Page 553 - Petrophysics

520 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

FRACTURES POROSITY AND APERTURE FROM CORES

Oil-bearing fractured granite is a major productive formation in some
parts of the world, such as in the Bach Ho field offshore Vietnam.
Fractured granite consists of three main elements: macrofractures, low
permeability matrix with microfractures, and tight non-permeable matrix
[33]. Tan et al. selected ten of the most representative whole cores
(D = 6.7 cm) from this oilfield, with total porosity from 3.03 to 9.93
percent, and permeability from 226 to 19,250 mD, as shown in Table
8.4 [33]. After trimming and cleaning, the cores were saturated with a
brine, then the total porosity (of) was determined. The saturated samples
were then loaded into a capillary cell (porous plate technique) and Pc
was increased in steps from 0.05 to 5 bars. The water saturation S, was
recorded at each step. They observed that the larger the fracture width,
the lower the capillary forces. From this observation, they demonstrated
that the sudden change in the slope of Pc versus &, as shown on Figure
8.16, corresponds to the volume of fractures. The fracture porosity and
porosity partitioning coefficients were then calculated from:

of = otu - SWS) (8.37a)

v = $f/$t = 1 - sws (8.37b)

where SWS is the water saturation corresponding to the sudden change
in slope of the PC curve. Values of of and v are shown in Table 8.4. This
table shows very high values of v, which indicates a very high density of
microfractures in the matrix.

TABLE 8.4
RESULTS OF CORE ANALYSIS OF FRACTURED RESERVOIR

1800 8.33 0.328 2.73 5.60
19250 8.63 0.302 2.61 6.02
15220 4.25 0.435 1.85 2.40
1704 4.67 0.4 1.87 2.80
8520 3.03 0.396 1.20 1.83
386 4.34 0.26 1.13 3.21
824 9.93 0.266 2.64 7.29
514 5.54 0.462 2.56 2.98
226 4.06 0.37 1.50 2.56
302 7.69 0.223 1.71 5.98