High-Silica Waters in Steamflood Operations

Abstract

Summary Most steamflood operations recycling water will experience silica scales at some location in the field operation. It is usually noticed in the steam generators or heat recovery steam generators (HRSG) used for cogeneration. Whether fresh water or recycled water is used, the hot water, high pH portion of the steam will affect the steam injection wellbore. Silica scaling in the formation around production wells, in the screen of the production wells, and in the pump in the production wells are also noticed due to flashing and deposits of silicates. Some of the silica scales can be controlled, some are beneficial, and some are very costly and difficult or impossible to control. In the formation the silica is in equilibrium with quartz so quartz solubility applies, while the more soluble amorphous silica solubility controls when not in contract with quartz rocks. Introduction Silica is a very complex chemical species with many different forms. In this article we will assume the silica is either the most insoluble quartz, or the more soluble, amorphous silica. The scale formed mostly is silicates—metal ion silicates such as sodium iron silicates or hardness ion silicates. In the oil field, deposits of silica or silica polymers are usually not found due to the long time required for them to form. Much of the literature on silica is concerned with geothermal operations. There are many U.S., German, Russian and Japanese articles that include studies of pH, temperature and total dissolved solids (TDS) waters vs. solubility. Equilibrium The silica in the water from a steamflood operation can be calculated from the bottomhole temperature of the well based on the quartz solubility curve. The solubility of the silica in the formation is thus controlled by the quartz solubility. Once the solution enters the wellbore the more soluble amorphous silica controls the solubility; otherwise the water would be supersaturated with silica as it cooled coming up the wellbore.1 Quartz solubility controls where there is quartz crystal material available. Thus the reservoir formation is all quartz solubility (or mixed solubility when other silica rock is also present but usually just quartz is considered). In the steam generator, the amorphous silica solubility controls and is very soluble at the high pH inside the unit. Once steam is generated and reinjected into the formation, the quartz solubility controls (which also increases in solubility with pH). Silica is only partially soluble in the steam itself. This is very important for steam turbines, but there is ample literature in this important area for those interested. It is important to know which solubility equilibrium on the silica is applicable. There are no really good silicate scale solubility numbers available. Some of the water solubility programs can predicate many of the silicate scales and some can even concentrate the water as if generating steam. Usually those programs associated with the geothermal operations are the best for silicates and steam generation. For amorphous silica, a limit of 75 ppm has evolved from the cooling tower industry. Experience has indicated that with 75 ppm silica in the feedwater only about 4 to 5 cycles or concentration can occur in a cooling tower prior to scaling. This has then been applied to the steam generation. Thus, the scale inhibitors which have been shown to inhibit silicate scale formation are limited to about 75 ppm silica in the feedwater and were developed from the cooling tower work. There are some new polymers on the market but they do not have any data on silicate inhibition at high temperatures and it is doubtful if they are useful at a high inlet water silica concentration. It would be a great benefit if a compound were developed. Steam Generators Internal in a steam generation, the alkalinity will decompose and form carbon dioxide and sodium hydroxide. The steam mixture for 70 to 80% steam quality will be pH of 10 to 11. (If steam is separated, the carbon dioxide goes with the steam and the water will contain the caustic and can indicate even higher pH.) Besides the 10 to 11 pH being beneficial to silica solubility, it is also the lowest point for iron solubility in hot water so iron is not put into solution. Also, it is known that a protective film of magnetite forms on steel when exposed to hot water. Thus pickup of iron, excluding an oxidizing agent such as oxygen, etc., should not occur in a steam generator. However iron contained in the inlet water will be deposited, depending upon the heat flux rate. Boilers have flux rates of about 100,000 Btu/hr/ft2 steam generators about 20,000 and heat recovery steam generators (HRSG's) about 1,000. Therefore boilers are the worst and HRSG's are the least problem on iron deposits from inlet water. Boilers can also develop a problem of caustic embrittlement or "free caustic gouging" but this requires high heat flux and is not normally seen in oil fields. In fact, the problems with the Foster Wheeler boilers at Shell's Mt. Poso field in the late 1970's was sodium iron silicate due to inlet iron and not caustic gouging related. Thus, with or without silica, iron can be deposited in heat equipment. Insoluble iron is usually deposited in the convection section, preheaters, etc. and soluble iron in the radiant section as the concentration of the ion is increased. Silica will not normally deposit in a steam generator. Amorphous silica becomes much more soluble with temperature and especially above pH 9 (Figs. 1 through 3)2 and can remain metastable at nearly twice the concentration for long periods without solid amorphous silica present. However silicates are a problem—mostly sodium iron silicates but also hardness silicates. Therefore the inlet iron must be limited to below 0.4 ppm soluble iron and it is better at 0.2 ppm soluble iron. No sodium iron silicates have occurred with water of low iron and 500 ppm silica, generating 70 to 75% steam quality at 1,000 psi in field tests. Shell, Mobil and Chevron have reported the problems that occur once the sodium iron silicates forms. However, the cause was the iron and/or hardness in the inlet water. Hot shutdowns will usually clean any sodium iron silicates from the tubes of steam equipment and sometimes is done rather than use of fluoride ion, but this can be very stressful mechanically. Preheaters usually need chemical cleaning.

The Effect of Silica Dissolution on Reservoir Properties during Alkaline-Surfactant-Polymer (ASP) Flooding

“…Introducing Mg 2+ and Ca 2+ ions to the brine in Sample A3, have further increased silica dissolution, and the silicate contents reduced by 7.32%. This is related to the depletion of the silicic acid by Mg 2+ and Ca 2+ minerals, resulting in dissolving more silica [37]. Sample A4 was treated with the ASP solution and indicated the highest reduction in silicate contents by 14.86%.…”

The Effect of Silica Dissolution on Reservoir Properties during Alkaline-Surfactant-Polymer (ASP) Flooding

“…Saturating sample A-4 in the ASP solution indicated a high silica solubility value of 48.2mg/L. The significant increase in the solution pH above 9, makes the silica more soluble (Bowman et al, 2000). This increase in the pH leads to depleting the silicic acid, and enforces the system to re-equilibrate, resulting in dissolving additional silica.…”

Interfacial Chemistry in Steam-Based Thermal Recovery of Oil Sands Bitumen with Emphasis on Steam-Assisted Gravity Drainage and the Role of Chemical Additives

“…The chemistry associated with SAGD operations can make this challenging, however, owing to the significant dissolution of reservoir minerals-e.g., silica-which increases substantially at temperatures above 100 • C. In turn, this increases the probability of silicate mineral deposition occurring on susceptible surfaces [62]. In the case of silica, solubility is greatly enhanced under high pH conditions, although by taking appropriate measures steam production is not necessarily compromised [63], as discussed in Section 5.…”