Acoustic and petrophysical relationships in low‐shale sandstone reservoir rocks

Abstract

A B S T R A C TThis paper describes the measurements of the acoustic and petrophysical properties of two suites of low-shale sandstone samples from North Sea hydrocarbon reservoirs, under simulated reservoir conditions. The acoustic velocities and quality factors of the samples, saturated with different pore fluids (brine, dead oil and kerosene), were measured at a frequency of about 0.8 MHz and over a range of pressures from 5 MPa to 40 MPa.The compressional-wave velocity is strongly correlated with the shear-wave velocity in this suite of rocks. The ratio V P /V S varies significantly with change of both porefluid type and differential pressure, confirming the usefulness of this parameter for seismic monitoring of producing reservoirs.The results of quality factor measurements were compared with predictions from Biot-flow and squirt-flow loss mechanisms. The results suggested that the dominating loss in these samples is due to squirt-flow of fluid between the pores of various geometries. The contribution of the Biot-flow loss mechanism to the total loss is negligible. The compressional-wave quality factor was shown to be inversely correlated with rock permeability, suggesting the possibility of using attenuation as a permeability indicator tool in low-shale, high-porosity sandstone reservoirs. I N T R O D U C T I O NAn understanding of the relationships between seismic (velocity and attenuation) and fluid-flow properties (porosity and permeability) of reservoir rocks under various stress regimes and with different pore fluids is the key to successful exploration and exploitation of hydrocarbon reservoirs. Obvious applications are the potential improvements in sonic wireline data interpretation, modelling and inversion of time-lapse seismic, and amplitude-versus-offset surveys. Variation of the in situ effective pressure in hydrocarbon reservoirs (for example, during production phases) directly affects the velocity and amplitude data determined from surface seismic and sonic log surveys. A good estimation of the magnitude of these changes over the life of a reservoir is important for better reservoir management. and others have investigated the effects of pressure and fluid type on the acoustic properties of sandstone rocks. This research project explored the effects of pressure and fluid type on the acoustic compressional-and shear-wave velocities (V P and V S ) and quality factors (Q P and Q S ) of two suites of sandstone samples from North Sea offshore reservoirs. These two suites of samples are referred to as PEGASUS Well 2 and PEGASUS Well 8. In total, the compressional-and shear-wave velocities of 19 sandstone samples were measured at ultrasonic frequency (about 0.8 MHz) under simulated in situ conditions of reservoir pressures (confining pressure and pore-fluid pressure). The measurements were conducted on air-dry and fully C 2005 European

Assessment of the structural representativeness of sample data sets for the mechanical characterization of deep formations

“…A fluid-dependency of shear moduli is also visible, although the Biot-Gassmann model predicts that the shear modulus will remain constant under different saturation. Several authors have, however, observed S-wave velocities for water saturation lower than for oil saturation (e.g., King, 1966;Khazanehdari and McCann, 2005). Other authors have observed increases in shear velocity with liquid saturation (e.g., Han et al, 1986;Khazanehdari and McCann, 2005).…”

Petrophysical models to describe the pressure dependence of acoustic wave propagation characteristics

“…Experiments denote, that the quality factors behave similarly to the velocities, a rapid nonlinear increase occurs at the beginning of loading (Toksöz et al 1979). The shale content, saturation and type of saturant, grain size influence the scale of pressure dependence (Khazanehdari and McCann 2005;Prasad and Meissner 1992;Prasad 2002;Domnesteanu et al 2002). With the increasing pressure the pore volume decreases (or the microcracks close), the contacts between the grains become better and better thus the measurable absorption coefficient decreases and the quality factor increases.…”

“…Walsh and Brace (1964) explain it with the closure of microcracks. Experiments demonstrate that beside other factors the type of pore fluid (Toksöz et al 1979;Khazanehdari and McCann 2005), the grain size (Prasad and Meissner 1992;Prasad 2002) have influence on the scale of pressure dependence. A nonlinear relationship was proved by several empirical equations (Eberhart Phillips et al 1989;Freund 1992;Jones 1995;Khaksar et al 1999;Wepfer and Christensen 1991), however in these equations only the regression parameters are given, they do not provide the physical explanation of the process.…”

Data-driven model for shear wave transit time prediction for formation evaluation

“…As far back as 1958, researchers like Doh and Alger (1958) perceived formation porosity estimation to be the major advantage of sonic logs. The transit arrival times of the sonic waves have evolved and now being used for formation, porosity determination, lithology identification, fluid saturation indication, formation strength characterization, hydrocarbon indication, and much more (Khazanehdari and Mccann 2005;Williams 1990). This is due to the fact that the sonic transit times are affected by reservoir properties that include compaction, porosity, anisotropy, density, lithology, cementation, consolidation, overburden stress and pore pressure (Khazanehdari and Mccann 2005;Krief et al 1990;Thomsen 1986;Toksöz et al 1976;Williams 1990).…”

“…The transit arrival times of the sonic waves have evolved and now being used for formation, porosity determination, lithology identification, fluid saturation indication, formation strength characterization, hydrocarbon indication, and much more (Khazanehdari and Mccann 2005;Williams 1990). This is due to the fact that the sonic transit times are affected by reservoir properties that include compaction, porosity, anisotropy, density, lithology, cementation, consolidation, overburden stress and pore pressure (Khazanehdari and Mccann 2005;Krief et al 1990;Thomsen 1986;Toksöz et al 1976;Williams 1990). A good understanding of how these properties change over the life of the reservoir is essential for proper reservoir planning, development and management (Dakhelpour-Ghoveifel et al 2019;Khazanehdari and Mccann 2005;Saboorian-Jooybari et al 2015).…”