(28b) Comparative Analysis of Gelled Fluids Through Large Amplitude Oscillatory Shear Rheology (LAOS)

Drilling muds have long been known to develop weak bonds between particles and emulsion droplets in suspension, commonly known as forming a gel structure.  Drilling fluid design has long hinged on the balance between providing sufficient viscosity and gel structure to suspend solids (both cuttings and barite) and minimizing the rheological impacts on equivalent circulating density (ECD), pump initiation, and swab and surge pressures.  A wide variety of products and solutions are offered to boost or thin viscosity, boost “low-shear” viscosity, and control gel strength; all of these aim to provide suspension and hole cleaning with minimal ECD and risk from pressure spikes associated with initiating fluid flow.  The prima facie supposition that increasing viscosity or yield point naturally leads to better solids suspension has been disproved through hard experience, while the idea that drillings fluids that appear significantly less rheologically “robust” can still provide suspension is now being observed.  The exact reasons for these apparent contradictions are as yet poorly understood.

Currently, the rheological impact of various products and treatments on microstructure formation are evaluated through a transient flow start-up test on a standard oilfield viscometer, with the gel strength recorded as the peak in this transient test.  Both its ease of use and ubiquitous nature in the industry have led to a long reliance on this instrument as the best, most convenient source of rheological information on drilling fluids.  Its shortcomings have been previously identified, and this paper endeavors to explore rheological differences through a variety of techniques on a lab-grade rheometer.  Questions as to how and why various typical products affect viscosity and gel structure in invert emulsion fluids are explored, along with the implications to suspension and transient pressure spikes in drilling operations.