Nature of the changes in clay minerals of the high temperature drilling fluids

The mineral reactions in the sepiolite- and palygorskite-based drilling fluids were systematically examined with X-ray diffraction and analytical electron microscopy before and after hydrothermal treatments. Both sepiolite and palygorskite were converted into smectites and other mineral phases in fluids containing either chlorides or hydroxide. The conversion rate usually increased with increasing temperature. Below 600''F, the smectite occurred as thin films with irregular outlines with a high layer charge, whereas discs or platelets with hexagonal outlines were formed above 600°F.

The conversion of sepiolite (or palygorskite) to smectite at low temperatures (<600ºF) was accomplished through epitaxial growth of smectite films around sepiolite (or palygorskite) fibers. At higher temperatures (>600ºF), smectites and other new mineral phases were formed through a dissolution-precipitation mechanism.

Smectite formed from sepiolite was chemically and structurally a trioctahedral variety; however, smectite formed in the palygorskite fluids consisted of trioctahedral and di-trioctahedral phases. The di-trioctahedral phase with approximately equal amounts of Al and Mg in the octahedral sheets was an unusual reaction product.

Other commonly observed reaction products, besides smectite, were feldspars, illites, cement minerals,zeolites, amphiboles, talc, and silica spherules. The formation of non-clay minerals resulted from reactions between additives and the chemicals released from the decomposition of original materials. The chemical properties of the additives apparently had profound effects on the formation of new mineral phases as well as on the stability of parent materials. The high alkalinity generated by hydroxides adversely affected the stability of sepiolite (or palygorskite) and favored the formation of framework silicates at lower temperatures.

The promoting effect of the added cations on the conversion rate of sepiolite and palygorskite to smectite seemed to be Ca>Mg>Na>K for both sepiolite/chloride and palygorskite/chloride systems, and Mg>Na>Ca>K for sepiolite/hydroxide, Mg>Ca>Na>K palygorskite/hydroxide systems. The rheological properties of the fluids, such as viscosity and fluid loss, were related to the mineralogical changes at elevated temperatures. The formation of smectite platelets, discs, and other new mineral phases obviously exerted a detrimental effect on the rheology of the fluids.