Matching the Performance of Saudi Arabian Oil Fields With an Electrical Model

W. L. WAHL L. D. MULLINS SOCONY MOBIL OIL CO., INC. R. H. BARHAM DALLAS, TEX. MEMBERS AIME W. R. BARTlETT ARABIAN AMERICAN OIL CO. MEMBER AIME DHAHRAN, SAUDI ARABIA

ABSTRACT devices, output devices, central control and a resistance­ Downloaded from http://onepetro.org/JPT/article-pdf/14/11/1275/2214599/spe-414-pa.pdf by guest on 02 October 2021 capacitance (RC) network. At times, the RC network This paper describes an electrical model and its appli­ alone is referred to as the "model". However, it should be cation to the analysis of four reservoirs in Saudi Arabia. evident from the text which meaning is attached to the The model has 2,501 mesh points and represents 35,000 word "model". A discussion of the equipment follows. sq miles of the Arab-D member. Details of modeling such as mesh size, control problems and standards of perfor­ THE RESISTANCE·CAPACITANCE NETWORK mance in matching reservoir history are discussed. The The RC network consists of 2,501 capacitance decades particular performance match achieved for the Arab-D interconnected through 4,900 resistance decades. The com­ member is presented. Details such as permeability barriers, ponents are arranged to form a rectangular network of aquifer depletion and interference between oil fields are 2,501 mesh points in a 41- X 61-mesh array. Imposing given. The performance match realized in the Abqaiq the mesh grid system on the continuous reservoir system pool is presented in detail. divides the reservoir into discrete areal segments. These discrete segments may be of various sizes. More precisely, INTRODUCTION the mesh size need not be uniform throughout the model. The resistor-capacitor network and associated control The RC network is fabricated in two sections which equipment described in this paper comprise an electrical are connected at the top. An inside view of the "tunnel" analog of a reservoir system. Similar equipment has been formed by the two sections is shown in Fig. 1. The height used to study the transient response of reservoirs for many and width of the tunnel are shown in the figure. Numerals years. The unique feature of the model and application appear along the bottom and along the back opening of to be described is the extremely large size of the model the tunnel. These numbers denote the x and yeo-ordinate and reservoir system, and the detail observed in simulating positions of mesh points. Fig. 2 presents a rear view of the reservoir with the model. one-half the model. The length dimensions of the model, as well as a rear view of the capacitor decades, are shown The Arabian American Oil Co. first became interested in this figure. The control dials used in adjusting the resist­ in analog computers for simulation of oil reservoirs in ance and capacitance values on the model can be seen 1949. Since that time, several models have been developed, in the enlarged portion of the model shown in Fig. 3. each more elaborate and refined so that the reservoir sys­ tem might be more closely simulated. The current model The electrical capacity at any mesh point can range is the latest in a series designed, built and operated by from 0 to 1.0 microfarads set to the nearest tenth of a the Field Research Laboratory of Socony Mobil Oil Co. microfarad. The electric resistance connecting any two in collaboration with Aramco. It has been and continues mesh points can range from 0 to 9,990,000 ohms set to to be used to study the regional performance of the the nearest 1,000 ohms. External capacitors may be added Arab-D member limestone reservoir. The Arab-D member to any or all mesh points if the need arises. The values of is one of the Middle East's most prolific producing hori­ electrical resistance and capacitance are adjusted manually zons .. by manipUlating the two types of decade units. INPUT EQUIPMENT THE MODEL A considerable quantity of equipment is used to control The theory of simulating a reservoir system with an the input to the RC network. These input devices are electrical system has been presented in the literature.'-5 Therefore, this paper will not discuss the theoretical as­ TABLE l-CORRESPONDENCE BETWEEN flUID AND ELECTRICAL SYSTEMS pect of the problem except to point out the correspondence Fluid System Electrical System between the fluid system and electrical system, as shown Item Units Item 'Units in Table 1. Reservoir Pressure psi Voltage Volts In general, the complete model is made up of input Reservoir Production Reservoir BID Current Microamperes Rate or I niection Rate Original manuscript received in Society of Petroleum Engineers office Fluid Capacitance :Reservoir bbl/psi EI&etrical Microfarads July 10, 1962. Revised manuscript received Oct. 3, 1962. Paper pre­ Capacitance sented at 37th Annual Fall Meeting of SPE held Oct. 7-10, 1962, in Transmissibility kh/ f.L. darcy·1t Electrical Mhos Los Angeles, Calif. /cp Conductivity 'References given at end of paper. SPE 414 Real Time Months Model Time Seconds

NOVEMBER, 1962 1275 necessary to control currents which are the analog of well from the memory unit to a time-dependent rate controller. production rates. For simplicity, the input equipment em­ It appears here as a step function of voltage to control ployed will be discussed briefly according to type. the current withdrawn from (or supplied to) a mesh point of the model. Upon completion of the data transfer Programmed Time-Dependent Rate Controllers from the memory unit to the controller, another card is read. The new rate is again stored in the memory unit for Controllers of this type are used primarily during the use at the next command from central control. history portion of a reservoir study. They generate step Fig. 4 shows 34 of the time-dependent rate controllers functions of current vs time from a predetermined pro­ (left side of the relay rack) together with 34 memory gram. Each 2.5 seconds (the basic model time interval), units (right side of rack). Also shown is an IBM repro­ a new rate can be generated. These rates are proportional ducer used to read the cards. A total of 102 time-depend­ to the average well production or injection rates for the ent controllers is available for use with the model. All period of time (weeks, months, etc.) equivalent to the can be used as either production or injection controllers. 2.5-second interval. The rate of production (or injection) for a particular Pressure-Dependent Rate Controllers well or group of wells is punched into IBM cards. Upon command from central control, * a card is read by an A relay rack containing 126 pressure-dependent rate IBM reproducer and the digital information is stored in controllers can be seen at the end of the "tunnel" shown a memory unit. Two and one-half seconds later (upon in Fig. 1. Of this number, 108 can be used only as production-rate controllers, while the remaining 18 can command from central control), the rate is transferred Downloaded from http://onepetro.org/JPT/article-pdf/14/11/1275/2214599/spe-414-pa.pdf by guest on 02 October 2021 be used as either production or injection controllers. «The central-control equipment is discussed later. For this type of controller, the current withdrawn (or injected) is a linear function of the voltage at a mesh point on the RC network. A variable resistor is placed between the mesh point and a point of zero potential.

FIG. 3- O:-lTR0L DIALS U E:D IN ADJUSTING RE [STANCl: AND APA ITANCf; VALUE ON THE 10nEL.

FIG. 4--TIME-DEPENDENT RATE CONTROLLERS AND MEMORY U NITS LOCATED ON THE RELAY RACK (LEFT), AND IBM REPRODUCER FIG. 2-REAR VIEW OF THE MODEL. U SED TO READ CARDS (RIGHT).

1276 JOURNAL OF PETROLEUM TECHNOLOGY Thus, as the voltage difference between the two points of a digital computer. The digital computer is programmed decreases, the current through the resistor decreases. The to convert the coded data in the cards to pressure, pro­ resistor value determines the slope of a graph of current duction rates and injection rates. These pressures, pro­ vs voltage difference. Also incorporated in the pressure­ duction rates, etc., may then be tabulated from the digi­ dependent controllers is a device whereby the intercept tal-computer output. of the afore-mentioned graph can be made greater than or less than zero. CENTRAL CONTROL This type of controller is very useful when predicting future reservoir performance. In such cases, the produc­ The central control (see Fig. 6) is the "brain" of the tion rates of existing and proposed wells can be described analog computer. It is here that the very accurate 2.5- by linear functions of static reservoir pressure. This func­ second time period is generated. During the first few mil­ tional relationship between production rate and static res­ liseconds of each time period, central control issues a ervoir pressure is represented and generated by the pres­ chain of commands to cemrol the "read-in" and "read­ sure-dependent controller. out" equipment. The read-in circuits are interlocked in such a way that most equipment malfunctions will be detected and the computer stopped. To provide further Constant-Rate Controllers observation of computer performance or reproducibility, Constant-rate controllers are practically self-explanatory. two additional potentiometers are used to monitor the The production or injection rate is constant with time total net current being withdrawn from the model. Any deviation from the regular curve drawn by these poten­ and independent of the voltage at a mesh point. Of the Downloaded from http://onepetro.org/JPT/article-pdf/14/11/1275/2214599/spe-414-pa.pdf by guest on 02 October 2021 79 constant-rate controllers available, 65 can be used as tiometers indicates a possible equipment malfunction. The either production-rate or injection-rate controllers. The control circuits also are interlocked. In the event that other 14 controllers will only inject at a constant rate. any portion of the read-out equipment fails, no output data will be punched into cards. Constant-Pressure Controllers Approximately 900 indicator lights have been incor­ Beginning at some desired time, this type of controller porated in the central control. These lights have several maintains a specific voltage at a mesh point. The current purposes. First, lights identify the mesh point at which (production or injection) required to maintain the con­ measurements are being made. This identification may be stant voltage is automatically supplied. Approximately in the form of an X -Y co-ordinate system, or some coding 200 mesh points can be controlled in this manner. All con­ system may be used. Second, lights provide a continuous trollers may be at one voltage, or each controller may check on read-in and read-out circuits. Malfunctions are maintain a different voltage. indicated in a manner which aids in diagnosing and lo­ cating equipment failures. Third, lights are used to identify OUTPUT EQUIPMENT the type of controller (time-dependent, or others) used at each mesh point at all times during an experiment. The output of the model consists primarily of (t) well pressure measurements made during the pressure-history Central control also is in charge of switching the type matching procedure and (2) well production rates and of controller at a mesh point and activating controllers pressures recorded during the prediction of reservoir at the proper time. Of the 2,501 mesh points on the performance. In addition, pressure measurements can be model, 300 may be selected as locations for production recorded throughout the RC network for the purpose of or injection controllers. Some 200 of these points can constructing isobaric maps and calculating influx data. be controlled by any of the four types of controllers Fig. 5 shows two relay racks containing eight poten­ already described. Furthermore, the type of control can tiometers. The potentiometers are used to: (1) measure production and injection currents flowing between the controllers and the mesh points of the mOdel; (2) measure the voltage at any of the 2,501 mesh points; and (3) measure the difference in voltage between any two mesh points. Each potentiometer is equipped with a scale a strip chart and a digitizing device. Therefore, a conti~u­ ous plot of scale reading vs time can be drawn, and/or the scale value can periodically be converted to a three­ digit number, transferred to a memory unit and then punched into IBM cards. Data can be obtained and punched into cards only at 2.5-second intervals. Since there are only eight potentiometers available for taking data, reservoir studies are repeated several times to obtain all the data required. The number of recorders used is a compromise between the cost of providing additional equipment and the time expended in gathering data. Using the punch-card system, processing output data is relatively fast and simple compared to the strip-chart system of recording data. With a card-reading device and an X-Y plotter, pressure data can be plotted directly in several ways. This method of plotting data is especially valuable during "history-matching" procedures. If field data also are put on cards, the deviation between model and field data can be observed directly. Data from studies FIG. 5-Two RELAY RACKS CONTAINING EIGHT predicting model performance are processed with the aid POTENTIOMETERS FOR RECORDING.

NOVEMBER, 1962 1277 be switched upon command. Control can be switched diately adjacent to 'Ain Dar is an eighth crestal closure back and forth in any order so long as the total number known as Fazran, which at the present stage of develop­ of switching operations does not exceed 100. Controller ment does not appear to be a continuous part of the switching on the remaining 100 mesh points is more Ghawar field. Most of Ghawar is 12 to 18 miles in width. limited. Some switching can be done, but all four types Production began in Ghawar-'Ain Dar in 1951. During of controllers cannot be used on these points. 1961, slightly more than 700,000 BOPD were produced from approximately 90 wells in the 'Ain Dar, Shedgum PHYSICAL NATURE OF THE RESERVOIR and Uthmaniyah areas. Abqaiq, the next largest pool, has a productive area Aramco has used the analog model to study simulta­ approximately 37-miles long X 7~miles wide. It is essen­ neously the future reservoir behaviors of the Abqaiq, tially a symmetrical anticline with a low, broad, slowly­ Ghawar, Dammam and Qatif oil pools, all of which plunging northern nose. At its nearest point, Abqaiq is produce from the Arab-D member of the Arab formation. located approximately 7 miles from the edge of the Gha­ Thus, pressure interference between the four fields could war field. Oil was found in the Abqaiq Arab-D member be taken into account. In the most recent study, approxi­ in early 1941, but the field was not put on full-scale pro­ mately 35,000 sq miles of Arab-D member were repre­ duction until Jan. 26, 1946. A secondary phase of develop­ sented on the model (Fig. 7). ment was begun in 1954 with the institution of gas in­ Rocks of the Arab formation are late Upper Jurassic jection in the southern part of the field. Pilot water injec­ in age. In this formation there are four obvious cycles, tion was started in northern Abqaiq in 1956, and was

each of which starts with more-or-less normal marine followed by full-scale injection in 1958. The current Downloaded from http://onepetro.org/JPT/article-pdf/14/11/1275/2214599/spe-414-pa.pdf by guest on 02 October 2021 sediments and closes with evaporites. The Arab-D member production rate of about 365,000 BOPD is supported by is the carbonate portion of the first or lowermost cycle approximately 300,000 BjD of injected water plus 176 and is also the principal oil-productive zone of the Arab MMscfjD of injected gas. Fig. 8 is a structure contour formation. map of the Abqaiq field. The carbonate rocks making up the Arab-D member Qatif and Damman are the smallest fields presently are all shallow-water types. They include calcarenite, fine­ producing from the Arab-D member. The oil accumula­ grained limestone, calcarenitic limestone, dJlomitic lime­ tion at Qatif is associated with an anticline, while that stone and dolomite. The most prolific production is de­ at Dammam is associated with a faulted dome. Production rived from calcarenites in the northeastern Ghawar and began at Dammam in 1938, and at Qatif in 1945. Abqaiq areas. These calcarenites are primarily of the pellet type, although there is considerable true oolite THE MATCH with well-developed concentric structure. There is a grad­ ual change from predominantly calcarenite in the north­ To place the entire Arab-D member region on the RC east of Ghawar to mixed calcarenite and fine-grained network, it was necessary to divide the region into various limestone in the southwest; true oolite is nearly absent sections. Sections containing the major oil pools and their in central and southern Ghawar. In the Dammam, Qatif, surrounding aquifer were further subdivided into 2,500- Bahrain and Qatar areas, the clastic carbonates have been and 5,000-m grid patterns as shown in Fig. 9. The out­ replaced by fine-grained limestones, and cleanly washed lying aquifer sections were divided into much larger calcarenites are present only in small amount. This trend squares which were multiples of 2,500 m. is more apparent in Qatif where the Arab-D member con­ It was assumed that the system was initially at equilib­ tains a great amount of lithographic limestone. rium and that the initial pressure in the Abqaiq pool was Ghawar, the largest pool, is a structural accumulation representative of the entire region under study. Ghawar, that stretches 140 miles along the north-south trending Dammam and Qatif pressures (three different datum lev­ En Nala anticlinal axis. Seven crestal closures have been els) were then corrected to the Abqaiq datum by adding found in Ghawar proper, but oil appears to be continuous from 'Ain Dar in the north to Haradh in the south. Imme- \

MANIFA ABU HADRIYA ~ OK_HURSANIYAH FADHILI'

o

KHURAIS~

FIG. 6--CENTRAL CONTROL-THE "BRAIN"· OF THE FIG. 7-AREAL EXTENT OF THE ARAB-D MEMBER REPRESENTED ANALOG COMPUTER. ON THE MODEL.

121.6 JOURNAL OF PETROLEUM TECHNOLOGY or subtracting the difference between the initial pressures of the oil well (or wells) it represented. The computed recorded in these oil pools and that recorded for the capacitance values of an area were seldom changed, inas­ Abqaiq pool. much as these values had been calculated from some of The pressure range necessary to cover anticipated oper­ the more-reliable basic data available_ Resistor adjustments ating conditions in any of the four oil pools was 1,500 psi. were made in a symmetrical pattern about and within the This pressure range was equated to its electrical equivalent, producing zones, and followed the general contour shape which is 200 v on this model. Electrical time of 2.5 sec­ of the structural contour maps and/or permeability-thick­ onds was made the equivalent of six months of field time. ness maps. It is desirable that the final resistor configura­ Therefore, the daily producing rates (or injection rates) tion represent a sound geological interpretation of perme­ of the individual wells were averaged over a six-month ability, within the limitations of the model, and this interval. Stock-tank barrels of production were converted configuration should not be contrary to any known geo­ to equivalent reservoir volumes by use of appropriate logic facts or physical properties of the system. formation volume factors. Gas injected into the Abqaiq The objective in this match was to make model pres­ field was converted to and handled as reservoir barrels sures at a particular mesh point agree with the observed of fluid through use of a gas formation volume factor. pressure history of the individual well (or wells) assigned The latter was possible because all of the gas had been to the mesh point. To be acceptable in most cases, the injected at essentially a constant reservoir pressure of pressures matched within 15 psi (from one week to the 2,470 psig. Water was injected on a barrel-for-barrel basis; next, model measurements repeat within 5 to 10 psi). that is, 1 STB of water was assumed to equal 1 reservoir There were time intervals when model pressures at a bbl of water. particular mesh point deviated more than 15 psi, but Downloaded from http://onepetro.org/JPT/article-pdf/14/11/1275/2214599/spe-414-pa.pdf by guest on 02 October 2021 Currents smaller than 2 or 3 microamps are difficult they were acceptable if the deviation was only for one to control. For this work, 3 microamps were made equi­ or two time intervals_ Also, there were three wells that valent to a 5,000-reservoir B/D withdrawal rate. It was did not meet the IS-psi criterion. These wells were judged felt that a 3,000- to 5,000-reservoir B/D rate was repre­ 011 an individual basis as to their importance to a satis­ sentative of the lower producing rates in Abqaiq and factory match. In this work, 50 controllers were matched Ghawar. in Abqaiq, 115 in Ghawar, 3 in Qatif and 1 in Dammam. With present equipment, the analog computer cannot We would not attempt to describe all adjustments made automatically handle the expansion or contraction of free­ to achieve a satisfactory pressure-history match of the oil gas volumes with changes in pressure. Inasmuch as south­ pools represented on this network; the final network con­ ern Abqaiq has been below the original bubble-point figuration was the result of approximately 150 match runs. pressure during recent years, it was necessary to devise However, we do wish to discuss briefly some adjustments a method of handling free-gas capacity. This was accom­ of major import by which we were able to achieve results plished by calculating the change in capacity of the free­ gas zone as the pressure of this zone varied with time, and subtracting or adding a correction to the production 11 II I rate from wells in the zone. The magnitude of the rate corrections for any particular time period depended on tmi the average pressure in the free-gas region during the time interval, and on the distribution of the oil and gas ttl l.! with depth. M ~i 1+ ~, , In history-matching, a trial-and-error procedure was -____·-r-<-u employed in which resistor values, and occasionally capac­ "- -+.1- --t-; i- itance values, were changed until the voltage history of ::t--1r each controller agreed with the recorded pressure history -t--l=-t-

~ - ffihr--

". " ~rn

". 00' , ,

CONTOUR INTERVAL 200 FEET ''-~~~_O'T(-'' __'' ". 50"00' " V FIG. 8-STRUCTURE CONTOUR MAP ON Top OF' THE ARAB-D MEMBER, ABQAIQ FIELD, FIG. 9-CONFIGURATION OF THE RESISTOR­ SAUDI ARABIA. CAPACITOR (RC) NETWORK,

NOVEMBER, 1962 1279 not possible in previous pressure-history matches. Also, Abqaiq Well 7, the nearest oil well. In the past, pressures ,the degree of reliability may be evaluated by comparing in Abqaiq Well 19 have, in general, been considered ano­ actual field pressure data in the two-year period beginning malous, and it was necessary to disregard them in pre­ Jan. 1, 1959, with the model-predicted behavior for the vious history matches. same time period. Other Abqaiq wells that previously had proven difficult to match were the water-injection wells located around ABQAIQ the periphery of the north Abqaiq nose. Model pressures Pressure histories of the majority of the Abqaiq wells invariably ran lower than the field-measured pressures were duplicated on the model with comparatively little in the six flank wells, and higher than observed pressures difficulty. However, considerable time and numerous ad­ justments were required in order to match the pressure in the three nose wells. histories of peripheral wells lying outside of the oil-water Two major adjustments were made to the initial net­ contact. work configuration in order to duplicate Abqaiq Well 19 and water-injection-well pressure histories on the model. Abqaiq Well 19, an observation well completed in the aquifer on the east flank of southern Abqaiq, has always First, a permeability barrier was installed between these exhibited a pressure much higher (150 to 300 psi) than peripheral wells in the oilfield proper (Fig. 10). Then an area of high permeability and expansibility was incorpo­ rated in the aquifer area between the Abqaiq field and the Ghawar field (Fig. 11). The permeability barrier had basis in fact, in that resi­ Downloaded from http://onepetro.org/JPT/article-pdf/14/11/1275/2214599/spe-414-pa.pdf by guest on 02 October 2021 68 dual tar had been observed below the oil-water contact in every Abqaiq well so completed. Unfortunately, this 76 physical evidence had been neglected in previous pressure-

2-7% OF AVERAGE OILPOOL KH 7-10% OF AOJACENT AQU!FERKH

50% OF 01 LPOOL E2lANO/OR AQUIFER KH

FIG. IO-PERMEABILITY BARRIER INSTALLED IN THE ABQAIQ FIELD BETWEEN PERIPHERY WELLS ! II AND THE OILFIELD PROPER. 2200~149 50 I 51 52 53 54 55 56 57 58 59 601 6: 62

FIG. I2-COMPARISON OF AVERAGE MODEL PRESSURE AND AVERAGE FIELD RESERVOIR PRESSURE, SOUTHERN ABQAIQ FIELD.

PSIG ! r,I 3200 -f~, ~ I 'II_1-L~ () '" \ --0--- FIELD DATA H 3000 MODEL DATA I ABOAIO --- i ! I T 2800 \ ! I 1 l I -L U \ fULL SCALE WATER INJ. I , 2600 , l;= ~ 1/ 1-1 ~ I I I 2400 PILOT WATER INJ. ~ 1 HISTORY PREDICTION l

2200 11 48 49 50 51 52 53 54 55 56 57 58 59 60 611621 FIG. ll--HIGH-CAPACITANCE AQUIFER AREA INCORPORATED BETWEEN THE ABQAIQ AND FIG. I3-COMPARISON OF AVERAGE MODEL PRESSURE AND AVERAGE GHAWAR FIELDS. FIELD RESERVOIR PRESSURE, NORTHERN ABQAIQ FIELD.

1280 JOURNAL OF PETROLEUM TECHNOLOGY history matches since the "tarry" material was believed to can be noted in each of the afore-mentioned wells; this have little or no effect on Abqaiq reservoir performance. pressure rise starts in mid-1958, and carries over into the At the time the present match was completed, there prediction period of 1959 and 1960. Although model were no data or physical evidence available to support pressures in Abqaiq Well 72 were low compared to field model requirements of a high-capacitance aquifer area pressure throughout the history period, plus two subse­ between Abqaiq and Ghawar. However, the area could be quent years, they duplicated very closely the incremental interpreted as (1) an undiscovered hydrocarbon accumu­ pressure changes for each time period. In the other three lation, (2) an aquifer in which the thickness of the wells, model pressures were in good agreement with Arab-D member had increased, (3) an aquifer in which measured-pressure performance in 1959 and 1960, as well the pore-volume compressibility of the rock had in­ as during the history period. creased, or (4) a combination of two or more of the GHAWAR foregoing. It also was necessary to incorporate a permeability bar­ Two years of field observations subsequent to the end rier (residual tar) around the entire Ghawar field, except of the history-match period indicate a high degree of reli­ in the Shedgum area, in order to match field pressures. ability for the present model, as can be seen by comparing Shedgum area field pressures simply could not be matched model pressure predictions for 1959 and 1960 with cor­ on the model using a barrier no matter how slight. Drill­ responding field pressure data for the same time interval. ing and core analysis near and on the Shed gum oil-water For data-analyses purposes, Abqaiq field has been div­ contact have substantiated the conclusion that there evi­ ided into two areas (see Fig. 8). Area A (or Southern dently is no tar in this area. Abqaiq) encompasses the main structure of the field, During the 1960-1961 period, a delineation program Downloaded from http://onepetro.org/JPT/article-pdf/14/11/1275/2214599/spe-414-pa.pdf by guest on 02 October 2021 while Area B (or Northern Abqaiq) is roughly limited was carried out on the east flank of north 'Ain Dar. As to the field's low, broad, northern extension. Figs. 12 and the result of the drilling of four wells (see Fig. 17), the 13 are graphs of the average model pressures compared oil-water contact on this side of the field has been moved to the average field reservoir pressure for these two areas. approximately 3 kilometers to the east of the contact (Note: average pressures are the arithmetic averages of used at the time of the pressure-history match on the individual well pressures in an area.) Fig. 14 is graph model, and an additional 1 billion bbl of recoverable oil of model pressure compared to field pressure for Con­ have been added to the 'Ain Dar reserves. The volume troller 4 in Northern Abqaiq. Figs. 12 through 14 are of oil "discovered" is not equivalent to the volume repre­ typical of the match achieved in Abqaiq. sented by the high-capacitance aquifer area between Gha- On a more specific basis, model pressure predictions have proven equally reliable. This is especially significant in those wells that determine Abqaiq boundary conditions. PSIG 3300 N As can be seen in Fig. 15, the model-predicted pres­ "\' --0--- FIELD DATA sures for Abqaiq Wells 19 and 7 are in satisfactory agree­ \ - MaDEL DATA ment with the pressures actually measured in these wells 3100 \ ~ ---- during the post-history period. \ ~ It is not possible to obtain comparative pressure data 2900 ~ for the water-injection wells subsequent to the start-up of ~ I' ABQAIQ WELL NO.. 19 injection, but predicted water-injection benefits can be " ...... \ b evaluated from the pressure performance of the three 2700 ""'" most-northern producing wells (Abqaiq Wells 67, 72 and r\ '" - 73), and a future water-injection well (Abqaiq Well 74). 1\ ABQAIQ WELL NO. 7 In Fig. 16, the oil-pool pressure rise due to water injection 2500 HISTO~Y p~EbICTION 2300 PSI G co:::---r----,---,----,--,-----;---,------r--,----r----,--,-----;-~ 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 6263 64 65 66

3300~ I FIG. IS-WELL PRESSURE VS TIME, ABQAIQ WELLS 7 AND 19. I f\ ! I : : 3200 ~_r~-~~-~-L-r--r~ I ---0-- FIELO OATA l PSIG i\ i ___ MODEL DATA j AW-72 3100t--~j__: : \ i ! ! i I I ~t--j--I,---l 2800 3000~_r~-~~--T-_+-r-~-~~~-+__+--1 2600 !: i i! II II 2900r---!--]-:.!\' , ! I HISTORY PREDICTION 2400 2600r--;! i I ~ I I I I HISTORY IPREDICTION HISTORY I PREDICTION 2700 - 1 , '>d-~-I-----t!-+--_+-I---+_-+___1

Ii'" ! I 2600~-r---t-,-~-+__T-~~-~·,-~-~~___1 I I !! I "I"'- I/~ 2500---jl--1 I b:::~

24001----~~-+_-4-+__+-~~-I----r___t-+__+___1

2300~4~6-r4~9-r5~0+-5-1~'-5-2+-53-+-54-+~55~5~6~5~7-r5~6+-5-9+-6~0+-6-i1 '57 '58 '59 '60 '55 '56 '57 '58 '59 '60

FIG. 14--COMPARISON OF MODEL PRESSURE AND FIELD PRESSURE FIG. 16---COMPARISON OF MODEL VS FIELD BOTTOM-HoLE FOR CONTROLLER 4, NORTHERN ABQAIQ FIELD. PRESSURES FOR FOUR ABQAIQ WELLS.

NOVEMBER, 1962 1281 PSIG

3200

3100 "" --0- FIELD DATA

'\. -MODEL DATA- ~ 3000 , 1'\ I I I 2900 I I I HISTORY PREDICTION 2800 \

2700 1\

2600 ~ ,-

82 -- " , e 2500 f-- I 61 I ~ o 2400 OIL ADDED GHAWAR IN 1960-61 2300 JDELINEATION Downloaded from http://onepetro.org/JPT/article-pdf/14/11/1275/2214599/spe-414-pa.pdf by guest on 02 October 2021 2200 . I 52 53 54 55 56 57 58 59 60 61

FIG. 17-DELINEATION OF THE NORTH 'AlN DAB FIG. 19-COMPARISON OF AVERAGE MODEL PRES· AREA, GHAWAR FIELD. SURE AND AVERAGE FIELD RESERVOIR PRESSURE, SHEDGUM AREA, GHAWAR FIELD.

PSIG

3100 I~ psig. The initial pressure measured in the injection well ! CAin Dar WeIl 61) was 2,500 psig, which compares fa­ 3000 \ --0-- FIELD DATA vorably with a model-predicted pressure of 2,466 psig. 290() \ ------MODEL DATA Here, Ghawar pressures are given at the Ghawar datum I level and Abqaiq pressures at the Abqaiq datum. 2800 1\ In summary, the results obtained to date lend increased confidence to model predictions of future reservoir per­ 2700 1\ I I formance. HISTORY PREDICTION 2600 ~ ACKNOWLEDGMENTS 2500 f------~ '- I The authors wish to acknowledge the many contribu­ 2400 tions made by others who have worked on the project. "- From Socony Mobil Oil Co., Inc., W. H. Wilson, E. E. 2300 "I Moreland and W. F. Baldwin have made major contribu­ "t tions. From Arabian American Oil Co., Q. Lowman and 2200 '"I ! A. Davis have been closely associated with the project. I 2100 i I The authors also wish to thank the managements of 51 52 53 54 55 56 57 58 59 60 61 Socony Mobil and Aramco for permission to present this paper. FIG. 18-COMPARISON OF AVERAGE MODEL PRES· SURE AND AVERAGE FIELD RESERVOIR PRESSURE, 'AlN DAR AREA, GHAWAR FIELD. REFERENCES 1. Bruce, W. A.: "An Electrical Device for Analyzing Oil·Reser· voir Behavior", Trans., AIME (1943) 151, 112. war and Abqaiq, but it does support model predictions 2. Liebman, G.: "Solution of Partial Differential Equations with that more capacitance was required in this area. It is a Resistance Network Analogue", Brit. Jour. App!. Phys. even possible that east flank delineation still may not be (1950) I, 92 and accompanying literature citations. complete. 3. Liebman, G.: "Resistance Network Analogues With Unequal Figs. 18 and 19 are graphs of the average model pres­ Meshes of Sub· divided Meshes", Brit. Jour. Appl. Phys. (1954) sure compared to average field reservoir pressure for the 5, 362 and accompanying literature citations. 'Ain Dar and Shedgum areas in Ghawar. Again, the aver­ 4. Patterson, O. L., Montague, K. E. and Wiess, Byron: "High Speed Electronic Reservoir Analyzer", Drill. and Prod. Prac., age model pressure curves for these two areas attest to API (1951) 47. the over-all accuracy of the match. The match of indi­ 5. Patterson, O. L., Dutton, C. G. O. and Ellis, H. E.: "The vidual well pressures was excellent. Determination of the Water· Injection Program for the Delhi An additional check on the validity of the match was Field by Means of the Automatic Multi·Pool Analyzer", Jour. obtained with the drilling of a water observation well Pet. Tech. (March, 1956) VIII, No.3, 73. (Abqaiq Well 82) in the saddle area between Abqaiq 6. Habitat of Oil, a symposium edited by Lewis G. Weeks, AAPG, and Ghawar and a future water-injection well CAin Dar Tulsa, Okla. (1958). Well 61) in 'Ain Dar (see Fig. 17). The initial pressure 7. Thralls, W. H. and Hasson, R. c.: "Geology and Oil Resources of Eastern Saudi Arabia", Paper presented at Symposium measured in Abqaiq Well 82 was 2,913 psig, which com­ Sobre Yacirnientos de Petroleo y Gas, 20th Int. Geo!. Cong., pares favorably with a model-predicted pressure of 2,883 Mexico City (1956). ***

1282 JOURNAL OF PETROLEUM TECHNOLOGY