Modeling of borehole washout effects and gas hydrate-filled fractures using NGHP-02 downhole data in Krishna Godavari offshore basin, India

Abstract

The purpose of this study is to look into the effects of borehole washouts on log measurements and the resulting error in predicting gas hydrate saturation using well logs. We employ logging while drilling (LWD) data from the Indian National Gas Hydrate Program's second expedition (NGHP-02) in 2015. The NGHP-02 expedition discovered a significant amount of gas hydrate in coarse grain sediments in the Krishna Godavari (KG) Basin while drilling, coring, and logging. Borehole collapse or washout at particular depths in the presence of loose sediments impacted downhole log data at a few sites. We chose Holes NGHP-02-22A and NGHP-02-23A drilled in Area B of the KG Basin for our investigation and attempted to compensate washout effects in density-derived porosity, sonic and resistivity measurements, and the corresponding effects in estimating gas hydrate saturation. We use the sand-shale porosity model to remove the washout effects from density-derived porosity at washed-out depths. The corrected porosities and washout parameters are then used in rock physics theory to remove the washout effects from resistivity and velocity measurements by assuming washed-out zones as vertical fractures filled with seawater. We also estimate gas hydrate saturations from resistivity and velocity logs, taking into account both pore-filling and fracture-filling distributions. Analyzing velocity and resistivity logs jointly, we obtain fracture-filled porosity as 7.5% and 8% at Hole 02-22A and 02-23A respectively. Estimated saturation compared with that of the pressure core measurements show good correlation.

Appendix A: Isotropic velocities

Bulk and shear modulus of the sediments using three-phase Biot-type equation (Lee and Collett 2009) are expressed as,

$$k={k}_{ma}\left(1-{\beta }_{1}\right)+{\beta }_{1}^{2}{K}_{av},$$

(11)

$$\mu ={\mu }_{ma}\left(1-{\beta }_{2}\right),$$

(12)

where $\frac{1}{{K}_{av}}=\frac{({\beta }_{1}-\varphi )}{{k}_{ma}}+\frac{{\varphi }_{w}}{{k}_{w}}+\frac{{\varphi }_{h}}{{k}_{h}}$, ${\beta }_{1}=\frac{{\varphi }_{as}(1+\alpha )}{(1+\alpha {\varphi }_{as})}$, ${\beta }_{2}=\frac{\varphi (1+\gamma \alpha )}{(1+\gamma \alpha \varphi )}$ and $\gamma =\frac{1+2\alpha }{1+\alpha }.$

The bulk (${k}_{ma}$) and shear modulus (${\mu }_{ma}$) of sediment matrix are calculated using Hill’s average equation (Hill 1952). $\varphi$ is the porosity of the sediment,${\varphi }_{as}={\varphi }_{w}+\varepsilon {\varphi }_{h}$ is the apparent porosity, ${\varphi }_{h}={S}_{h}\varphi$ is the gas hydrate saturated porosity, ${\varphi }_{w}=\left(1-{S}_{h}\right)\varphi$ is the water-saturated porosity and ${S}_{h}$ is the gas hydrate saturation. The value of $\varepsilon$ is used here as 0.12. The consolidation parameter $\alpha ={\alpha }_{o}{({d}_{o}/h)}^{1/3}$, where ${\alpha }_{o}$ is chosen as 65 at Hole NGHP-02-22A and 45 at Hole NGHP-02-23A by calibrating the theoretical velocity of water-saturated sediment with the observed velocity of no gas hydrate zones,${d}_{o}$ is the maximum depth of investigation below seafloor and $h$ is the depth below seafloor.

The P-wave velocity (${V}_{P})$ for the pore-filling gas hydrate distribution is written as,

$${V}_{P}=\sqrt{\frac{k+4\mu /3}{{\rho }_{b}}},$$

(13)

where ${{\rho }_{b}={\rho }_{s}\left(1-\varphi \right)+{\rho }_{w}\varphi (1-{S}_{h})+{\rho }_{h}\varphi {S}_{h}}$ is the bulk density of sediment, ${\rho }_{s}$ is the matrix density, ${\rho }_{w}$ is the density of water and ${\rho }_{h}$ is the density of gas hydrate (see Table 1).

Table 1 Various constants used in rock physics modeling (Helgerud et al. 1999; Lee et al. 1996)