Warm ice at temperatures close to the pressure melting point is often encountered in deep ice-core drilling. The heat generated by rotary cutting can melt ice chips, which seriously threatens the safety of drilling if the chips refreeze on the drill bit or barrel. Lowering the cutting heat is an effective method to reduce the melting of ice chips. In this study, a general theoretical model was established based on heat transfer theory and the cutting mechanism to calculate and analyze the cutter temperature during the circulation of the drilling fluid. The model was validated by a series of experiments, which demonstrated reasonable agreement between the calculated data and experimental results, with a maximum error of <16%. The factors that contribute to the rise in the cutter temperature during warm ice drilling were investigated. Results suggest that the drilling fluid has excellent cooling performance, and its type and flow rate have minimal impact on the cutter temperature. To mitigate the cutter temperature rise, maximizing the rake angle and thermal conductivity of the cutter, while minimizing the rotation speed of the drill bit, cutting depth, cutter width and friction coefficient between the ice and cutter is recommended.

In the petroleum industry, one of the most important functions fluids must perform is to carry cuttings efficiently. In this context, the contamination of drilling fluids represents a challenge to the industry because it alters the properties of these fluids. This work compares two drilling fluids (one palygorskite-based and the other polymer-based), evaluating the effects of contaminants on these drilling fluids and their ability to carry cuttings. A contaminant is considered to be any material that causes undesirable changes in the properties of the drilling fluid. Contamination of an aqueous drilling fluid by calcium hydroxide (Ca(OH)2) and brine can increase the filtrate volume, decrease the perforation rate and alter the rheological properties of the fluid. This work analyses the effects of contamination by Ca(OH)2 and brine on the properties of salted water-based drilling fluids viscosified with palygorskite. For comparison, tests were performed with a polymeric drilling fluid. The cutting-carrying capacity of these fluids was verified using the empirical correlations of Moore (1974), Chien (1994) and Walker & Mayes (1975). The results show that the drilling fluid with palygorskite is resistant to contamination with Ca(OH)2 and brine, presenting low filtrate volume values. The drilling fluid with palygorskite presented greater slip velocities than the polymeric drilling fluid. Regarding the cutting-carrying capacity, the drilling fluid with palygorskite presented positive transport ratio values (>0), making it suitable for carrying rock cuttings. However, the polymeric drilling fluid showed better performance in terms of cutting-carrying capacity.

The rheological and filtration properties of a Tunisian clay are presented and compared with those of Wyoming bentonite. The study was part of an evauation of Tunisian mining resources in general, and of the bentonitic deposits in particular. Qualitatively, the two clays exhibit the same rheological and filtration behaviour but quantitatively the properties of purified Wyoming bentonite are distinctly better at the same concentration. The difference can be related to the mineralogical structure and the presence of 13% illite in the Tunisian clay. Standard tests show that the rheological behaviour of the purified Tunisian clay is close to that of the raw Wyoming bentonite and conform to the American Petroluem Institute (A.P.I.) standards for medium-depth drilling formulations.

The sedimentation characteristics of two commercial bentonites, Tixoton (organically treated) and Montigel-F (untreated), were investigated using a 3% w/v clay suspension at different concentrations (1, 3.5 and 10%) of NaCl and pH values (2, 7 and 12). Settling rates, floc diameters and sediment volumes were derived from changes in light transmittance using a Turbiscan Ma 2000 instrument.

Both bentonite suspensions were unstable (flocculated) in NaCl solutions. The settling rate increased with increasing concentration of NaCl and was directly related to floc diameter. The sediment volume reduced with increasing NaCl concentrations, a result of greater double layer compression caused by increased ionic strength. At comparable salt concentrations, the organically-treated bentonite (Tixoton) settled at a much slower rate and had a greater sedimentation volume. The suspensions of both organically-treated and untreated bentonites were stable (dispersed) above pH 7 and unstable in acidic conditions. The settling rate for Tixoton under acid conditions was much smaller than that for the Montigel-F. Differences in sedimentation characteristics between the two bentonite samples are probably due to the presence of an anionic polymer (carboxymethyl cellulose: CMC) in Tixoton.

The viscosity of the bentonite suspensions was also studied. The viscosity of the clay suspension is closely related to clay dispersivity in solution. The CMC was highly effective in increasing the viscosity of the bentonite suspensions, but only under neutral and alkaline conditions.