New Bentonite Extension Test Provides A Better Gel Quality Evaluation
Authors: Warren B.K. , Cavanagh P. , Woodland D. 
Published: March 2010

A bentonite extension (BEXT) lest has been developed as a bentonite quality control procedure. Unlike the API bentonite quality control test, the HEXT test will differentiate between poorer and better grades of bentonitic ores. The BEXT lest generates an ultimate yield point for a bentonite based upon that bentonite's maximum extendability in fresh water. Beneficiating polymers commercially added to bentonite samples do not affect ultimate yield point values. Bentonite samples with ultimate yield paints greater than 30 pascals are shown to produce fresh water bentonite muds with acceptable rheological properties. Poorer quality bentonites having ultimate yield points of less than 16 pascals do not give adequate yield points or gel values.

Data presented from the BEXT test correlates closely to western Canadian drilling performance. Field data demonstrate that faster drilling rates and fewer hole problems have been realized when good quality bentonites, as specified by the BEXT test, have been used. A review of case histories for foothills wells indicates better hole cleaning and improved suspension properties have been observed when poorer quality bentonites were replaced by better quality gels. Drilling costs have been significantly reduced when better quality bentonites are used for fresh water drilling operations.

Introduction

Improved hole cleaning and suspension characteristics or drilling fluids have commonly been associated with reduced drilling problems. This is especially true when fresh water systems normally associated with significant hole washouts are used. Most of Shell Canada's rheological problems in recent years have been associated with poor quality bentonite. Therefore, a quality control program was initiated ror bentonitic clays to reduce drilling problems and mud costs in the field. This paper discusses testing procedures as well as field results for fresh water drilling fluids. Gel/XC polymer costs have been reduced by at least 37% in areas where the program has been implemented.

Bentonite Quality

Most bentonitic days used in the drilling industry have beneficiating agents added during milling operations. .Although the agents have qualified interior clays to API specifications, problems meeting our rheological specifications in the field have been observed where poor quality bentonite has been used(1). With additions of poor quality bentonite minimal increases in yield point and initial gels have been observed, while methylene blue test values, plastic viscosity, gel progression and % sounds increase significantly making the drilling fluid more susceptible to contamination as well as more costly. In contrast, with higher quality bentonite additions both yield point and initial gel values improve significantly.

The current approved use of extending polymers to upgrade bentonite has led to concern about true bentonite quality(2). A new method to ensure bentonite quality is required to replace the current API procedure. Alternate quality procedures for bentonite have been proposed, however, none of these methods quantitatively measure raw bentonite quality(3,4).

Bentonite Extension Text (BEXT Test)

The BEXT test was developed to quantitatively measure bentonite quality. This procedure measures the maximum yield potential of bentonite samples for a standard bentonite extender Alcomer 1773.