Chemical Engineering Science 62 (2007) 5880 – 5896 www.elsevier.com/locate/ces Reaction of citric acid with calcite M.H. Al-Khaldi a , H.A. Nasr-El-Din b, ∗ , S. Mehta b , A.D. Al-Aamri b a The University of Adelaide, Adelaide, SA 5005, Australia b EXPEC Advanced Research Center, Box 62, Dhahran 31311, Saudi Aramco, Saudi Arabia Received 19 July 2006; received in revised form 20 May 2007; accepted 13 June 2007 Available online 19 June 2007 Abstract The reaction of citric acid with calcite was investigated using the rotating disk apparatus. The effects of disk rotational speed, system pressure, and presence of magnesium and ferric ions on this reaction were examined. Scanning electron microscope (SEM) was also used to characterize the surface of the calcite disks at the end of each experiment. The reaction of citric acid and calcite is mass-transfer limited up to 500 rpm. The reaction rate of citric acid–calcite is limited by the precipitation of calcium citrate on the surface, especially at atmospheric pressure. Increasing the system pressure from 1000 to 1500 psi has no significant effect on the dissolution rate of calcite. The diffusion coefficient of 7.5 wt% citric acid in the presence of calcium citrate and calcium ions is 4.5E - 6 cm 2 /s at 50 ◦ C and 1000 psi. The presence of magnesium ions in citric acid solutions results in higher concentration of calcium ions in solution. Calcium citrate forms only on the disk surface, but not in the bulk solution. The morphology of the calcium citrate layer is dependent on the rotational speed of the calcite disk. More calcium citrate precipitation occurs at high disk rotational speeds, above 500 rpm, and at system pressures less than 100 psi. Calcium citrate appears to precipitate as feathery aggregates of radiating platy crystals on the surface of calcite.  2007 Elsevier Ltd. All rights reserved. Keywords: Citric acid; Calcium citrate; Dissolution rate; Rotating disk 1. Introduction Organic acids have been used to stimulate carbonate reser- voirs for more than four decades (Harris, 1961; Chatelain et al., 1976; Crowe et al., 1988; Fredd and Fogler, 1998a,b,c; Fredd, 2000; Huang et al., 2000; Nasr-El-Din et al., 2001). The two main organic acids that are frequently used to stimulate the carbonate reservoirs are formic acid (HCOOH) and acetic acid (CH 3 COOH). These acids are less reactive with carbonate rocks than hydrochloric acid (Nierode and Williams, 1971). Acetic and formic acids are less corrosive than mineral acids and can be inhibited. For example, acetic acid can be inhibited against all types of steel at elevated temperatures for extended periods of time (Harris, 1961). They are mostly used in high temperature reservoirs, where the fast HCl spending rate can cause severe tubing corrosion and poorly etched fractures (Crowe et al., 1988). ∗ Corresponding author. Tel.: +966 3 872 3567; fax: +966 3 872 3926. E-mail address: hisham.nasreldin@aramco.com (H.A. Nasr-El-Din). 0009-2509/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2007.06.021 Organic acids are more expensive than HCl per unit volume of rock dissolved and cannot be used at high concentrations. Typically, the concentrations of acetic and formic acids used are less than 13 and 9wt%, respectively. This is because the reaction products (especially calcium formate) can precipitate at higher acid concentrations (Robert and Crowe, 2000). In addition, the reaction of organic acids with calcite, CaCO 3 , is reversible and thermodynamically limited by the presence of the reaction products (Chatelain et al., 1976; Buijse et al., 2004). In other words, the reaction is controlled by the diffusion of the reaction products away from the rock surface (Fredd and Fogler, 1998a; Fredd, 2000). Mixtures of organic acids have been used to stimulate high temperature/pressure wells in the Arun limestone field in Indonesia ( Van Domelen and Jennings, 1995) and to remove calcium carbonate scale in gas wells in the Merluza field (Da Motta et al., 1998). Citric acid (C 6 H 8 O 7 ) has historically been used in oilfield treatments as an iron-control agent (Hall and Dill, 1988). It is an -hydroxy carboxylic acid with three carboxylic (–COOH), and one hydroxyl (–OH) groups. Because of its highly stable