Laboratory MRI Investigation in the Effects of Invert Oil Muds on Primary MRI Log Determinations
Authors: Marschall D.M. , Coates G. 
Published: December 2007

Emulsifiers are common to invert oil mud systems since they are an aid to drilling mud stability. When filtrate from such muds invade, they are likely to include some of these emulsifiers and result in altering the rocks wetting characteristics.

If so, the NMR response will change since the relaxation rate of an oil's hydrogen protons, when contacting the rock, is typically much slower than the hydrogen protons of capillary bound water. Therefore, as the wetting fluid changes from water to oil, the relaxation rate, being measured by a MRI logging tools, changes. The consequence of this to the NMR interpreter, being unaware that a change in wetting has occurred, is that they are likely to under-estimate the irreducible (BVI) water volume, and, when using a calculation of permeability dependent upon the FFI/BVI ratio, will overcall permeability.

When an under called BVI scenario was identified from the comparison of core-to-log permeability's in a series of wells drilled with invert emulsion mud's, it was speculated that the rocks wetness had been altered.

To investigate, core samples were obtained from one of the suspect wells for a laboratory evaluation of the reservoir's NMR properties. A detailed laboratory protocol, focused on the influence of invert emulsion filtrates, was then developed based on the use of rock samples form the troubled area in order to see if, and how, the mud system alters the NMR characteristics.

A clear demonstration of wetting alteration was found, and, the observed effect on BVI, and permeability, was confirmed, closely replicating the MRI BVI observations as given by the MRI log thus offering one explanation for the observed log behavior.

Presented here are the procedures, methods and findings, as well as, a new process for determining reliable BVI and permeability values when MRI logs are run in invert oil mud systems.

Introduction

Several MRI logs performed in the North Sea area were called into question when core measurements showed the formations permeability to be generally of poorer quality than predicted by MRI log results. Figure 1 is an example log from the area. It shows that MR permeability (MPERM) is generally higher than the core's, meaning the MRI log's FFI/BVI ratio is too low.

A comparison of core BVI to MBVI was made as a point of reference. CoreBVI* values were computed using the correlation shown in figure 2. This was accomplished using the samples from the troubled area. They were measured for air permeability (Ka) and desaturated to irreducible water saturation (Swi) using an air/brine displacement (100 psi). CoreBVI* values were then calculated from,

(1)

Comparing MBVI to the computed CoreBVI* tends to verity that, for a water wet rock, BVI is underestimated and a likely cause of the overestimation of MPERM. A closer inspection of fig. 1 shows MPERM to correctly reflect the relative amplitudes, but the magnitude is too high.

The under-called BVI problem was conjecturally linked to these wells since they had all been drilled with invert oil mud systems. And other wells in the area, drilled with water base muds, did not appear to have the same difficulties. Potential Reasons for Anomalous Results There are three potential reasons for this set of anomalous responses. First, the relaxation time used to separate bound from free fluid may be incorrect for these formations. P. 203