Recognizing the potential for, mitigating, or preventing weight material sag plays a critical role in the safety and success of many wellbore configurations. Accessing hydrocarbon reserves in deeper and ever extending reaches presents new challenges of narrower operating windows and hostile environments (higher pressures and temperatures). As a result, drilling fluids for these applications must possess an elevated degree of barite sag minimization. This paper reviews barite sag testing methodologies and discusses their relationship to field applications.
While it has been demonstrated that wellbore conditions, characteristics of the weight material used, and fluid type were all factors affecting settling rates in any given drilling fluid, there are still discussions on what single evaluation method or test might best qualify its potential to sag.
Various barite sag evaluation methods and techniques have been developed, used, and some even recognized as industry standards to assess the potential of weight material settling in drilling fluids. Because of the increasing variety of evaluations methodologies, it is timely to review the relationship between these actual laboratory measurements and field engineering of drilling fluids to recognize the potential for, minimizing, and managing weight material sag.
In this review, synthetic-based drilling fluids engineered to exhibit various types of rheological profiles were tested using a variety of methodologies for dynamic and static weight material settling. Relationships between the composition, rheological properties, and predisposition of the fluids tested to sag were established.
While the data from the tests detailed in this paper suggest clear differentiation between the key properties that most affect static and dynamic settling, the interpretation regarding the selection of a particular testing methodology were more subtle. It was observed that some of the results correlate better with actual wellbore conditions in which either static or dynamic sag would occur than with common drilling fluids engineering best practices. It was also confirmed that sag remains a very complexe phenomenon with a multiplicity of variables making it challenging to indubitably qualify the sag potential with just one single laboratory test.