water con tan~~na t~on to the 011 zone, ~t is not good practice to precede the cement with water or water-base mud, although a more positive cement job would probably result.

Gun perforating does not present a problem. I t can be done wlth a s much ease a s in water-base fluids. There 1s no known effect on scraping and wash work.

Experience has shown that considerable packer trouble IS encountered in making casing tests of hole ~ntervals. The llght refined oils used in compound~ng the oil-base fluid attack the packer rubbers and cause failure, or the weighting lnater~al niasks the slips to such an extent thada good hold is not obtained. Newer packer-ring designs and use of drillable bridge plugs have improved packer seals.

Effect of Use of Oil Base on Drilling and Completion Costs

As shown hereinbefore, the use of oil base can materially increase the cost of completiiig a well. Added shallow-hole cost is very small, as it IS difficult to find any difference in dri l l~ng and r ~ g time. Cost per hole for shallow drilling will run from $2,000 to $3,000 per well for nlater~als and handhng A deep hole of 10,000 f t or better, with 500 f t of oil horizon, will show con- siderable added rig t ~ i n e using oil base, as outlined previously, to make the same hole. Costs w ~ l l likewise go up. As an example, assume heavy oil base required to hold down a well and 15 extra days rig time to drill the 500 f t of hole, run casing, gun-perforate, and do the necessary cement work for secondary shutoffs The heavy oil base will average $13 50 per barrel to

CORRELATION SECTION

COMMING OIL 84SE WITH WATER BASE ELECTRIC LOGS SOUTH CENTRAL SAN m U I N VbLLCI FIELD

Scale: 1 in. = 100 ft vertical; 1 in. = 200 ft horizot~tal. Date: March 12, 1946.

FIG. 9

easy a s wlth water base The expenses of completion a r e usually increased

froin $3 00 per foot on shallow holes to possibly $30.00 per foot on deep 'holes where necessary secondary shutoffs a r e made. Improvements In technique and handllng should reduce costs In the future.

The operator first uslng oil base is llkely to esperl- ence excessive costs untll his men a r e properly edu- cated a s to technique of handhng and saving the oil base.

66 DRILLING PRACTICE

ACKNOWLEDGMENT

mix and maintam ~n usable shape. As it was esti- lnated previously t h a t 1,000 bbl would drill G holes, the cost per well would be $2,250. Fifteen days ex t ra r lg t ~ m e a t $650 per day 1s $9,750. Ext ra tools and fuel a r e estimated to be $3,000. Total is $15,000, o r $30 per foot added cost due to use of oil base. Experience and improved technique in handling and use should reduce future costs.

0

CONCLUSIONS

The use of oil base appears to improve the initial abllity of a well to produce 011 Inadequate history ~ndlcates tha t the improved p ~ s i t l o n of the well-~roduc- tivity ~i idex is maintained.

Well-completion practices a r e improved. Swabbing, washing, and scratching a r e seldom necessary.

The effect of use of 011 base On drilllng tends, ~n general, to Increase r ig time Precautions are llecessary to prevent fires, water contalnination, and unnecessary fluld losses.

The effect of 011 base On problelns Is

noticeable where the Program calls for Inore than a drilled hole with a liner. Cement work on blank set- tions and squeeze work on gun perforations a r e not so

Thanks a r e gratefully expressed for the help and criticism extended by t h e varlous con~panies and com- pany personnel which contributed data. The author also thanks company personnel f o r a ~ d in preparation, and the Western Gulf Oil Coinpany for perniiss~on to prepare and publish

the llner. I n such cases it might be advantageous t o attempt to remove the mud sheath by scratching and washlng \nth 011 before hanglng the Ilner.

The data shown in Fig. 7 indicate in general better productivity lndlces fo r wells completed with 011-base mud than wells completed with normal water-clay muds or colnpounded water-base muds which in this case a r e water-clay starch muds. In defense of^ the low in- dlces shown for three wells completed with the starch muds, i t should be stated t h a t these three wells were drilled a t low structural positions, and produced oil of unusually low gravlty. The effect of d ~ f f e r e ~ c e s In gravity and viscosity on productlvlty Index was not considered in Mr. Kersten's paper.

The da ta fo r oil-base completion shown in F i g 7 a r e for wells drilled a n a rea in there had been no other wells drilled f o r over 38 years. During the Intervening period there had been a substantial decrease 1, reserv01, pressure. Ful.thermore, the colnpletlon practice in regard to washlng and reniovlng the mud sheath had been changed, and, therefore, the produc- t~vity-lndex da ta fo r the earlier wells completed with normal clay mud a r e not directly comparable with the productlvlty-index data for recellt wells completed with

muds.

REFERENCES

1 X o r n s Tohns ton ,~u t l C.irrol 31 Bersoo "Water Permeahillty ~f Reservoir Sands. AIIZ I118t d l l n f ? ~ ! ~ M e t E18grs , Petro lez tm D f ~ ~ e l o p a l c l ~ t atld Teclrjtologu 160, 43-5 (1945) , origlnal pre- sented a t Am I n s t Mlulng Met Engr s meeting, Los Angeles, Calif 111 Oet 1044

3 A ' W Alexander, "011-Base Drl lhng F l u ~ d s Often Boost Pro- duct~on." 115 111 36-40 (1941)

3 W H Far rand and W fi Clark. "Drilling F l u ~ d fo r Com- pletlon of Shallow 011 Wells, Oil Weckl11 113 191 24 (1944)

DISCUSSION

A. W. Gentry (Long Beach 011 Developnlent Company, Long Beach, Calif.) ( w r ~ t t e n ) : Mr. Kersten points out tha t swabbing, scratching, o r washlng is seldom neces- sa ry when oil-base mud is used However, when very fine perforations a r e used or when a prepacked liner 1s used, i t is possible t h a t the mud sheath a s it dis- Integrates is transferred to, but cannot pass through,

Theoret~cal considerations, laboratory experiments, and some field experiences lead t o the conclusion t h a t it is desirable to prevent water f rom drilling fluids from entering the formation to be produced. The charts presented by Mr Kersten certainly tend to confirm this conclusion, although the polnts used a r e so widely scattered t h a t some doubt still remains a s t o the value of the specla1 muds.

Drill-pipe wlpers which clean the outside of the drill pipe have been in use f o r some time. It is of interest t h a t a wiper has been developed for cleaning the inside of the drlll pipe a s the drill pipe is pulled from the hole, bu t thls wlper is not yet in general use. Thls wiper helps prevent waste of the costly oil-base fluid and keeps the derrlck floor cleaner and safer.

In the relatively shallow depths drilled in the harbor area of the Wllinington Field the use of oil-base mud makes no apparent difference in drilllng speed

Mr Kersten ( w r ~ t t e n ) . The comment on posslble plugging of fine mesh perforabons or prepacked liners is very well taken, a s I have personally seen hard, gummy 011-mud sheaths around cores taken when using oil in a n area where clay shales a r e common.

Product iv~ty ~ndices, a s commonly used, a r e related to quantities and pressure differentla]. They a r e Ilke- wise related to sand thickness, permeability, viscosity, and ratlo of well bore t o drainage radius Therefore, it follows that, fo r any particular field and sand body, the relatlon should automatically correct fo r vlscoslty. A sand zone in one field differing in characteristics from a sand zone in another field cannot be compared without some correct~on factor. Sands of the same character can, however, be compared. If a plot were made of all wells of fields "A," "B," and "C," which

IRILLING AND COMPLETING WELLS 67

a r e all Stevens Zone wells, ~t would be found that, although not so definite, the same conclusion a s shown separately wlll emerge.

I an1 not a t all conv~nced t h a t use of oil base 1s a cure-all fo r well work. I belleve, however, t h a t certaln conditlons, such a s low, permeable, t ight sands con- taining hydratable material, wlll probably result 111

better wells through the use of oil flulds. Where there a r e data and knowledge of conditlons of a n area, each well should be analyzed to judge the value of using 011 base.

J. P. Shea (Richfield Oil Corporation, Tupman, Callf ) (written) * : I find t h a t the presentation of the sub- ject encompasses all of my experience with oil base a s a d r ~ l l i n g fluld.

I a m In agreement with the conclusions a s drawn. However, the following notations from a n operat~onal standpoint nlay be of interest:

1. Water content of over 5 per cent tends t o create a high gel strength-3 per cent o r less water IS more des~rable.

2. Shaker screens of 20-mesh a r e desirable. I t has been found advantageous to scrub the screen wlth sol- vent each time t h a t flow over the screen is stopped. T h ~ s is to prevent excessive loss of 011-base fluld over the mud screen.

3 Low pump suctlons In plts and storage decrease losses In nlovlng.

4. Use of neoprene rubber m pumps plus a steel rotary hose has glven the best results with the least trouble

5. In 6 wells where approximately 600 bbl of fluld were Osed in the circulating system, a n average of 150 bbl of 011 base was lost per well in drilling, completion, and transfer operatlons.

H E Radford (Shell Oil Company, I n c , Ventura, Cahf.) ( w r ~ t t e n ) : In all of the fields examined by Mr Kersten the evidence Indicates t h a t Initial productivity 1s Increased by using oil-base mud while drilling i n the 011 sand Similar da ta f o r three wells In a coastal divis~oil field using oil-base mud in a zone above the bubble-point pressure falled t o show any advantage for 011-base mud. Wells drilled in coinparatlvely low- pressure zones In the same field generally had better Initial productions than expected; but, a s no completions were made In these zones with waterLbase muds, no dlrect comparison can be made.

I n the case of field "A" in Mr. Kersten's paper there is evidence that , when present, the superiority in pro- ductivity Index of the oil-base completions a s compared to water-base completions, continues for a t least a substantlal portion of the well's h~s tory .

Assuming tha t the improvement continues throughout the life of the well, ~t follows t h a t the economlc 11fe would be extended and additional oil would be recovered It is also to be expected tha t the flowlng period would

* Presented by John 0 H ~ l l s , General Petroleum Corp. Los .111gcler. C'allf

be lengthened-thereby decreasing production expense. It is unfortunate t h a t sufficient data were not avail- able to evaluate these factors to show a n actual payout on the added cost of using 011-base mud

One of the most inlportant items which determines the cost of using oil-base mud IS the amount of extra r ig time required. Some data recently collected on drllllng speed with various muds a r e presented for com- parison with Mr Kersten's experience.

Oil-base and clay-base con~pletions 111 a seml-depleted zone a t a depth of 8,000 f t to 9,000 f t in a coastal dlvl- won field were examined. Two wells, drilled whlle using 011-base mud, averaged 96 f t per day in the oil zone. Drllling speed for "5 nearby water-base complet~ons averaged 105 f t per day in the same section. Thus, f o r a n average 1,000-ft productive ~nte rva l , it IS expected tha t use of 011-base nlud would reclulre only one extra day's drilling time

In a shallower undepleted zone a t approsimately 7,000 f t in the same field, 2 other groups of wells were studied It was found tha t the average r a t e of pene- t r a t ~ o n of the oil zone f o r 3 wells uslng oil-base mud was 104 f t per day. This compares with 118 f t per day for a slnillar group of 6 wells, whereln water-base mud was used Here again, for a n average 1,000-ft section, approsimately one extra day's r ig time would be requlred for penetration of the zone

Study of a group of wells in a still shallower zone a t 4,000 f t to 5,000 f t in thls field showed that the produc- t ~ v e interval was actually drilled faster while using 011-base than with water-base mud.

Two additional wells in the first zone mentioned furnished a n interesting comparison Both penetrated the same product~ve Interval whlle circulating 011-base mud, but the average drilling speed In the oil zone of one was 118 f t per day a s compared with 46 f t per day for the other. I t was found af ter completion tha t the bottom-hole pressure of the well which drilled so slowly was very low; and, a s a result, the differentla1 between the pressure of the mud column and the forma- t ~ o n pressure was unusually large. This large differ- ential may have contributed materially to the slow . drilling progrkss Some addltlonal experience has been gained, which also indicates that drilllng speed 1s affected by weight of nlud used.

The following data a r e available from two Los Angeles Basin fields. In one field, where the zone to be pene- trated IS p redon~~nant ly shale, i t was found that, fo r comparable bit sizes, drilllng rates were reduced ap- proximately 18 per cent by using 011-base mud; but, when reduction in bit size accoinpanled its use, the loss of drllling speed amounted to 52 per cent. In compari- son, a deep zone composed of hard sand in another field was drilled with a n estimated reduction i n drilling speed of 15 per cent a s a result of using 011-base nlud

I n the foregoing discussion of drilling rates, most of the wells ment~oned were normal completions with a water s t r ing and preperforated llner, so t h a t the question of loss of time due to reduction in hole slze o r to cementing and testing operatlons was not involved.

For such completions, it appears that the extra cost of using oil-base mud would be of the order of $2,000 to $5,000 including the cost of the mud. Thls IS in co111- parlson wlth the $15,000 mentioned by Mr. Kersten for wells wlth more expensive completion methods.

Mr. Kersten (wntten) - I should like to thank Mr. Radford for his criticism and addit~onal data. In as- sembling the data for the paper, I trled to keep an open mind as to the value of the muds compared, as use of oil base lnvolves extra expenditures, especially when operating in deep holes. Wlth clean sands giving high-pressure gassy flow, i t is often d~fficult to find any difference in completion between 011 base and water base.

Wlth dirty sands containlng hydratable material, I'd llke to again state that I think a study should be made to determine the value of using oil fluids.

In general, dnll~ng-rate variation through use of 011 fluids does not materially Increase costs. Speaking in terms of overall hole cost and a normal slotted liner type of completion, the addit~onal cost of using oil base is not excessive. If blank hners or blank sect~ons are necessary to carry out a specific program, the costs wlll mount due to the extra rig time, to cement, to test, etc.

I t IS indeed unfortunate that there are insufficient data to show some actual payouts, as i t is often difficult to show a "dollar-wise" management the necessity of using such fluids where i t would possibly result in better well completion.

Read Wlnterburn (Union Pacific Railroad Company, Los Angeles; Calif.) (written) : I have one or two polnts which inight supplement Mr Kersten's excellent paper. First, In regard to the electric logglng, I have had an opportunity to examine some 40 electric logs run In oil-base fluld a t Wilmington, and the improvement in a short space of tlme In these logs has been very marked and much more pronounced In some types of formation than in others. One thing I noticed was that, even though the potential curve does not look the same as it does in water-base logs, there seeins to be some con- sistent relation to the resistivity which m ~ g h t yield some information as to the nature of the sand and its fluid content.

Recently I have Inspected the results obtained with drilling fluid in the Rangeley Fleld in Colorado, where there are certainly the most obvious benefits I have seen. The wells are drllled into a zone about 600 f t thick of very hard impermeable sand of which 25 per

cent is productive. Nlnety per cent of the holes drllled with water-base mud were shot with nitroglycerin in order to make them flow, and to obtaln satisfactory rates of production. There have been 12 to 15 wells drllled wlth 011-base mud. All but one of these wells flowed, and they have yielded rates of production com- parable to those obta~ned in other wells after shooting. The expense of uslng oil-base mud is only a f rac t~on of the cost of shootmg.

H. T. Olsen (The Superior 011 Company, Los Angeles, Calif.) (written) : I am in agreement wlth Mr. Wlnter- burn regarding the improvement in the character of electrlc logs run in 011-base drilling fluid, and also that it is definitely possible now to determne the amount of productive sand In a well. In regard to counting sand In a well drilled w ~ t h an 011-base drllling fluid, one must be famillar with the depositional characteristics of the productive sands. Thls is necessary because the poten- tial curve of the electrlc log has a tendency to reverse potentla1 within the sand body, and then sometimes wi th~n even a short dlstance to reverse back to normal potentla1 recording.

I t is practically impossible to determine the crude- oil saturat~on of a core which has been cut with oil- base fluid. There are some qualitative methods of determining whether such a core contains crude 011 or just 011-base drilling fluid. This method entails the examination of the core under ultraviolet light. The U. S. Bureau of Mines, In conjunction with the Insti- tute, has a project in progress a t present to determ~ne the amount of oil-base fluid which has filtered into the core. This procedure en ta~ l s the use of an 011-soluble tracer which IS msed Into the oil-base fluid. 'By a separate d~stillatlon analysis, that portion which is oil-base fluid of the total oil In the core is determined.

011-base flulds have another advantage in the coring of wells, which is the determination of the interstitial water of a core; that is, all of , t he water whlch is determined by an analysis of the core may be consid- ered a s interstitlal water except that which might be water of crystallization of the sand grains. Recently I had the opportunity of checking the determination of the interst~tial water of a sand In a pool agalnst the method which was described by P. H. Jones and asso- ciates of the Union Oil Company presented before the American Institute of Mining and Metallurgical Engl- neers. These rhsults checked remarkably close