DRILLING

Application of Drilling Performance Data to OverpressureDetection

J. R. JORDENMEMBER A/MEO. J. SHIRLEY

Abstract

The previously recognized effects of pressure on rateof penetration have been adapted to an overpressuredetection technique for the Texas-Louisiana Gulf Coast.It is postulated that, under specified conditions, a plotof incremental rate of penetration vs depth will definean ever-decreasing trend in the normal pressure sectionand that this trend will reverse when overpressures areencountered, thus permitting the detection of overpres-sures from drilling performance data. It also is postu-lated that a relationship between rate of penetration anddifferential pressure exists. Relationships between pres-sure and rate of penetration as developed by several in-vestigators are reviewed. Methods are developed to nor-malize rate of penetration data with respect to some ofthe more significant drilling variables (bit weight, rotaryspeed and bit diameter). Using these methods, drillingperformance data are analyzed to determine if the postu-lated correlations mentioned can be recognized from ac-tual field data, and results of these analyses are presented.

Introduction

The presence of overpressured formations (formationswith abnormally high fluid pressure) is a significantcharacteristic of the Texas-Louisiana Gulf Coast geologicprovince. Dickinson' originally outlined the occurrenceof overpressured formations in this province, and bothDickinson' and Hubbert and Rubey' presented theorieson the origin of these high-pressure zones. Subsequently,drilling practices and mud and casing programs havebeen developed which permit the detection and controlof overpressures, thus enhancing the chances of success-fully drilling these zones. Many of these techniques,which are not in the literature, have been used extensive-ly by various operators in the Gulf Coast for severalyears. Recently, log analysis methods have been reportedby Hottman and Johnson' using the transit time and

Original manuscript received in Society of Petroleum Engineers officeMarch 31. 1966. Revised manuscript received Sept. 13, 1966. Paper(SPE 1407) was presented at SPE Symposium on Offshore Technologyand Operations held in New Orleans. La. May 23-24. 1966; and at SPE41st Annual Fall Meeting held in Dallas. Tex. Oct. 2-5. 1966. Copy-right 1966 American Institute of Mining. Metallurgical, and PetroleumEngineers, Inc.

lReferences given at end of paper.

NOVEMBER, 1966

SHELL DEVELOPMENT CO.HOUSTON, TEX.SHELL OIL CO.META/R/E, LA.

resistivity of shales to (1) identify the first occurrenceof overpressures, and (2) estimate formation pressuregradients.

This paper presents a technique developed for identi-fying the first occurrence of overpressured formationsfrom interpretation of drilling performance data. Specif-ically, rate of penetration data, by virtue of its depend-ence on differential pressure (the bottom-hole pressuredifference between the mud column and the formation),can be used to identify overpressures. Data from thistechnique are immediately available as a well is drilledwhich is an obvious operational advantage over the loganalYsis methods reported earlier.

Studies- in the industry have shown that rate of pene-tration is considerably reduced by an increased mudcolumn pressure. Assuming an inversely proportional re-lationship between rate of penetration and differentialpressure, consider drilling (under constant conditions) agiven rock in the normal pressure section at ever-increas-ing depths. The total differential pressure will increasewith depth, and rate of penetration should decrease.Consider drilling the same rock under the same condi-tions as the overpressure section is entered. As the for-mation pressure gradient increases, the differential pres-sure decreases and the rate of penetration should improve.

Thus, for constant rock properties and drilling condi-tions, it is postulated that a plot of incremental rate ofpenetration vs depth should define an ever-decreasingtrend in the normal pressure section, and that the trendshould reverse when drilling into overpressures (Fig.1). Such a plot would show characteristics similar to theshale resistivity and shale transit time plots currentlyused for overpressure detection, and would be a valu-able supplement to these log analysis methods because itwould be available as a well is drilled.

Furthermore, there should be a relationship betweenincremental rate of penetration and the differential pres-sure existing between the mud column and the formation.Knowledge of such a relationship would provide a meth-od to (1) maintain minimum differential pressures andthus improve drilling efficiency, and (2) predict forma-tion pressures while drilling.

If rate of penetration is proportionally related to pres-

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125

STRENGTHEFFECT

25 50 75 100MUD PRESSURES, kg/cm 2

32 RPM ATMOSPHERIC PORE PRESSUREo OBERNKIRCHENER SANDSTONE BIT LOAD 300 kg.X VAURION LIMESTONE BIT LOAD 500 kg./! BELGIAN LIMESTONE BIT LOAD 700 kg.

O+----.,..-----,------r---:---r------,o

~ 4-.....

Ee

zoI-

~2I-wZWD..

Applieation of Drilling Fundamentals

they affect rate of penetration. These data indicate thatchip hold-down affects rate of penetration more signifi-cantly than does rock strengthening.

Most of the work cited suggests that a relationshipdoes exist between differential pressure and rate of pene-tration. However, the work of Bingham" has shown arelationship between rate of penetration and pressure,but not necessarily a proportional relationship betweenrate of penetration and differential pressure. Bingham'sfindings imply that drilling under laboratory conditionsmay be directly influenced by differential pressure, butdrilling under field conditions is not necessarily influencedby differential pressure in the same manner.

In summary, all investigators recognize a strong rela-tion between rate of penetration and pressure, differentialor hydrostatic; however, definition of the exact nature ofthe relation has not yet been made. It was the originalpremise of the present study that overpressures can bedetected in a well by a change in penetration rate andit is implicit that a relationship between rate of pene-tration and differential pressure must exist for this prem-ise to be valid. Therefore, this investigation is directedtoward developing a reasonable means to represent theeffect of differential pressure, recognizing that it mayormay not be the only physical phenomenon involved.

TheoryThis investigation is to relate rate of penetration be-

havior to differential pressure behavior and thus developa tool to detect overpressures. It has been shown byseveral investigators'" that a recognizable relationship be-tween differential pressure and rate of penetration shouldobtain under constant drilling conditions. An equationof the general form R/N =a(W/ D)d has been shown" torelate penetration rate to bit weight, rotary speed andbit size, provided that all other drilling variables are

Fig. 2-Penetration rate as a function of mud pressure atatmospheric pore pressure (after Garnier and van Lingen4).

- RATE OF PENETRATION"d" EXPONENT -LOG 6tsh PREssuRE tiP

GRADIENT

\ '"\ ...\ 5~ I\ >:;i I\ o~

( :ORMAL~COMPACTION

TREND

LOG Rsh

Fig. I-Schematic comparison between shale resistivity,shale transit time, differential pressure and rate

of penetration.

sure differential, the foregoing postulations are valid inthe ideal case where all other drilling variables are con-stant. However, the rate of penetration-differential pres-sure relationship is obscure, and actual field conditionsnecessarily include variations in rock properties and drill-ing mechanics. Therefore, it is the purpose of this studyto examine actual field data to determine if these postu-lated correlations are recognizable and, if so, to establishmeans of applying these relationships to improve cur-rent overpressure detection and drilling techniques.

Influence of Differential PressureOn Drilling Performance

The influence of pressure, either differential or hydro-static, on drilling performance has not been clearly de-fined. Murray and Cunningham' concluded from labora-tory experiments that rate of penetration is decreasedby an increased confining pressure in most formations,and suggested this decrease is caused by rock strengthen-ing due to the confining pressure. They further concludedfrom field data that rate of penetration is decreased byan increased mud column pressure, and that mud columnpressure affects rate of penetration in the field approxi-mately the same as confining pressure in the laboratory.Subsequently, Eckel" reported laboratory work on lime-stones which indicated the pressure differential betweenthe mud column and formation is the only pressureparameter which affects rate of penetration.

Further laboratory work by Cunningham and Eenink'showed that overburden pressure has practically no ef-fect on rate of penetration and confirmed that rate ofpenetration is dependent on the difference between mudcolumn and formation pressures. These authors foundthat rate of penetration decreased when mud column pres-sure was greater than formation pressure. They attributedthe decrease primarily to the redrilling of a layer ofcuttings and mud particles held to the hole bottom bythe difference in pressure, and secondarily to the strength-ening of the rock by the differential pressure.

Garnier and van Lingen' found that differential pres-sure affects both rock strength and chip hold-down. Fig.2 shows an example of the relative influence of differen-tial pressure on rock strength and chip hold-down as

1388 JOURNAL OF PETROLEUM TECHNOLOGY

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24

6

20

3

8

fO

30

40

50

BIT WEIGHTW,IOOO LB.

6"6'/i'8 '/i'9 7/8"

I~"\

\

BIT SIZED,INCHES

12W106D

\~

\\

.010

.050

.040

.006

003

.020

.004

"d" ..200RATE OF PENETRATION

lLR, FT. PER HOUR EXAMPLE60N Ro20NIOO

250 W=25,OOOD=97/8200 d=1.64

.001150

100

.002

ROTARY SPEEDN, REV. PER MIN.

50 250200 ~

.004159/~O

/" .006/" 50

.008

30.010

20

10 10 R8 .020 60Nd= ---

12 W6

10 6 D.0305

Fig. 3-Nomogram for d exponent determination.

constant and certain other ideal conditions are met. Ithas also been shown' that this equation does not de-scribe drilling performance under field conditions. How-ever, as an empirical approximation, it is suggested thata recognizable relationship between differential pressureand d exponent (exponent of the general drilling equa-

2000

1600(/J0-

Wa:::::>en 1200enw0::a.

...J

DEPTHflUID PRESSURE AUXILIARY DATA

SHALE RESISTIVITY SHALE TRANSIT "d" EXPONENT DIFFERENTIAL PRESSURE GRADIENTFEET TIME -MUD --- FORMATION CASING POINTS, K!CKS, ETC.X MEASURED F. P G

1\ I. HAD DRLG. BREAK 13205'-10', STARTE'I~ GAINING MUD VOLUME WHILE CIRC.6(l(X) OUT. WELL FLOWED W/PUMPS OFF

CLOSED HYDRIL. INCR. Wl. IN PITS\

\ ITO IZ.O. CIRCULATED ON CHOKE~ I 7 HRS. a RETURNS STABILIZED AT8000 IL8. 'BOTTOMS UP HAD BEEN SALT-

\ \ I WATER-CUT TO 9.8 MIN. OPENED~ I HYDRIL. INCREASEO WT. IN PITS TOI 12.5. CIRCULATED 6 HRS. a RETURNSSTABILIZED AT 12.5. MADE SHORT10,000 \ TRIP O.K.I~ I \ 2. INCREASED WT. FROM 12.5 TO 13.0, I AT 13232' TO CONTROL CAVING12,000---- I '-lL HOLE a EXCESS SHALE.TOP \OVERPRESSURES ~ ~--'"( 1/ '-. ,.) \,14,000 9 5,,'\.

lc",,~ 14157'

/ 1\ ~ ,_...._,~~ 3 INCREASED WT, FROM 10.0 TO 17.0

16.000

\ ? 'i AT 15066' TO CONTROL CAVINGI HOLE 6 EXCESS SHALE.I 000 I \

0' 05 '0 '0 70 100 150 1 2

'000 2000 .4 06 'OB 1.0RSh, ohm-m li.t.,aS~(:/fl 'd" dp,psi FP.G. , psi.lft

Fig. 5-0verpressure data sheet, Well A.

FLUID PRESSURE AUXILIARY DATADEPTH SHALE RESISTIVITY SHALE TRANSIT "d" EXPONENT DIFFERENTIAL PRESSURE GRADIENTFEET TIME -MUD --- fORMATION CASING POINTS, KICKS, ETC

X MEASURED fPC>

\1:\'1,"

I I 250u'II I l. HAD EXCESS SHALE AT 12035:4000

1 \ I 2, HAD GAS-CUT MUD AT 12160:I AfTER LOG AT 12160' HADMOOERATE GAS-CUT MUD6000 ~\ \ WHILE REAMING BACK TOI BOTTOMI 1 3. AFTER TRIPS AT 13202: 13325'8000 6 13436', RETURNS CUT fROM~ f s I 165 TO 154 MIN.I 4. AFTER TRIP AT 13718: RETURNSI CUT FROM \6.5 TO ] L

----

,l__) AFTER LOG AT 13718', RETURNSw ~ CUT FROM 16.5 TO 15.5 MIN.OVEflP~E~S 1P- ~i;4~~"12,000' '\ ..-- ~ 1"~ , /} ~ 1. AT 11948'. W/12.3 MUD,WELL lOCKED. CLOSED HYDRIL WI8000 \~ If ~ \ I 300p,i SIOPP. INCfl. WT. IN PITS TOI 14.0. CIRCULATED 3.5 HRS. ON CHOICE} I l.5 HRS, OPEN 6 RETURNS STABILIZED

10,000 ~ ~ i LJ AT !4.0, RESUMED DRLG.TOP -~ i \ 1--'OVE~P~ESSU"ES 7~-

l:.L_ /' HB60'

12.000 ~ \ I I ) :3- ../ (,.~ \, 4. HOLE SWABBED ON TRIP AT 12.015'./ P. ,~ -=: 5. HAD INTERMITTENT MILD GAS"""

(- CUTTING THROUGHOUT INTERVAL14.000

I \ I,

"L 14013'-14355'.hlFPG AT II'UTO.6$3 eA~,EO ON

,,~ i \ 12.3 PPG jUO a :soo"r' SIOPP0' 05 '0 20 '0 '00 '50 , 2 0 '000 200 4 0.6 , 10RtM, ohm-m l1t.,A,St(:,/Fl "d" Ii ,j:l$' Ff'G, psi/F!

Fig. 7-0verpressure data sheet, Well C.

lil90 JOURNAL OF PETROLEUM TECHNOLOGY

*This equation is not a rigorous solution of the equation R/N=a (W / D)' in that the d exponent as used here actually represents d +]oga

RWhen 60N is less than unity, the absolute value of log

Rate 01 Penetration

During the collection and analysis of d exponent datait was recognized that another approach, possibly betterthan normalizing drilling data, would be to maintain alldrilling variables constant and simply record uncorrectedrate of penetration. Since this practice cannot prudentlybe followed over the entire hole due to changes in for-mation characteristics with depth, the following methodwas developed.

1. Preselect an interval of 1,000 to 1,500 ft immediate-ly above the expected top of overpressures based on avail-able data in the area of the well of interest.

"d"EXPONENT MUD WT. ppg..0 2.0 3.0 10 12 14 16

C/)w~ \ ~zt: :r.l, \ u~~\ Uiq 1~c::;

t:::

~

~ -~f-----~ l-

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~~

-~

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.,

~{ 1-~ '--

=

oI---+==~-~ g

S.P.-H~20MV g~---r-.,...--4 0

o

ool-__-L-J._--' 0C\J

2. When the well reaches this preselected depth, in-crease mud weight sufficiently (up to 12.0 Ib/gal) toavoid a kick at the top of overpressures.

3. Maintain (as nearly as practical) constant rotaryspeed, weight on bit, bit size and type, pump pressure,etc., throughout the selected interval. The rotary speed,weight on bit, etc., should be optimum based on experi-ence in the well of interest or nearby wells.

(I)"log [6~ ]-[12W]'log lO"D

d=

6~N varies inversely with R; therefore, the d exponentvaries inversely with rate of penetration. Calculations ofthe d exponent were facilitated by use of the nomogramshown in Fig. 3.

4. d exponents and differential pressure between themud column and the formation were plotted vs depthfor each bit run (Figs. 5 through 7). To calculate thedifferential pressure, a constant forrgation pressure gra-dient of 0.465 psi/ ft was used in the known normalpressured section, and the formation pressure gradient inthe overpressured section was estimated from shale re-sistivity-formation pressure gradient relationships developedby Hottman and Johnson: Recorded mud density datawere assumed to occur at the lowest depth of each bitrun and any changes in mud density were assumed tooccur linearly between these depth points, unless other-wise noted.

5. To evaluate the reliability of d exponent plots asan overpressure detection method, these plots were thencompared with shale resistivity and shale transit timeplots and the known drilling history of the wells understudy. To obtain a relationship between d and differentialpressure, d exponent data for each bit run were plottedvs the average differential pressure occurring during thebit run.

6. Additionally, a program of data collection was un-dertaken on then-current offshore exploratory wells. Datacollection methods were similar to those previously out-lined except that rate of penetration, bit weight, rotaryspeed, mud weight and circulation rate were recordedfor short intervals (some every 30 ft and some every 10ft drilled). These data were then analyzed as previouslydescribed. The d exponents in shale sections were selectedand plotted for comparison with the shale resistivity andtransit time plots and drilling history. Short interval datawere available from two wells, and the analysis of oneis discussed herein.

Fig. 3-d exponent vs depth compared with SP log,Well C.

NOVEMBER, 1966 1~91

4. Record and plot rate of penetration in lO-ft incre-ments throughout the interval until overpressures are in-dicated by the plot.

5. Log to confirm overp~essures by shale resistivity ortransit time plots.

Evaluation of Results

Normalized Rate of PenetrationResults of the original study show that definite corre-

lations between d exponent and differential pressure canbe recognized from field data. Of the 17 original casesstudied, four showed a very consistent d exponent trendin the normal pressure section, nine showed a reason-ably consistent trend and four showed a poor trend. Thetwo wells for which closely spaced data were availableclearly showed the top of overpressures. Of the 11 wellswhich drilled deep into overpressures, nine showed a defi-nite decrease in d exponent. Several of the wells studiedshowed an excellent correlation between d and differen-tial pressure. in cases where the differential pressurechanged abruptly due to abrupt changes in mud weight.These correlations were noted in both the normal andoverpressure sections. Fig. 4 shows a differential pres-sure-d exponent relationship obtained as previously de-scribed. Although a trend is indicated, the scatter ofdata is too great for a quantitative field application ofthis relationship. To note in some detail the character-istics and limitations of drilling performance data as anoverpressure detection technique, a discussion of repre-sentative well studies follows.

Well A, Offshore St. Mary Parish, La.A very consistent d exponent trend occurs in the nor-

mal pressure section of this well, with the top of theoverpressures indicated at 12,450 ft (Fig. 5). Shale re-sistivity data are difficult to analyze in this well with twopossible interpretations of the overpressure top; i.e., at12,200 and 14,000 ft. The preferred interpretation placesoverpressures at 12,200 ft which seems to be confirmedby d exponent data and the well behavior. There areexcellent indications of a decreasing pressure differentialbelow 15,300 ft. As the mud weight was held constantthrough this interval, d exponent data give a direct in-dication of the increasing formation pressure gradient,which is confirmed by shale resistivity data.

Note that the d exponent is higher between 13,000to 14,000 ft (in the overpressure section) than any-where in the normal pressure section. Even though theformation pressure gradient is higher in the overpressureinterval than in the normal pressure interval, the re-spective mud column pressures are such that a higherdifferential pressure occurs in the overpressure section.This emphasizes the belief that drilling performance isdependent upon differential pressure, and that d exponentdata must be interpreted in thc light .of existing mudcolumn pressures.

Well B, Terrebonne Parish, La.

The d exponent data form a poor trend in the normalpressure section of this well (Fig. 6). The sharp in-crease in d at 10,450 ft appears related to the significantincrease in differential pressure at about the same depth.Below protective casing there is an outstanding correla-tion between d exponent and differential pressure. Notethat shale resistivity data show an increasing formationpressure gradient from 11,450 to 12,700 it and then adecreasing formation pressure gradient below 12,700 ft.The resulting differential pressure is clearly reflected bythe d exponent.

Well C, Offshore Iberia Parish, La.Considerable scatter of d exponent data is evident in

the normal pressure section of this well (Figs. 7 and 8).Much of this scatter is undoubtedly due to varying toothwear which becomes a major variable affecting drillingperformance when several data points are taken fromone bit run. A series of individual trends are apparentin the individual bit runs. However, the over-all trendis one of consistent increase with depth in the normalpressure section. Note the abrupt shift in the trend atabout 10,400 ft due to the abrupt increase in diffe'rentialpressure. This was the first well in which overpressureswere detected from drilling performance data before ex-periencing a kick.

Between 10,000 and 10,400 ft the mud weight was in-creased rapidly from 10.3 to 12.1 lblgal and held at thatvalue to 11,860 ft. Drilling performance data were re-corded at lO-ft intervals. Fig. 8 shows a plot of d ex-ponent vs depth on an expanded depth scale. Based onthe slow, consistent decrease in d exponent between11,730 and 11,860 ft, the decision was made to stop

FLUID PRESSUREDEPTH SHALE RESISTIVITY SHALE TRANSIT RATE OF DIFFERENTIAL GffADIENT AUXILIARY DATAFF:"ET TIME PENETRATION PRESSURE -MUO - - - FOMIATIC* CASING ~OIHTS. I(I(:KS, (TC.

X MIE ... SUltED .... ~.G.

~ ,0>}4'.000

4000 ~ .r B>T \""NS I""

I6000

t J \~ I8000 I WELl. KICkEO WHILE P"EPARING TO\ I,{-- ..... I IUI([ TIltP AT 9610' INCIt. MUD........ ---- - - --C --~-- --) - - -- --- l ..FROM 10.0 TO ll,7 PPG,-LOGGED

10000[;:., ~~ ST 1 5/S" CS6. H8"

rop OVl"'__55""

12000

14000

16000 o. os 'D 2. 70 100 LSO "0 100 .0 00 1000 200

FLUID PRESSUREDEPTH SHALE RESISTIVITY SHALE TRANSIT RATE OF DIFFERENTIAL GRADIENT AUXILIARY DATAFEET TIME PENETRAflON PRESSURE --MUD - - -FQltMATlOM CASING POINTS, KICKS. ETC

X MEASURED F.'.G.

~.lO3/4"'""TI48

4000 l 1/I6000 ~ ~ \ III8000 ~ ( I1I10000 MUD GAS CUT WHILE DRLG 12030 INCR~ POSSIBLE if ( ~~ I J"OM :'0 '0 " . ,eo '" ,"co m~OVERPRE7SURE~ I, 12287 -LOGGEDlli SET CSG12000 ~ ,",L I,f(~ ~ l2: ~ 12287 DALD OUT W114.0 PPG MUD, INCR TO\ I~,O PPG WHILE DRLG.\ I DEFINITE

14000,OVERTEssurs

16000"

0' '.0 w w '00 "0"

'0"

" 0 '000 200< ~. 0.0 0.' LO

As~, o~m-m 0.1,# Sec:./FI Ft/H,.Il.P,I"i FPG.,l's;!Fl

Fig. IO-Overpressure data sheet, Well E.

RATE OF PENETRATIONFTiHOUR

150 100 50 0.,

.!BIT CHANGE - -Z

from an automatic rate of penetration recorder on anoffshore exploratory test. Comparison with the induction-electric log shows very good correlation with sands en-countered in the interval. This example also shows theeffect of bit wear on rate of penetration. (Note the sawtooth profile with continually decreasing average pene-tration rate with depth.) Overpressures are clearly in-dicated by continuous increase in penetration rate below11,750 ft. Note that the penetration rate reaches 125ft/hour near total depth, being approximately eight timesgreater than expected from extrapolation of the expectedrate of penetration.

Conclusions

Based on the original study from which this new tech-nique was developed and over two years of subsequentexperience in field application, the following conclusionscan be made relative to the Texas-Louisiana Gulf Coast.

1. Drilling performance data can be used to detectthe top of overpressured sediments in areas where theapproximate depth of overpressuring is known. A plotof normalized rate of penetration will show a trend ofcontinually decreasing penetration rates with depth anda reversal in this trend as overpressures are penetratedby the drill bit. This technique can be used as a meansto avoid taking a kick and to identify overpressures priorto logging.

2. Rate of penetration data can be normalized by atleast two methods with sufficient validity for use withthis technique: (a) by using a general drilling equation(d exponent method), or (b) by maintaining all drillingvariables constant in the field. Experience has shown thelatter method to be preferable. Experience has also shownthat analysis of short footage increments gives morediagnostic results.

3. A correlation between normalized rate of penetra-tion and differential pressure is recognizable from theavailable data. Although a trend is indicated in the dexponent-differential pressure curve shown in Fig. 4, thescatter of data is too great for quantitative application.However, results are sufficiently encouraging to merit fur-ther investigation under controlled drilling conditions spe-cifically designed to investigate this relation.

Nomenclature

a = constant in general drilling equationD = bit diameter, in.d = exponent in general drilling equationN = rotary speed, rpm

6.p = differential pressure between mud column andformation, psi

1394

R = penetration rate, ft/ hrR'h = shale resistivity, ohm-m'/m

6.t'h = shale transit time, microsec/ftW = bit load, Ib

References

1. Dickinson, G.: "Geologic Aspects of Abnormal Reservoir Pres-sures in the Gulf Coast Region of Louisiana, U.S.A.", Proc.,Third World Petroleum Congress, The Hague, Netherlands(1951) 1.

2. Hubbert, M. King and Rubey, W. W.: "Role of Fluid Pressurein Mechanics of Overthrust Faulting. Part I", Bull., GSA(Feb., 1959) 70.

3. Hottman, C E. and Johnson, R. K.: "Estimation of FormationPressures from Log-Derived Shale Properties", lour. Pet. Tech.(J une, 1965) 717-722.

4. Garnier, A. J. and van Lingen, N. H.: "Phenomena AffectingDrilling Rates at Depth", Trans., AIME (1959) 216, 232-239.

5. Murray, A. S. and Cunningham, R. A.: "Effect of Mud Col-umn Pressure on Drilling Rates", Trans., AIME (1955) 204,196-204.

6. Eckel, J. R.: "Effect of Pressure on Rock Drillability", Trans.,AIME (1958) 213, 1-6.

7. Cunningham, R. A. and Eenink, J. G.: "Laboratory Study ofErrect of Overburden. Formation and Mud Column Pressureson Drilling Rates of Permeable Formations", Trans., AIME(1959) 216,9-17.

8. Bingham, M. G.: "A New Approach to Interpreting RockDrillability", Oil & Gas IOllr. (Nov. 2, 1964-April 5, 1965).

***

J. R. JORDEN (left) is a senior petrophysical engineerwith Shell Development Co., Houston. He received a BSdegree with honors in petroleum engineering from the U.of Tulsa in 1957. Since returning to Shell in 1960 frommilitary duty, he held various engineering positions inSouth Louisiana prior to his present assignment. O. J.SHIRLEY (right) holds a BS degree in petroleum engineer-ing from The U. of Oklahoma. He joined Shell in 1948as an exploitation engineer and has held various engineer-ing assignments for Shell Oil and Shell Development inCorpus Christi, Houston and New Orleans. He is current-ly assigned to Shell's Offshore West Div. as staff petro-physical engineer.

.1011 II:'iA I. OF PETROLlWM TECHNOLOGY

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