GB2013941A - Apparatus and method for determining characteristics of subsurface formations - Google Patents

Abstract

The determination of a "composite" parameter of the formation water in formations surrounding a borehole, for example the composite conductivity of the formation water, is used to obtain a relatively accurate determination of formation characteristics, such as water saturation. The determined values are meaningful even in shaly regions of the formations. In contrast to past approaches which attempted to determine the volume of shale or clay present in the formations and then introduce appropriate factors which often involve substantial guesswork, the disclosed technique determines a composite water parameter, for example a composite water conductivity, which represents the conductivity of the bulk water in the formations, including both free water and bound water. Bound water trapped in shales is accounted for in this determination, so unlike prior techniques, the shales can be considered as having a porosity. Having determined the composite water conductivity, water saturation can be directly obtained using relatively straightforward relationships which do not require estimates of the volume of shale in the formations. Shale effects are accounted for by the different conductivities (or other parameter such as capture cross sections) of the formation water constituents (free and bound) which make up the total water. In apparatus for determining the composite conductivity of the formation water in formations surrounding a borehole, a first quantity is derived as being representative of the free water conductivity in the formations. A second quantity, representative of the fraction of bound water in the formations, is also derived. A third quantity, representative of the conductivity of the bound water in the formations is also derived. The composite parameter is then determined as a function of the first, second and third quantities.