Rock physics research with application to CO<sub>2</sub> geological storage II: Relationship between CO<sub>2</sub> saturation and P-wave velocity in a porous sandstone.

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  • Nishizawa Osamu
    Research Institute of Innovative Technology for the Earth
  • Zhang Yi
    Research Institute of Innovative Technology for the Earth
  • Xue Ziqiu
    Research Institute of Innovative Technology for the Earth

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Other Title
  • 岩石物性研究とCO<sub>2</sub>地中貯留 II：砂岩におけるCO<sub>2</sub>飽和度とP波速度変化
  • 岩石物性研究とCO₂地中貯留(2)砂岩におけるCO₂飽和度とP波速度変化
  • ガンセキ ブッセイ ケンキュウ ト CO ₂ チチュウ チョリュウ(2)サガン ニ オケル CO ₂ ホウワド ト Pハ ソクド ヘンカ

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Abstract

  In the first article, we described flow properties and flow mechanisms of CO2 in porous sandstones in most of the potential reservoirs. The flow and trapping of CO2 in porous sandstones are primarily controlled by capillary pressure and the different scales of inhomogeneity in geomaterials: from pore networks to geological structures. Following the view points in the first article, we present our experimental investigation on the relationship between P-wave velocity and CO2 saturation in a porous sandstone. We conducted a simultaneous measurement of P-wave velocity and mapping of CO2/brine saturation in a rock sample by employing a medical X-ray CT scanner and a pressure vessel that was designed for velocity measurements during fluid injection into a rock sample. The P-wave velocity and CO2/brine saturation relationship differs between CO2 drainage and brine imbibition, suggesting that CO2 saturation in rock cannot be uniquely determined by P-wave velocity. We tried to interpret the non-unique relationship by applying a continuous random patchy saturation model, which is essentially a combined model of poroelasticity and the theory of seismic wave scattering in random inhomogeneous media. To understand the model, we first present the basic relationships for describing elasticity in fluid-saturated porous media. Then we present an example which is interpreted by employing the random patchy saturation model. In the last, we discuss applications of the relationships for CO2 storage reservoirs.<br>  Followings are the points of this article: 1. Clusters of CO2 exist in pores and form P-wave velocity inhomogeneity in brine saturated rock. 2. The relationship between P-wave velocity and CO2 saturation depends on the sizes of CO2 clusters in pores. 3. The scattered Biot slow wave (the second-kind wave) appears due to the velocity inhomogeneity, which is responsible for velocity changes in CO2/brine saturated porous rock. 4. The relationship between P-wave velocity and CO2 saturation in pore depends on the relationship between characteristic cluster size and the wave number of P wave. 5. When considering the relationship between CO2 saturation and P-wave velocity in a reservoir, all scales of inhomogeneity, from pore size to geologic scales, should be considered because of the regional bias of the CO2 saturation.<br>

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