Analytical and Numerical Analysis for Estimation Hydrocarbon in Place for Fauqi Oil Field

The Fauqi oil field is located about 50Km north-east Amara town in Iraq. This field has two producing reservoir units, the Asmari and the deeper Mishrif. Fauqi field is an anticline crossing the IraqiIranian border and approximately 15 km long and 6 km wide. The Fauqi anticline is most probably segmented by several faults due to its location on Zagrous mountains area. Since, it is not possible to get full knowledge on the extent, orientation and segmentation of the field. Production data used for the Material Balance analyses is comprised of a record, by well, of monthly cumulative oil production for the period 01 May 1979 – 01November 2007. The field was shut in due to the Iran-Iraq war between September 1980 and August 1998 and due to Gulf war between March to December 2003. Twelve major modules, with different degrees of analytical sophistication have been used to prove oil initially in place (STOIIP) for Fauqi oil field throughout analyzing single well production history of FQ-8A. The accuracy for the obtained results has been investigated which show an acceptable degree of reliability. Journal of Petroleum Research & Studies E 187 Performing the twelve of different analytical and numerical calculation for the production history of Fauqi oil well Fq-8A, may assist to reduce the uncertainties in the reservoir calculation of the STOIIP especially in Asmari reservoir which show a considerable degree of uncertainty between the analytical (multiple column model) and the (full tank model) and also with the numerical calculation صـخلـــملا خفبغًث خكفنا ممز غمٚ 50 قاشؼنا ٙف حسبًؼنا خُٚذي قشش لبًش ىك . كهزًٚ ممسنا ازْ خًُٛكي ٍٛرذزٔ , فششًنا ْٙ كًػلاأ ٘شًعلاا . مخاد ذزًر خجل ٍي ٌٕكزٚ خكفنا ممز ٙنإز دبؼثأ ّنٔ خَٛاشٚلاا دٔذسنا 15 ٔ لٕؽ ىك 6 عشػ ىك . لبجخ ؾٚشش ٗهػ ِدٕخٔ تجغث قٕمشٔ كنإف ٍي ٌٕكزٚ لبًززلاا تهغا ٗهػ خكفنا ممز طٔشكاص , ٌٔد ٍي ممسنا ٙف قٕمشنا دبْبدرأٔ لإؽأ خفشؼي ٍكًًنا شٛغ َّبف كنر غي خٛناضنضنا دبزٕغًنا ءاشخأ . حدبًنا خَصإي مٛهسر ٙف ذهًؼزعأ ٙزنا خٛخبزَلاا دبيٕهؼًنا ٍي حشزفهن 1 ظٚبي 1979 خٚبغن 1 َٙبثنا ٍٚششر 2007 . خَٛاشٚلاا خٛلاشؼنا ةشسنا تجغث كهغأ ممسنا ٌأٔ لٕهٚأ ٍي حشزفهن 1980 ةأ لا 1998 ٌَٕبك ٗنا سارأ ٍي حشزفهن حٛهخنا ةشز تجغث كنزكٔ لٔلاا 2003 . دبجثلا خٛهٛهسزنا دداذزيلاا فهزخًث خغٛئس خمٚشؽ حششػ بُثأ ذهًؼزعأ ٙؽبٛززلاا خكف شئجن ٙخبزَلاا خٚسبزنا مٛهسر للاخ ٍي خكفنا ممسن ٙنٔلاا 8 . بٓهٛهسر ىر حئبزُنا خلد دبًزػلاا ٍي خنٕجمي خخسد دشٓػأٔ . مٛهمر ٗهػ ذػبع جبزَلاا خٚسبزن خٚدذػٔ خهٛهسر قشطث خٛثبغز خٛهًػ حششػ بُثأ صبدَأ خطبخٔ ممسهن ٙنٔلاا ٙطفُنا ٖٕزسًنا ةبغز دبمفإر وذػ للاخ ٍي ٘شًعلاا ًٍكًن ا ي بٓزَسبمئ خمجؽ مكن ؾفُنا دبؼجشر لبًؼزع ممسنا مكن غجشزنا لذؼي لبًؼزعأ غ . Journal of Petroleum Research & Studies

Performing the twelve of different analytical and numerical calculation for the production history of Fauqi oil well Fq-8A, may assist to reduce the uncertainties in the reservoir calculation of the STOIIP especially in Asmari reservoir which show a considerable degree of uncertainty between the analytical (multiple column model) and the (full tank model) and also with the numerical calculation

Introduction
Accurate determination of oil in place in a reservoir is important when decisions are being made regarding development of a field; it is even more important later when decisions are made regarding installation of fluid injection projects when less of the oil remains; and it is extremely important in considering the recovery of additional oil by tertiary methods [1].
Frequently, interpreting pressure-production performance of the reservoir through material balance techniques helps to establish the reliability of volumetric estimates. In some very heterogeneous reservoir rocks or in some reservoirs of limited areal extent, a material balance estimate is superior to the volumetric estimate. Uncertainties exist in all factors involved in both types of estimates [2].
Because of alteration of cores during coring, handling, and analysis, volumetric estimates of oil in place in unconsolidated reservoirs are subject to added uncertainties [2].
Errors in the oil, gas and water production data are unavoidable.
It has always been a matter of concern that these errors may have a serious effect on the results of model studies to determine the original oil-in-place [3].

Results and Discussion
Twelve major modules, with different degrees of analytical sophistication have been analyzed to provide reasonable confidence for estimation STOIIP for Fauqi oil field, throughout analyzing single well production history of FQ-8A; the pressure measurements already corrected to datum of 3030 mSL for the Asmari and 3950 mSL for Mishrif reservoirs. These major models have been illustrated in Appendix.

1-Asmari Reservoir:
The analytical and numerical simulation for the production history matching of well FQ-8A shown in figs. (1)(2)(3)(4)(5)(6) and the analysis of flowing material balance shown in fig. (7), in addition to that models depends for type curve matching, provide the STOIIP for the entire Asmari formation of Fauqi oil field as listed in Table (1); while, the detailed analysis of multilayer reservoir has been listed in Table   (2) to provide more detailed analysis of the STOIIP for the individual layers of Asmari reservoir.
The results show large uncertainties (58%) in the Asmari STOIIP between the analytical and numerical methods. The STOIIP [1.01 MMMbbl] for analytical (multiple columns model) to [1.6 MMMbbl] (full tank) numerical modeling. Moreover, the analytical multiple oil columns provide about (30%) difference than other analytical of radial, water drive and material balance modeling.

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Since, it could be concluded that the analytical models may it could be seen the early observed pressure decline is less than predicted suggesting the need for pressure support from an aquifer.
Since, because the reservoir has a circular geometry, suggestion for limited pressure support of edge-water drive aquifer model was selected; this was done in order to represent the vertical communication that exists between the individual reservoirs; this modeling of water drive has been shown in fig. (3).
However, figs. (4)(5)(6) show also the predicted pressure response versus the pressure history in three different analysis for STOIIP, as could be seen the quality of the history match is acceptable indicating that Asmari reservoir of Fauqi oil field is surrounded by active drive aquifer. The strength of the aquifer is also may be due to fractures that may exist in Asmari formation.    Fig. (14), in addition to that models depends for type curve matching, provide the STOIIP for the entire Mishrif formation as listed in Table (3).   of uncertainty between the analytical (multiple column model) and

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the (full tank model) and also with the numerical calculation.
4. The calculated STOIIP for Mishrif is 244 MMbbl. has more reliability than that of Asmari reservoir in spite of limited production history data got for this reservoir.
5. Assess the information from 3D seismic survey to obtain faults and fracture information. In addition to running detailed production logging test (PLT) to understand intervals contributing in fluids production, will be very important to provide full detailed study for this field.

Traditional Modeling
These analytical modeling Decline curve analysis is a graphical procedure used for analyzing declining production rates and forecasting future performance of oil and gas wells. A curve fit of past production performance is done using certain standard curves. This curve fit is then extrapolated to predict potential future performance.
Decline curve analysis is a basic tool for estimating recoverable

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reserves. Conventional or basic decline curve analysis can be used only when the production history is long enough that a trend can be identified.
It is implicitly assumed, when using decline curve analysis, the factors causing the historical decline continue unchanged during the forecast period. These factors include both reservoir conditions and operating conditions. Some of the reservoir factors that affect the decline rate include; pressure depletion, number of producing wells, drive mechanism, reservoir characteristics, saturation changes, and relative permeability. Operating conditions that influence the decline rate are: separator pressure, tubing size, choke setting, workovers, compression, operating hours, and artificial lift. As long as these conditions do not change, the trend in decline can be analyzed and extrapolated to forecast future well performance. If these conditions are altered, for example through a well workover, then the decline rate determined pre-workover will not be applicable to the post-workover period.
Decline curve analysis is derived from empirical observations of the production performance of oil and gas wells. Three types of decline have been observed historically: exponential, hyperbolic, and harmonic. All decline curve theory starts from the definition of the instantaneous or current decline rate (D) as follows;   type curves for estimating gas or oil in place. q/P is plotted against dimensionless cumulative production.

Flowing Material Balance
The Flowing Material Balance uses the concept of stabilized or "pseudo-steady-state" flow to evaluate total in-place fluid volumes. In a conventional material-balance calculation, reservoir pressure is

Wattenbarger Type curve Analysis
Long linear flow has been observed in many gas wells. These wells are usually in very tight gas reservoirs with hydraulically fractures designed to extend to or nearly to the drainage boundary of the well. Wattenbarger et al. (1998) presented new type curves to analyze the production data of these gas wells. They assumed a hydraulically fractured well in the center of a rectangular reservoir.
The fracture is assumed to be extended to the boundaries of the reservoir.

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Numerical (Multi-phase) Modeling: The assumption of the analytical models for production data analysisx is single phase flow in the reservoir. In order to accommodate multiple flowing phases, the model must be able to handle changing fluid saturations and relative permeabilities. Since these phenomena are highly non-linear, analytical solutions are very difficult to obtain and use. Thus, numerical models are generally used to provide solutions for the multi-phase flow problem.
The advantages of numerical method approach are that the reservoir heterogeneity, mass transfer between phases, and forces/mechanisms responsible for flow can be taken into consideration adequately, for instance, multiphase flow, capillary and gravity forces, spatial variations of rock properties, fluid properties, and relative permeability characteristics can be represented accurately in a numerical model. In general, analytical methods provide exact solutions to simplified problems, while numerical methods yield approximate solutions to the exact problems.
The Numerical modeling assumes a cylindrical reservoir model used for single-well studies. Cylindrical grids are used in the reservoir (see fig. 17). The grid block size increases logarithmically in size outward from the well. Small grids near the wellbore can effectively simulate the well behavior. In current version of Rate Transient Analysis software, numerical model is a one-dimension radial model,

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and gas is modeled by single-phase model, oil can be modeled either by single-phase model (pressure above the bubble-point) or by multiphase model.