Acidization of Analcime-Cemented Sandstone, Gulf of Mexico

ABSTRACT In a number of offshore Louisiana oil wells a white powdery, finely granular siliceous material was swabbed from wells and found on screens after acidizing with mud acid. The white material was analyzed as silica gel or aluminosilicate gels. Analysis of the formation sands which produced the white material after acidization shows the sands to be extensively cemented with analcime. Analcime is a hydrated sodium aluminosilicate related to zeolites. Analcime is a low density (2.3 gm/cc) mineral which is highly soluble in HF and will form hydrated gels with concentrated HC1. Analcime is associated with volcanic ashes, altered volcanic sandstones and salt domes. There are many salt dome related oil fields in the central Gulf where volcanic ash beds are locally common. Therefore, the occurrence of analcime cemented sandstones may be more widespread in the Gulf than previously suspected. A significant production decline was noted in a number of wells in the South Pass Blk. 61 field, a complex salt dome related structure, after acidization with HF or concentrated HC1. In many cases, production decline is attributed to damage by mineral precipitation or gel formation from acid spent on analcime-cemented sands. This paper describes the petrographic and fluid/rock studies done to optimize pre and/or post gravel pack acid stimulation system for analcime- cemented sandstones from South Pass Blk. 61. SEM photos show the effect of various acid systems on analcime and an effective acid system for these types of formations is discussed.