**6. Microcrack Estimation**

Based on approximated reflection coefficients [53], a pre-stack AVA inversion [54–56] is adopted to obtain the Poisson ratio and P-wave impedance and build the templates. Then, the microcrack porosity is estimated, with data points outside the template considered non-reservoir.

Figure 15 shows the P-wave impedance and Poisson's ratio obtained from a seismic survey line. Well M shows high P-wave impedances at the top and low P-wave impedances at the bottom of the target layer, and high Poisson's ratio in the middle. On the other hand, Well F shows the low impedance and Poisson's ratio. Figure 16 shows the total and microcrack porosities predicted for the survey. Both porosities are high at Well M and low at Well F. The gas production reports of wells M and F are 100.5 × <sup>10</sup><sup>4</sup> m3 per day and 4.4 × <sup>10</sup><sup>4</sup> <sup>m</sup><sup>3</sup> per day, respectively, indicating that the predictions are consistent with the reports.

**Figure 15.** 2-D seismic profiles of P-wave impedance (**a**) and Poisson's ratio (**b**).

**Figure 16.** Inversion results of total (**a**) and microcrack (**b**) porosities corresponding to the data of Figure 15.

Figures 17 and 18 show results for another seismic survey line. Wells C and P show high P-wave impedances in the middle section, while Poisson's ratio is low throughout the target layer around Well C. This well exhibits low total and microcrack porosities (see Figure 18). Although Well P shows a higher total porosity, the microcrack porosity is low. Well J shows the best potential for gas production with high total and microcrack porosities. The reports show that Well C produced almost no gas with non-industrial gas production flow, while Wells P and J produce 2.14 × 104 and 6.4 × 104 m3 gas per day, respectively. The predictions of the porosities are consistent with the production status.

**Figure 17.** 2-D seismic profiles of P-wave impedance (**a**) and Poisson's ratio (**b**) corresponding to a second survey.

**Figure 18.** Inversion results of total (**a**) and microcrack (**b**) porosities, corresponding to the data of Figure 17.

3D seismic slices of the work area of ~471 km<sup>2</sup> are produced from the RPTs. According to the daily gas production rate, ten wells are classified into three categories, namely, extremely low (less than 1 × 104 m3), low (1 × 104 − <sup>7</sup> × 104 m3) and high (higher than <sup>7</sup> × 104 m3) gas production wells. Figures <sup>19</sup> and <sup>20</sup> show the corresponding maps of total and microcrack porosities, respectively.

**Figure 19.** 3D slice of reservoir total porosity. Black, white, and red circles indicate extremely low, low, and high gas production wells, respectively.

**Figure 20.** 3D slice of reservoir microcrack porosity. Black, white, and red circles indicate extremely low, low, and high gas production wells, respectively.

The predictions show that Well A is located in an area with extremely low total and microcrack porosities. The total porosity of Wells C and D are higher than that of Well A, but their microcrack porosities are low. Wells F, G, J, and P are located in an area with low total and microcrack porosities, whereas Wells E, N, and M show high total and microcrack porosities. According to the production test report, Well A is a water well with no gas

production, and Wells C and D produce almost no gas. The three wells are classified as the lowest production. Wells P, F, G, and J produce 2.14 × 104, 4.4 × 104, 5.76 × 104, and 6.4 × <sup>10</sup><sup>4</sup> <sup>m</sup><sup>3</sup> gas per day, respectively, classified as low production. Wells E, N and M produce 17.66 × 104, 25 × 104, and 100.5 × <sup>10</sup><sup>4</sup> <sup>m</sup><sup>3</sup> gas per day, respectively, and are classified as high production. The predictions basically agree with the reports, so that the RPTs can discriminate between low and high gas production reservoirs.
