Evaluation of mangenese tetraoxide as a weighting material for water-based drill-in fluids

Abstract

Drilling Unayzah-B reservoir (shale and sandstone) in Saudi Arabia requires high mud density (plus/minus 95 pcf). To formulate this mud, calcium carbonate particles were used because of their high acid solubility. However, when drilling the 5-7/8' hole, sticking occurred, which resulted in expensive fishing and/or sidetracks operations. To minimize these problems, barite was added with CaCO( 3) to reduce the amount of solids needed to formulate the drill-in fluid. However, barite is acid in-soluble and may cause formation damage. To enhance productivity in such case, the weighting solids invading the reservoir should have the ability to flow naturally since no other means such as acidizing are effective. But, conventional weighting solids used in drill-in fluids when entered the reservoir formation will not easily flow back into the wellbore when production resumes. Formate drill-in fluids with low CaCO(3) content were used to drill some wells in this reservoir, however these fluids are expensive. In this study, experimental effort was done to formulate drill-in fluids that are based on manganese tetraoxide particles to drill deep gas reservoirs. The experimental work included measuring the rheological properties, thermal stability, API and HT/HP filtration of the three drill-in fluids. The results obtained showed that several polymers can be used to design KCl/Mn(3)O( 4)--drill-in fluids. The developed fluid showed better thermal stability compared to the typical drill-in fluids that are currently being used. Also, the KCl/Mn(3)O(4) formation damage were compared to the two typical fluids above. The experimental work in this paper involved Unayzah-B cores flood testing (linear flowing and tangentional) to learn about return permeability, flow initiation pressure and filtrate loss during drill-in exposure (dynamic and static). Also, the study involved HT/HP Filter Press dynamic experiments to determine the filter cake thickness, filter cake composition (solids, polymers and mineralogy) using TGA, XRD and XRF, spurt loss and chemical analysis of filtrates. And finally, filtrates compatibility with produced brine from Unayzah-B gas reservoirs at 300degF and 300 psi. The data generated from this study showed that the KCl/Mn(3)O( 4) showed the least formation damage compared to the other two fluids. It showed higher return permeability (50% more) than potassium formate/CaCO( 3) and BaSO(4)/CaCO(3) and similar filter cake thickness. The filtrate was compatible with the brine compared to potassium Formate where high precipitation was observed when its filtrate was mixed with the formation brine. The use of the developed fluid in this study should result in an increase in gas production from Unayzah-B reservoir.