C. Cosmo, M.R. Spalburg and W.J. Looyestijn
Koninklijke/S hell Exploratie & Pmduktie Laborasorium, Shell Research B.V., Rijswijk, The Netherlands
ABSTRACT
The laterolog and induction-log resistivity tools are commonly used to derive in-situ hydrocarbon saturation. Induction logs, however, are notorious for their poor vertical resolution. In thinly bedded formations or inclined boreholes a similar resolution problem also exists for laterologs, leading to inaccurate evaluations. Efforts to improve the tools’ resolution by deconvolution have been mainly focused on induction logs, The development of such resolution-enhancing procedures for laterologs, however, is less advanced.
A simple and fast method of direct inverse filtering for deconvolving a laterolog is described here. Whereas an induction log can at present only be deconvolved by iterative forward modelling and only if the tool was run in a vertical hole through horizontal layering, the robust and rapid single-pass method presented here enables the laterolog to be routinely deconvolved even if the tool was run in a deviated hole through dipping layers.
The method arose in connection with the development of software to simulate the effect of borehole deviation on the laterolog response for various resistivity profiles, excluding the effects of borehole and invasion. Following numerous simulations, it became apparent that the perturbing effect of the non-linear tool response could be approximately corrected for by means of simple convolution filters, depending on the dip and average conductivity contrast over a formation interval These inverse filters make the direct deconvolution of the measured log possible, thereby correcting the laterolog for thin-bed effects and apparent formation dip.
Prototype software, based on the new deconvolution method, was tested on real field logs. The results indicate a possible large impact on the prediction of hydrocarbon reserves.