Fluid flow and compressional wave propagation measurements were made on fractured samples prior to and after the chemical deposition of calcium carbonate. We observed that the initial void geometry (aperture and contact area) controlled the amount and spatial distribution of mineral deposition within the fracture. The most reliable seismic indicator that the fracture had been altered is a reduction in the width of the distribution of the most probable frequency in the received signal. A reduction in the width of the distribution indicates that the fracture is homogenized by mineral deposition in the fracture voids. Homogenization occurs because the mixing predominantly takes place in the dominant flow paths within the fracture which tend to have low fracture stiffness. The results indicate that acoustic imaging techniques are required in this characterization because they provide a statistical indicator to monitor changes in fracture geometry caused by mineral deposition.
1. INTRODUCTION
The construction of underground structures can alter the underground environment by changing the local stress distribution as well as the local hydrogeology of a site. Depending on the purpose of the underground structure, long-term monitoring of underground environments may be required because fractures, joints and faults can be altered over the lifetime of the engineered structure. Because rocks are part of the tectonic and hydrogeologic cycles, the geometry of the voids and contact area in a fracture can be altered by time-dependent processes such as normal and shear displacement, weathering, chemical precipitation, and chemical dissolution. For example, CO2 injection into a subsurface reservoir can initiate a complex set of reactions that involves interactions between aqueous solutions and the minerals in the host rock. CO2 can transfer across a gas-water interface to become an aqueous ion by the reaction:
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This is followed by rapid dissociation of carbonic acid:
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The formation of bicarbonate ion, HCO3-, and production of acidity by release of H+ leads to a series of secondary reactions. These reactionscomplicate estimates of CO2 storage volumes as well as measurements of the geophysical characteristics of the rock. If minerals precipitate in a fracture or if the fracture void geometry is dissolved because of CO2 injection, the geometrical properties of the fracture will change and may alter the hydraulic and seismic properties of the fracture. In this study, we used acoustic measurements to monitor the deposition of calcium carbonate in a racture and its effect on hydraulic properties of single fractures.
2.1. Sample Preparation
Granitic samples were used to study wave propagation across a fracture prior to and after mineral precipitation within a fracture. The samples were made from Barre granite which had no connected matrix porosity. The compressional wave velocity for an intact sample (i.e., no throughgoing fractures) was 3520 m/s and the shear wave velocity was 2570 m/s. An intact ample, GI04, was used for control experiments. Four fracture samples, GF01, GF03, GF05 and GF08, were created for this investigation by inducing a single fracture in each using a technique similar to brazil testing [1].
