Petroleo y gas

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Petroleum Reserves Estimation Methods
© 2003-2004 Petrobjects www.petrobjects.com

Introduction
The process of estimating oil and gas reserves for a producing field continues throughout the life of the field. There is always uncertainty in making such estimates. The level of uncertainty is affected by the following factors: 1. Reservoir type, 2. Source of reservoir energy, 3. Quantity andquality of the geological, engineering, and geophysical data, 4. Assumptions adopted when making the estimate, 5. Available technology, and 6. Experience and knowledge of the evaluator. The magnitude of uncertainty, however, decreases with time until the economic limit is reached and the ultimate recovery is realized, see Figure 1.

Figure 1: Magnitude of uncertainty in reserves estimates The oiland gas reserves estimation methods can be grouped into the following categories: 1. Analogy, 2. Volumetric, 3. Decline analysis, 4. Material balance calculations for oil reservoirs, 5. Material balance calculations for gas reservoirs, 6. Reservoir simulation.

© 2003-2004 Petrobjects www.petrobjects.com

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Petroleum Reserves Estimation Methods
© 2003-2004 Petrobjects www.petrobjects.comIn the early stages of development, reserves estimates are restricted to the analogy and volumetric calculations. The analogy method is applied by comparing factors for the analogous and current fields or wells. A close-to-abandonment analogous field is taken as an approximate to the current field. This method is most useful when running the economics on the current field; which is supposed tobe an exploratory field. The volumetric method, on the other hand, entails determining the areal extent of the reservoir, the rock pore volume, and the fluid content within the pore volume. This provides an estimate of the amount of hydrocarbons-in-place. The ultimate recovery, then, can be estimated by using an appropriate recovery factor. Each of the factors used in the calculation above haveinherent uncertainties that, when combined, cause significant uncertainties in the reserves estimate. As production and pressure data from a field become available, decline analysis and material balance calculations, become the predominant methods of calculating reserves. These methods greatly reduce the uncertainty in reserves estimates; however, during early depletion, caution should be exercisedin using them. Decline curve relationships are empirical, and rely on uniform, lengthy production periods. It is more suited to oil wells, which are usually produced against fixed bottom-hole pressures. In gas wells, however, wellhead back-pressures usually fluctuate, causing varying production trends and therefore, not as reliable. The most common decline curve relationship is the constantpercentage decline (exponential). With more and more low productivity wells coming on stream, there is currently a swing toward decline rates proportional to production rates (hyperbolic and harmonic). Although some wells exhibit these trends, hyperbolic or harmonic decline extrapolations should only be used for these specific cases. Overexuberance in the use of hyperbolic or harmonic relationships canresult in excessive reserves estimates. Material balance calculation is an excellent tool for estimating gas reserves. If a reservoir comprises a closed system and contains single-phase gas, the pressure in the reservoir will decline proportionately to the amount of gas produced. Unfortunately, sometimes bottom water drive in gas reservoirs contributes to the depletion mechanism, altering theperformance of the non-ideal gas law in the reservoir. Under these conditions, optimistic reserves estimates can result. When calculating reserves using any of the above methods, two calculation procedures may be used: deterministic and/or probabilistic. The deterministic method is by far the most common. The procedure is to select a single value for each parameter to input into an appropriate...
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