Thermoelastic relaxation and its effects on the compressibility of pore fluid and P wave velocities
- PDF / 1,503,857 Bytes
- 11 Pages / 595.276 x 790.866 pts Page_size
- 80 Downloads / 155 Views
ORIGINAL PAPER
Thermoelastic relaxation and its effects on the compressibility of pore fluid and P wave velocities Perveiz Khalid
Received: 19 February 2014 / Accepted: 22 September 2014 # Saudi Society for Geosciences 2014
Abstract In poroelastic models, the effective compressibility of rock saturating fluid is conventionally taken as the adiabatic compressibility. This compressibility is valid for seismic waves having frequencies large enough to neglect heat exchanges between the rock matrix and the pore fluid, which are subjected to undergo temperature variations in response to seismic wave propagation. For low enough frequencies, heat flows back and forth across the rock/fluid interfaces, which leads the fluid to smaller temperature variations resulting in a higher compressibility than in the absence of the rock. An analysis of the characteristic heat conduction time over the pore and grain lengths shows that the rock-fluid composite should be considered under thermal relaxation in the surface seismic frequency band rather than the conventional highfrequency (unrelaxed) limit. Our analysis for a rock-fluid system shows that in the low-frequency limit, the effective compressibility of the saturating fluid is neither isothermal nor adiabatic but is equal to the average of the adiabatic and isothermal fluid compressibilities weighted by the volumetric heat capacities of the fluid and rock, respectively. The resulting bulk moduli and compressional velocities of fluidsaturated rock are lower than the conventional values. P wave velocity dispersion, in poorly consolidated rocks, is about 15– 20 % subject to the type of saturating fluid is rich in acidic gases (CO2, H2S).
Keywords Thermoelasticity . Velocity dispersion . Thermomechanical properties . Fluid-saturated rock . Low-frequency limit . Rock physics
P. Khalid (*) Institute of Geology, University of the Punjab, Quaid-i-Azam Campus, Lahore 54590, Pakistan e-mail: [email protected]
Introduction Many researchers have investigated the effects of thermomechanical properties of rock-forming minerals and fluids on the seismic wave propagation in terms of energy dissipation, velocity dispersion, and decay of reflection amplitudes with some other reservoir parameters (McTigue 1986; Wang 2001; Martinez et al. 2012). Table 1 summarizes some important parameters influencing seismic signatures of saturated rocks. However, thermomechanical properties like thermodynamics, acoustic, and elastic properties of pore fluids mainly govern the acoustic and seismic behavior of fluid-saturated rocks (Batzle and Wang 1992). Newton-Laplace equation (Rowlinson and Swinton 1982) relates the thermodynamic properties of pure fluids to the acoustic properties of the fluids, whereas the Gassmann’s (1951) or Biot’s (1956) equations relate the thermodynamic properties of fluids to the seismic properties of saturated rocks. In 1687, Newton developed the mathematical theory of acoustic propagation in pure fluid. In this theory, the compressibility of the fluid was taken as isothermal wi
Data Loading...
