Characterising geothermal resources using passive seismics

In the evaluation of a geothermal resource it is critical to know the reservoir geometry, temperature, saturation, state of saturants, pore pressure, porosity and permeability. These are the parameters which will determine the production feasibility and cost effectiveness of a geothermal prospect. The increasing sophistication of seismic wave data collection and processing and recent experimental work on factors governing wave propagation in rocks has stimulated increased interest in the use of seismic techniques to determine the on site physical state of crustal rocks for engineering applications.
Passive Seismic Tomography (PST) can also be successfully used to investigate geothermal reservoirs. The Vp/Vs ratio observed in active geothermal areas depends upon the rock matrix, porosity, pore fluid content, pore pressure, temperature, and pore shape.

A low Vp/Vs ratio is thought to be caused by abundance of fractures filled with hot water. In fact, the mechanism of water steam transition has a stronger effect on compressibility than shear modulus, leading to low Vp/Vs ratios.

The relationship between low Vp/Vs ratios and geothermal areas is so sound that the study of the Vp/Vs ratio is a promising technique to identify geothermal resources and monitor their exploitation. Contrarily, melt inclusions reduce S-wave velocity more than P-wave velocity, resulting in high Vp/Vs ratios.

In general the application of a PST survey in a geothermal region can lead to the following results:

  • Tomography from wave propagation can image reservoir for seismic velocity and attenuation, geologic structure, and lithology‏
  • Accurate plotting of microseisms can delineate fracture zones
  • Interpretation of tomography results (rock physics) can identify altered or permeable zones, phase states of fluids, crack density, and saturation
  • Improve resolution for drilling targets
  • Improve success estimates for drilling targets – Monte Carlo Coupled inversion
  • Monitor existing wells and production
  • Variations in lithology observed in elastic constants
  • Increase of velocity and decrease in attenuation with depth due to closing of small cracks because of pressure
  • Decrease in velocity and increase in attenuation due to fracturing
  • Decrease in velocity due to chemical alteration
  • Extreme temperature gradient works to decrease velocity with depth
  • Fluid saturation acts to stiffen the pores to deformation; affects P-waves, but not shear-waves
  • Saturation increases the density of the material and increases both compressional- and shear-wave velocity, and increases attenuation
  • Dilatency can cause expansion and permeability

In addition to determination of velocity and Vp/Vs volumes LandTech has developed the technology to obtain 3D volumes of seismic attenuation (or alternatively the Qp quality factor). Increasing pressure closes pores and decreases attenuation.

Seismic attenuation is again very low in dry rocks whereas in partially saturated rocks (Sw =: 95%) attenuation is significant probably due to intracrack fluid flow, with P attenuation about twice as large as S. In fully saturated rock, intercrack flow is eliminated, Qp is sharply lowered and again shear attenuation due to intercrack fluid flow is maximized. The following figure illustrates schematically how the change from partial to full saturation changes the types of pore fluid flow induced by passing strain waves which may occur within rocks and hence changes the relative amounts of attenuation in compression and shear.

In fully saturated rocks first order intercrack flow may occur only in shear, not in compression. Since the P wave is strongly affected by bulk compression it is less attenuated than an S wave. In partially saturated rock flow will occur within all pores, induced by bulk compression; whereas shear induces flow only in properly oriented pores. Consequently S attenuation is less than P. In rocks containing no liquid phase, no flow occurs and attenuation is low for all waves. Hence, the ratio of P to S attenuation may allow us to distinguish between a partially saturated and dry or fully saturated rocks.