A Novel Procedure for Screening Drilling Fluid Compatibility
Authors: Gupta A. , Santos H. 
Published: March 2010

Abstract
This paper presents a new methodology to screen drilling fluids using cuttings, that may be used in the field. This methodology would help operators and mud companies in selecting and designing the drilling fluid, and in verifying the suitability of the fluids being used. Well-preserved shale samples were immersed in brines (NaCl, KCl, CaCl2) with varying salinities, as well as typical oil-based muds. The alterations in structure and composition of the rock were characterized with thermogravimetry (TGA) tests. This paper presents the results of immersion tests, showing the impact of different cations on various shale samples. It is shown that not only free water, but also interlayer, bound, and crystalline water, as well as calcite content may be altered depending on the cation present in the brine solution. In addition to TGA tests, the strength of corresponding shales was measured with a penetrometer that provided an index to assess the weakening of the material. The study indicates that different ions have minimal impact on the strength of the rock, even though the distribution of water content in a shale is changed. The paper provides a description of different scenarios suggested for the use of the methodology proposed herein, to design the drilling fluid, and to continuously check the suitability of the fluid with the drilled formations.

Introduction
Shale alterations due to interaction with the drilling fluid are believed to cause severe instability problems which, in turn, are responsible for huge losses for the oil industry. For many decades, shale swelling has been thought to be the main mechanism responsible for the majority of the complications encountered during drilling. The cost associated with drilling fluid design, its loss during drilling, along with the cost related to instability, constitute a significant percentage of the overall cost of a well. The availability of a methodology to design better drilling fluids and check their suitability for the formations being drilled can drastically reduce the well costs. Standard procedures have focused on reduction of shale swelling. However, recent studies have suggested that minimization of structural modifications and reduction of the internal energy are the more important issues. A recent study(1) suggests that an improved understanding of instabilities derives from a mechanical approach rather than a physico-chemical one.
The majority of tests currently used to design an optimum drilling fluid do not pay much attention to the reliability and preservation of the samples, which is critical to the validity of such tests. Also, most of the tests investigate the rock alteration only superficially. This paper presents a methodology to investigate shale-fluid interaction, based on thermogravimetry, that is able to detect even a slight variation of the rock structure as a function of immersion time in a drilling fluid. It can be used with cores, or even with cuttings, provided the cuttings are large enough to avoid the portion contaminated with the drilling fluid. The possibility of using cuttings opens the option for the methodology to be used in near-real-time during drilling.