Date of Award

January 2023

Document Type


Degree Name

Master of Science (MS)



First Advisor

William Gosnold

Second Advisor

Adesina Fadairo


The harsh temperature in which some reservoirs are geologically located has presented a major drilling fluids challenge to the geothermal industry. Therefore, the design of drilling fluid that will be effective and cost-friendly in high-temperature high-pressure (HPHT) environments has become an age-long threat to the economic development of geothermal resources. A geothermal drilling fluid should possess high thermal stability while performing the basic functions of drilling fluid all through the drilling process. Exerting sufficient hydrostatic pressure, suspension, and effective removal of cuttings, shale stabilization, minimum filtrate loss, and thin filter cake thickness. Ilmenite was adopted as a weighting material due to its advantages over barite. This study evaluates the effectiveness of fly ash buffered with an industrial standard copolymer to eliminate ilmenite sag, filtrate loss, and rheological instability encountered with ilmenite-densified water-based drilling fluids at HPHT environments when accessing geothermal wells. Four laboratory barrels of water-based drilling fluids were formulated for this study. Fly ash of 130 µm particle size was added to mud samples at 0, 1, 2, and 3 lb./bbl ratios. The influence of fly ash on drilling fluids properties was thoroughly evaluated by measuring static and dynamic sagging at vertical and inclined (30, 45, 60, 70, 80, and 90 deg) conditions over a range of temperatures (250F, 300F, 350F, and 400F). A high sag factor (SF) occurs between 45-75 deg, with 61 deg as the peak regardless of the temperature alteration. Additionally, it was observed that the inclusion of fly ash improves the sagging phenomena, the rheological and filtration behavior of the water-based mud samples to satisfy the industrial API standard value.