Fracture and stress evaluation using well logs and microseismicity, in the exploitation of Los Humeros geothermal field, Mexico
Por:
Urban E., Lermo J.
Publicada:
1 ene 2017
Resumen:
Throughout the years of seismic monitoring in the Los Humeros geothermal field, Puebla, Mexico, a growing number of earthquakes have been observed around the area of exploitation, in the reservoir permeable horizons closer to the injection wells, in the highest pressure and temperature area of the field. The installation of seismographs began after an important event, which caused damage to the superficial infrastructure, on November 25, 1994 (duration magnitude, Md = 4.6). In 2002 (Md = 3.2) and 2016 (Md = 3.6), major events were accompanied by numerous aftershocks and a considerable increase in production, probably by the reopening of fractures due to the release of confinement stresses; A phenomenon that favors the reopening of natural and induced fractures and the increase of permeability near the wells. However, the mechanisms involved in these phenomena are not clear, the principal objective of this paper is the characterization of the fractures involve in the triggered seismicity and increase of production. In 2008, two important production wells were drilled (well H-42 and H-43), after a cooling process, these wells were enabled to take petrophysical logs. In this paper, the dipolar sonic log measured in well H-43 was used to determine the principal geomechanical properties of the reservoir, identified and characterized the fractures in the well: the magnitude of the principal insitu stresses and the failure conditions, considering the Mohr-Coulomb criterion, the pore pressure necessary to activate the fractures and maintain their conductivity is obtained, in addition to supporting the closure by the overburden stress. The analysis in the well H-43, allowed to observe that the fractures related with the tectonic activity are active and conductive; fractures considered open but without permeability indicators, were also observed to be active and conductive, whereas fractures induced by perforation are sealed and do not allow the passage of fluids. The geomechanical characterization and the microseismicity of the reservoir, allowed to observe the percolation of fluids into lower strata by the conduit of open fractures related to the active faults, from the injection interval in the reservoir, this also affected by the instability in the seal between the reservoir and the basement. These fluids produce changes in the pore pressure, which has direct implications in the effective confinement stresses that can trigger earthquakes of greater amplitude, based on microseismic events scaling. Finally, the energy generated by the earthquakes is correlated with possible zones of breakage and certain fractures and fault patches, related to the stress drop and the shear modulus. This liberated energy is interpolated to identify the zones with energy accumulation, which are related to active fractures characterized with the sonic log. This analysis, in conjunction with reservoir boundaries, tectonic faults, and activity zones, characterizes areas of interest for future drilling and field development objectives.
Filiaciones:
Urban E.:
University of Calgary, Canada
Lermo J.:
Instituto de Ingeniería, UNAM, Mexico
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