A Decline Curve Analysis Model Based on Fluid Flow Mechanisms
- Kewen Li (Stanford U.) | Roland N. Horne (Stanford U.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2005
- Document Type
- Journal Paper
- 197 - 204
- 2005. Society of Petroleum Engineers
- 5.8.7 Carbonate Reservoir, 5.1.2 Faults and Fracture Characterisation, 1.6.9 Coring, Fishing, 5.4.1 Waterflooding, 5.6.4 Drillstem/Well Testing, 5.7.2 Recovery Factors, 5.9.2 Geothermal Resources, 5.5.8 History Matching, 6.5.2 Water use, produced water discharge and disposal, 1.2.3 Rock properties, 5.3.1 Flow in Porous Media, 5.5.2 Core Analysis, 5.6.5 Tracers, 4.6 Natural Gas, 5.8.6 Naturally Fractured Reservoir, 1.6 Drilling Operations, 5.3.2 Multiphase Flow, 5.7 Reserves Evaluation
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Decline-curve-analysis models are used frequently but still have manylimitations. Approaches of decline-curve analysis used for naturally fracturedreservoirs developed by waterflooding have been few. To this end, adecline-analysis model derived on the basis of fluid-flow mechanisms wasproposed and used to analyze the oil-production data from naturally fracturedreservoirs developed by waterflooding. Relative permeability and capillarypressure were included in this model. The model reveals a linear relationshipbetween the oil-production rate and the reciprocal of the oil recovery or theaccumulated oil production. We applied the model to the oil-production datafrom different types of reservoirs and found a linear relationship between theproduction rate and the reciprocal of the oil recovery as foreseen by themodel, especially at the late period of production. The values of maximum oilrecovery for the example reservoirs were evaluated with the parametersdetermined from the linear relationship. An analytical oil-recovery model wasalso proposed. The results showed that the analytical model could match theoil-production data satisfactorily. We also demonstrated that the frequentlyused nonlinear type curves could be transformed to linear relationships in alog-log plot. This may facilitate the production-decline analysis. Finally, theanalytical model was compared with conventional models.
Estimating reserves and predicting production in reservoirs has been achallenge for many years. Many methods have been developed in the last severaldecades. One frequently used technique is the decline-curve-analysis approach.There have been a great number of papers on this subject.
Most of the existing decline-curve-analysis techniques are based on theempirical Arps equations: exponential, hyperbolic, and harmonic. It isdifficult to foresee which equation the reservoir will follow. On the otherhand, each approach has some disadvantages. For example, the exponentialdecline curve tends to underestimate reserves and production rates; theharmonic decline curve has a tendency to overpredict the reservoir performance.In some cases, production-decline data do not follow any model but cross overthe entire set of curves.
Fetkovich combined the transient rate and the pseudosteady-state declinecurves in a single graph. He also related the empirical equations of Arps tothe single-phase-flow solutions and attempted to provide a theoretical basisfor the Arps equations. This was realized by developing the connection betweenthe material balance and the flow-rate equations on the basis of his previouspapers.
Many derivations were based on the assumption of single-phase oil flow inclosed-boundary systems. These solutions were suitable only for undersaturated(single-phase) oil flow. However, many oil fields are developed bywaterflooding. Therefore, two-phase fluid flow (rather than single-phase flow)occurs. In this case, Lefkovits and Matthews derived the exponential declineform for gravity-drainage reservoirs with a free surface by neglectingcapillary pressure. Fetkovich et al. included gas/oil relative permeabilityeffects on oil production for solution-gas drive through the pressure-ratioterm. This assumes that the oil relative permeability is a function ofpressure. It is known that gas/oil relative permeability is a function of fluidsaturation, which depends on fluid/rock properties.
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