Quality of Bentonite and Its Effect on Cement-Slurry Performance

Abstract

Summary This paper shows the results of a 2-year study on the quality of cement-grade bentonite now available commercially. A comparison of laboratory-designed slurries with field-blend samples shows drastic alteration of cement-slurry performance from poor blending procedures and/or poor-quality bentonite, which could cause catastrophic consequences. Evidence from microscopic examination, chemical-quality procedures, and cement-slurry performance data shows a failure rate approaching 50% among more than 150 samples tested from field stocks. Laboratory data on cement-slurry performance with bentonite that passed and failed the current API specification test are presented. Introduction Problems with bentonite quality and difficulties in determining Problems with bentonite quality and difficulties in determining quality variations among manufacturers' plants and service-company stock points continue to occur. The important considerations in the use of a premium bentonite are the properties that good bentonite gives a lightweight cementing system: zero free water, good fluidloss control, good rheological properties, adequate compressive strengths for filler cement, and cost-effectiveness. The API Specification for Materials and Testing for Well Cements defines bentonite as "a natural material consisting principally of the clay mineral, montmorillonite. It is dried and ground, principally of the clay mineral, montmorillonite. It is dried and ground, but otherwise untreated in processing. No beneficiating agents or other material shall be added to bentonite used in well cements." Additives have been used widely to improve the performance of the bentonite temporarily in drilling fluids to meet API specifications. It has become apparent that treated bentonite is also being used in cementing applications as API cement-grade bentonite. Past API specifications on cement-grade bentonite were violated by the addition of polyacrylamides to increase the yield of lowgrade bentonites. The polyacrylamide concentrations were so small that detection by sophisticated analytical techniques was impossible, but the yield of bentonite was beneficiated so that the bentonite appeared to pass the specification test. The current API specification test fails this type of beneficiated bentonite. API Specifications on Bentonite Beneficiating agents have long been forbidden by the description of cement-grade bentonite. In the past, enforcement of this specification has been difficult because of the shortcomings of classic analytical techniques mentioned earlier. Because of viscosity and filtration problems resulting from the use of beneficiated bentonite, the API Committee on Drilling Fluids (Committee 13) developed a test procedure that uses sodium hex-ametaphosphate to screen bentonite for beneficiation. This screening test has been accepted by the Committee on Well Cements. The test does not detect beneficiating agents directly but involves measuring properties of a bentonite slurry (such as plastic viscosity, yield point, and fluid-loss control) both before and after treatment with a solution of sodium hexametaphosphate to ensure that they fall within specified ranges. The sodium hexametaphosphate negates the beneficiating effects of added polymers and allows the detection of treated bentonite.

Effect of Bentonite Prehydration Time on the Stability of Lightweight Oil-Well Cement System

“…The sodium bentonite is very useful in cement composites because of its ability to swell when dispersed in water and also to form stable dispersions [7,10]. As an extender in lightweight cement systems, bentonite acts as a free water controller, fluid loss additive, or antisettling agent [11][12][13].…”