Elton Head, Darrel Cannon, David Allen, and Leif Colson
Schlumberger Well Services, Houston, TX
ABSTRACT
Resistivity logs are acquired in most wells. In addition to the traditional parameters derived from log analysis, resistivity data can supply new information based on invasion character to assist prediction of hydrocarbon presence and well performance. Historically, such attempts have been restricted. Available logs contained limited radial information; in addition shallow investigation laterolog devices and deep investigation induction tools respond differently to resistivity contrast in the radial and depth dimensions. These complications are resolved by a multiarray induction tool that produces five resistivity logs with closely matched vertical resolution and progressive depths of investigation.
Invasion has traditionally been defined for log interpretation as the “diameter of invasion” of mud filtrate, which presumes a piston-like displacement of reservoir fluid by the filtrate. The associated filtrate saturation profile, referred to as a “step profile,” represents the simplest possible invasion model. A more realistic model which includes a transition zone is presented, giving a more detailed and consistent description of invasion character.
The simplest invasion interpretation is intuitive, based on visual inspection of separation between logs. Subsequently, resistivity differences are quantified and converted into dimensions of length and saturation. Finally, radial and resistivity information is transformed into filtrate volume because quantitative invasion analysis must be done in the volume dimension to allow well-to-well comparisons and comparison of hydrocarbon zones to water zones. Quantitative determination of filtrate volume may be used to identify lost circulation zones and to estimate relative cleanup time for multizone tests. A profile of filtrate volume vs. depth can contain information about vertical permeability, If the approximation m = n is acceptable for the zone of interest, the volume computation can be done without porosity definition.
Each stage of invasion interpretation is illustrated by field examples. Oil-base mud and the water-base mud domain where Rxo < Rt: are particularly of interest, since those conditions do not allow invasion interpretation with conventional induction/focused electric log tools.