SPE-175882-MS
Eliminate Pre-Flush and/or Post-Flush Acid Stages During Hydrofluoric
Acid Treatments: Experimental and Field Cases
Ahmed M. Gomaa, Sergey Stolyarov, and Jennifer Cutler, Baker Hughes
Abstract
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/ft
2
for N–80
and less than 0.02 lb/ft
2
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.
