One Discipline, Two Arenas - Reservoir Engineering in Geothermal and Petroleum Industries

The similarities and differences in reservoir engineering in the geothermal and petroleum industries are not familiar to many. This unfamiliarity frequently leads to aberrant perception of the risks and rewards of geothermal development in the minds of developers and financiers who are accustomed to the petroleum industry but are new to geothermal. This paper is a comparative survey of the state-of-the-art of reservoir engineering in the two industries. This survey leads to the following conclusions. First, compared to petroleum, geothermal reservoir engineering is more challenging in that conceptual modeling has more complexity, parameter estimation has more limitations, and volumetric reserve estimation has more uncertainty. However, the saving grace of geothermal reservoir engineering is numerical simulation, which allows one to overcome the above limitations and produce estimates of reserves and forecasts of reservoir and well behavior that are at least as reliable as in the petroleum industry. Second, the term “reserves” in the geothermal industry has no standard definition, and in fact is a misnomer compared to the usage in the petroleum industry. Third, unlike in the petroleum industry, reservoir engineering and geoscience are intricately intertwined in geothermal. Finally, empirical knowledge about the nature of geothermal reservoirs is minimal and the body of literature on geothermal case histories is minuscule compared to petroleum. This imposes a higher premium on the practical experience of the engineer in geothermal reservoir engineering. INTRODUCTION Reservoir engineering as a distinct discipline had a hazy beginning in the petroleum industry in the 1930s, whereas the entry of reservoir engineering in the geothermal industry can be precisely dated to 1969 (Whiting and Ramey, 1969). By the mid 1970s geothermal reservoir engineering was an established discipline, the Second U.N. Symposium on the Development and Use of Geothermal Resources in 1975 being the first presentation of a multitude of papers on geothermal reservoir engineering at a single forum. Since then, several hundred papers have appeared in the literature marking the advance of the state-of-the-art of geothermal reservoir engineering. Over the last three decades geothermal reservoir engineering has evolved and diverged from its petroleum counterpart. Today, many individuals in both petroleum and geothermal industries are unaware of the similarities and differences between reservoir engineering in the two industries. This unfamiliarity frequently leads to aberrant perception of the risks and rewards of geothermal development in the minds of developers and financiers who are accustomed to the petroleum industry but are new to geothermal. Such parties often consider geothermal reservoir engineering to be inherently less reliable than its petroleum counterpart. This perception is born of the contrast between the familiar and tangible nature of petroleum and the seemingly exotic and nebulous aspects of geothermal energy, being associated with such disquieting images as violent volcanoes, gushing geysers and seeping sulfurous gases. On the other hand, a greater and abiding faith in petroleum reservoir engineering prompts certain parties to believe that petroleum reservoir engineering can be readily applied, unaltered, to geothermal. Neither view is reasonable. The author