A New Method of Evaluating the Hydrate Saturation

Acoustic and electrical methods are commonly used to evaluate hydrate saturation based on the P-wave velocity and resistivity, respectively. This study measured the hydrate saturation using petrophysical parameters that are directly related to the presence of hydrates. Five petrophysical parameters sensitive to hydrate saturation were analyzed using the equivalent medium rock physical model (EMTL), logging intersection plots and petrophysical parameter inversion. The inversion results and the logging intersection plots of the sensitivity parameters related to the elastic modulus are consistent with the hydrate reservoir, however, the sensitive parameters related to the shear modulus are not consistent with the hydrate reservoir. And σ may be more sensitive to free gas than hydrate. The simulated annealing global optimization method was used to estimate the hydrate saturation profile in the Shenhu area. The petrophysical parameters of Vp, λρ and λμ are associated with the rock modulus (both the elastic modulus and shear modulus), which are sensitive to hydrate saturation with an estimated saturation range of 0.1-0.45 that is highly consistent with the original well diameter curves. The parameters of Vs and μρ are only related to the shear modulus of the rock, which had a high hydrate saturation range of 0.15-0.48 that is overall greater than the logging saturation data. Among the many sensitive parameters of λρ, λμ, Vp, Vs, μρ and σ, the estimation error of the saturation of the gas hydrate are 0.11, 0.15, 0.19, 0.26, 0.29 and 0.33, respectively. The sensitivity of the Lemma coefficient λ is greater than that of the shear modulus μ by two reasons. First, although the hydrate saturation is high, the gas hydrate of Shenhu area is deposited in the surrounding rock with pore filling, the shear modulus is sensitive to the rock skeleton composition but not to the pore medium. Second, when the hydrate saturation is low and the porosity is greater than 40%, the hydrate may exist in both the rock skeleton form and fluid form.