Abstract This paper deals with the possibility of using usual oil well transient pressure testing methods in hot water wells when flashing occurs in the wellbore. Multiple rate analysis techniques have been successfully applied to available data on a well drilled by B.R.G.M. in the former French Territory of Afars and Issas. Practical recommendations concerning such tests are also included. Introduction Well established pressure transient analysis techniques are routinely used in oil and gas wells for the determination of reservoir parameters. In recent years, improvements in these techniques have allowed a better understanding of complex reservoir and well behavior, as in the presence of skin, wellbore storages, or fractures. Only recently have these methods been applied to geothermal systems. For a number of years, geothermal well testing was based on empirical methods, that, although useful for estimating geothermal field production potential, gave limited knowledge on the reservoir itself. Pressure transient techniques have been applied successfully in vapor dominated geothermal reservoirs. It was found that gas well analysis methods apply to steam wells and that wellhead measurements were generally adequate for analysis, thus eliminating the need for downhole instrumentation. In water dominated systems, on the contrary, transient well testing can be far more complex, depending upon whether two-phase flow develops, with flashing in the wellbore or in the formation. Usual oil well methods have already been shown to apply when no flashing develops. On the other hand, they apparently cannot be used when flashing occurs in the formations. Very little information was available until recently on the case of hot water wells in which flashing occurs at some depth in the wellbore. This is mainly due to the difficulty of bottom hole data gathering, that often results in mechanical damage of the measuring devices because of the high temperatures involved, and of the high production rates of boiling geothermal fluids. A two rate flow test, conducted on one well in the Cerro Prieto field was recently presented by Rivera and Ramey. The data were satisfactorily interpreted by means of models suggested by Russell, Selim, and Odeh and Jones. To the author's knowledge, no other example of transient test in a two-phase well has been published in the literature. Unpublished data, published in the literature. Unpublished data, however, were available from a well drilled in 1975 by B.R.G.M. in the French Territory of Afars and Issas (now Republic of Djibouti). Analysis of these data, which include pressure build-up as well as two rate flow tests is presented hereafter. presented hereafter. ASAL WELL The Asal rift, located 80 km west of Djibouti, is one of the active "rifts-in rift" structures of the Afar depression, a transition between the Gulf of Aden and the Red Sea ridges. Attention was drawn to this zone because of the presence of a graben structure, and of geochemical particularities of various hot springs. Two wells were drilled on the S-W margin of the rift, at locations chosen mainly from geological considerations. The first hole (Asal 1) reached a hot water geothermal reservoir, the second, one kilometer away, was dry. Both wells found, from top to bottom, a recent basaltic series, then a thick rhyolitic volcanic series, and finally an old tectonic, tilted, basaitic series, where the reservoir is located. The first well was drilled to a depth of 1130 m, where heavy mud losses occured, while the second well reached 1550 m. An important normal fault appears to separate the two wells. A schematic of Asal 1 wellhead is presented in figure 1.