1. n. [Enhanced Oil Recovery]
A fluid stage, normally hydrochloric acid [HCl], pumped ahead of the main treating fluid (mixture of hydrofluoric [HF] and hydrochloric [HCl] or organic acids) in a sandstone matrix-stimulation treatment. One of the purposes of a preflush is to displace formation brines that contain K, Na, Ca ions away from the wellbore, decreasing the possibility of crystallizing alkali-fluosilicates that could plug the pores. The other purpose of a preflush is to dissolve calcareous materials to minimize calcium fluoride [CaF2] precipitation, and to dissolve iron scale or rust to avoid the precipitation of the gelatinous, highly insoluble ferric hydroxide [Fe(OH)3]. Multiple preflush stages using brines such as ammonium chloride [NH4Cl] or solvents are used when multiple damage types are present. A preflush is sometimes called a spearhead.
See: hydrochloric acid, matrix stimulation, overflush, precipitate
2. n. [Enhanced Oil Recovery]
In chemical flooding, a fluid stage, normally low-salinity water, pumped ahead of the micellar or alkaline chemical solution. One of the purposes of the preflush is to displace reservoir brine containing potassium, sodium, calcium and magnesium ions from the near-wellbore area, avoiding adverse interactions with the chemical solution. The other purposes are to adjust reservoir salinity to favorable conditions for the surfactant (chemical solution) and to obtain information about reservoir flow patterns. Sometimes a preflush stage is not necessary, especially when brine-tolerant chemical systems are used.
