The effect of drilling mud properties on shallow lateral resistivity logging of gas hydrate bearing sediments

Resistivity logging is one of the most important ways of identifying and estimating the saturation level of gas hydrates in permafrost and ocean regions. In practical drilling operations, resistivity loggings, especially shallow lateral resistivity logging in gas hydrate bearing sediments (GHBS), are likely to be affected by drilling mud invasions. Here, we use available data from site measurements to construct a two-dimensional model for hydrate reservoirs around the borehole SH2, a hole that was drilled during the first expedition in Shenhu area of South China Sea to examine and drill into gas hydrates. We then use this model to investigate the characteristics of drilling mud invasions and the effect of drilling mud properties (e.g., temperature, density, and salinity) on resistivity logging using a numerical simulation. This simulation and associated calculations indicate that shallow lateral resistivity logging is significantly affected by variations in drilling mud temperature, which leads to hydrate dissociation and the formation of secondary hydrates. Increasing drilling mud salinity accelerates hydrate dissociation, and has a greater effect on shallow lateral resistivity logging than the free gas produced during drilling and any potential mud density influence, which is generally dependent on the depth at which the drilling mud invasion occurred. This means that future drilling operations should focus on ensuring that the temperature, salinity, and density of drilling muds remain within a reasonable range in order to minimize the effect of mud invasions on resistivity logging data. (C) 2014 Elsevier B.V. All rights reserved.