The Appalachian Basin presents one of the most challenging production and stimulation problems because of the iron content of its hydrocarbon producing formations.
A variety of iron compounds in the producing formations present problems that have to be considered to effectively stimulate these formations. These iron compounds present three major problems:
Relatively insoluble iron compounds that may be released by acidizing fluids and have a plugging effect when they migrate;
Acid soluble iron compounds that may reprecipitate when the acid spends; and
Aerated fracturing fluids being incompatible with formation waters that contain ferrous iron.
A research program was initiated in the later part of 1980 to determine methods of more effectively controlling the iron problems in the Appalachian Basin. Results of this study provide data for comparing the effectiveness of various iron control systems that are used in acid stimulation or breakdown techniques that minimize the release of acid insoluble solids and stabilizes them to decrease the detrimental effect caused by fines migration. Also developed in this study was an iron control system that helps the compatibility of the treating fluid with ferrous iron in the formation water.
Flow test data and field results indicate the effectiveness of these iron control systems and treating techniques.
Production and stimulation problems have been encountered in the Appalachian Basin because of the iron content of some of the hydrocarbon producing formations
Iron control studies previously have dealt primarily with iron control in acidizing operations. Acid primarily with iron control in acidizing operations. Acid solutions progressively dissolve iron and iron compounds as they are mixed in the acid tanks, pumped through tubular goods, and into the formation. If the acid does not contain an effective iron control system, the dissolved ferric (Fe) iron will start to precipitate when the acid spends to a pH of 2.5 and will precipitate when the acid spends to a pH of 2.5 and will approach complete precipitation when the pH increases to 3.5.
Iron compounds such as magnetite and hematite are slowly soluble in acid, while pyrite is relatively insoluble in acid. The acid may spend on rapidly soluble material in the formation and release the relatively insoluble or slowly soluble compounds. The released solids may migrate, bridge flow channels, create cleanup problems, decrease production, and increase the rate of production decline after stimulation.
Fracturing fluids, other than the breakdown acid, are usually non-acidic or weakly acidic solutions that dissolve very little iron (less than 5 mpl). Iron had not been recognized as an important factor in fracturing treatment design prior to about 1977.
Ferrous (Fe ++) iron in a formation water will remain in solution at the pH of the natural formation environment. However, an aerated treating fluid may mix with the formation water and oxidize the ferrous iron to ferric iron. Ferric iron will then precipitate at the pH of the natural environment. This precipitate at the pH of the natural environment. This precipitate is considered detrimental to the stimulation precipitate is considered detrimental to the stimulation operation.
Ammonium citrate has been used as a iron control agent in fracturing fluids. Ammonium citrate chelates iron and was effective to some extent. However, like other metal chelating agents [citric acid, ethylene-diamine tetracetic acid (EDTA), nitrilotriacetic acid (NTA) and modifications thereof], it is incompatible with most of the metallic crosslinking systems that are used to form cross-linked gels. Cross-linked gels are popular because of their proppant carrying ability.
A research project was initiated to obtain an improved iron control system for fracturing fluids and to develop treating techniques that would improve production stimulation and production decline. production stimulation and production decline. P. 203