Geostatistical and Physical Analysis of Temperature Distribution in the Akutan Geothermal Reservoir

Ohren, Mary; Vigier, Louise; Siffert, Deborah; Garcia, Michel H.; Bonnetblanc, Paul; and Bailey, Alan
[email protected]

New exploration studies were carried out during summer 2012 to better characterize and assess the geothermal resource on Akutan Island, Alaska. The geothermal resource is considered a possible alternative energy source to offset the use of diesel fuel on this remote island. The studies included geological mapping, a magnetotelluric survey and a gravity survey. Previously, several conceptual models of the Akutan geothermal field were proposed, primarily based on surface observations. They all consider the same hydrothermal fluid upflow area from a high-temperature deep source, followed by a shallower and lower temperature, sub-horizontal outflow further from the heat source. One way in which the models differ is the outflow path. Fluid flow in the geothermal system is controlled by fractures and faults; therefore, defining the flow paths based on what is currently known about the geologic structure may reveal how temperature is distributed in the field.

We propose a numerical approach, using the TOUGH2 reservoir simulator, to quantify the impact of regional flow and heat transport on the temperature field. The already established conceptual models, aerial images, gravity survey results, modeled 3D resistivity, and limited well data are used to build simplified structural and geological models using GOCAD/SKUA® software. These models include the preferential outflow paths, and define relevant flow and temperature boundary conditions. The numerical flow and temperature solutions are compared with spatial trends derived from the available temperature data.

For each conceptual model, the flow properties, attached to rock types, and the flow and temperature boundary conditions, are considered as model calibration parameters. By adjusting these calibration parameters, their quantitative impact on the temperature distribution can be studied. Comparison of model results with measured data allows for validation of the numerical and conceptual models. The numerical models also provide useful information regarding the regional flow velocity field, which is consequential to reservoir production performance.

Finally, using the most appropriate numerical temperature model, a suitable geostatistical method is used to estimate a temperature field that is consistent with the observed temperature data and reproduces the numerically simulated spatial trends. Thorough understanding of temperature distribution allows better definition of drilling targets, thereby reducing geothermal energy development risk.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013