Paper: Acid-Soluble Drilling, Completion Workover Fluids

Mud solids can cause severe productivity impairment which can only be prevented by the use of acid-soluble muds. Conditions justifying the use of such muds are as follows: 1) Highly permeable formations (say above 500 md). 2) Carbonate reservoirs of low matrix permeability whose productivity depends on a fracture network. 3) In production repair wells where the risk of lost circulation into depleted reservoirs and of plugging established flow channels is high. 4) When it is advantageous to have an acid-soluble filter cake, e.g. when gravel packing, shot perforating, or if a good cement bond is required. Acid soluble, water-base fluids are composed of graded carbonate bridging particles whose maximum size must be equal to at least one third the largest expected pore opening. Filter loss control is best provided by an acid-soluble lignosulfonate and viscosity by hydroxyethyl-cellulose. Sodium chloride or calcium chloride brines and/or ground carbonates (44-2 microns) are used then higher weights are required. One type of acid-soluble oil-base fluid is an invert emulsion stabilized by treated carbonate fines. In others an oil-soluble colloid is used. In both cases ground carbonates are used for bridging and weighting. One problem that arises when drilling with acid-soluble fluids is the incorporation of insoluble drilled solids. This is minimized by not introducing the fluid until the top of the productive horizon is reached and by the greatest possible use of mechanical separation equipment. In production workover jobs, the most common problem is loss of circulation. To prevent or remedy this, the fluid is applied as a pill containing a high concentration of bridging solids.