A modified rock physics model for analysis of seismic signatures of low gas-saturated rocks

Abstract

International audienceIn seismic applications, the bulk modulus of porous media saturated with liquid and gas phases is often estimated using Gassmann's fluid substitution formula, in which the effective bulk modulus of the two-phase fluid is the Reuss average of the gas and liquid bulk moduli. This averaging procedure, referred to as Wood's approximation, holds if the liquid and gas phases are homogeneously distributed within the pore space down to sizes well below the seismic wavelength and if the phase transfer processes between liquid and gas domains induced by the pressure variations of the seismic wave are negligible over the timescale of the wave period. Using existing theoretical results and low-frequency acoustic measurements in bubbly liquids, we argue that the latter assumption of "frozen" phases, valid for large enough frequencies, is likely to fail in the seismic frequency range where lower effective bulk modulus and velocity, together with dispersion and attenuation effects, are expected. We provide a simple method, which extends to reservoir fluids a classical result by Landau and Lifshitz valid for pure fluids, to compute the effective bulk modulus of thermodynamically equilibrated liquid and gas phases. For low gas saturation, this modulus is significantly lower than its Wood's counterpart, especially at the crossing of bubble point conditions. A seismic reflector associated to a phase transition between a monophasic and a two-phase fluid thus will appear. We discuss the consequences of these results for various seismic applications including fizz water discrimination and hydrocarbon reservoir depletion and CO2 geological storage monitoring

An application of AVO derived attributes to analyze seismic anomalies of gas hydrate bearing sediments in Makran offshore, Pakistan

“…Different input parameters are derived by using modified effective medium theory (Ehsan et al 2015). FSM is a fundamental step in AVO analysis (Russell et al 2003;Khalid et al 2014;Ahmed et al 2015), which leads us to interpret the gas hydrate reservoirs quantitatively. In first step, we have performed Gassmann's (1951) FSM by using available petrophysical parameters for gas hydrate-bearing sediments in the Makran area of Pakistan.…”

“…For computation of intercept I, gradient G, Poisson reflectivity PR, fluid factor, etc. we have used effective medium theory (Khalid et al 2014) and Gassmann fluid substitution approach (1951). Theoretical mapping of gas hydrate reservoir for different AVO derived attributes provide an intuitive approach to identify BSR and quantify weather BSR is related gas hydrate bearing sediments (Ojha and Sain 2009;Ojha et al 2010).…”

Rock physics modeling to assess the impact of spatial distribution pattern of pore fluid and clay contents on acoustic signatures of partially-saturated reservoirs

“…saturation pattern. A problem with Gassmann-Wood (GW) approach is that, it only valid if fluid phases (in case of mixed phase) are uniformly distributed at small scale but in contrast if the saturation is patchy over large scale the above approach fails to model the exact seismic response at the passage of seismic waves (Mavko and Mukerji 1998;Khalid et al 2014a). …”

“…, pore aspect ratio (Wang 2000), pore shape (Kuster and Toksoz 1974), overburden pressure (Korneev and Glubokovskikh 2013), pore fluid type (Ahmed et al 2014;Khalid and Ghazi 2013), fluid saturation (Khalid et al 2014a) and fluid distribution pattern within the pore space (Dvorkin et al 1999). For porous rock saturated with two phase fluids, one is liquid (oil, water or brine) and other is gas, two saturation patterns within in the pore space: uniform and patchy are considered.…”

“…In quantitative seismic, the effect of partial saturation on seismic parameters has taken lot of attention Knight et al 1998;Mavko and Mukerji 1998;Dvorkin et al 1999;Toms et al 2007;Khalid and Ghazi 2013;Khalid et al 2014a). All these studies have a following common argument.…”