The goal of sandstone matrix acidizing is to remove siliceous particles such as formation clay, feldspar, and quartz fines that are blocking or bridging pore throats. This is accomplished by injecting acid formulations containing hydrofluoric (HF) acid or its precursors, as HF is the only common acid that dissolves siliceous particles sufficiently. Standard treatments include pre-flush and/or post-flush stages of HCl acid to minimize the potential for calcium fluoride and other secondary precipitation. However, there is no guarantee that the HF acid stage will follow the path of the preflush stage; therefore, precipitation could still occur. In addition, the fast reaction of HF acid with clay minerals presents another challenge to effectively stimulating deep sandstone formations.

This paper presents experimental and field case studies with a sandstone acidizing treatment designed to retard the HF reaction rate and enable single-stage treatment -- eliminating the pre- and post-flush HCl acid stages and thereby reducing treatment complexity and treatment/rig time.

Extensive laboratory testing was performed using a variety of quarried sandstone cores with varying amounts of clay minerals, feldspar, and carbonate to confirm the ability to stimulate a wide range of sandstone formations. Also, formation core material was evaluated to confirm the results. Static solubility tests indicated that around 80% of the treated formation was soluble in the new acid system formulation. Coreflood testing noted an improvement in permeability of as much as 326%. Also, corrosion tests conducted using different metallurgy showed a very low corrosion rate (less than 0.005 lb/ft2 for N–80 and less than 0.02 lb/ft2 for Cr–13). Using coiled tubing, the new design was injected into three deep offshore wells, resulting in injectivity increases of 227%, 236%, and 256%. This paper will describe in detail the experimental result and the successful field treatments.

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