Abstract

A new laboratory procedure has been developed to study formation damage mechanisms and Improve acid stimulation designs using an Environmental Scanning Electron Microscope (ESEM) coupled with core flood tests. An ESEM has the unique capability of observing a sample, wet or dry, in its natural, uncoated state. The core plugs used In core flooding experiments can be observed at the same locations, before, during and after treatment with workover and completion fluids, without any cleaning, drying or metal coating processes. The direct effects of the stimulation and completion fluids on the formation minerals can be seen along with any changes to the initial porosity. The ESEM is combined with energy dispersive X-ray spectroscopy(EDS) to allow elemental analysis of precipitates or observation of changes in the elemental composition of the clay minerals.

This paper describes both the analytical procedures and the results of acidizing sandstone cores. The core material was observed by ESEM before, during and after laboratory core flood tests. This has resulted in a more effective acid stimulation treatment design through a better understanding of the fluid/rock interactions.

Introduction

The new Environmental Scanning Electron Microscope (ESEM) has become a valuable research tool for performing laboratory acid stimulation studies. It provides the capability of observing core material In its natural, uncoated state, before and after core acidizing experiments. The direct effect of specific treatment fluids on the formation rock can be observed while eliminating any changes to the core due to sample preparation techniques.

There are many problems associated with using a conventional scanning electron microscope (SEM) to identify changes in core material produced by core acidizing experiments. The problems are caused by the internal design of a conventional SEM which prevents the observation of wet and uncoated core material. The problems are as follows:

  1. The core plugs used in the acidizing experiments cannot be directly observed in a conventional SEM because they are saturated with varIous treatment fluids. Before imaging, small pieces must be broken from the core plug, cleaned, dried, and metal coated. Because observations are made from different samples before and after treatment, interpretation is very subjective.

  2. The cleaning, drying, and coating processes may cause changes to the delicate clay structure or movement of the fine material which may lead to erroneous conclusions.

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