P.F. Rodney, S.G. Mack, M.S. Bittar, and R.P. Bartel
Sperry-Sun Drilling Services
Abstract
An MWD resistivity device with multiple depths of investigation using several transmitter-receiver antenna spacings provides information about formation conductivity which was previously not attainable. By recording three independent resistivity measurements at different radial distances from the borehole, the log presentation has the same function and appearance as a dual induction wireline log and Rt can be estimated with a high degree of confidence even in a Measurement-After-Drilling (MAD) mode.
Resistivity is calculated both by measuring the phase shift and amplitude attenuation of a transmitted electromagnetic wave at each of three different receiver-antenna pairs. Extensive fluid tank tests and finite element modeling are used to define the radial depth of investigation and borehole corrections for each of the six measurements.
Field tests conducted in several different localities having different rock types, mud salinities, and formation exposure times are compared with wireline dual induction logs. Logs made while drilling were frequently noted to have different resistivities and invasion profiles compared with subsequent logs recorded progressively later in time after bit penetration. Field test results provide evidence of decreased accuracy and impaired vertical resolution of amplitude—based measurements at higher resistivities.
While the deepest reading phase-based measurement requires a modest shoulder bed correction for thin beds, all three measurements are of substantially the same axial (vertical) extent of rock, in sharp contrast to the wireline logging alternative. Consequently, for the first time, the petrophysicist is afforded a tool more amenable to invasion modeling.
The merits of an MWD multiple depth of investigation resistivity tool include determination of Rt, Rxo, and the extent of mud filtrate invasion while drilling. In difficult drilling situations requiring expensive and time consuming wireline logging attempts, the well may now be logged on any bit trip with the confidence that R, can be measured or estimated, depending on the degree of invasion. Measurement-After-Drilling runs can also be made with greater confidence, even in highly permeable zones.