Mud Filtrate Effects On The Laterolog In Alberta Foothills Carbonates
Authors: Dziuba T.T. 
Published: March 2010

This report proposes a new set of invasion correction curves for the laterolog that can be used quantitatively for a variety of mud systems and various diameters of invasion to obtain the true resistivity of the formation.

Present resistivity tool designs do not appear to allow measurement deep enough to provide resistivity values of the undisturbed formation; nor do present laterolog invasion correction charts appear to correct sufficiently for invasion. At Shell Canada, the laterolog has been the primary resistivity tool used in the evaluation of deep, 3000 m to 5500 m Alberta Foothills carbonates. Low water saturations, calculated after the high resistivities have been corrected for invasion using existing charts, have led to costly production testing of wet zones.

In this report, the Resistivity Index(I) was used to develop new invasion corrections. By assigning an I value to each of the flushed, transition and uninvaded zones and establishing the resistivity contribution of each zone to the laterolog reading from industry-published pseudo-geometrical factors, invasion corrections were determined. The effects of changing the Rmf/Rw value and the diameter of invasion were included in the design of the invasion correction curves.

Open-hole logs of actual field cases are used to demonstrate the new invasion corrections for the laterolog. The proposed invasion corrections have resulted in the calculation of more accurate water saturations as substantiated by production test results.

Introduction

The laterolog is strongly affected by invasion, therefore, the ratio of the mud filtrate resistivity to the formation water resistivity is crucial because invasion often adds a resistive medium to the formation. Most Shell Canada wells are drilled with muds having a different resistivity than the formation water. Invasion occurs to some unknown depth and to obtain a true resistivity, the effect of the invading mud filtrate on the resistivity must be removed.

Logging companies have attempted to minimize the effects of invasion on their resistivity tools through tool designs that utilize geometrical factors(1). A resistivity tool's reading can be considered to be the sum of the weighted values of the resistivities of the borehole, the invaded and non-invaded zones (Fig. 1). The deep reading resistivity tools are designed to obtain the major portion of their resistivity reading from as far away from the borehole as possible. For cases where invasion is shallow to moderate, this is usually the undisturbed zone. In deep invasion cases the resistivity reading must be corrected using service company charts to provide a value closer to the true resistivity.

In foothills evaluation, very obvious gas zones with high resistivities (e.g. &gt; 1000 ohm-m) and very obvious wet zones with low resistivities (e.g. &lt;50 ohm-m) do not usually pose evaluation problems because they are readily identified from resistivity logs. Zones that appear to exist at some intermediate water saturation (e.g. Sw 30% to 60%) can be extremely difficult to evaluate for a variety of reasons. Today's resistivity tools run in situations with high resistivity mud systems (Rmf/Rw &gt; 1.0) and large diameters of invasion will provide resistivity readings reflecting this and in turn will result in incorrect Sw calculations unless properly corrected for invasion.