Читать книгу Geophysical Monitoring for Geologic Carbon Storage - Группа авторов - Страница 74

A series of seismic measurements with X‐ray CT imaging was conducted on two Carbon Tan sandstone core samples (Kocurek Industries, Inc., http://www.kocurekindustries.com/) with various fracture configurations (Table 5.1). The sandstone has a porosity of 16.7% and a matrix permeability of 34.5 md (measured only for the Carbon Tan #2 core. However, similar dry densities of the #1 and #2 cores (2,380 kg/m³ and 2,375 kg/m³, respectively) indicate that the #1 core has a similar porosity. The permeability was measured using the falling‐head permeability test method (e.g., Head, 1982) under unconfined conditions, using tap water at room temperature. The diameter of the cores was 38 mm, and the length varied slightly among the cores, from 98 mm to 101 mm. A single fracture was introduced in each core via extensile splitting (Brazilian loading) either parallel or perpendicular to the core axis.

Table 5.1 Descriptions of test samples with different fracture configurations

Sample	Description	scCO2 injection rate (mL/min)	Configuration
Carbon Tan #1 core	Intact I	Intact core	0.035
Frac Ia	Mated core‐parallel fracture (no offset)	0.037
Frac Ib	Sheared core‐parallel fracture; with spacers	0.017
Frac Ic	Sheared core‐parallel fracture; without spacers (shorter core)	0.027
Frac Id	Same as Frac Ic sample, but with a vertically oriented fracture	0.043
Carbon Tan #2 core	Intact II	Intact core	0.042
Frac IIa	Sheared (rotationally offset) core‐perpendicular fracture; with a mylar wrapping	0.023
Frac IIb	Sheared (rotationally offset) core‐perpendicular fracture	0.020

Note: The core numbers were redesignated in this paper and can be different from what are seen in the photographs.

The first core sample (Carbon Tan #1) was tested for: intact state (Intact I); a mated, core‐parallel fracture (Frac Ia); a sheared (linearly displaced) core‐parallel fracture with end spacers (Frac Ib); and a sheared core parallel fracture without end spacers (Frac Ic). The end spacers (thickness 3 mm) in the Frac Ib test, which were coupled to the sample and the metal rods using thin lead foil disks, were made from the same sandstone and were used to apply and maintain a shear offset along the fracture surface. These spacers were later removed, and the protruding sample ends were ground flat for use in the Frac Ic test. These tests were all conducted while the fracture was oriented horizontally. In addition, the same test as Frac Ic was conducted while the fracture was oriented vertically to examine the effect of buoyancy‐controlled scCO₂ migration within the fracture on seismic waves (Frac Id).

For the second core sample (Carbon Tan #2), tests were conducted for: intact state (Intact II); a sheared (rotational offset) core‐perpendicular fracture with a Mylar wrapping (Frac IIa); and the same sample as Frac IIa without the Mylar wrapping (Frac IIb). Note that for Frac IIa test, scCO₂ followed a preferential flow path along the Mylar sheet, resulting in premature breakthrough across the core. This test can be viewed as a special case where both core‐perpendicular and core‐parallel fractures are present. However, the permeability of this fast‐pass‐providing core‐parallel “fracture” was thought to be small.