Performance of a Heavy-Oil Field Under CO2 Injection, Bati Raman, Turkey
- Kayhan Issever (Turkish Petroleum Corp.) | A. Necdet Pamir (Turkish Petroleum Corp.) | Ali Tirek (Turkish Petroleum Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1993
- Document Type
- Journal Paper
- 256 - 260
- 1993. Society of Petroleum Engineers
- 5.1.1 Exploration, Development, Structural Geology, 5.1 Reservoir Characterisation, 5.4.6 Thermal Methods, 4.1.9 Tanks and storage systems, 5.4 Enhanced Recovery, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.1.2 Separation and Treating, 5.1.2 Faults and Fracture Characterisation, 5.7.2 Recovery Factors, 5.5 Reservoir Simulation, 5.4.2 Gas Injection Methods, 1.6 Drilling Operations, 4.6 Natural Gas, 5.4.10 Microbial Methods, 5.2 Reservoir Fluid Dynamics, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.6.4 Drillstem/Well Testing, 5.2.1 Phase Behavior and PVT Measurements, 5.4.1 Waterflooding
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Batt Raman heavy-oil (12API [0.986 g/cm ]) field, with 1.85 x 109 STB (294 x106 stock-tank M3) original oil in place (OOIP) and poor primary recovery (1.5%OOIP), was a good candidate for EOR applications. Detailed studies resulted inapplication of an immiscible CO2 injection in a 33-well pilot area. Aninitially planned huff 'n' puff became a successful continuous CO2 flood andencouraged us in puff became a successful continuous CO2 flood and encouragedus in our decision to widen the project to the central part of the field.Incremental oil production of 5.742 x 10 STB [913 x 10 stock-tank m ] resultedfrom 28-MMscf [7.9x 105 std m ] CO2 injection. Oil production increased from900 STB/D C 143. 1 stock-tank m /d] production increased from 900 STB/D C 143.1 stock-tank m /d] primary production to 10,500 STB/D [1669 stock-tank m /d] inthe primary production to 10,500 STB/D [1669 stock-tank m /d] in the treatedarea.
The Bab Raman heavy-oil field (Fig. 1), discovered in 1961 in southeastTurkey, is the largest oilfield in Turkey. It is a low-pressure fieldcontaining low-gravity (12API [0.986-g/cm3]), high-viscosity (592-cp [0. 592-Pas]) oil at an average depth of 4,300 ft [1311 m] with a gross thickness of 210ft (64 m) and 1.85 x 10 STB [294 x 10 stock-tank m ] OOIP. The producingformation of the field is Cretaceous Garzan limestone, an elongated cast/westasymmetric anticline measuring - 10. 5 x 2.5 miles [ - 17 x 4 km]. Thereservoir is limited in the north and west by an oil/water contact (OWC) at1,970 ft (600 m) subsea, in the southwest by a fault, and in the south andsoutheast by a permeability pinchout. The Garzan limestone has a reefal originand a fractured, vuggy character exhibiting areal and vertical heterogeneities.The structure becomes chalky and thus tighter to the east. Average porosity is18% and is mainly vugular and fissured in type. porosity is 18% and is mainlyvugular and fissured in type. Average matrix permeability determined from coresis - 16 md [0.016 m2]. Well tests indicate 200- to 500-md [0.20- to 0.48-m ]effective permeabilities, confirming the contribution of secondary porosity.permeabilities, confirming the contribution of secondary porosity. In thewestern and central ponions of the field, a secondary vugular porosityinterconnected by fissures appears to be superimposed over porosityinterconnected by fissures appears to be superimposed over a low primary matrixporosity. The main (primary) production mechanism is rock and fluid expansion.Waterdrive appears to be insignificant except for a very weak aquifer influenceat the central north flank wells. The solution GOR is 18 scf/STB (3.24 std m/stock-tank m ], resulting in a low bubblepoint pressure of 160 psi [1103.1kPa]. Before EOR application, the reservoir pressure did not decrease below thebubblepoint pressure; therefore, in practice, there had been nosolutiongas-drive mechanism. The original reservoir pressure was - 1,800 psi [- 12 41 1 kPa], which dropped to an average pressure of 400 psi [2758 kPa)after cumulative production of 30 million STB [4.77 x 10 stock-tank m ) beforeCO2 production of 30 million STB [4.77 x 10 stock-tank m ) before CO2application. The field originally was developed on 62-acre/well (25-ha/well]spacing. Before the project began, 65 active producers were pumping, with atotal production rate of 1,600 STB/D [254.4 stock-tank m /d] compared with the1969 peak rate of - 9,000 STB/D [ 1431 stock-tank m3/d]. Initially, wellproduction rates were up to 400 STB/D (63.6 stock-tank m /d]; this decreased toan average of 25 STB/D [3.97 stock-tank m /d] before treatment. Table 1summarizes reservoir and fluid characteristics. Primary recovery prospects werelow because of unfavorable oil properties (such as low API gravity, lowsolution gas, and high properties (such as low API gravity, low solution gas,and high viscosity), low reservoir energy, and the type of driving mechanism.It was estimated that ultimately 1.5% OOIP could be produced without an EORapplication. The reservoir history and unfavorable properties caused rapiddeclines in reservoir pressure and production. which suggested (he need for asuitable EOR method to increase ultimate recovery from this vast reservoir.Since 1968, several EOR pilot tests have been conducted, including steam huff'n' puff, steamdrive, air injection, and waterflooding. The results ofextensive laboratory, modeling. and engineering studies; the presence of a CO2reservoir 55 miles (89 km] from Bati Raman; and econon-dc considerations ledTurkiye Petrolleri AO to apply an immiscible C02 huff-'n'-puff-type EOR methodPetrolleri AO to apply an immiscible C02 huff-'n'-puff-type EOR method to thefield. The reservoir gas in Dodan field was almost pure CO2. Table 2 gives theaverage composition of Dodan gas. The present field production capacity is 60MMscf/D [ 1.7 x 106 std m /d].
Because of the high molecular weight of Bati Raman oil, the miscibilitypressure is considerably higher than the reservoir pressure. Therefore, thesuccess of CO2 injection in the Bati pressure. Therefore, the success of CO2injection in the Bati Raman field should rely primarily on the oil-swellingeffect of CO2 after it goes into solution and its viscosity-reduction effect.The efficiency will increase if maximum contact takes place between theinjected gas and the reservoir oil, causing the gas to go into solution aspressure increases. Primary production performance of the Bati Raman fieldshowed that the oil in the reservoir is in the matrix and oil flow is throughthe fractures, which indicated a probable fracture/matrix system. In such asystem, we would expect injected gas to move into the fractures first and thenmove toward the matrix. For this reason, in a fractured system. fracture/matrixdiffusion works as a basic mechanism to enhance oil recovery in an immiscibleCO2 injection application. Diffusion occurs in various ways between thefracture and the matrix. Gas in the gas or oil phase can diffuse into the oilphase in the matrix. and gas in the gas or oil phase in the fractures candiffuse into the gas phase in the matrix. To increase oil production, however,diffusion of gas in the fractures into oil in the matrix is the most importantfactor. Laboratory studies showed that the diffusion constant varies from0.00372 to 0.00706 ft2/D [0.04 x 10 to 0.076 x 10 m /s]. A more importantfactor in diffusion is the area through which the diffusion process iseffective. For this reason. the effectiveness of the fracture/matrix diffusionmechanism depends strongly on the number of fractures and the distance betweenfractures. The difficulties in defining and evaluating these parametersquantitatively result in process performance uncertainties. Therefore,calculations or approaches not supported by a pilot application could givemisleading results when evaluating the success of the method. In the simulatorused to monitor this work, the rate of diffusion between the fracture and thematrix is related to mass-tmnsfer coefficients defined for both the liquid andvapor phases. Mass-transfer coefficients in turn depend on the matrix phases.Mass-transfer coefficients in turn depend on the matrix shape factor, whichcharacterizes the matrix block surface area per unit volume. The rate ofdiffusion was controlled by changing the matrix shape factor in different runs,and its significance in the recovery mechanism was observed. Matrix shapefactor is solely a function of the distance between fractures and is defined inthree dimensions. Increasing the distance between fractures decreased thecontact area between the matrix and fractures and thus the diffusion rate. Theheterogeneous character of the reservoir made defining the matrix shape factorcomplex. Because of reservoir heterogeneity, different matrix shape factors hadto be matched for different parts of the reservoir.
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