Matrix Acidizing of Carbonate Formations: A Case Study

Formation damage is the impairment of permeability of rocks inside a petroleum reservoir occurs, during drilling, production, workover, stimulation and enhanced oil recovery operations, by various mechanisms such as chemical, mechanical, biological and thermal mechanisms. Near wellbore formation damages have a great impact on productivity or injectivity of the damaged formation. Acidizing is a stimulation method to remove the effect of near wellbore damage through reacting with damaging materials or the formation rocks (carbonate or sandstone rocks) to restore or improve permeability around the wellbore.Acid can be injected into the damaged formation below the fracture gradient of the formation which is called matrix acidizing. Acidizing operations is not only for treating the damaged formation, but also could be used for cleaning wellbore, production tubing and perforations from deposited scales prior to the matrix or fracture acidizing. The main objective of this paper is to achieve a homogeneous removal of formation damage through the entire pay zone of one of the Iraqi oil fields in order to increase well productivity index.The most common acid used in matrix acidizing of carbonate rocks (limestone or dolomite rock) is Hydrochloric acid (HCl); however, organic acids are preferably consumed in deep and high temperature wells due to high cost and uncontrolled corrosion problems with (HCl) acid. Various types of additives are added to the acid such as corrosion inhibitors, clay stabilizer, iron control agents and suspending agents.During the injection, the acid will flow into the least resistant (higher permeability) intervals in the formation due to heterogeneity or non-uniform damage in the formation and this prevents the damaged or low permeable sections from being stimulated. For achieving uniform acid treatment and diverting acid to the damaged zone, acid can be diverted mechanically or chemically. Each diversion technique has right conditions limitations to be applied depending on the formation situation.The name of the field is not mentioned in this work due to the confidentiality of publication and the well is named as Koya. Koya well is being drilled and completed at the carbonate pay zone, then the entire perforation zone being acidized to improve productivity. It has been observed that the production rate increases to 75% of the oil production rate.