Acidizing high-temperature wells has always been a challenge due to problems associated with corrosion of tubulars and loss of acid carbonate dissolving efficiency. Typical acid treatments of offshore wells with extreme downhole temperatures are carried out using retarded acids, which often degrade, form insoluble salts, and require further neutralization of flowed back spent acid, resulting in loss of production days. Recently there has been a growing interest in using amino-polycarboxylic acids such as glutamic acid, N, N'-diacetic acid (GLDA) to treat carbonate or sandstone formations with high bottomhole temperatures. The benefits of using such acid for high-temperature acidizing are lowered reactivity, inherent chelation chemistry, better tubular protection and biodegradability.
Extensive laboratory testing was conducted before a field trial of the acid. A corrosion test conducted at 395°F using corrosion-resistant G-3 and G-50 coupons (identical to the metallurgy of the tubular) without corrosion inhibitor had a corrosion rate of 0.0068 and 0.0069 lb/ft2, respectively; thus, no corrosion inhibitor was included in the acid used for field treatment. About 2.38 pore volumes of the acid broke through a carbonate core, dissolving 452 mg of calcium at 300°F. A computed tomography scan of the treated core showed a highly branched wormhole distribution network. The acid was found to be compatible with brines and different elastomers often used in downhole completions.
Flowback samples of spent acid analyzed by ICP-OES contained a high amount of dissolved calcium, iron, and magnesium. Further neutralization of the flowed back acid was not required, saving the operator two or more days of production per well.
The successful removal of wellbore scale and formation damage in four offshore wells with bottomhole temperatures ranging from 395 to 410°F with an amino-polycarboxylic acid is presented. Detailed laboratory testing that supported the treatment design and field application will be reported. Production enhancement as a result of the treatments, statistical analysis of flowed back spent acid physico-chemical testing, challenges with the treatment process, and treatment design will also be presented.