Better Completion by Controlled Fracture Placement Limited-entry Technique

K. W. LAGRONE* AND J. W. RASMUSSEN* ABSTRACT

Shell Oil Company, in Texas and New Mexico, has experienced results f rom a n improved well-stimulation method called "limited-entry treatment." This t reat- ment is accomplished by: 1 , limiting the number of perforations in a well; and 2, providing injection rates sufficient to require the restricted flow capacity of the perforations to divert the treatment to a greater portion of the perforated interval. This method has proved more effective than any other in diverting treating fluids to nlultiple horizons.

From December 3, 1960, to January 1, 1962, Shell Oil Company has treated 201 wells by this technique. The initial procluction performance of wells treated by lim- ited-entry conlpletions is superior to t h a t of conven- tionally treated wells. Gamma ray tracer logs indicate most of the pay is being treated even though not cov- ered by perforations. The limited-entry technique. has been successfully used in treating two separate reser- voirs simultaneously in dually conlpleted wells. Results of these silnultaneous treatments have been gratifying both in well perforlnance and in reduced costs.

GENERAL REVIEW O F MULTIPLE-ZONE limited-entry treatnlents liad been performed in a vari- TREATMENT ety of reservoirs (see Fig. 1 ) . No mechanical difficulties

Conventional Treatment Techniques The efficient simultaneous treatment of multiple

porous intervals in a reservoir has been a long-standing problem in well stinlulation. Various methods have been used to t reat multiple zones, with greater o r lesser de- grees of effectiveness. The bridge plug and packer method is effective, but is relatively expensive. Futher, the injection rates a r e considerably reduced, and this method is sometilnes mecl~anically llazardous. Tem- porary plugging agents to divert the treatment have been used with apparent success. The main disadvantage of temporary plugging agents, such a s 1110th balls o r gel blocks, is the difficulty in determining the proper quan- tity of agent required to divert the treatment. Ball sealers a re often ineffective because of: 1 , fluid com- municating behind the casing between closely spaced perforations; 2 , failure of the balls to seat on the perforations; and J", abrasion of the ball sealers allow- ing fluid to bypass. Limited-entry Technique

Shell Oil Company, in Texas and New Mexico, has experienced excellent results from a n in~proved well- stilnulation method called "limited-entry treatment." Based on data obtained to date, this method i s much superior to the other methods of obtaining simultaneous treatment of multiple zones. The treatment is performed by: 1, limiting the number of perforations in a well; and 2, providing injection rates sufficient to require the restricted capacity of the perforations to divert the treatment to a greater portion of the perforated in- terval.

The first limited-entry treatment in this region was performed in Shell TXL M-3 (TXL-Tubb Field), Ector County, Texas, following a review of a paper by W. B. Murphy and A. H. Juch of Coinpania Shell de Vene- zuela.' From December 3, 1960, to January 1, 1962, 201 *Shell Oil Company, Midland. Texas. IReferencw are at the end of the paper.

have been encountered tha t can be attributed to this method of treatment. Of the treatments performed to date, 94 percent were successful, without "sandout." This is considered to be a norlnal success ratio. Of the 13 sandouts, only 6 required re-treatment to obtain top allowable production. Treatments have been successfully performed in carbonate, sandstone, conglomerate, and chert reservoirs. These reservoirs range in depth from 3,100 f t to 9,500 f t , with bottom-hole pressures varying from 1,000 psi to 3,600 psi.

BASIC THEORY O F FRACTURING PROCESS General

The basic objective of all well treatments is to get the best well simulation co~npatible with cost. To get a n effective treatment, i t is desirable to t reat a s nluch of the perforated interval a s possible. Also, the treatment should be proportioned into the perforated intervals. Both of these objectives can be better fulfilled by a properly designed limited-entry treatment than by con- ventional treatments. Conventional Treatment

The sinlultaneous treatment of multiple porous inter- vals by conventional methods is depicted in Fig. 2. Three zones with different bottom-hole fractur ing pres- sures a re opened up in the same well bore. The zone which offers the least resistance to fractur ing will take the treatment. This zone will continue to take the t reat- ment until a diverting method is utilized.

Limited-entry Treatment To t reat more than one porous interval, the bottom-

hole treating pressure must be raised above the fracture- initiation pressure of each successive zone. This can be accomplished by limiting the number and diameter of the perforations i n the casing. As seen in Fig. 3, the perforation friction pressure varies directly with the pumping rate through the perforation. Therefore, by

increasing the injection rate , the perforation friction will be increased. In other words, the perforations act a s individual bottom-hole chokes. They create a n in- crease in available bottom-hole casing pressure a s the injection rate is increased. The accompanying increase in pressure in the casing will then break down o r frac- tu re the next zone, a s indicated i n Fig. 4.

The process of breaking down each successive zone occurs rapidly, because inaxin~um pressure and rates a r e established early in the treatment. Assunling ade- quate injection rate a t the surface, this process is con- tinued until either all the perforated zones a r e being fracture-treated or the maximum permissible pressure on the casing is reached.

SPECIFIC FACTORS AFFECTING DESIGN OF LIMITED-ENTRY COMPLETIONS

Total Pay Treatment Best results a r e obtained by maintaining perforation

friction a t a maximum during treatment. This insures treatment of all perforated intervals tha t will accept fluid (within the permissible casing-pressure limita-

tions). I t is recognized tha t all the perforations could be treated a t a lesser injection rate. However, this would not be t rue if the bottom-hole fractur ing pressure of the individual porous members varies to any degree. Therefore, to have the most assurance tha t all zones a r e being treated, a n injection rate tha t will give a inasimunl pern~issible casing pressure is necessary.

Small-diameter perforations a r e preferred in limited- entry treatments to: 1 , increase perforation friction; and 2, lower hydraulic horsepower requirements. Fig. 3 shows that, fo r the same perforation friction, approsi- mately twice a s much fluid can be injected through a %-in. hole a s through a ?&-in. hole. Therefore, by using the smaller perforations, less hydraulic horsepower is required to deliver a n injection ra te adequate to main- ta in a inaxiinunl perforation friction. Few difficulties have been encountered to date in fracture-treating through K-in. jet perforations.

Experiments have been performed by S tekol lhnc l Halliburton Co. where a variety of treating fluids with sand were pumiped through %-in. and %-in. perfora-

-

0

ZONE A 4 2 0 0 pst ZONE A 4 2 0 0 p s 1

BEFORE TREATMENT START FRACTURE 8

fn 1 S T E P 3

CONTINUE TREATMENT AT FRACTURE PRES- SURE OF ZONE B

STEP 4

TEMPORARILY PLUG ZONE B AND T R E A T Z O N E C, ETC.

ZONE B 3 8 0 0 p s i NOTE

PRESSURE I N EACH ZONE C 4 0 0 0 p s i ZONE EQUALS BOTTOM

HOLE FRACTURE PRES- F IN ISHED T R E A T M E N T SURE

Fig. 2 - Fracturing Process - Conventional Treatment

tions. During the tests, small irregularities in the per- forations were quickly smoothed out and the perfora- tions altered from sharp-edged to round-edged orifices. The hole diameters, however, remained essentially un- changed within the normal pumping times of a fractur- ing treatment. Proportioning of Treatment

Limited-entry treatments can be designed so tha t the desired anlount of fluids will be injected into each porous zone. This is a n inlportant advantage where thick zones,

Fig. 3 - Flow Rate vs. Friction Loss Laboratory Measured (Halliburton)

which require more treatment, a r e treated i n conjunc- tion with thin zones. It is assun~ecl t h a t each perfora- tion will accept approximately the same amount of fluid. Therefore, by proportioning the nuinber of per- forations according to the thickness of the zone, each zone will be given the desired anlount of treatment.

A word of caution - the foregoing nlethod of propor- tioning fluids through perforations depencls upon the bottom-hole fractur ing pressures being similar. Where i t is recognized tha t considerable variation exists in the bottom-hole fractur ing presures of the zones, the treatment design should be altered. The zone with the lowest bottom-hole fractur ing pressure would norinally receive the inost treatment per perforation. Therefore, the number and /or size of the perforations should be

0-2 ZONE A 4 2 0 0 p s i

0-:: ZONE B 3 8 0 0 p s i

0- ZONE C 4 0 0 0 p s i

BEFORE TREATMENT .- rn

111 1 S T E P 4 I o-IP ZONE A 4 2 0 0 p s i ZONE B 3 8 0 0 psi lo--

ZONE C 4 0 0 0 p s i lo* - LARGER RATE = INCREASED . PRESSURE

0

ZONE A 4200 p s i

0- ZONE 8 3 8 0 0 p s i

ZONE C 4 0 0 0 p s i START FRACTURE AND FINISHED TREATMENT BUILD UP PRESSURE-

= BREAK ZONE 8

S T E P '3 NOTE -

PRESSURE I N EACH ZONE. EQUALS BOTTOM HOLE FRACTURE PRESSURE

LARGER RATE = INCREASED PRESSURE Fig. 4 - Fracturing Process - Limited-entry

Treatment reduced in this zone. I n the zone with the highest bottom-hole fractur ing pressure, the converse would be true.

DESIGN OF A LIMITED-ENTRY TREATMENT A s stated before, the main reason for limiting the

number of perforations is to maintain control of the placement of the fractur ing fluids. Therefore, i t is im- portant to know the number of perforations to use f o r a desired injection ra te t o obtain maximum perfora- tion friction.

Fig. 5 - Aid to Limited-entry Design

The equation for perforation friction is: P,, = P, - ZSIP - P, (1)

Wi~ercin : P, = surface injection pressure, psi.

ZSZP = instantaneous shut-in pressure, psi. Pf = casing o r tubing friction loss, psi.

This equation was derived by substitution in the follow- ing equations :

BHFP = P, + P,, - Pf - P,, (2 ) BHFP = ZSZP + P, (3) TVI~erei?z :

BHFP = bottom-hole fractur ing pressure, psi. Ph = hydrostatic pressure, psi.

A s a n aid to the design and analysis of a limited-entry treatment, Fig. 5 has been prepared. Perforation fric- tion pressure from Fig. 3 and average treating condi- tions were assumed. The bottom-hole fractur ing pres- sure used is 0.65 psi per f t . This has been obtained by averaging the bottom-hole pressures of various forma- tions in the Permian Basin. Few horizons deviated ~nater ial ly fro111 this average figure. These assunlptions have been made so tha t the number of holes accepting

treatnlent can be determined by the observed surface treating pressures.

Fig. 6 shows a comparison between the design of a limited-entry completion vs. a conventional completion. This well has 5%-in. casing cemented through lnultiple

1 4 6 P E R F O R A T I O N S 10 P E R F O R A T I O N S 73 F E E T PAY U " 1 1 U 73 F E E T PA!

Fig. 6 - Comparison of Completion Design Limited Entry vs. Conventional

porous zones. I n the limited-entry design, 10 holes were distributed into the various porous n~embers to t rea t all of the pay and to properly proportion the treatment. Using Fig. 5 with 3,600 psi surface treating pressure (the pressure limitation on the casing) and %-in. jet perforations, the expected injection rate would be 2.85 bbl per inin per hole. Therefore, if a n injection ra te of 28.5 bbl per min were obtained a t a surface treating pressure of 3,600 psi, all 10 perforations should be tak- ing treatment. In the conventional completion a s shown, with two perforations per foot of pay, any one zone could accept all of the treatment unless diverting agents were successfully used.

Treatment Analysis from Field Data The limited-entry technique provides field data tha t

can be used to determine the number of intervals tha t were treated. If this analysis indicates tha t all zones a r e not being treated, the conlpletion design can be altered. In any other method i t would be difficult, if not impos- sible, to recognize tha t a change in completion design is warranted.

The three essentials necessary to determine the num- ber of perforations accepting fluid a re : 1 , accurate in- jection rates; 2, accurate surface injection pressures; and 3, and instantaneous shut-in pressure ( ISIP) a t the beginning of the job. Injection rates obtained by averag- ing over prolonged periods of the treatment a r e not generally adequate for this method. A continuous ra te recorder is considered most helpful. Based upon experi- ence, i t is necessary to use the instantaneous shut-in pressure a t the s ta r t of the treatment. This is rec~uired to obtain a perforation friction pressure tha t will agree with the laboratory-measurecl data.

I f a perforation friction calculation is to be made while a sand-oil mixture is being injected into the forma- tion, the instantaneous shut-in pressure a s measured a t the surface must be corrected for the change in hydro- static pressure resulting from the addition of sand. A sample calculation of the number of perforations taking treatment is given in the Appendix.

OPERATIONAL TECHNIQUES Opening Perforations Prior t o Treatment

The major difficulty tha t has been encountered in limited-entry treatment has been insuring tha t all holes a r e open prior to the fractur ing treatments. Seldom a r e all of the perforations able to accept fluids without being aciclized. I t is believed this problem exists with conven- tional completions, but usually remains unnoticed. Where the number of perforations a r e greatly limited, it becomes obvious if any a r e not open to the formation.

An acidizing technique has been adopted tha t is only practical f o r limited-entry coinpletions. The procedure involves staging the acid in small slugs separated by a maximum of two ball sealers in a n oil spacer. The num- ber of stages is determined by the number of ball sealers required to "ball out" the perforations. This allows a better estimate of the number of perforations tha t a r e open a t the end of the acid treatment. After "ball-out"

occurs pressure is held on the remaining acid in t h e casing to provide every opportunity fo r additional per- forations to be opened. Fracturing Treatment

Limited-entry fractur ing treatments have been per- forined with injection pressures, rates, treating-fluid types, and volumes similar to those of conventional treat- ments. Soinetimes i t is not desired or possible to have injection rates sufficient to insure treatment of all the perforations. In this case ball sealers can be effectively used a s a diverting agent. Experience indicates ball- sealer action to be 100 percent effective in limited- entry treatments. This may be the result of higher injection rate per hole and greater separation between perforations. Ext ra precaution should be taken to avoid excessive pressure surges caused by the excellent ball- sealer action.

Individual perforations sonletiines sand out during treatment. A decrease in injection ra te is indicative of the time and number of perforations affected when sand-out occurs. A continuous-rate recorder is necessary for observing the loss of perforations taking treatment. I t is most helpful in determining the proper number of ball sealers to drop during the job.

A COMPARISON O F LIMITED-ENTRY VS. CONVENTIONAL COMPLETIONS

The following data from wells in the TXL-Tubb Field (Lower Clearfork formation) a re offered a s a compari- son of initial performance of similar wells in a n a rea where the comparison is available. The wells a r e com- parable in the feet of pay developed, the size of f ractur- ing treatments, and the expected ultimate recoveries.

Table 1 is a tabulation of all Shell TSL-Tubb wells in which pressure-buildup tests have been taken. The data a r e considered to be of good quality because of the exceptionally good pressure-buildup curves.

A s indicated on Table 1, the average KIL (pern~ea- bility-feet) was measured a f te r completion by conven- tional methods to be 41 md-ft. In the wells t h a t were completed by the limited-entry technique, the average KIL n~easuren~ent was 854 md-ft. This represents a n increase of 6.2 times greater fo r the limited-entry completions. Productivity index, a s calculated from pres- sure-buildup data, has been increased a n average of 4.3 times over tha t of conventionally treated wells. Specific productivity index ( P I per net feet pay) has been increased a n average of 4.4 times. The average official potential test (OPT) has been 31 BOPD higher fo r the limited-entry completioi~s. It is to be pointed out that the foregoing PI data a r e based upon initial meas- urements taken shortly a f te r the recovery of all the load oil. However, if the P I measurements remain higher during the lives of the limited-entry conlpleted wells, a n increase in their ultimate recoveries would be expected.

Table 1 A Comparison of Initial Well Performance Limited-entry Techniques vs. Conventional Treatment, TXL-Tubb Field - Lower Clear Fork Formation

Ector County, Texas Fracture Treatment Potential Test Productivity Indices Data

Number/- - A ---- .A > Lease and Net Perfora- Ball Specific Well No. P a y tions Oil, Gal Sand, Lb Sealers BOPD BWPD Choke K H I

AND J.

GAMMA RAY SONIC

GAMMA RAY LOG BEFORE TREATMENT - GAMMA RAY LOG AFTER TREATMENT 0- SINGLE PERFORATIONS

NET PAY

Fig. 8 - Simultaneous Treatment of Separate Horizons by Limited-entry Treatment

El Cinco Detrital and Devonian Fields, Crockett County, Texas

than that of the other, allowing the zone with the lower bottom-hole fracturing pressure to accept more of the treatment than was originally anticipated [see deriva- tion of Equation ( I ) ] . These problenls can be solved by recognizing that they exist and by varying the design of the treatment. The well was conlpleted a s a dual producer flowing 392 and 538 BOPD froill the El Cinco Detrital and Devonian zones, respectively. Even though the performance of this well is good, the information obtained indicates that future remedial operations call be justified. With the combination of the limited-entry designed completion and the radioactive tracer log, future remedial operations are greatly simplified.

With limited-entry treatment, close proximity of two formations is not necessarily required for a successful simultaneous treatment. An example of this is shown in Fig. 9. The Tubb (LCF) and the Devonian horizons, separated by about 1,350 f t , were successfully treated by fracturing in one operation in the Shell TXL L-26, TXL Field, Ector County, Texas. Potential tests showed the Tubb (LCF) and the Devonian zones flowing 132 and 435 BOPD, respectively.

GAMMA RAY SONIC

L E G E N D --- - GAMMA RAY LOG BEFORE TREATMENT

- GAMMA RAY LOG AFTER TREATMENT SINGLE PERFORATIONS N E T PAY

Fig. 9 - Simultaneous Treatment of Separa Horizons by Limited-entry Treatment

TXL - Tubb (LCF) and Devonian Fields, Ector County, Texas

CONCLUSIONS 1 6. The limited-entry technique is not devoid of prob- 1. Lim~ted-entry treatments have proved more effec-

tive than other nlethods in diverting fluids to multiple horizons. No mechanical failures have occurred t h a t can be attributed to this technique.

2. Performance to date of limited-entry con~pletions is superior to tha t of conventionally treated wells.

3. The number of perforations accepting fluid a t any - -

time during a treatment can be determined by calcula- tlons made from field observations. I n order to estimate the proportion of the treatment received by the various perforations, a continuous injection-rate recorder is desirable.

4. Gamma r a y t racer surveys of radioactive sand used during fractur ing treatments have provided a graphical record of: 1, the effectiveness of the limited- entry technique in diverting the t reatment; and 2 , the amount of the porous interval treated through one perforation.

5. The sinlultaneous treatment of dual horizons offers great potential saving in conlpletion costs.

lems. Sometimes portions of the pay remain untreated. However, by the use of the information gathered during the treatment, this problem can be recognized and 1111- provenlents in design can be made. In any method other than the limited-entry technique, ~t would be clifficult, if not impossible, to recognize tha t pay intervals a r e being left untreated.

ACKNOWLEDGMENT The authors wish to express appreciation to the

management of Shell Oil Company for permission to publish this paper, and to the Halliburton Company for laboratory measurements of fluid-flow characteristics through single perforations.

REFERENCES 'Murphy, W. B. and Juch, A. H: Pin-point Sand-

fractur ing - A Method of Sin~ultaneous Injection into Selected Sands, J. Petr . Tech. 12 Clll 21, Nov. (1960).

2Stekol1, Marion H : New Light on Fracturing through Perforations, Oil Gas J., 95, Oct. 9 (1956).

APPENDIX SAMPLE CALCULATION OF DESIGN AND ANALYSIS OF A

LIMITED-ENTRY TREATMENT Sample Calculation of Design I casing with a maximum permissible casing pressure - Data

Well number: TXL K-18, TXL-Wolfcaml) Field, Ector County, Texas.

Casing: 7-in., 23-lb. Gross pay interval : 7,509-7,682 f t . Net pay: 63 f t . This well is to be treated by fractur ing down 7-in.

of 3,600 psi. From Fig. 5 a surface treating pres- sure of 3,600 psi gives a n injection rate per perfora- tion of 3.1 bbl ver min f o r a 3/ir-in. hole- 7-in. casing. Based upon the pay distribution a s shown by the porosity log (Fig. 7 ) , a total of 9 holes was chosen to effectively proportion the treatment over the pay interval. Therefore, the expected injection rate would be:

(3.1 bbl per I ? L ~ ? L per perforation) (9 11erforc1,tions) = 28 bbl per m.in at 3,600 psi The calculated injection ra te was acceptable. If this

injection ra te had been undesirable, the number and placement of the perforations would have been reviewed.

Sample Calculation of Treatment Analysis Data

Perforations: top 7,509 f t , bottom 7,656 f t ; average depth, 7,550' f t , holes 9.

Breakdown fluid: oil, 36 A P I gravity = 0.365 psi per f t .

Fracturing fluid: oil + 1% Ib per gal sand = 0.415 psi per f t .

Surface treating pressure (P , ) : 3,600 psi.

Injection rate: 31 bbl per min. Instantaneous shut-in pressure, surface (ZSIP) :

1,700 psi. Casing friction (P , ) a t 31 bbl per min a t 7,550-ft

depth: 315 psi. This well was treated by fractur ing down 7-in. casing

through nine %-in. perforations. Instantaneous shut-in pressure of 1,700 psi was measured during breakdown of formation wlth lease crude. The fol- lowing calculation was made from data obtained while fracturing with lease crude and sand. There- fore, i t is necessary to correct ZSIP fo r increase in hydrostatic pressure resulting from add~t ion of sand a s follows :

[ZSIP surfa,ce - ( f r a c t ~ ~ r i n g fluid, psi per ft - breakdown fluid, psi per f t ) ] [rnverc~ge depth] [ZSZP (corrected)] = [l ,700 psi - (0.415 - 0.3651 [7,550] = 1,320 psi

Perforation friction (Ppf) = 3,600 - 1320 - 315 P,, = P , - ZSIP - P , PPf = 1,965 psi

190 I(. W. LAGRONE A N D J. W. RASMUSSEN

I From Flg. 3 the injection ra te per perforation, barrels

per minute, a t a perforation friction of 1,965 psi is 3.6 bbl per n ~ i n per perforation. Therefore, the theoretical rate through all perforat~ons would be: 9 S 3.6 = 32 bbl per w~i?~.

This compares favorably with the observed injection rate of 31 bbl per mln or 3.45 bbl per nun per perforation. From this comparison, i t is concluded that all 9 holes

were treated. See Fig. 7 fo r confirmation of t reat- ment analysis.

DISCUSSION Morton C. Roman (The Atlantic Refining Co., Mid-

land, Texas): After the various perforated intervals a r e broken down, do you feel they a r e all accepting fracturing fluid a t the same rate?

Mr. Lagrone: The rate a t which the perforations will accept fluid 1s in relation to their individual bottom- hole fracturing pressures. If their hottonl-hole fracture pressures vary to any great degree, there wlll be some difference in the rate a t which the individual perfora- tions take treatment. If however, the individual bottom- hole fracturing pressures a r e known, these differences can be compensated for by the treatment design to properly proportion the treatment.

E. G. Hayes (Humble Oil & Refining Co., Midland, Texas) : I wonder if you have a feel fo r the possibly minimum perforation hole sizes tha t would be desirable - 114-ln. diameter o r possibly 3116-in.?

Mr. Lagrone: We have used %-in. perforations be- cause it has been found t h a t the number of holes can generally be llmltecl to t rea t all of the pay interval while maintaining a maximum perforation friction dur- lng treatment with ~njection rates and pressures similar

to our past operations. Your minin~um would probably he what your foreman could stand while he was t reat ing the well. We have used some smaller jets, which reduced the ~njection rate to about a barrel per m ~ n u t e per hole. We treated satisfactorily. We fincl that by using %-in. perforations, we can quite easily limit our perforations to around 15 which limlts the injectloll ra te to some- where around 30 or 40 bbl per n ~ i n down a 7-in. casing.

Jack Hellinghausen (The Atlantic Refining Co., Dallas, Texas) : I notice you often use 7-in. casing. W a s it necessary to use thls rather large size to get the desired rate, o r did you have another reason f o r using 7-in. casing?

Mr. Lagrone: The reason we had 7-in. in this par- ticular case was tha t the example we used was from the T S L F ~ e l d , which were dual completions and ~t is our normal practice there to use 7-ln. casing.

Mr. Hellinghausen : You can get the rates f o r smaller- size casing? I i Mr. Lagrone: Jn our C r o s ~ e t t Field, we use 4%- anrl 5%-in. casing. Generally, we t r y to limit perforations to about 10, which gives us a n Injection rate of around 25 bbl per min. In our 2%-in. con~pletlons, we tried to limit our perforat~ons to a n order of 5.