The objective of this work is to present a novel methodology – Hybrid Method - to estimate reserves and resources in mature fields. The Hybrid Method consists of combining the probabilistic reserve estimation by volumetric method with the Arps´ decline curve analysis, also including a probabilistic approach. The understanding and managing of uncertainties in reserves and resources estimation are two of the most important problems in the oil industry, because the final figures might add value to the capital of a company and might influence the economy of a country.

The best way to deal with uncertainties in hydrocarbon in place and EUR estimations is to assign a probability distribution function to each variable involved in the computation and to apply the Monte Carlo approach. The Hybrid Method starts with the estimation of reserves obtained by the probabilistic volumetric method including a drilling portfolio. Porosity, initial water saturation, area, net pay, formation volume factor are the main variables used. Three levels of reserves are obtained: 1P, 2P and 3P. Then, the Arp´s decline curve analysis is applied. A probability distribution function is assigned to the Arp´s curve parameters, "b" and "D". Again, three levels of production performance are obtained and the proved reserves 1P (base curve + drilling portfolio) are defined using the cumulative production principle. The final step consists of normalizing, correlating and integrating the two distributions independently obtained by the two methods in the 1P and 2P zones. In this way, a unique distribution function, its percentiles and the location of "boundary reserves 1P/2P" (minimum value to be considered proved reserves) are determined. The "boundary reserves 1P/2P" might differ from the PRMS definition (2011) [6], that uses 90 % certainty to classify 1P proved reserves.

The described methodology is applied to a mature field in Talara Basin (Perú) using the production history and average petrophysical properties from Pariñas Inferior and Mogollón formations. A portfolio of four infill wells is incorporated in the calculation. In this case, the Hybrid Method obtains a better estimation of reserves in mature fields through risk reduction of up to 80 % compared to traditional methods.

The results show that this methodology has a great potential to provide consistency, transparency and reliability to the financial decisions [13].

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