Development of Non-Iron Crosslinked Self-Diverting Gelled Acids for Carbonate Acidizing

It is often necessary to apply diversion techniques in the acid treatment procedures to ensure homogeneous coverage of treatment zones and maximize the acidizing effectiveness. Due to the heterogeneous properties of carbonate formations, it is even more critical to design diversion properly in carbonate acidizing than sandstone acidizing. This paper will describe the process of developing a special non-iron crosslinked self-diverting gelled acids for efficient carbonate acidizing. During acidizing operations, acids tend to flow into less flow resistance areas because these areas have higher permeability or less damage, leaving the low permeability areas non-treated. In order to control the placement of treatment fluids, a number of diversion techniques have been developed for carbonate acidizing treatment, both mechanical and chemical. Although mechanical techniques such as inflatable or straddle packers and ball sealers are very effective, they are more expensive and operationally time consuming than chemical diversion techniques, and furthermore not applicable in open hole and gravel packed well completions. The study identifies the non-iron crosslinked self-diverting gelled acids as the most cost-effective chemical diverter which are self-diverting and less damaging. There are mainly two types of polymeric self-diverting gelled acids, iron-crosslinked and non-iron crosslinked. The ferric ions in iron-crosslinked self-diverting gelled acids are chemically active and tend to form precipitations and create secondary formation damage especially in treatment of sour gas or oil reservoirs. Therefore, it is necessary to develop non-iron crosslinked self-diverting gelled acids in acidizing treatment of carbonate reservoirs. The results indicate that novel non-iron crosslinked self-diverting gelled acids, both fresh and spent acids, are stable at atmospheric and bottom hole conditions. Minimal corrosion (PI≤1, corrosion rate≤0.05 lbs/ft2) was obtained by introducing the unique corrosion inhibitor package. Up to 3000 ppm ferric ion was controlled properly in fluid design. The viscosity buildup of spent acids at pH 2-3 increases the fluid flowing pressure and provides effective diversion. The de-crosslink mechanism at higher pH (4-5) breaks the acidic gels, leaving less damage to the formation. This paper describes the development of non-iron crosslinked self-diverting gelled acids. A novel non-iron crosslinking chemical mechanism is introduced for the first time. Fluid properties of diversion, stability, iron control, corrosion and rheology are evaluated in acidizing laboratory.