Summary
This paper discusses the effects of Ca2+, Mg2+, and Fe2+ on inhibitor retention and release. Better understanding of phosphonate reactions during inhibitor squeeze treatments has direct implication on how to design and improve scale inhibitor squeeze treatments for optimum scale control. Putting various amounts of metal ions in the inhibitor pill adds another degree of freedom in squeeze design, especially in controlling return concentrations and squeeze life.
Phosphonate reactions during squeeze treatments involve a series of self-regulating reactions with calcite and other minerals. However, excess calcite does not improve the retention of phosphonate due to the surface poisoning effect of Ca2+. The squeeze can be designed so that maximum squeeze life is achieved by forming a low solubility phase in the formation. Addition of Ca2+, Mg2+, and Fe2+ in the pill solution at 0.1 to 1 molar ratios significantly improves the retention of phosphonate. Alternatively, these metal ions can be dissolved from the formation while an acidic inhibitor pill is in contact with the formation minerals. Both BHPMP and DTPMP returns were significantly extended by the addition of metal ions (e.g., Ca2+ and Fe2+). The addition of Mg2+ may increase the long-term return concentration, which is important for some wells where a higher inhibitor return concentration is needed.
The laboratory squeeze simulations were compared to return data obtained from squeeze treatments performed on two wells located in a sandstone reservoir in Saudi Arabia. The sandstone formation contains significant amounts of iron-bearing minerals.
Introduction
Mineral scale formation is a persistent problem in oil and gas production, especially in older reservoirs with increased water production and drawdown. Inhibitor squeezes are commonly used to deposit a suitable scale inhibitor in the formation. During an inhibitor squeeze treatment, a predetermined volume of the inhibitor solution is pumped into the formation and followed by injecting another volume of brine or diesel to place the inhibitor further away from the wellbore and allowing it to react with the existing rock. During production following a squeeze treatment, the inhibitor is slowly desorbed or dissolved into the formation water.
Earlier efforts have focused on describing what happens and when to resqueeze (Hong and Shuler 1988; Rogers et al. 1990). More recent papers have advanced the knowledge of inhibitor reactions under various production conditions (Benton et al. 1993; Sweeney and Cooper 1993; Lawless et al. 1993; Sorbie et al. 1994; Jordan et al. 1994; Jordan et al. 1995; Jordan et al. 1997; Lawless and Smith 1998; Smith et al. 2000; Collins 2003). The primary conclusions from several previous studies (Al-Thubaiti et al. 2004; Kan et al. 2004a; Kan et al. 2004b; Tomson et al. 2006) of NTMP(aminotri(methylene phosphonic acid))-calcite reaction are:The extent of NTMP retention by carbonate-rich formation rock is limited by the amount of calcite that can dissolve prior to inhibitor-induced surface poisoning;calcite-surface poisoning effect is observed after approximately 20 molecular layers of phosphonate surface coverage that retards further calcite dissolution; andthe consequence of retarded calcite dissolution is that less basic ion, CO2-3, is released into solution, leaving the solution more acidic; therefore, more soluble calcium phosphonate solid phases form.
The inhibitor return concentration can be altered by changing the inhibitor concentration in the pill. The ability to control the high inhibitor return may be useful in initial water breakthrough where high inhibitor return is desired. Kan et al. (2005) also compared the retention of NTMP, DTPMP (diethylenetriamine penta (methylene phosphonic acid)), BHPMP (bis-hexamethylenetriamine penta (methylene phosphonic acid)), and PPCA (phosphinopolycarboxylic acid) with pure calcite, a calcite-rich chalk rock, a calcite and clay-rich formation rock from Guerra Ranch, McAllen, Texas, and a quartz sandstone with very little calcite from Frio formation, Galveston County, Texas. Similar inhibitor returns were observed in both calcite-rich and low-calcite rock, suggesting that calcite is the primary solid responsible for phosphonate retention. Clays or other minerals play a secondary role in phosphonate retention. The retention of the polymer-based inhibitors is much lower than phosphonates. The data show that BHPMP provides the highest squeeze life at MIC > 50 mg/L. DTPMP is the preferred inhibitor at MIC between 1 and 50 mg/L and NTMP is the preferred inhibitor at MIC < 0.3 mg/L.
Calcium ion (Ca2+) is the predominant divalent metal ion in most oilfield produced waters. Previously, several reports indicated that Ca2+ and Mg2+ have a strong effect on inhibition of barite by common inhibitors (Fernandez-Diaz et al. 1990; Boak et al. 1999; Collins 1999). Collins (1999) observed a clear change in crystal habit between barite growth in the presence and absence of Ca. Xiao et al. (2001) noted that Ca significantly enhanced the inhibitor efficiency; however, Ca had no effect on barite nucleation time in the absence of scale inhibitor. Collins (1999) reported a similar effect of Ca with polyaspartate as a barite inhibitor. The enhanced inhibition efficiency may be attributed to the reduction of net negative charge of the polyion due to complexation of the polyaspartate with divalent cations (Tomson et al. 2003).
In the present paper, the influence of metal ions, e.g., Ca2+, Mg2+, and Fe2+ on the inhibitor retention and release was evaluated in both laboratory simulation and field case studies. These metal ions were either originally added to the inhibitor pill solutions or generated in-situ because of the dissolution of reservoir minerals by acidic inhibitors.
The state of the art in scale inhibitor squeeze treatment
