An Engineering Study of the Magnolia Field in Arkansas

Transcription

1 AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS Technical Publication No (CLASS G. PETROLEUM DIVISION. NO. 168) DISCUSSION OF THIS PAPER IS INVITED. Discussion in writing (z copies) may be sent to the Secretary, American Institute of Mining and MetaUurgical Engineers. 29 West 39th Street. New York. N. Y. Unless s ecial arrangement is made, discussion of this paper will close Dec. I Any discussion offered thereafter sfould preferably be in the form of a new paper. An Engineering Study of the Magnolia Field in Arkansas TKE history, development, subsurface geology, production, economics and estimated reserves are discussed in this paper. The Magnolia structure is an anticline with a known maximum structural relief at 372 ft., and the areal extent of the producing acreage is 4494 acres. Production is obtained from the Smackover oijlitic lime and 116 successful wells have been drilled. The field, which was developed on 40-acre spacing, has been under strict control. It is believed to be producing with a strong water drive. To date it has produced more than eighteen million barrels and the estimate of ultimate recovery is 220 million barrels. (New York Meeting. February 1942) The Magnolia field is in Columbia County, Arkansas, 'T. 17 S., R. 19 and, 20 Ur. Various geologists had long been interested in this general area, and as early as 1921, John F. Magale was able to define a surface structure. The first wildcat, one of the first in Columbia County, was drilled in 1923 by Mid-States Oil Co., in sec. 10, T. 17 S., R. 20 W. This well was about 1)4 miles northwest of the Kerlyn Oil Company's discovery well and was based on surface geological work done by Jewel and Dobie. Subsequently, several shallow wells that were not carried below the Upper Cretaceous beds were drilled in the immediate area of the discovery well, but none of them contained any commercial showings of oil or gas. In the latter part of 1935, Elam and Manuscript received at the office of the Institute Fgb. 9, Production Department. Texas-Gulf Area, Shell Oil Co.. White Castle, La. Magale (or Southwood) Garrett No. I was located in sec. 13, T. 17 S., R. 20 W., 3i mile east of the discovery well. It was drilled to a total depth of 4018 ft. in the lowermost Glen Rose beds but failed to obtain any showings of oil or gas. Interest had been stimulated in this general area, however, by the discovery of several oil. fields, mainly Glen Rose oil ~roduction in the Rodessa field in Louisiana in 1935, Smackover lime production in Snow Hill in May 1936, production from the Cotton Valley formation in Schuler in 1937, and Smackover lime production from the Buckner field in November With this background and subsequent seismic work, which was carried on partly because the old shallow wells drilled in the township had shown evidence of closure in the Upper Cretaceous beds, the discovery well, Kerlyn Oil Company's Barnett No. I, sec. 14, T. I ~S., R. 20 W., was originally projected to a total depth of 6300 ft., to test the equivalents of the Morgan sands of the Cotton Valley series, then producing at Schuler. The well was carried to a total depth of 6325 ft. and tested the objective beds without finding commercial production. In the meantime, however, one well had found production in the Jones sand and another in the Smackover lime at Schuler. Kerlyn immediately made arrangements to deepen the Barnett well to the Smackover lime. The top of the Smackover lime was encountered at 7618 ft. and the well was cored and drilled to a depth of 7740 ft. Copyright. 1942; by the American Institute of Mining and Metallurgical Engineers, Inc. PETROLEUM TECHNOLOGY. September Printed in U. S. A.

2 2 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS Showings of oil and gas were encountered in porous oolitic lime from 7647 to 7652 and from 7664 to After somc difficulties in completing the well, some production was obtained during March 1938, and the well finally was completed on Apr. 2, 1938, producing 298 bbl. per day on a x-in.'choke. Further activity at Magnolia was held in abeyance until the completion of another Smackover lime test 6 miles to the east, which later proved to be the discovery well of the Village field. This well was completed June 26, 1938, and added considerable stimulus to the development of the whole area. Three wells-atlantic-harrington No. I, Atlantic-Baker No. I and Petroleum Finance-Baker No. I-were all started in the latter part of July, located in a straight east-west line just north of the discovery well on 40-acre locations. Just prior to the starting of these wells, the Atlantic Refining Co. had acquired the Magnolia 13scovery well and all of the holdings of the Kerlyn Oil Co. Shortly thereafter, Grady Vaughn started his Nannie Garrett No. I, a southeast offset location to the discovery well. All four of these wells were carried to the Smackover lime, but the first three were dry holes and only the fourth one was a good producer. The Vaughn well encountered the top of the Smackover lime I 27 f t. higher than the discovery well, whereas the other three wells encountered the Smackover lime from 58 to 83 ft. lower. The northern productive limits of the field, therefore, were immediately established and the structure was proved to be rising to the south. After this, orderly and uniform development, on 40-acre spacing, was continued in east, south and westerly directions. During the remainder of 1938, devdopment in the Magnolia field was confined to immediate offsets to the two producers, with six wells producing at the end of the year. By that time the lease owners had sufficient justification to embark upon a drilling campaign, and development was stepped up considerably. At one time, 15 strings of tools were in operation in the field. During July and August of 1939, the peak rate of development was attained, 12 w.ells being completed during July and 14 during August. For the most part, the development was limited to offset locations or stepping out only one location, but during June of 1939 a 3000-ft. outstep to the east and a 5000-ft. outstepping well to the west were drilled. Each of these wells encountered the top of the Smackover lime at points higher than generally expected and proved that the field was much larger than anticipated. The development of the intervening territory was carried on at a rapid pace, and 87 wells were producing by the end of From this time onward, development slackened somewhat but by the end of January 1941 there were 17 operators in the field and 116 wells had been completed. The total number of wells drilled in the field, including dry holes, is 124. It is notable that the eight dry holes drilled on the Magnolia structure are all very close to the productive limits of the field and consequently have served to establish these limits accurately. The orderly and even development of the Magnolia field can be attributed to two factors, both of which were brought about through the control exercised by the Arkansas Oil and Gas Commission. As soon as the field was discovered, this body set up rules for development and prevailed upon the operators to adhere to them, which resulted in the 40-acre spacing of the field. This fact, in turn, caused each well to step out at least )d mile, which was a limiting factor on the prudent operators to await the time until development had approached their leases.

3 The Magnolia field was drilled, with but few exceptions, to the 40-acre spacing pattern, the wells being located in the approximate center of the drilling units. The few exceptions not drilled in the center of a 40-acre spacing were permitted by the Oil and Gas Commission only after open hearings at which all facts and requests were presented for consideration. This body is to be commended on its diligence in insisting that the operators adhere to the prescribed spacing scheme. The small tracts scattered throughout the producing area were unitized into 40-acre drilling units, thus permitting all owners of acreage in the producing area to recover an equitable share of the oil produced. The rules of the Arkansas Oil and Gas Commission also provided that three strings of casing be set in all wells. These were required to be a surface string of not less than IOO ft.; an intermediate string, set in hard chalk at about 2200 ft.; and a producing string set on top of or through the producing horizon. Two different casing programs were utilized by the various operators: the larger called for a 16-in. surface casing, 10%-in. chalk string and a 7-in. oil string and the smaller one utilized 1396-in. and 538 or 6-in. casing. During the early stages of development the larger design was the more popular, but as soon as the reservoir characteristics had been adequately determined the smaller program was used in almost every case in the interest of economy. As soon as rapid development commenced, it was apparent to those engaged in the supervision of drilling that accurate data must be secured as soon as possible concerning the gas-oil contact and the water-oil contact of the main producing reservoir. The great majority of the wells were cored from the top of the producing horizon to the limits of their penetration. A gas cap was discovered as soon as development had progressed far enough up-structure to penetrate it. The gas-oil contact was determined by coring and drill-stem testing to be 7140 ft. subsea. This point has been coniirmed by almost every well drilled through the gas cap into the main oil-saturated zone. Variations from this point will be discussed 1ater.under Subsurface. ' The water-oil contact, however, remained rather obscure, owing to mechanical difficulties in the discovery well and, although originally reported at 7348 ft. subsea, considerable doubt was later thrown upon this fgure because of a possible crooked hole and reported hole-depth corrections. It was not until May 1939 that the oil-water contact in the main body of the field was penetrated, drill-stem tested and the contact accurately determined to be 7318 ft. subsea. This point has been fairly well established throughout the field except for a small portion on the southwest flank, which also will be discussed under.subsurface. With the establishment of the gas-oil contact and the water level, the completion of the wells- became more or less routine. Wells were completed both by cementing casing through the gas-oil contact with open hole below or by drilling to total depth, setting casing on bottom and gunperforating the producing intervals. Owing to the fact that the pain oil column is approximately 180 ft. thick, with a gas cap having a maximum thickness of 120 ft. on the crest of the structure, some operators were reluctant to drill the wells close to the water level when they could secure good producing wells with shallower penetration. However, a considerable number of wells were drilled to a safe penetration (10 to 15 ft.) above the-water-oil contact. Cement jobs were successful in shutting off the gas from the gas cap and in only a few wells was it necessary to squeeze ce-.merit after they were brought into production in order to repair the ihutoff because of high gas-oil ratio. The Oil and Gas Commission again displayed its strict con-

4 4 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS trol by testing each well upon completion, plotted on the same strip log, it became to determine that the gas-oil ratio was evident that, although the producing below the allowed limit of 2000 CU. ft. formation as a whole has a thickness of per barrel and, in all cases where this figure porosity in excess of the goo-ft. combined Permcabll1h~-Mlllidarcyr -Horij. 6 Ved. low 1500 low 5m 0 %Effective Porosity - Hori3. ( Vert I was exceeded, the operator was required to repair the well before an allowable was assigned to it. It was suspected from the very start that the Magnolia field would produce by means of an active water drive and, although it was necessary to study the pool data for some time in order to prove this point, preparations were made in many of the wells to take advantage of the hydraulic energy. As shown in Fig. I, the wells were cored and tested thoroughly, the cores analyzed in the laboratory and results of the porosity and permeability analysis plotted on the strip log. In addition to this, accurate drilling time was observed, so that in the event of missed cores a reasonably accurate estimate could be made of the relative porosity and permea-p bility of the formations penetrated. After the information listed above had been gas cap and oil column, the impervious streaks that occur within this goo-ft. interval are of ' sufficient magnitude to prevent the vertical migration of the oil. As yet, correlation of the impervious streaks observed in the individual wells has not been established for any reasonable distance. With these points in mind, an interesting procedure was utilized in some wells as follows: For wells that had casing cemented through the formation, perforating intervals were selected, so that each individual streak of permeability would be open to production. If the streak was relatively thin, it was perforated throughout its entirety. However, if the vertical permeabilities indicated that the streak had considerable thickness, approximately the upper 10 ft. of this streak was perforated. This procedure was established in order to drain the

5 lowermost producing layers first and, as water encroached to the top of any one permeable streak, the latter could then be shut off by plugging back or setting a tion forwarded to the laboratory and the results returned to the field during the three-day interval the casing was standing cemented. bridge plug in the casing and continuing the production from the next higher producing zone, in this manner progressively depleting the producing horizons from the bottom toward the top. By handling the wells in that manner, it is believed that less oil will be trapped in the formation by encroaching water. It should be pointed out here that close cooperation was required between the field and the laboratory, so as to have the cores from the producing forma- Owing to the consolidated nature of the producing formation, no screens or perforated liners were necessary. Although this paper is not designed to give a detailed treatise upon the geological section in the Magnolia field, a short discussion covering the general features of the

6 6 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS

7

8 8 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS beds penetrated is essential to give a age, and, in a few localities, a small amount complete picture of engineering problems of Upper Glen Rose lime.* In the Rodessa encountered during the development of field, 45 miles to the southwest, less than the field. In Fig. 2 is illustrated a generalized 500 ft. of the Washita-Fredericksburg had columnar section of the Magnolia-Village been removed before Gulf time, and the area in Columbia County, Arkansas, and Trinity remained intact. the following constitutes a brief summary Below the Comanchean series lies a of the stratigraphy. group of beds, the age of which is still The series of Cretaceous strata encoun- somewhat doubtful. For convenience, they tered in this field is similar to that found are called pre-comanchean, but it is to the southwest in the main part of the believed that they are probably of upper Woodbine, or East Texas Basin. The most Jurassic age. This series of beds includes noticeable exceptions to this occur above the Buckner, Smackover, and Eagle Mills and below the major Gulf-Comanchean formations. The last formation has not unconformity, which truncates successively been penetrated in this field, but is placed older beds in the Comanchean series pro- in the section on the basis of near-by ceeding from East Texas to Arkansas. deep tests. From the Upper Cretaceous, or Gulf Production in this field, to date, has Series, all the beds of Eagle Ford and been found only in the Smackover lime, Woodbine age - are missing, -. and from the -- Comanchean nearly 2500 ft. of Washita- The field nomenclature used to describe the various formations has been carried out in Fredericksburg and Upper Glen Rose this paper. For this reason the variou: lim~; strata have been eroded, leaving only the Stone bodies have been called simply lime with the term "limestone" used to indicate Massive Anhydrite of Middle Glen Rose only the hard, dense, crystalline formations. EOCENE Claiborne 700 Disconformity Wilcox 540 Midway 450 Disconformity Navarro ~ao Arkadelphia ( ago Nacatoch 70 Saratoga Upper Taylor 155 Marlbrook Gulf 220 Ozan Lower Taylor 1 20 Buckrange 180 Brownstown CRETACEOUS Austin 310 Tokio a5 Massive Anhydrite Lower Glen Rose 940 ( Gloyd lime James lime Travis Peak r,aao Comanchean Unconformity Morgan sands Big sand Cotton Valley Massive red sand Jones sand Unconformity JURASSIC? Buckner I 10 Anhy. and shale 1 10 Dolomitic shale Pre-Comanchean itic lime 400 C stalline lime Eagle Mills ( 800 SZ Anhydrite, etc. Major Unconformity PALEOZOIC

9 although shows have been reported and tested in the Glen Rose and Cotton Valley formations. The stratigraphy of the Smackover lime, as outlined in the accompanying columnar section, consists of dolomitic shale, oolitic lime and crystalline lime. The dolomitic phase is hard, dense, and nonporous, and is directly underlain by the oolitic facies, sometimes known as the Reynolds lime. The latter is a porous oolitic and pisolitic lime with hard, dense, firmly cemented streaks varying in thickness from a few inches to 60 ft. Occasional streaks of cavernous, coral-reef material also occur, as well as layers of dense, crystalline lime, with inclusions of anhydrite, gypsum, calcite, pyrite, and lignite. Stylolites and sections with asphaltic matrix are not uncommon. The porous and permeable zone in the oolitic member usually commences near the top, but may be found as much as 60 ft. below the base of the dolomitic shale. This porous and permeable zone in the Reynolds lime of the Smackover is the source of the Magnolia field production. Attempts have been made to correlate the dense streaks and coral reefs, but as yet have not met with success. Unfortunately, these layers are not always recognizable on the electrical logs, and core logs on some wells are not complete. The greatest thickness of oolitic lime drilled, or cored, in the Magnolia field is about 340 ft. Deeper exploration indicates dense, crystalline lime below this point, and there is no evidence from other areas that other deeper porous zones exist. The main body of the crystalline lime, and lower strata, have not been penetrated in this field. Markers As development in this area rapidly evolved with the Smackover lime as the primary objective, it was not necessary to follow the detail markers of the strati- graphic column too closely. The chalks at 2000 to 2300 ft. were characterized by hard drilling, hence the casing point for the intermediate string was easily determined. Except for the testing of the various sands throughout the Comanchean series, where minor showings occasionally were encountered, drilling usually progressed rapidly to the Buckner formation, which was easily distinguished from the Cotton Valley series. It became the common practice, therefore, to drill the wells to or almost to the top of the Smackover lime before coring. The latter formation in most cases was completely cored by conventional methods with a large-sized core barrel. Electrical logs were run and the exact position of the upper markers determined after the total depth had been reached. Skucture and Accumulation The subsurface structure of the Magnolia field (Fig. 3) is based upon the top of the Smackover lime, which occurs at an average depth of 7150 ft. subsea in an anticlinal fold. The anticline is elongated in an eastwest direction, the axis swinging slightly to the south on the eastern end. The average dip on the north flank is about 635" and on the south flank about 4". The crest of the structure is in the vicinity of the NE>Q of the NEfQ of sec. 23, where the top of the Smackover lime was encountered at 6986 ft. subsea in Frank Frankel et al., Levi Garrett No. I. The lowest structural point is along the north flank in Atlantic Refining Company's Baker No. I, a dry hole that encountered the top of the Smackover lime at minus 7358 feet. The areal extent of the producing acreage in the field is estimated to be 4494 acres (Fig. 3). The limits of the field have been determined by dry holes on the north and east flanks and by the establishment of the water level in producing wells on the south and west flanks. The field is peculiarly free from faulting of any consequence. The

10 I0 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS

11 known maximum structural relief is 372 than the top of the Smackover lime. feet. Core analyses showed that this electrical The top of the Smackover lime was log marker did not always represent the selected as a marker on which to contour true top of the porous and permeable zone. FIG. 4.-NORTH-SOUTH SECTION THROUGH MAGNOLIA FIELD. the structure, because of its relative proximity to the producing' zone (about Soft.), and also because it isa true lithologic break, which could easily be determined in the field from drilling time, coring and electrical logs. Contours were also drawn on an electrical marker at the top of or close to the Smackover lime porosity. This datum proved very satisfactory in most instances, and gave a slightly more accurate picture of the top of the porosity but, because of some variation, it was less accurate in predicting structural position of new wells It was labeled, for practical purposes, "Top of the Pay," and the structure closely resembles that shown on Fig. 3. The largest variation between this marker and the actual top of the porous and permeable zone does not exceed 15 ft. On the subsurface map in Fig. 3, the contours are shown on top of the Smackover lime because it was the most widely used horizon during the development of the field, but it nfust be noted that this datum is approximately 30 ft. above the actual top of the producing formation. The average position

12 I2 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS of the gas-oil contact on the top of the main porosity, and the average position of the water level on top of the porosity, as shown on the map, correspond, therefore, with the Smackover contour 30 ft. shallower. Fig. 4 is a schematic, north-south cross section through the approximate center of the Magnolia field. The details of the wells shown on this cross section have been eliminated so that pertinent data could be better illustrated. Originally this section was made of standard strip logs, which resemble the type of columnar section shown in Fig. 2, and gave all the details of electrical logs, coring, testing and paleontological markers. The top of the porosity is shown on Fig. 4 as well as the top of the Smackover lime, and the gas cap, oil column and water levels are indicated. The deeper structure is not reflected through the upper beds and the first semblance of it appears on the top of the Buckner anhydrite, which lies directly above the Smackover lime and is composed principally of red shales and anhydrite. The cross section indicates the thinning of the anhydrite over the crest of the structure; although not shown on these illustrations, the anhydrite continues to thin regionally updip to the northeast. As demonstrated on the columnar section, the top of the Buckner anhydrite is an erosional feature, which varies throughout this general area. In the Village field, 6 miles to the east, the maximum recorded thickness is 135 ft., whereas at Magnolia the maximum thickness is 155 ft. Six miles to the southeast, in the Atlanta field, and in the Schuler field, 15 miles southeast, no Buckner zone is found. The Buckner zone forms a complete impervious cap over the entire structure and could have acted in the capacity of a cap rock. Although not shown on the cross section, there is some evidence to indicate that what little structural anomaly appears in the succeeding younger beds has shifted to the south or southwest with depth. This is a feature that has been recognized throughout the general region in other fields. Until the top of the Buckner anhydrite was encountered, through the drilling operations, therefore, the structural position of any well was indefinite, and the true position was not determined until the top of the Smackover lime had been encountered. Producing Horizons Smackover Lime.-The top of the Smackover lime is readily discernible by the abrupt change from red shales and anhydrite to dense crystalline limestone. Although it varies from well to well, the top 30 ft. of the Smackover lime is dense and contains no porosity, permeability or showings of oil and gas and generally increases in hardness from the top downward. After about 30 ft. of penetration in the Smackover lime, the character of the limestone grades from a crystalline to an oolitic lime; a gradational feature that is one of the reasons for the variation in distance from top of the Smackover lime to the top of the productive zone. The top of the oolites has at times been designated as "top of the Reynolds lime," but this nomenclature has not been widely used. The remainder of the section penetrated in the Smackover lime (or Reynolds lime) consists of oolitic lime, pisolitic lime, streaks of coquina and some streaks of crystalline limestone. As mentioned under Development, no general correlation can be established between the impervious streaks encountered, and the main porous zone must be considered as continuous from top to bottom; even if this is not true in one well, the connection is believed to exist elsewhere in the structure. In general, the impervious streaks in the producing horizon are attributed to secondary deposition of calcareous material in the porous spaces. However, some crystalline limestone streaks have been observed in cores where oolitic structure is absent. Black materials (called

13 asphaltic) also form impervious streaks in the porous section. In Fig. I a typical bottom-hole detail is shown, which demonstrates the existence of a hard, impervious streak in the producingformation. To the left of the bottomhole detail are also shown analyses of the cores taken from the well, which indicate the varying porosities and permeabilities. The cutting time observed during the progress of coring is also plotted on the well log, in order to indicate the occurrence of hard, impervious streaks. As shown in the schematic cross section of Fig. 4, the maximum thickness of the gas cap is 120 ft., whereas the thickness of the oil column varies from 178 to 188 ft. The variance indicated in the water level is due to the lack of data on the north flank concerning the exact position of the water level. The figure of 7318 ft. subsea for the oil-water contact was taken from several wells that have penetrated the water level in the main portion and southern flank of the reservoir, and, although it is applicable for the main porous section, the discovery well may be producing oil from as low as 7328 ft. subsea. The accuracy of the data for the discovery well is questionable, but it is altogether possible that this well may be producing from a porous and permeable streak in the Reynolds lime, above the main producing horizon on the flanks of the structure. A detailed study of the cross sections across the field indicates that on the south flank there is definitely an upper porous and permeable streak that is not connected in its upper portions with the main productive zone and, as indicated in Fig. 4, carries a separate gas cap with a gas-oil contact at 7249 ft. subsea. The existence of this secondary gas cap on the south flank has been definitely proved in several wells, therefore it is altogether reasonable to expect that the same condition could exist on the north flank of the structure with a separate water level. This point has yet to be proved, so for all purposes of estimation the water level has been assumed to be at 7318 ft. subsea throughout the field. So far as is known, the water level in the upper zone on the south flank is the same as in the main body of the reservoir but on the southwestern flank of the structure, an exceptionally high water-oil contact, varying from 7297 to 7308 ft. subsea, has been determined by both electrical logs and production data. The reason for this condition has not been thoroughly explained, but it does exist and has given considerable trouble during the producing life of some of the wells drilled on that flank. Undoubtedly the existence of impervious streaks close to the water level has obscured the true oil-water contact in places, and water was not encountered until the well had been carried to the next permeable streak. Owing to the fact that the 7318 figure was established by electrical logs and drill-stem testing in a continuous porous section, during the early stages of development in the field, it is felt that this determination can be considered the most accurate. A detailed study of the cross section across the field reveals that, in the 30-ft. interval between the top of the Smackover lime and the top of the main porosity, an oolitic streak is developed on the north and south flanks of the field, which diminishes in thickness toward the crest and disappears entirely on the highest portions of the structure. Within this upper porous and permeable streak the secondary gas cap was developed on the south flank. The upper streak of porosity is not as well developed on the north flank as on the south flank but there is sufficient evidence of its existence to indicate that it may be productive in a few of the wells. The interpretation of some of the earlier electrical logs in the producing horizon was difficult at first because of lack of experience with this type of reservoir. The selfpotential curves in particular indicated

14 14 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS relatively high permeability throughout the It has been estimated that the. effective > reservoir section, although it was known producing section of the main porosity is that impervious streaks existed. However, approximately 125 ft., or roughly 70 per Shel I Aisy G/ssgow Nf 3 with the development of a more sensitive self-potential reading (Fig. I), the permeable streaks were brought out more clearly. cent, of the porous section throughout the field. The asphaltic and dense streaks encountered in almost every well accounted

15 for most of the impervious portions in the main reservoir. The amount of penetration of the producing section taken in each well varied according to the policy of the various millidarcys, varying from o as a minimum to 16,ooo as a maximum. Connate water has been taken at 20 per cent, based on laboratory tests. Shell Southwood Oil Co essu ~ SL t R 7192 (Po) y (10) -7257(15) o -7288C6) h. O 748 W CV4' TD ma TSL tr ) R$ (90) -T262CIO) -7v, -8b * tR P-W-39 M~J: 8aik-y GIdayow No Wac. Log -TOSO t.r h. 0 W(YW'> *.3S Gd/@ Vaughn $ Hunt 1 ""!& TSL T.R " 460w wc43 &-39 4.S Crisp 23 f.... Well Nb- Sca It : I in ft. El...a* TSL.... Top Smsckover Limo Tp... TOP -7101(8)&,+.... Perforation lnkrvel. 7/43-7'.... Oil String-Si3e ~7&l(h~?s3+~.....&;" ).tnitiei ~ rodu&ion-hrn~o,@~~~ "" Total Depth Wtr: Prod. at compl TD l(30) 100Qts:...a*.. Shot Interval- NO- Qta Used 1-2- &,IS.... Number of Acid Treatments-Total Gals. operators. It is believed that wells bottomed as much as IOO ft. or more above the known water level will have to be deepeped eventually in order to drain the reservoir effectively. The characteristics of the producing horizon, based upon all core analyses available and as submitted to the Oil and Gas Commission, have resulted in the establishment of average figures as follows: porosity, 18.5 per cent; permeability, 1500 Fig. 5 has been included to demonstrate a method of presenting pertinent information graphically on maps and electrical logs. By this, or similar methods, the data are available in a convenient and usable form on maps and cross sections. Other Indications oj Accumulations.-In general, recompletion possibilities in upper zones of wells drilled in the Magnolia field are not attractive. Various small showings were reported in the Rodessa, Cotton

16 I 6 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS Valley and Travis Peak sands and although these showings were not of sufficient magnitude to warrant the testing of the wells at that time, some interest has developed in shallow production because of the appearance of oil and gas between the casing strings of three of the Smackover lime producers. It is significant that, whereas the oil and gas from the Smackover zone carries a high concentration of hydrogen sulphide, the showings found between the strings of the three wells are sweet; that is, they do not contain hydrogen sulphide in appreciable quantities. A shallow test slightly north of the limit of Smackover production was drilled on the strength of these showings and was designed to test the Rodessa zone, which was considered to be the most likely source, but cores and electrical surveys of this section failed to indicate any productive possibilities. It appears now most probable that the source of this sweet oil and gas is some shallow, porous zone of small magnitude that is not recorded on the electrical survey. Type of Oil As indicated in the foregoing pages, the Magnolia field represents a normal type of accumulation; i.e., the oil has accumulated on top of water and a gas cap has formed in the crest. The oil varies in gravity from 38' to ~ I O A.P.I., with an average slightly above 39O. The characteristics of the crude are shown in Table I, which is a brief analysis of the oil from three random wells in the field. One unusual feature of the Magnolia crude is its salt content, which varies from 30 or 40 lb. to 400 Ib. per 1000 bbl. Some concern was expressed early in the life of the field concerning this characteristic but satisfactory means of de-salting the crude at the refineries was established. The gas produced with the oil is composed of the normal lighter fractions of hydrocarbons TABLE I.-Analysis of Oil from Three Wells PER CENT Crude Gravity, deg. A.P.I. at 60 F Sulphur, per cent by wt..... Pour point, deg F..... Saybolt Universal viscosity: At 40 F At60 F At 80 F s I WeU ( we11 I well No. I No. a So. 3 I.o3 o Laboratory Yields Gasoline (net). 4oo F. EP, percent 32.6 Kerosene, per cent Furnace oi (max.). per cent Gas oil, per cent Residue, per cent Water, per cent Loss, per cent contained in the oil under its reservoir conditions; it is not the gas that has accumulated in the gas cap. This gas has an average gasoline and butane content of about 3.10 gal. per rooo cu. ft. A shrinkage factor of 34 per cent in volume of the original crude has been determined when it is removed from reservoir conditions to standard temperature and pressure. Proration Before establishment of proration orders for the Magnolia field by the Arkansas Oil and Gas Commission, production from the discovery well was restricted to 200 bbl. per day for several months. During December 1938 the Arkansas Oil and Gas Commission established proration regulations for the Magnolia field. Provisions were made to measure bottom-hole pressures at regular intervals and the Commission o I 17. I qa. a 0. I 0.1 ordered production allotted on the basis of 40-acre units. The allocation of production between wells was as follows: the allowables were based half on each well's share of the sum of bottom-hole pressures and half of each well's share of the sum of producing acreage when applied against the total field allowable for a given month. It was also provided that wells with gas-oil ratios greater than 2000 CU. ft. per barrel

17 be penalized. At that time the average per \vcll allowable was set at 2 jo bbl. per day. Thc per well allomable fluctuated from month to month, depending upon market after which the ratc was held more or less constant. Thc schemc of proration just described remained in cbcct until April 1939, when 121ti. 6.-,\IosT~~LY-ASD demand and pipe-line capacity, and for a period during February 1939 it was increased to a maximum of 375 bbl. per well per day. This was reduced to 300 bbl. per day the following month and remained at this amount for several months. As more wells were completed and the market demand supplied, allowables per well were gradually reduced until December I 941, when the allowable was approximately 165 bbl. per day. The trend of production during the period of development is illustrated in Fig. 6. The total field production was allowed to increase with development until it had reached approximately 21,000 bbl per day, CL'JIL.LAT~~E~PXODL.CTIOS, BUTTOY-HULL PRESSIVRE..\\'F:R.\(:E XATIO AND NI:UBEX OF PXOI)CCISG NELLS. GAS-OIL. the method of allocation was based on acrcage times reservoir pressure. This was accomplished by measurement of reservoir pressures in key wells at stated intervals under the supervision of the Arkansas Oil and Gas Commission; results were then contoured on a map and each well's pressure determined. The estimated pressures and acreage units assigned to individual wells then formed the basis for each well's share of the field allomables. It was anticipated that this method of proration would be more satisfactory than the one previously used, but under the existing conditions it had very little effect on the allowables assigned to individual wells

18 I 8 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS because the pressures over the field did not vary any considerable amount. However, if a certain area was receiving too great an allowable, which, in turn, caused the bottom-hole pressures to be lowered more rapidly than in other parts of the field, this area automatically received a cut in allowable tending to equalize the bottom-hole pressures throughout the field and minimize the amount of drainage across lease lines. As both plans of allocation in the Magnolia field placed considerable emphasis on acreage, they served to encourage unitization of small tracts into 40-acre drilling units, even though the Oil and Gas Commission had other machinery set up to provide for this unitization. The original bottom-hole pressure was found to be 3465 lb. at 7100 ft. subsea. The results of subsequent periodic measurements are shown in Fig. 6. As of December 1941, the bottom-hole pressure had declined a total of 336 lb.; that is, to 3129 lb. per sq. in. The cumulative production for the same interval amounted to 18,377,000 bbl., from 116 oil wells. Based on these figures for the reservoir, a productivity of 54,694 bbl. of oil produced per pound drop in pressure has been calculated. In recent months, however, the productivity of the reservoir has increased. From February 20, 1941, to July 15, 1941, the productivity averaged 65,500 bbl. of oil per pound drop. More recent indications are that its rate of productivity will be continued or may still further increase. These data, although not conclusive, indicate the reaction to withdrawals of the various factors affecting the productivity of the reservoir and serve to show the ability of the reservoir to sustain production. The increase in productivity probably is due to a better balancing of pressures throughout the field since complete development, and a consequent spreading of withdrawals over the entire reservoir. The original gas-oil ratio observed on the discovery well averaged approximately 700 cu. ft. per bbl. but increased throughout the succeeding months to 1150 CU. ft. per bbl. by January With the completion of additional wells the average ratio declined to approximately 850 cu. ft. per bbl. and has fluctuated between this amount and 950 cu. ft. per bbl. The present ratio for the field is 889. Based upon the relatively small decline in bottom-hole pressures and the stationary position of the average gas-oil ratio of the field, it may be concluded that the Magnolia field as a whole is being controlled according to the better principles of conservation. Even though the oil accumulation is overlain by a large gas cap, there has been no appreciable change in the average gas-oil ratio, and it appears that the field is being produced at an efficient if not an optimum rate of production. The strict control exercised by the Arkansas Oil and Gas Commission over the gas-oil ratios, with its rigid penalties for wells that exceed the gas-oil ratio of 2000 cu. ft. per bbl., has stimulated the use of remedial measures to correct excessive ratios. Judging from the record of bottomhole pressures and the performance of the wells, it is probable that the field has benefited by the presence of a strong water drive, which should serve as an unlimited source of energy and, if controlled properly, will increase the recoveries from the field. Disposal Facilities, Oil and Gas The Magnolia field is served by six pipe lines, which take all of the crude produced in the field. All of the gas is processed in a gas plant in the central part of the field, which handles an average of 17,000,000 cu. ft. per day. After passing through the plant, approximately 12,ooo,ooo cu. ft. is sold for commercial use and the rest of the gas is returned to the operators in the field for their use or for use as fuel in the plant. This plant is designed to carry a maximum capacity of 20,000,000 cu. ft. per day. It is of the high-pressure, absorption type and

19 operates at about zoo lb. pressure. The gasoline and butane content of the gas averages 3.10 gal. per 1000 cu. ft. The yield of the plant, therefore, is approximately 52,000 gal. per day, of which 30,000 gal. is natural gasoline and 22,000 is butane. The products of the gasoline plant are sold to the trade in railroad tankcars. ECONOMICS The cost of drilling wells in the Magnolia field approached the average for wells of this depth in this type of formation. The earlier wells, which of necessity were of an exploratory or semiexploratory nature and therefore entailed a larger amount of coring and testing, as well as the larger casing scheme, cost in the vicinity of $75,000 to $80,000. As drilling proceeded and contract prices were lowered because of the familiarity of the various drilling companies with the formations to be penetrated, the average cost of completing a well to tanks was reduced to approximately $65,000, even though some operators were still utilizing the larger casing scheme. Later, wells utilizing the smaller casing scheme were drilled and completed to the tanks for approximately $55,000. With the allowables as they existed during the period of drilling, and depending upon the size of the casing scheme chosen by the individual operator, pay-outs of from I to 1% years were possible. During the early life of the field the posted price of crude oil was approximately 936 per barrel. With the increased demand for crude oil, prices have now been raised to $1.06 and $1.08, dependent on the gravity of the oil. The maximum posted price at the time of writing is $1.08 for 40" A.P.I. oil, with stipulated reductions of price as the gravity of the oil decreases. The cumulative field production from March 1938 through December 1941 is approximately 18,377,000 bbl., based on figures furnished by the Arkansas Oil and Gas Commission. Taking the formation characteristics mentioned earlier, of 18.5 per cent average porosity, zo per cent connate water and 34 per cent shrinkage, and an assumed recovery factor of 60 per cent, it has been estimated that the original reserves were about 180,000,000 bbl. Based on these figures, the estimated recovery per acre-foot is about 455 bbl. However, there is considerable reason to believe that these figures are conservative, inasmuch as ample allowance has been made for connate water and shrinkage. In view of the high permeability, and an efficient water drive, recoveries as high as 555 bbl. per acre-foot are now considered possible, which would increase the estimated original reserves to 2zo,ooo,ooo bbl. From these figures, it may readily be seen that more than 93 per cent of the producton to be obtained from the Magnolia field is yet to be produced. Because of the type of reservoir and the water drive, as indicated by pressure performance as well as the encroachment of water into the flank wells, it is anticipated that some wells will be forced to employ artificial lift early in their productive life, and subsequent abandonments will take place as the production continues. The fvst abandonment of a producing well in the Magnolia field occurred during August This well, which was on the southwest flank and had produced water since completion, had declined until it was producing 125 bbl. of salt water and 6 bbl. of oil daily by gas lift. According to the latest reports available, 16 wells in the Magnolia field are producing a total of 2392 bbl. of salt water per day, in amounts varying from 3 to 760 bbl. per well. Salt-water disposal in this field has not yet become a serious problem, and no disposal facilities other than surface pits have been provided. The field is of such magnitude that, if conditions remain as they have been, it is reasonable to expect

20 20 AN ENGINEERING STUDY OF THE MAGNOLIA FIELD IN ARKANSAS that the flowing life of the majority of the wells will be from 10 to 15 years, and that the productive life of some of the wells will extend over a period of from 25 to 30 years, or longer. In preparing this paper I wish to thank the Management of the Shell Oil Co., Inc., as it was with their permission that I was granted time and assistance to prepare the paper. I have drawn freely on their 61es of information, and particularly wish to thank the members of the Shell Exploration Department for their assistance. The Production Department staff of Shell Oil Co., Inc., have also furnished considerable information; and where applicable data have been taken from earlier Shell reports. The Arkansas Oil and Gas Commission, especially its director, Mr. A. M. Crowell, contributed data; Mr. John F. Magale, of Shreveport, furnished information concerning the early exploration of the area, and Mr. D. A. McGee, of the Kerlyn Oil Co., furnished information concerning the drilling of the discovery well.