A Kinetic Model of CO2 Generation and Mineral and Isotopic Alteration During Steamflooding
- L.M. Cathles (Chevron Oil Field Research Co.) | Martin Schoell (Chevron Oil Field Research Co.) | Ralph Simon (Chevron Oil Field Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1990
- Document Type
- Journal Paper
- 524 - 530
- 1990. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.8.5 Oil Sand, Oil Shale, Bitumen, 4.1.5 Processing Equipment, 2.4.3 Sand/Solids Control, 6.5.2 Water use, produced water discharge and disposal, 1.8 Formation Damage, 5.5 Reservoir Simulation, 5.4.10 Microbial Methods, 5.9.2 Geothermal Resources, 5.4.6 Thermal Methods
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CO2 is commonly produced during steamflooding. Inhigher-temperature steamfloods the amount of CO2 produced is suchthat the only viable source is carbonate minerals. We show by reference toCO2 concentrations in natural geothermal systems that in most commonrocks and sediments, CO2 is generated naturally under hydrothermal(~572°F) conditions by the dissolution of carbonate minerals. CO2solubility is a strong positive function of temperature and, at 572°F, is morethan sufficient to account for high CO2 concentrations like thoseencountered at Chevron's Buena Vista (BV) Hills steamflood pilot test. A linearkinetic model is developed to describe chemical equilibration andCO2 generation in the hot parts of the reservoir. The model alsodescribes CO2 removal through carbonate precipitation in the coolerparts of the rock formation where the fluid moves ahead of the thermal front.Changes in carbon isotope concentration are included in the model. The smalldifference between the 13C in the produced gas and the sourcecarbonates at BV Hills suggests that little CO2 is precipitated ascarbonate within that reservoir, a prediction that can be tested bypoststeamflood coring.
The steamflooding process for oil recovery often produces large quantitiesof CO2. Over 1 year at Chevron's BV Hills, CA, steamflood pilotproject, for example, 80×106 scf of CO2 was produced atan average ratio of 31 scf of CO2/bbl steam injected [cold waterequivalent (CWE)]. At Texaco's steamflood in the San Ardo Field,1gas that was 80% CO2 was produced at the rate of 3.5×106scf/D (35 scf/bbl steam injected). CO2 production duringsteamflooding was also documented at the Lacq Supérieur field in southwesternFrance.2 CO2 production appears to occur in allsteamfloods, with greater quantities occurring in higher-temperaturesteamfloods.
CO2 production is important because it can accelerate corrosionand because in-situ generation and precipitation of CO2 can affectmass-balance and pressure calculations in reservoir engineering models of thesteamflood process. To the best of our knowledge, there has been no previousattempt to account for quantitatively or to explain the CO2 producedduring steamflooding with an integrated physical/chemical kinetic model.
The purpose of this paper is to present a model of CO2 productionduring steamflooding that identifies the principal variables controlling theamount of CO2 produced and that provides insight into theconsequences of the CO2 production for reservoir simulation of thesteamflooding process.
BV Hills Steamflood
From May 1985 through May 1986, 65 acres of the 120-ft-thick oil-bearingPliocene Wilhelm formation at BV Hills was subjected to steam injection. Steam(80 wt%) and hot water (20 wt%) entered the Wilhelm at about 2,500-ft depth and572°F under a driving pressure of 1,700 psi. Injection into 12 wells was steadyover the year at about 7,000 B/D (CWE). Water, oil, and gas were produced from20 production wells, drawn down to atmospheric pressure. The wells werearranged in five-spot patterns with about 340 ft between injection andproducing wells. At the start of the test, the reservoir pressure in the testarea had been reduced by previous oil production to about 130 psi. Reservoirtemperature was about 113°F. At the end of the first year, the temperatures inthe producing wells had risen to 140 to 180°F.
In the first year about 2.5 million bbl of steam (CWE) was injected andabout 1 million bbl of water and 105 million scf of gas were produced. Gasproduction began a few months after the start of the test and then increasedlinearly to about 700,000 scf/D by the end of 1 year. Initially, most of thisgas was methane (and higher hydrocarbons), but, during the year, CO2in the gas steadily increased until it was about 90 mol% at the end of theyear. On average for the year, the gas produced ran about 75 mol%CO2.
Several lines of evidence indicated that the produced CO2 derivedfrom the breakdown of inorganic carbonate in the Wilhelm formation. First, thiswas the only source of carbon large enough. The water injected contained ~133ppm HCO3, but this accounts for only 0.9% of the 80 million scf(~100 million g mol) of CO2 produced in the first year. Formationwater could have contributed at most 5%, and organic acids in all the Wilhelmformation oil in the test site could have contributed at most 16%. The amountof CO2 that could have been produced from organic sources isactually much less than this. Because of channeling, only a fraction of theWilhelm formation in the test area is believed to have been heated sufficientlyto cause the breakdown of organic acids to CO2.
On the other hand, the Wilhelm formation contains a few wt% calcite and 4 to21 wt% dolomite. One wt% carbonate in a 6-ft sand layer within the 65-acre testarea could generate 92 million mol of CO2.
The isotropic composition of the Wilhelm carbonate is unusually heavy [15 to28 o/oo vs. PDB (Belemnitella Americana from the Cretaceous Peedeeformation, SC) as shown in Table 1]. The isotropic composition of theproduced CO2 matches this unusual composition almost exactly (19 to26 o/oo vs. PDB, as shown in Fig. 1). CO2 generation from CH4is unlikely because of the very slow kinetics of the reaction and the lack ofan obvious oxidant. In any case, CO2 generation from CH4at 392 to 572°F would have the wrong isotopic composition (-5 o/oo) if thesource CH4 had the isotopic composition of the CH4 produced (Fig.1).3 Field and chemical evidence thus strongly suggests that theCO2 produced at BV Hills came from carbonate breakdown.
The question that must be addressed is how this breakdown is achieved duringsteamflooding. Further questions of interest are how much CO2 isprecipitated ahead of the thermal front and what do the chemical changes in thereservoir imply for the evolution of the steamdrive and the physical propertiesof the rock formation?
CO2 Generation in Natural Geothermal Systems
A clue to the processes that may be involved is provided by data fromnatural geothermal systems. Waters in natural geothermal systems contain largeamounts of CO2. As shown in Fig. 2, the CO2content of unboiled geothermal waters is relatively independent of themineralogy of the host formation and increases strongly with temperature.
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