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FORMATIONFRACTURING

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Pseudolimited Entry: A Sand Fracturing Technique for Simultaneous

Treatment of Multiple Pays

LOUIS C. STIPP

R. A. WILLIFORDMEMBERS AIME

Abstract

Tenneco Oil Co. has u.~ed the pseudttlimited emry tech.nique to simultatwously and effectively sand-water fracturethe mt41tiple, hard, tight Dakota gas pays in the San Juanbasin. Application of this technique involves (1) the use O/ball sealer.v t,) insure tha[ a satisfactory number of per-j[n-ations are open prior to fract14ring. (2) employment ofa low (approximately 300 psi) perforation differential r[~insure simultaneous, ef?ective stimulation of all perforatedpays (If approximately equal fracturing pressure and {3)the use oj bridge plug.r when necessary to insure stimula-tion of zwtes having significantly di#erent fracturing pres-sures.

The use of the pseudolimited entry technique essentiallyhas elitninated sand. ot41s, poor treatment coverage andother prohletns a,wociated with other fracturing tech-niqt4es. As of March, 1966, the absolute open flow poten-tials (A OFP) of the nine wells completed by this melhodaveraged twer 4,000 Mcf/D or 78 percent above tho.~eAOFP’s of con ven(ionally treated oflset wells. Cotrslant-rate prrsst4re drawdown tesring’ also has supported theresults attained.

The rest41t.r and contint4ed use of this techniq14e haveproved it 10 be an eflective method for stimt.dating (he Da-kota pays of Northwest New Mexico.

Introduction

Fracturing multiple pays with conventional techniques(densely perforated zones, treatment control through theuse of bridge plugs or ball sealers) can result in expensiveor marginal completions. Fracturing the Dakota formationin the San Juan basin with conventional methods otTers fewexceptions to this.

The Dakota formation in the San Juan basin is ap-proximately 200 to 300 ft thick. As shown by Fig. 1, itoften is composed of multiple, hard, low-porosity, gas-bear-ing sands separated by shale. At a given location, the lowerDakota usually is composed of a series of sands of ap-proximately equal fracturing pressures, These pressuresoften vary markedly between locations. The middle andupper Dakota sands are less numerous. They have approxi-mately equal fracturing pressures significantly differentfrom those of the lower sandx those of the middle and

Ork’inal manuscrkt receiwl In SocIetYof Petroieum Engineers offlreAug. S. 1967.Rwlsed manuscript r.scekedMarch 2Q.196S.Paner (SPE]s0s) was nwented at SPE 42ndAnnual Fall Meeting held In Houston.Tex., Oct. 1-4, 19S7.@ Copyright 196S American InstltuW of MInlnx.Metallurgical. and Petroleum Entrineers, inc.

preferences uiwn at end of mum’.

TENNECO OIL CO.BAKERSFIELD,CAIIF.

DLSRANGO, CO1O.

upper sands also can vary over a short distance. Fractur-ing pressures of all the Ditkota sands are difficult to pre-dict quantitatively.

The Dakota frequently is sand-water fractured with con-ventional multistage treatments, employing bridge plugsor ball sealers for treatment control and diversion. Normal-1y, the amount of propping agent used is between 50,000and. 100.000 lb of sand. Treating rates usually are between35 and 50 bbI/minute. Jn many cases individual sands willbe perforated with so many holds that the treatment will beaccepted into the sand that has the lowest fracturing pres-sure.

317550‘ ‘==176501+11

TD 7695’

Ffg. l—Dakota section, San Juan basin.I

Ball sealers generally are used during a conventionaltreatment to open additional perforations and/or divert thefracturing fluid. However, they have not proved reliable forthese purposes. Ball sealer leakage. communication behindthe casing between closely spaced perforations and main-tenance of an effective seal on upper perforations in small-diameter casing due to the bypass of fluid and additionalballs could contribute to poor ball sealer performance. Useof ball sealers during a Dakota treatment is complicatedfurther by the treating pressures necessary to (1) achievebreakdown of additional pays, (2) open additional per-forations and (3) achieve sufficient rates to prevent sand-outs and effectively stimulate the Dakota sands, Usuallysuch pressures are near the maximum safe internal yieldpressure of the casing.

Other problems often associated with the conventionalDakota fracturing techniques are poor treatment coverage.tiand-outs, low fracturing rates at high treating pressures,ciicessive hydraulic horsepower usage and the associatedextra costs. In general, poor treatment coverage can be at.tributed to an excessive number of perforations, or un-dependable ball sealer performance for treatment diversion,or to other problems associated with the use of balls. Allof these problems can occur when an excessive pressuredrop exists across the open perforations as a result of poorperforating efficiency. In this case, the fracturing rate willbe restricted because of a high surface-treating pressure.

Development of the Technique

The pseudolimited entry technique was developed toeliminate the foregoing problems and those associated withother fracturing techniques to be discussed later. It alsowas developed (o meet the following completion objectivesand to accomplish them at reasonable cost: (1) effectivelystimulate all reservoir beds, (2) obtain maximum produc-tivity, (3) produce maximum reserves, and (4) eliminate orreduce remedial operations.

Several methods were considered. They were (1) a frac-turksg technique dependent on ball sealer performance todivert the treatment: (2) limited entry and (3) staging—treating each isolated zone—using only bridge plugs,

The use of ball sealers is not satisfactory for reasons dis-cussed previously.

The second method, limited entry, employs a limitednumber of perforations to provide a certain pressure dropacross the perforations (perforation differential) at a de-signed treating rate. The perforation differential is designedto be high enough to cause the bottom-hole treating pres.sure to exceed the various fracturing pressures of the per-forated zones. In this case, the zones will be effectivelytreated—assuming that all perforations accept fluid as in-tended. However, to assure successful application of thetechnique, the treatment must be designed properly. To dothis, the fracturing pressures of the perforated zones mustbe known. Also, the effectiveness of the simultaneous lim-ited entry treatment of zones of different fracturing pres-sures becomes more reliable with the employment of highperforation differentials (1,000+ psi ).

Limited entry per se is not justifiable for effective stimu-lation of the various Dakota pays for several reasons. Oneis the high perforation d~erential that is necessary to in-crease the reliability of the technique. High perforation dif-ferentials and the associated high treating pressures couldnecessitate an increase in casing costs and hydraulic horse-power requirements. Also, the accurate design of such atreatment, particularly necessary if lower perforation dif-ferentials are employed, is ditTicult because of the signi-ficant and unpredictable variation of the fracturing pres-

sums of the various Dakota zones. Finally, lhnhed entrytreatment of an extensive group of pays, as usually foundin tht~ Dakota, can result in low rates into an individualpay }Iith a corresponding reduction in fracture extensionor a wutd-out.

The third method, staging, employs a bridge plug forseparate treatment of each isolated zone. Using this methodexclusively is not practical because of the number ofstages necessary for effective treatment of the usuallynumerous Dakota pays.

By combining the advantage; of the limited entry andstaging techniques, the pseudolimited entry fracturing tech-nique was developed.

Pseudolimited Entry Technique

This technique is (1) the employment of a limited num-ber of perforations to provide a low (approximately 300psi) perforation differential for simultaneous treatment ofpays of approximately equal fracturing pressureq (2) theuse of the minimum number of stages necessarY (usuailYtwo) for positive separation of zones of significantly dif-ferent fracturing pressures; and (3) the use of acid and ballsprior to the actual treatment to open the perforations.

Item 1 is accomplished through the use of a number ofperforations that, when all are open to accept fluid, willprovide approximate] y a 300.psi perforation diRerential atthe designed treatment rate. This perforation differentialis sufficient to insure that all perforations in pays of ap-proximately equal fracturing pressure are accepting fluid atan effective rate. This amount of perforation differentialcan be obtained at reasonable hydrauiic horsepower cost.Treating pressures associated with perforation differentialsin this range do not require special casing, nor cause treat-ing rates to be restricted significantly.

Incorporation of Item 2, staging, is necessary for theeffective treatment of zones of significantly different frac.turing pressures, using low perforation differentials. Forreasons given previously, this technique is used rather thana high -perforation differential. Staging also is incorporatedinto the technique to provide concentration of the treatingrate and treatment per zone. This can be necessary for sat-isfactory rates and fracture extension in the individualpays. As discussed, this can be an advantage over simul-taneous limited entry treatment of an extensive group ofpays. Two stages usually are required for effective Dakotatreatment coverage.

Item 3 usually is a ne=ssary step for achieving the de-signed perforation differential, or for having a satisfactorynumber of perforations open to accept fluid. This is becauseof the low perforating efficiency experienced in the Dakotaformation. By insuring that a satisfactory number of per.forations will accept fluid prior to the actual treatment,effective stimulation of all reservoirs in a given stage prac-tically is assured. There is no dependence on ball sealersduring the actual treatment to break down zorws, to openadditional perforations or to divert the treatment.

The cost of a pseudolimited entry treatment is approxi-mately the same as that of a conventional treatment ofthe same size.

Application

A pseudolimited entry treatment normally is done in twoseparate stages. The first stage is the most involved and isdevoted to fracturing the lower Dakota, typically a seriesof thin sands separated by shale with all sands having ap-proximately the same fracturing prmsure (Fig. 1). Initially,the sands justifying stimulation are selected. A number of

—

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perforations are chosen that, when all are open to acceptfluid, will provide adequate treatment coverage, desiredtreatment distribution and a perforation differential of ap.proximately 300 psi at an acceptable rate.The perforations(usually 15 to 20) normally are grouped in the most poroussections of the sands to be fractured. Vertical fracturingshould result in effective treatment of each zone. The treat-ing rate is governed by the casing pressure limitations, hy.draulic horsepower cost, the number of perforations select-ed and the desired perforation ditIerential, For Dakotafracturing, the treating rate necessary to satisfy all condi-tions usually is between 50 and 55 bbMrrhrute.

By obtaining the 300-psi perforation dtierential at theanticipated pressure and rate, all perforations will take fluidand all porous intervals will be stimulated. If a perforationdifferential is obtained in excess of that designed for, allpays still will be treated if the excess is due to a treatingrate higher than anticipated. However, if this is not thecase, all perforations probably are not open. Treatmentcoverage then is reduced proportionally and sand-outs as

, well as other associated problems can occur. Therefore, thetreatment should not be started unt i] a sufficient number ofthe perforations are calculated to be open — a sufficientnumber being enough open perforations to insure satisfac-tory treatment of all sands in a given stage. As previouslymentioned, ball sealers and acid are used to open the per-forations prior to the treatment.

rhe second stage of the pseudolimited entry techniqueas applied to Dakota stimulation usually involves the treat-ment of no more than two separate sands having approxi-mately the same fracturing pressure. I’hese sands normallyhave a fracturing pressure different from that of the sandstreated by the first stage. For reasons given previously, abridge plug is used for zone isolation. The second stageapplication is simplified due to a reduction in the numberof sands requiring stimulation. However, design considers-t ions are the sam~ for each stage.

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h0.95

0)9 ,

0.85

I0.8

0.75

0.7

0.65

I0.6

PERFORATIONCOEFFICIENT

(1)

0.2

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I10.0 0,7

0.811)0 0.$

12.01J

PERFORA1II

ExampleThe treatment is designed and evaluated by using the

fo!Iowing equation and the various charts and nomographyincluded inthe limited entry manuals of the various servicecompanies.’”’

P*= LSIP+PI+PPJ. . . . . ~ . (1)

where P, = surface injection pressure

l!UP=instantaneous shut-in pressure, psi

P, = pipe friction, psi

P,, = perforation friction. or perforation dfier.ential. psi.

Also.

ISIP=BHFP– P,,. . . . . . . (2)

where BHFP = bottom-hole fracturing pressure

P. = hydrostatic pressure.

In Dakota fracturing in the San Juan basin, the surfacetreating pressure P, is limited to approximately 4,000 psi.primarily by the 4% -in. J-55 casing normally set. Due toeconomic considerations, this casing is considered stand.ard for these completions. For illustration, there are as-sumed to be 10 individual sands to be treated by the firststage. It also is assumed that 15 perforations will distributethe treatment appropriately. From analogy, an L$JP of ap-proximately 1,600 psi for each of the sands is expected.Sand concentrati~n is 1 lb/gal of water.

From Fig. 2 it is determined tha~ a rate of 3.6 bblfminutelperforation will provide a perforation differentialof 300 psi. A 0.55-in. perforation diameter and a 0.95 per-foration coefficient are assumed. Because 15 perforationshave been selected, a total treating rate of 54 bbVminuteis required (3.6 bblfminute x 15 perforations).

T100

t DIAMETER I ~ 2000IES

RATE / PERFORATIONBPM FRICTION LOSSACROSS

A PERIH)[ATION

Fig. 2-Nomogrnph for determining friction !OSS through a perforation (courtesy of Halliburton Servkes).

The effest of the propping agent on the treatment &signis minor, and it has been omitted for purposes of ilhsstra-tion, Actually, if the treatment is designed without consid-ering the propping agent, the actual treatment should bemore effective than the designed treatment. The sand USU-

ally will result in an increase in bottom-hole treating pres-sure without a corresponding increase in surface treatingpressure at the designed rate, Also, a higher surface-treatingpressure will occur prior to the actual treatment whiledetermining the number of perforations open with fracturewater only. Therefore, consideration must be given to de-termining the pressures to be expected without the sand.

The use of relatively high (0.95) perforation coefficientin treatment design usually results in the calculation of themaximum treating rate (thus surface pressure) necessary totreat a given number of perforations at a desired perfora-tion differential. lf the perforation coefficient used in thedesign is less than actual, the calculated treating rate willnot produce the desired perforation differential.

From service company friction charts: pipe friction as-suming a 6,000-ft completion is determined to be approxi-mately 1,750 psi. This is based on fresh water containing2.5 lb of synthetic polymer friction reducer per 1,000 galand a rate of 54 bbl/minute.

Substitution into Eq. 1 results in a calculated surfacetreating pressure of 3,650 psi. This is within the casingpressure limitations. Therefore, the diameter and numberof selected perforations and the calculated treating rate aresatisfactory. If the surface treating pressure had been above4,000 psi (considered to be the maximum safe internalyield pressure of the casing), the diameter or number ofperforations would be modified to result in a satisfactorysurface-treating pressure and fracturing rate. Additionalfriction reducer also could be used. A perforation differen-tial of approximately 300 psi would be maintained. A satis-factory number of perforations to provide adequate treat-ment coverage would be retained. If this were impossible,a second stage would be employed.

PerforatingPerforating plays an important role in successfully ap-

plying the technique. Because a relatively small number ofperforations are employed. it is necessary and basic to theapplication of the technique to have all perforations, or atleast a majority, open to accept fluid prior to pumpingaand, This is accomplished in two steps, The first is theuse of the best perforating charges available for this typeof completion. The second step is the use of acid and balls.(This is discussed in detail in the Procedure section.) Todate, the most successful jobs, in terms of average produc-tivity increase above that of offset completions, were thoseperformed with at least 80 per cent of the total perfora-tions calculated open to accept fluid. Two pseudolimitedentry completions were perforated with bullets (0,63-in.hole size) and another was perforated with a burr-freeshaped charge (0.57-irr. hole size). These wells and most ofthe other successful completions in which a high percentageof the holes were open prior to fracturing were perforatedwith cased carrier charges designed to create hole sisesgreater than 0.55 in. in diameter.

Good ball sealing action is imperative for opening allperforations prior to the actual treatment. Therefore, theburr-free characteristics and consistent hole sizes obtainedwith cased carrier charges are very important. The con-sistency of the hole size obtained with these charges alsocontributes to the reliable calculation of the number of-.

—, perforations open. In addition, when large holes are per-= forated fewer perforations are necessary to provide the de-

sired perfor tion dfierential. Therefore, fewer balls are

.

needed, the number of holes to be opened is reduced, thetreatment is concentrated, the poasibili~~ of ball sealerleakage is reduced and application of the technique is sim-plified. For these reasons, the cased-carrier, large-hole,burr-free type of perforating charge is preferred.

Procedurefor ApplicationFollowing is the usual procedure used for application of

the pseudolimited entry technique.

F?rst Stage Procedure

1. Displace the hole with fracture water. Spot HCL acidacross the interval to be perforated.

Comment — This lowers pipe friction and facilitatesbreakdown.

2. Perforate.3. Run tubing with a full-opening retrievable packer.

Comment — The tubing and packer permit better con-trol of acid, as used in Step 4, and maximum pressures inStep 5.

4. Set the packer above the perforated interval; matrixacidize with mud acid.

Comment — Matrix acidizing facilitates the opening ofadditional perforations by ball sealers (Step 5). It is doneat low rates and pressures necessary to prevent prematurefracturing prior to maximum perforation coverage.

5. Drop nylon core rubber-covered balls in groups oftwo or three with a fracture water spacer between eachgroup. Drop balls until the perforations ball off at themaximum safe tubing pressure. Hold this pressure on thetubing to allow additional perforations to open. If thisoccurs, reball off to the above pressure until no furtherpressure breaks occur. (lf ball-off occurs before as manyballs as there are perforations have reached the perforatedinterval and no additional break is seen, it may be neces-sary to drop balls in an acid spacer to facilitate additionalbreakdown.) Knock off balls by lowering packer throughthe perforations.

Comment — Significant pressure breaks (perforationsopened) have been observed to occur above 5,0Q0 psi.Therefore. packer, tubing and maximum pressures are be-lieved essential for optimum success. Holding pressure andreballing off is done to insure that the maximum numberof perforations are open and will accept fluid. The nyloncore diameter of the ball sealers must exceed that of theperforations. Attempting this step down casing has notproved satisfactory for the multiple pay lower Dakota(first stage).

6. Leave HCL acid spotted across the perforation pullout of hole,

Comment — HCL left over perforations should preventobserved plugging of perforations (by undissolved solids)prior to fracturing.

7, Using fracture water only, obtain a maximum stabi-lized rate and premure for 1 minute, then shut down andread the lSIP. calculate the perforation differential, andthus the number of” perforations taking fluid. If the de-signed parameters are obtained, or if a majority of theperforations are taking fluid so that adequate treatmentcoverage and a trouble-free stage can be expected, proceedwith the treatment,

Comment — A maximum stabilized rate and pressureshould be obtained for approximately 1 minute to insurethat no further breakdown is occurring, and that the LVPis obtained under consistent conditions.

8. Set a cast ‘iron bridge plug.

* .

Second Stage Procedure

I. Perforate.2. Establish a maximum stabilized rate and pressure

by pumping down the casing using fracture water. Obtainthe ISIP. Calculate the perforations open. If the designparameters are satisfactorily met, proceed with the treat-ment.

Comment — Because. of the generally lower fracturingpressure, the smaller number and increased porosity of theupper Dakota sands, fewer perforation opening problemsare encountered. This and the increase in costs associatedwith the use of the tubing and packer preclude the runningof them. If necessary, acid or balls, or both, are pumpeddown the casing prior to the second stage as described underStep 5. However, beeause balls are used through theeasing, the associated pressures necessarily will be lower.

In summary, the pseudolimited entry procedure is basedon having a satisfactory number of perforations open toaeeept the fracturing fluid prior to pumping sarid, By doingso, adequate treatment coverage is virtually assured. Thereis no dependence on ball sealer action during the actualtreatment.

Results

To evaluate the result of the pseudolimited entry teeh-nique, initial Dakota well productivityy (in terms of AOFP)was compared with the average of the conventionallytreated offset Dakota wells.

The initial application of the technique was made dur-ing the seeond half of 1965 on the last three wells of aneight-well Dakota program at the Angel’s Peak area, lo-cated in the central portion of the San Juan basin. Thethree wells have an average AOFP of 16,900 Mcf/ D (Fig.3). This is an 89 percent increase in AOFP over theaverage of the nine offset wells. The average AOFP ofthe remaining five wells in the program completed withconventional two-stage treatments is only 29 percent higherthan that of the 15 offset completions,

The other area of concentrated application of thepseudolimited entry technique was Blanco Canyon, In this

I

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Fig 3-Angef’s Peak area completion results

area, approximately 6 miles east of the Angel’s Peak area,an 11-well Dakota development program was conducted&sring 1965. Three of the wells completed using the de-scribed technique were successful in that over 80 percentof the total perforations were calculated to bt: open. Theremaining wells had a majority of the perforations openprior to treatment such that all significant pays were indi.cated to have been treated. The three most sueeeasfulpseudolimited entry completions had an average AOFP73 percent higher than that of the 12 offset wells (Fig. 4).A similar comparison shows that the remaining threepseudolimited entry completions had a 59 percent increasein average AOFP over the 11 offset wells. Over all, thesix Blanco Canyon pseudolimited entry completions hadan average AOFP 65 percent higher than the 17 offsetwells. Constant-rate pressure drawdown testing’ of twopseudolimited entry completions in the Blanco Canyonarea indicated that large effective fracture systems wereestablished in both wells. The testing also indicated thatproductivity and recoveries could be expected to be aboveaverage.

During 1965 there were two other separate completionsusing the pseudolimited entry technique. In both cases theinitial productivities were exceptional.

The average Dakota AOFP obtained on the aforemen-tioned nine completions by using the pseudolimited entrytechnique was 78 percent, or over 4,000 Mcf/D above thatof the offset Dakota wells.

Conclusions

1. Definite productivityy increases, and thus allowabks,eiin be expected over that of Dakotii wells conventionallyfractured.

2. Due to cffeztivc treatment of all Dakota pays withthe pseudolimited entry tcehnique, it is reasonable to con-clude that such completions will have sustained produc.tivities and ultimate recoveries higher than conventionalcompletions.

3, Poor treatment coverage, low treating rates at highpressures and sand-outs practically have been eliminated.

‘~

-L—

—

—

Fig 4-Blanco Canyon area completion results

.

4. Costs are approximately equivalent to those of con-ventional Dakota treatments of the same A.

5. Multiple pays of approximately equal fracturingpressure can be stimulated effectively with one fracturingstage.

6. Some areas previously considered uneconomical nowcan be reconsidered for development.

7. This technique or a modification has application insimilar formations where mechanical or treatment cove~age problems are encountered in the fracturing phase ofa completion.

8, The pseudolimitcd entry technique is effective forDakota stimulation in the San Juan basin.

Acknowledgments

The authors wish to express their appreciation m themanagement of Temeco OiJ Co. for permission to publishthis paper, Special thanks are extended to R. H. Byers.R. E. Siverson, Joe Murray, Les Plumb, Jerry Lacey, Dar=rell Brown, Ken Dowel] and other Tenneco personnel fortheir help and encouragement in developing the pseudo-Iimited entry technique. The assistance provided by Dow-ell Div., Dow Chemical Co., Halliburton Services andWestern Co. during development of the technique also isappreciated.

References

1. Millheim, Keith and Cichowicz, Leo: “Testing and AnalyzingLow Permeabilityy Fractured Gas Wells”, J. Per. Tech. (Feb.,1968) 193-198.

2. Webster, K. R., Coins, W. C:, Jr., and Berry, S. C,: ‘“AContinuous Multistage Frsscturmg Technique”, J. Pet, Tech.(June, 196S ) 619-625.

3. Lagrone, K. W. and Rasmussen, 1. W.: “A New Develop-ment in Completion Methods—rhe Limited En[ry Tech-nique”. J. Pet. Tech. (July, 1963) 695-702.

4. Halliburton Services Technical Report: Limited Enrry jorHydraulic Fracturing, Bull. F.3077 (June, 1964).

5. Dowell Div. Technkal Report: Limited Entry Well Com-pletion Technique, (Sept.,1964 ).

6, 130well JXv.: Frac Gufde Data Book (1965).7. Western Co.: Engineered Limited Entry ( 1964). ***

L(tubsC Stipp (right) is district geological engineer inTenneco Oil Co.’s Thermal District, BakersjieId, Calif, Hereceived BS degrees in petroleum and geological engirreer-ing from Texas A&M f.1. in 1959. After working for ShellOil Co. man exploitation engineer in the Gulf Coast, Stippjoined Termeco in 1964 as a geoktgical engineer in Duran. “go, Co!o. He transferred to Houston in 1966 and servedos stafl enginee~ bejore moving 10 Bakersfield in April.1968. Stipp is a registered professional engineer in Texas.R. A. WMforiJ[Ie ‘t) is a 1956 graduate of Texas A&M U.with BS degrees in petroleum and geological engineering.He worked as a petroleum and reservoir engineer for GtdjOil Corp. before joining Tenneco in 1961 as a petroleumengineer. J+esendy district geological engineer in Durango,Colo., Willijord is a registered ptvjessional engineer inTexas.