Radial Filtration of Drilling Muds

By MILTON WILLIAMS, * JUNIOR MEMBER A.I.M.E.

(Galveston Meeting, October 1939)

IT is generally recognized that fluid is lost from rotary drilling muds to permeable strata during normal drilling operationsj l.2,3 but that this fluid is the filtrate from the mud, rather than the mud itself, has been shown by various workers. 1 ,4,5

The presence of this filtrate in oil-bearing or gas-bearing strata is undesirable and harmful. The decreased permeability of the stratum to oil effected by the presence of water has been pointed out by several investigators,6,7 and since, in the production of the reservoir fluid, the greatest pressure drop occurs immediately adjacent to the borehole, it follows that infiltered water is particularly objectionable in this region. Again, this filtrate tends to make electrical logs less reliable, since oil in the proximity of the borehole is probably flushed out to an appreciable extent by the water.8 Observation of side wall cores may be misleading for the same reason. Drill-stem tests in low-pressure areas may be of doubtful value where no oil is recovered, unless sufficient time is allowed for production of water lost from the mud. The in filtered water may hinder drilling operations by softening shales and causing sloughing; the "tight hole" occasioned by excessive deposition of filter cake in the hole is a common occurrence. The filter cake remaining.on the face of the producing formation may itself sometimes be detrimental to production.9

In view of these implications of the importance of filtration of mud, an investigation was undertaken to determine the factors involved in the infiltration process, and to correlate these in such a manner that the values obtained in routine filter tests employing a conventional filter of the "wall-building tester" type,1° together with data on size of hole, drill pipe, and rate of mud circulation, could be used to estimate water loss and distance of penetration of the filtrate into strata.

Quantitative relations of filter-cake permeability, filtrate viscosity, and rate o~ filtration have been developed for filtration in the simple laboratory filter. 4a ,l1,12,13 It is obvious, however, that the conditions

Manuscript received at the office of the Institute May 13, 1939. Issued as T.P. 1112 in PETROLEUM TECHNOLOGY, November 1939.

* Humble Oil and Refining Co., Houston, Texas. 1 References are at the end of the paper.

57

58 RADIAL FILTRATION OF DRILLING MUDS

under which the mud is tested in this procedure are not similar to those prevailing in drilling wells, where the mud is circulated past the filter cake deposited on the face of the strata. Consequently, in this work filtration under drilling conditions was simulated as closely as was feasible

Mud Inlet

Cylinder Head

Gasket Ring

t" e Duprene

Gasket -......;~~~~

t" . 16 F,ber

Gasket

t" 4,t" Flat

Ste.IS~

FIG. I.-PRESSURE FILTER.

with the laboratory equipment available, and the results were correlated with data from the simple laboratory filters.

FILTRA TION WITH AxIAL MUD FLOW

Apparatus, Procedure and Results

Apparatus.-The construction of the filter and the arrangement of apparatus used in all tests are shown in Figs. 1 and 2, respectively. The filter consisted of a porous "Filtros" cylinder (of a siliceous synthetic

MILTON WILLIAMS 511

material) contained in a steel holder: the inside diameter of the cylinder was 1.5 in. (3.8 cm.); the length was 5.9 in. (15.0 cm.). The permeability was determined to be 1.3 darcys.

A 2~ by 4-in. pump, powered with an electric motor, was used to circulate the mud; pressure on the filter was adjusted by use of a by-pass, with gate valves in the by-pass and below the filter. Pressure was read from a gauge teed into the line just above the filter; the rate of mud flow was determined by measurement of the volume of mud discharged from the filter in unit time.

Oischcrge Llne ___

By-Poss

Li ne ........ l

FIG. 2.-FILTRATION APPARATUS.

The evaluation of "filtration constants"l1 for each mud sample run was carried out in simple brass filters, using filter paper as a filter­ing medium.

Procedure.-Runs were made on: (1) a typical Gulf Coast mud, (2) a sample of that mud treated with 0.1 per cent by weight of sodium tannate from quebracho extract, (3) a sample treated with 1.0 per cent by weight of salt, (4) a mud containing 6.5 per cent Wyoming bentonite in tap water, and (5) a Gulf Coast mud weighted with a commercial weighting material to 15.0 lb. per gal. The filtration constants for these muds are shown in Table 1.

The pressure range covered was from 30 to 75 lb. per sq. in.; rate of mud flow was 100 to 600 c.c. per sec. (corresponding to about 0.5 to 2.5 ft. per sec. mean linear velocity).

60 RADIAL FILTRATION OF DRILLING MUDS

TABLE I.-Filtration Constants of Muds Used in Radial Filtration Determined in linear filters at 80· F.

Density, Viscosity. R' Sample Ltay~r Sec •• • • Cpo Recip;ocal

Marsh Darcys

----1. Gulf Coast No. 1. ............. 9.5 36 0.127 0.833 0.89 8.8 X 10' 2. No.1, 0.1 per cent sodium tan-

nate added .................. 9.5 30 0.127 0.833 0.89 1.0 X 106

3. No.1, 1.0 per cent salt added ... 9.5 44 0.222 0.850 0.91 2.2 X 10' 4. 6.5 per cent bentonite .......... 8.6 37 0.11 0.833 0.89 3.6 X 106

5. Gulf Coast No.2, weighted ..... 15.0 36 081 0.50 0.89 7.8 X 10'

Results.-The results of a representative series of runs (on the untrea.ted Gulf Coast mud) are shown in Fig. 3, where are plotted cumu-

u c.;j

...

240

200

160

:. 120 IZ to­.J ;;: ... o 80 ...J o >

40

o

I I ! I LEGEND I

Pr ... ur. Mud Flow I I--

" 20 Lbl,llq.in.- 120 c.C,/S'C. ! " 20 .. 300 ..

~ c 33 200 .. ..

~ x 35 .. 150 .. o 70 .. 150 .. V I-- + 50 .. 210 .. • 55 .. 600 / ,/

'/

I ! I /1 ~ ~ I

V /' ,~ / /y // .",~ r::--- I

P ~ ~ ~ ...... i ~ ~ V

~

?8 ~ ! I /P, ! I I i

10 20 30 40 50 60 70 80 TIME: MIN.

FIG. 3.-FILTRATION OF UNTREATED MUD IN CYLINDRICAL FILTER.

lative volume of filtrate against time for each of the pressures and rates of mud flow.

In all runs, it was observed that the rate of filtration became constant after a short time. It was found that the value of the constant rate attained in any particular run depended on the pressure, rate of mud flow, and character of the mud. From these results, it is apparent that constant rate of filtration obtains because the thickness of the filter cake becomes constant; as fresh cake tends to form, it is carried away by the hydraulic action of the mud stream.

MILTON WILLIAMS 61

Correlation

Since the volume of filtrate-time curves for all runs become linear soon after filtration is initiated, only the steady state of constant rate of filtration will be considered.

The equation for filtration against a plane medium with no mud agitation is given byll

in which

v = V mfJ + c2 - c A

2P1-.

m = -p,v'R""'"( 1;-----8')

Ap'f(P) c = vR'P'

where V = cumulative volume of filtrate, c.c., A = area of filter, sq. cm.,

fJ = time, sec., P = pressure drop across cake, atm., JJ. = viscosity of filtrate, centipoise, v = ratio of volume of filter cake, c.c., to volume of filtrate, c.c., 8 = a compaction function of the cake,

R' = a function of the resistivity (reciprocal permeability) of the cake

p'f(P) = a function of the resistance of the filtering medium (detailed discussion in ref. 11).

In this equation, c is ordinarily negligible.

Empirically, it was found that the steady-state filtration rate (~~). of these runs varied linearly with the square root of the rate of axial mud flow Q, and with the square root of the m value of the mud as determined by routine evaluation; or

(dV) = cIvQm dfJ c

The plot of (~~)c against v'Qm for all runs made is given in Fig. 4.

Since the rate of filtration varies directly as the filtering area a, this may be written

(j2 RADIAL FILTRATION OF DRILLING MUDS

It should be pointed out that pressure, cake compaction, viscosity of filtrate, and cake resistance are included in the m term; consequently, when it is desired to apply this equation to a particular case, it is only necessary to obtain the m value from a single run made on the" wall­building tester" at the desired temperature and pressure differential. Where this is not feasible, an approximation may be made, as will be shown later.

0.5

1 V

V v 04

/' V

0

~ V

~ 0.3

....... Wx " V. LEGEND

0 o Unlrealed Mud 0

~02 -

o. V • Treated 0.1.'" Sodium Tannal •

" x Treated I'" Noel I -<" ° 6.5 '" Benlanll. 0v---II Mud Weighled To 15.0 LBS/GAL._

./ V I I I I I I I

o 0.0001 0.0002 0.0003 00004 0.0005 0.0006 0.0007 0.0008 (dV/d9Ic

a FIG. 4.-CORRELATION OF RATE OF FILTRATION AT STEADY STATE, WITH RATE OF MUD

FLOW AND PROPERTIES OF MUD.

Variation of Filter Diameter

The effect of one important variable, size of filter, has not been investi­gated experimentally. Accordingly, the extension of this equation to filters of other sizes has been made by dimensional analysis.

Recalling the equation

V A = ym() + C2 - C

it will be seen that m has the dimensions (length)2(time)-1. The dimen­sions of Q are (length)3(time)-1, so that the dimensions of Qm are (length) 5 (time) -2 •

In the equation

(~~)c = c2yQm a

the left-hand member has the dimensions (length)(time)-l. For a dimensionally sound equation, both members must have the same dimen-

MILTON WILLIAMS

sions. In order to satisfy this condition, a term having the dimensions (length)3 must be introduced into the denominator of the term under the radical. This is done by writing the equation

where M is the hydraulic radius of the filter; or, in the case of annular flow between a pipe and the inner wall of the filter, the hydraulic radius of this annulus.

The introduction of the hydraulic radiu~ in this manner appears to be satisfactory, inasmuch as it is almost equivalent to the quite reasonable assumption that, for a given mud, the same rate of filtration per unit area will be established in any size filter when the velocity of axial mud flow is such that the same fluid pressure drop per unit length is maintained.

Although, strictly speaking, the hydraulic radius should be based on the inner diameter of the filter cake itself, no appreciable error is intro­duced by use of the inner diameter of the filtering medium, since at moderate rates of mud flow the thickness of the cake is relatively small.

Evaluation of k

Inspection of the filter at the completion of each run indicated a large accumulation of mud cake at the ends of the filter, through which, at steady-state conditions, only a small part of the total filtrate passed. In the evaluation of the constants in these equations correction should be made for these end effects. This has been done by assuming that the effective length of the filtering surface was approximately two-thirds the actual length. Although it is recognized that this assumption will introduce some error into the computations, the equation is probably sufficiently precise for practical purposes.

From the plot given in Fig. 4, for the filter used,

(~~)c = O.0019VQm a

Introducing the hydraulic radius M, this becomes:

(~P, ~ O.0017~ Although centimeters-grams-seconds were used in evaluating k, its

value will remain unchanged as long as' all variables are expressed in the same system of units. However, m is customarily expressed in square centimeters per second, while the other variables are usually determined

64 RADIAL FILTRATION OF DRILLING MUDS

in English units. Accordingly, a more convenient form of the equation for practical application is

(~I). ~ O.12D~~": in which the terms have the following units:

or

(~~). = rate of filtration, barrels of filtrate per hour,

L = length of filter, ft.

(~). ~ ,..t. offilt,..lion, ban.1s of filt,..te pe' hom pedool of fill",

D = internal diameter of filter, in. Q = rate of axial mud flow, bbl. per min.,

M = hydraulic radius of annular space between filter and pipe, in., m = filtration constant of mud under existing conditions, sq. cm.

per sec.

Application

In the application of this equation to particular cases, it is found con­venient to employ graphical methods to facilitate computation.

Computation of m.-Since the c value in routine evaluation is usually negligible, a plot of V at given lengths of time against m may be employed to obtain m values for muds run in the usual type of wall-building tester at 100 lb per sq. in. Such a plot is given in Fig. 5.

Although m should preferably be determined at the pressure differ­ential and temperature existing in the well, it is possible to obtain a fair approximation of this value from routine wall-building tests at 100 lb. per sq. in. and atmospheric temperature.

The value of 8, the cake-compaction function, has been determined from a large number of runs on Gulf Coast muds. From the averages (8 decreases with increasing mud weight) of these values, Fig. 6 has been constructed. Computation has further been simplified by introduction of normal Gulf Coast formation pressures14 to give differential pressures between the hydrostatic head of mud' and formation pressures directly. Thus, the pressure-correction factor for m for mud of any weight at any depth is readily found.

Because of the erratic variation of m with temperature, only approxi­mate rules can be formulated. Generally, in the Gulf Coast area, increased temperature will cause m to double for every 5000 ft. of hole drilled with unweighted muds, and with heavily weighted muds m will double every 10,000 ft. of hole.

MILTON WILLIAMS 65

Computation of (~~) ;-Graphs may be constructed for a given

hydraulic radius, showing the relation of m to (~~). at various rates of

mud flow. Such a graph, for the particular case of 6-in. drill collar in 8%;-in. hole is shown in Fig. 7. The use of this is obvious. "

Distance of Penetration of Filtrate.-The relation of distance of penetra­tion to time for various rates of filtration is shown in Fig. 8, with the

800 600

400

200

'" ~ ~ ...

! ,00 t- 80

Filleri"; Area. 49 Cm_2 L ~ ~~ :c 60

... 40 t-o( II:

~ 20 "-

~ '0 , 8

> 6

4

,/ ~

./

/ 1/ V

~ ~ f; );' );' V

~ ~ V- ~ V

....-: ./ V~ I-' .......-

/ 1/ ~V/ /

V~ f; ~ V );' \~~ ~~{~ ~~ ry>0 y. ~

.....-: "'~ -.........

~ ./ V

- - - -IXIO 72 4 6 8 1)(10 6 2 4 6 8 IXIO 52 4 6 B IXIO 42 4 6 8 1XIO 32 4 6 8 I

m FIG. 5.-RELATION OF VOLUME OF FILTRATE AT TIME (J TO m VALUE OF MUD IN

COMMERCIAL FILTER.

permeable formation assumed to have a porosity of 25 per cent, and SO per cent of the reservoir fluid assumed to be displaced by the filtrate.

Example of Use

To illustrate the use of the equation, an example met in practice may be cited. It is desired to compute the distance of penetration of the filtrate into a producing sand approximately 6000 ft. deep, the hole being bottomed at this depth. The data are:

Size of hole: 8% in. Size of drill collar: 6 in. Pump: 20-in. stroke, 7%;-in. liner, 37

cycles per min.

Mud: weight: 9.8 lb. per gal.; filtration: 29 c.c. in 30 min. in wall-building tester at 100 lb. per sq. in. and 80° F.

The rate of mud circulation, assuming 85 per cent volumetric effi­ciency for the pump, is readily computed to be 12 bbl. per minute.

From Fig. 5, the m value of the mud is 2 X 10-4• By reference to Fig. 6, the pressure-correction factor is found to be 1.2; the temperature-

66

'" ...J

~ 08

... 0.6 o ,..:

" 04 II:

" II:

'" Q.

.... 0.2

'" o ...J

UJ

~ O. II:

I

..... 08

...J

;;: .06

...i

'" '" .04 ...J

"

RADIAL FILTRATION OF DRILLING MUDS

10

9

8

7 E 0 6 .... ~

a: 0 5 .... 0 .. ...

4 z 0 35 .... 0

'" 3 a: a: 0 0 25 ,

Ell 2

15

./ ,/'

~

\~ V /"

,/' V

"..-

P \~ ...... /" "..-V

V·' " .. ,/' \~ V ,;"'" i-' ~,'~ .,. "..-Q_fo V ,/' ~ ,;"'" ,.;' V

~fr,/' ,;"'" ./ V -----%VV,;"'" ......-~ V ,.;' f-"'"

~ ,;""'/ ,;"'" 9 ~ ..--

~

~ %~v::: p ~

,.;'

~

~ -----~ v --~ f..-..-- v ....

-----\\.0 V -~

~ ~ r:: ~ --fo?-v -- f..-V - ~ ~

I--

V - J2E-I--~ E:::= ~ v --- ~ ~ f-

l-I---I I 1.5 2 2.5 3 3.5 4 5 6 7 8 9 10

DE PTH - THOUSAND FT

FIG. 6.-ApPROXIMATE CORRECTION FACTORS DUE TO PRESSURES FOR m. Normal Gulf Coast formation pressure only.

'0""":::: ~ ~

~ ~ t:::: ~ ",,~.~~\o. /' Y:::::: / "..-

i~~~~/ /

'G"o~ Vk L ~~~X V V

\,G~~//o.'l. ,/"

~ ~ V V;/ .// V ~ ,/

~ ~ ~ ./ V V ,/"

/ "..-V

V

~ V I ,/" .0 10 -. 2 -4 4 6 8 10 2 4 6 8 10 -3

m- SQ. eM. PER SEC.

FIG. 7.-RATE OF FILTRATION. 8%-inch hole, 6-inch drill collar.

2

~ ~ ,/

V

MILTON WILLIAMS 67

correction factor, from the empirical rule cited previously, may be assumed to be 2.4 (from the depth of hole, 600%000 X 2). The cor­rected m then is 5.8 X 10-4•

From Fig. 7, the rate of filtration is found to be 0.15 bbl. per hr. per foot of hole. Assuming that one volume of filtrate displaces the reservoir fluid from five volumes of sand, the distance of penetration is found from Fig. 8. Thus, when 8 = 4 hr., Z, the distance of penetration, = 1.9 ft.; when 8 = 8 hr., Z = 2.9 ft.; when 8 = 24 hr., Z = 5.4 ft.; when 8 = 168 hr., Z = 14 ft. and when 8 = 720 hr., Z = 30 feet.

FILTRATION WITH No MUD FLOW

When there is no axial mud flow, as when drill pipe is withdrawn from open hole, the hydraulicking effect of the mud stream is obviously absent. 100

t­IL

8

.6 \OJ t-.. II: 4 t--' ;;: .. .. o 2 \OJ t-.. II: t-\OJ ZIO \OJ

"- 8

'" ~ 6 .. 0-Il>

<5 4 I

'" 2

I

V ./ ....

~ V V

~ / V V

",/ v ./ V V

/ ....

,~~/~v~ ~ V V

./ I---'<!l;J....... o~ L....- / [7 t...,... »,.

"oy'6;y V ./ ~i--'" V / /' ~

./ V./"/ ~ ~/ / ~o'·./ Y 0'1.

~V VV:~V /v:V ././ ././

,/

~ V ",-V '/ / / ./

V / I 2 46810 2 4681002 4681000

8- TIME, HOURS

FIG. 8.-DISTANCE OF PENETRATION OF PERMEABLE STRATA BY FILTRATE ASSUMING 25 PER CENT POROSITY AND 80 PER CENT FLUID DISPLACEMENT.

In this case, the area of the filter cake exposed to the mud becomes smaller as the cake becomes thicker, until finally, when the hole is completely filled with filter cake, the exposed area becomes zero.

Filtration Equation.-By a mathematical treatment similar to that used in the development of the filtration equation for the simple labora­tory filter with no mud agitation, it is possible to arrive at an approximate relation for the "radial flow" case, This involves only the factors involved in the "linear flow" case, and of course, the dimensions of the cylindrical filter. This relation is

V + (1I"ro2L _ V) In (1 _ ~) + 41rLp'f(P) = 41rLpl-I 8 v 1I"ro2L R'P' }lR'(l - 8)

68 RADIAL FILTRATION OF DRILLING MUDS

where ro = radius of cylindrical filter, L = length of cylindrical filter, and the other symbols are as previously defined.

It can be shown that this equation reduces to that of the "linear" case when the cake thickness is small compared to the radius of the filter.

Experimental.-The filtration curve of a Gulf Coast mud, with no axial mud flow, as compared with the calculated curve, is shown in Fig. 9. In calculating the theoretical curve, the values of the constants were obtained from simple "linear" evaluation; the deviation of the theoretical from the observed is due to the neglect of the resistance of the por­ous cylinder.

10' e 6

4

2

1_104

8 6

4

2

IxlO 2 J o

..... V ,.... ,....

~ ~ VV

8.·2.7 alO' SIC. .,'t. C"'rv~:"oIo Calc. CUrYI

~ t/ V V. -1130 C.C

J /

V V;V

IIV

I I

100 200 300 400 ~ 600 700 800 900 1000 VOL. OF FILTRATE. C.C.

FIG. 9.-RADlAL FILTRAU'ION OF GULF COAST MUD WITH NO AXIAL MUD FLOW.

Complete Filling of Hole with Filter Cake.-From this equation, it is possible to obtain the relations giving the time necessary for complete filling of the hole with filter cake, as well as the cumulative value of filtrate at this time. Denoting these values by the subscript c, it is found that

(J = J.lR'(l - s)ro2 c 4VPl •

ro2 = 2mv2

Vc = 7rro2L v

For the Gulf Coast mud previously illustrated, v was found to be 0.13; the time for complete filling of the hole by the cake would be

(Jc = 6.6 X 106 sec., or 1.8 X 103 hr.

MILTON WILLIAMS 69

For a mud of poor quality, the time for complete filling may be eoo­'derably less; for a mud from West Texas having m = 5.0 X 10-2 and

51 d' . _ 015 under the same con ItlOns, ,,- . ,

8. = 4.7 X 104 sec., or 13 hr.

It follows that complete filling of the hole during normal ol'erations is liable to occur only with muds of unusually bad filtration characteristics.

CONCLUSIONS

It is evident that infiltration may present a serious problem in certain cases; a study of the factors involved suggests the best remedial measures.

Since the dimensions of hole and rate of mud circulation are usually fixed by drilling conditions, variation of these to reduce filtration is impractical. Mud control obviously is the solution.

While each mud presents a different problem with regard to securing optimum filtration properties, certain generalities can be stated; (1) tll.at dispersing agents reduce filtration, while coagulating agents increase filtration; and (2) that the predominance of particles of colloidal dimen­sions ordinarily is conducive to low filtration rates.

ACKNOWLEDGMENT

The author wishes to acknowledge his indebtedness to Dr. W. B. Lewis aod Mr. R. W. Jenkins, of the Production Research Department of the Humble Oil & Refining Co., for the execution of the greater pa.rt of the experimental work.

REFERENCES

1. P. H. Jones and E. C. Babson: Amer. Petro lnst., Drill. and Prod. Practice (1935) 22.

2. R. A. Silent: Oil and Gas Jnl. (1936) 35, No.1, 72. 3. W. A. Sawdon: Petro Engr. (1936) 7, No.5, 27. 4. Drilling Mud (Baroid Sales Dept.,): a, April 1937; b, May 1938. 5. W. L. Horner: Oil Weekly (1935) 78, No.3. 6. E. N. Dunlap: Trans. A.I.M.E. (1938) 127,215. 7. R. J. Schilthuis: Trans. A.I.M.E. (1938) 127, 199. 8. R. H. Fash: Bull. Amer. Assn. Petro Geol. (1934) 18,265. 9. C. P. Bowie: U. S. Bur. Mines R.I. 3354 (1937).

10. P. H. Jones: Amer. Petro lnst., Drill. and Prod. Practice (1937) 24. 11. M. Williams and G. E. Cannon: Oil Weekly (1938) 90, No.2, 25. 12. B. F. Ruth, G. H. Montillon and R. E. Montonna: Ind. and Eng. Chem. (1933) 215,

76,153. 13. B. F. Ruth: Ind. and Eng. Chem. (1935) 27, 708, 806. 14. G. E. Cannon and R. C. Craze: Trans. A.I.M.E. (1938) 127,31.

70 HADIAL FILTRATION OF DRILLING MUDS

DISCUSSION

(M. T. Halbouty presiding)

R. W. WILSON, * Houston, Tex.-Mr. Williams has presented a timely contribu­tion to the art of drilling wells. It is a method of interpretation that makes much more valuable the standard static mud-filtration test that has already proved extremely useful and describes an application of this test to many of the problems of drilling, logging and completing oil and gas wells.

The results on the distance to which water from drilling muds may penetrate permeable bodies are startling and certainly emphasize the importance of maintaining drilling muds that will allow a minimum amount of filtrate to penetrate the formations.

The empirical relationship that was developed to estimate the steady rate of filtra­tion is simple but takes into account most of the important variables related to the problem except the texture-toughness and brittleness-of the cake deposited, which is notably absent from consideration. Mud cakes vary widely in texture and it may be expected that these properties will affect the rate of erosion of the cake by the mud stream, which is important in determining the equilibrium filtration rate.

W. E. WINN, t Dallas, Texas.-In several instances we believe the filtrate from drilling muds has served to give erroneous results from drill-stem tests. We believe that this filtrate has not only pushed back into the capillaries, forcing the oil ahead of it, but that also it has caused the small amount of bentonite present in most southwest Texas sands to swell and partly plug the hole. Consequently, when the tool is opened on a drill-stem test, the oil is delayed so long by the necessity of breaking down the oil­water interface, opening the partly plugged capillary channels, and discharging the filtrate from the sands, that the tool is closed before the oil reaches the hole.

Have you studied the effect of filtration of drill-stem tests? Also, have you studied the filtration effect on the fluid contents of cores?

M. WILLIAMS (author's reply).-Although I have no positive knowledge of any case in which excessive filtration has prevented oil from entering the hole on a drill-stem test, that this could and has happened is a distinct possibility. In this connection, the difficulty of bringing in wells in many fields may be mentioned. In many cases, wells cannot be brought in until much water, presumably lost from the drilling mud, has been swabbed out of the producing formation.

The displacement of the original fluids from cores is apparently quite a different process from the radial flow of filtrate into strata around the well bore; the factors affecting contamination of cores by the filtrate have not been evaluated. Regardless of what other factors may be operative, however, this contamination would be expected to decrease with decreasing mud filtration as determined in routine "wall-build­ing" tests .

.. Chief Chemist, Baroid Sales Department, National Lead Co. t In charge of chemical work for Production Division, Sun Oil Co.