Decline curves are among the oldest and most commonly used tools in analyzing petroleum production. Arps's (1,2) analysis is the basis for numerous methods currently in use from hand calculations to computer studies. The successful adaptation to geothermal problems of pressure testing methods developed in the petroleum and ground water fields suggests that decline curve analysis should be looked at more closely. Rivera (12) presented a paper at the 1977 GRC conference in which he analyzed 3 wells from Cerro Prieto and found fits to Arps's hyperbolic, harmonic, and exponential models. A quick check using Gentry's (6) method confirms his observations. A test of some Wairakei data indicates exponential decline. A detailed comparison of all the available methods was severely hampered by a lack of good field data. The methods have all been thoroughly verified on oil and gas fields, but little has yet been published on their use in geothermal reservoir engineering.

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