Abstract: A composition, especially suitable for use as a mud sweep and a spacer. The composition includes a petroleum-based hydrocarbon, a biopolymer, a thickening agent, a surfactant and potassium chloride. Other optional ingredients in embodiments of the present invention include a lower alkanol, sodium salts and aluminum stearate.

Abstract: A fluid system has been discovered using a distribution of graded calcium carbonate particle sizes, a biopolymer and a polymerized starch. The biopolymer is a non-ionic polysaccharide of the scleroglucan type. It is important that the calcium carbonate particles be distributed across a wide size range to effectively prevent filtration or fluid loss into the formation. Since the filter cake particles do not invade the well bore due to the action of the biopolymer and starch, no high pressure spike occurs during the removal of the filter cake. This high pressure spike indicates damage to the formation and well bore surface, which damage typically reduces overall permeability of the formation. The rheological properties of the fluid allow it to be used in a number of applications where protection of the original permeable formation is desirable.

Abstract: Oil or water based drilling muds, together with other wellbore contaminants such as pipe thread sealants and lubricants are displaced from the well and the wellbore surfaces are cleaned by a system of four serially pumped fluid pills. The first stage fluid pill is a weighted spacer having an alkyl polyglycoside surfactant, a dispersant, a weighting agent and a viscosifier. The second stage pill is a drive fluid formed by water and a viscous polymer gel such as clarified xanthan gum having a viscosity/shear rate characteristic which minimizes mixing of the drive fluid pill with the displacement fluid pill. A buffer fluid pill comprising completion brine is interposed between the drive fluid pill and a fourth stage comprising a wash fluid pill using an organic solvent or a caustic alkyl polyglycoside as a final cleaning stage. The densities of the pills are controlled to minimize mixing and fingering of one pill into the other.