Bi-linear models for simultaneous estimation of a formation pressure and lithological characteristics in interbedded sands and shales

Abstract

A new method for analysis of lithology variations in multi-layered geological formations and suppression of lithological components in formation pressure indicators is described. The key element of the methodology is the suggestion that several curves indicating variations with depth in formation pressure and in lithology may be presented as a sum of three basic components: (a) smooth components, slowly changing with depth; (b) a fast-changing component, and (c) random components, mutually uncorrelated at different curves. The smooth components in the recorded curves are assumed to be those strongly correlated with actual variations of the formation pressure. They may be either common or unique to each recorded indicator, depending on the type of model. Each of the recorded curves is described by two groups of parameters: variables defining properties of the basic components and factors with which the components are included in a given indicator. The models for formation pressure and lithological indicators are built as bi-linear functions of variables from those two groups. The methodology includes estimation of smooth components in all curves, estimation of factors, and extraction of the fast, lithological component via joint processing of several formation pressure and lithological indicators. This allows for correction of formation pressure indicators distorted by changing lithology in interbedded formations and for better detection of zones of abnormal pressure, characterized by an abrupt change in the corrected formation pressure indicator. An example of this methology is a joint use of information on drilling rate taken in the form of d c -exponent and information on the lithology taken in the form of the shale content in interbedded sands and shales.