Geophysical Monitoring for Geologic Carbon Storage. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: География
Год издания: 0
isbn: 9781119156840
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Fracture aperture (core perpendicular) h 0.26 mm

      Note: Unlisted poroelastic parameters such as α , B, K U , K D , E D , M are derived from these parameters using Gassmann relationships for isotropic poroelastic media. The fluid bulk modulus K f is given as a function of the scCO2 pore saturation upper S Subscript s c upper C upper O 2 via

1 slash upper K Subscript f Baseline equals upper S Subscript s c upper C upper O 2 Baseline slash upper K Subscript upper H 2 upper O Baseline plus left-parenthesis 1 minus upper S Subscript s c upper C upper O 2 Baseline right-parenthesis slash upper K Subscript s c upper C upper O 2 Baseline period

      The rock’s effective stress coefficient α is computed from undrained bulk modulus K U = E U G / 3(3GE U ), K f , and the porosity φ via

1 slash alpha equals 1 slash left-parenthesis 1 minus upper K Subscript upper U Baseline slash upper K Subscript s Baseline right-parenthesis plus 1 slash phi left-parenthesis 1 minus upper K Subscript s Baseline slash upper K Subscript f Baseline right-parenthesis period

      The abrupt changes in the attenuation can be explained by coexisting two attenuation mechanisms. The first mechanism is the effect of the heterogeneous and patchy scCO2 distribution in the rock matrix, as seen for the intact cores. As assumed by the patchy‐saturation model (e.g., Azuma et al., 2013), pressure in these patches does not equilibrate with the water in the surrounding rock if ultrasonic waves are used for the measurements (i.e., there is no fluid flow across the boundaries). However, with the current, sonic‐frequency measurements, seismic waves cause higher pressure within the stiff, water‐saturated part of the rock, which drives the water toward the softer, scCO2‐saturated part. With increasing volume of the rock where the two fluids coexist, the overall attenuation of the sample increases as scCO2 is injected into the sample.

      The second mechanism is the attenuation caused by the interaction between a high‐porosity, high‐compliance fracture and a lower‐porosity, low‐compliance rock matrix. At the initial, water‐saturated state, attenuation in the sample is large. This is because seismic waves induce enhanced pressure changes within the compliant fracture, which drives dynamic fluid exchange with the matrix and dissipates a large amount of energy. This attenuation becomes small once the compliance of the fluid in the fracture increases, and the fracture‐driven motions of the water in the rock matrix diminish.

Schematic illustration of Young's modulus E and its related attenuation aE compared with the scCO2 distribution in Frac IIa and Frac IIb cores.

      (5.13)minus p Subscript f Baseline equals left-parenthesis upper B slash 3 right-parenthesis dot tau 33 comma

Schematic illustration of the comparison between the scCO2 saturations in Frac IIb sample within only the fracture versus the entire core.

      where the fractor 1/3 is due to the fact the radial total stresses τ 11 and τ 22 are zero. B can be expressed as (e.g., Mavko et al., 1998)

      (5.14)upper B equals 1 slash left-parenthesis 1 plus phi StartFraction upper K Subscript s Baseline slash upper K Subscript f Baseline minus 1 Over upper K Subscript s Baseline slash upper K Subscript upper D Baseline minus 1 EndFraction right-parenthesis comma

      where K f is the bulk fluid modulus for mixed water and scCO2. Also, from equations (5.5) and (5.6), by assuming no fluid motion in the fracture (i.e., w Subscript n Superscript plus Baseline equals w Subscript n Superscript minus Baseline equals 0 ),

      (5.15)minus p Subscript f Baseline equals StartFraction alpha Superscript upper F Baseline eta Subscript upper D Baseline Over eta Subscript upper M Baseline plus left-parenthesis alpha 


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