Method of removing dissolved iron in aqueous systems

Abstract

Oilfield completion, drilling and workover fluids containing iron are treated to remove the iron by passing them through a cavitation device together with an oxidizing agent. The cavitation device intimately mixes the oxidizing agent with the fluid while increasing the temperature of the fluid, thus promoting the oxidation reaction. Ferric hydrate and other solids or colloidal iron are removed in a filter capable of removing particles as small as 0.5 micron. The system may be enhanced by the addition of a bed of activated carbon capable of catalyzing the oxidation reaction.

Description

TECHNICAL FIELD [0001]Dissolved iron is removed from an aqueous solution by passing the solution through a cavitation device while feeding an oxidizing agent into the solution, mixing and heating the solution in the cavitation device to oxidize ferrous iron to ferric iron, optionally increasing the pH to form solid iron oxide, and separating the solid iron oxide from the solution in a filter. The process is particularly useful for removing iron from oilfield completion, drilling, and workover fluidsBACKGROUND OF THE INVENTION [0002]Iron dissolved in various kinds of aqueous solutions has caused many undesirable effects, and its removal has long been a vexing problem. As applied to workover and completion fluids used in hydrocarbon recovery, sometimes called clear completion brines, used in oil recovery, the background of the problem has been well described by Qu et al in U.S. Pat. No. 7,144,512:[0003]“High density brines (completion brines) have been widely used in well completion a...

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Benefits of technology

[0016]The cavitation device has a distinct advantage in the common situation where polymeric viscosifiers, or other polymers, are present in the fluid to be treated for iron removal. Water-soluble polymers of almost all varieties are notorious for their tendency to plug filters, and this is especially true where the pore size of the filter is small. Subjecting the viscosity-enhancing polymers to the cavitation process and its accompanying temperature increase, however, will physically destroy the polymer molecules and render their remnants filterable without plugging the filters. The heat generated within the cavitation device during its normal operation also assists in reducing the detrimental effects of polymers via breakdown and / or viscosity reduction.

[0017]Our invention benefits from the additional use of certain types of activated carbon which have been found to rapidly decompose peroxides or otherwise catalytically enhance the oxidation rate of the iron species in the liquid. The liquid is beneficially contacted with the activated carbon immediately downstream from the cavitation device, but may be used anywhere in the system to enhance the reaction of a peroxide with the iron species in the liquid.

Problems solved by technology

Iron dissolved in various kinds of aqueous solutions has caused many undesirable effects, and its removal has long been a vexing problem.

Such fluids may be contaminated with any or all of the following: water, drilling mud, formation materials, rust, scale, pipe dope, and viscosifiers and bridging agents used for fluid-loss-control pills.

Depending on their composition and level of contamination, these fluids may or may not have further practical or economic value.

Unfortunately, the costs associated with the initial purchase and subsequent disposal of such brines has been a hindrance to their universal acceptance especially since the “use once and dispose” means of disposal is neither prudent nor economically sound.Because of the relatively high cost and limited worldwide natural mineral resources available for producing medium- and high-density completion / workover fluids, it is essential that their used fluids be reclaimed.

Simple filtration processes, wherein the brine is filtered through a plate and frame type filter press with the use of a filter aid such as diatomaceous earth and then through a cartridge polishing filter, are effective to remove solid contamination but they have no effect on removing other types of contamination such as colloidal or soluble species.

This is the case since colloidally dispersed and soluble contaminants cannot be removed by filtration without first treating the fluid to change the chemical and / or physical properties of the contaminants.

Almost all used clear completion fluids, and also many drilling fluids, contain iron, which has historically been extremely difficult to remove in the process of cleaning and preserving the fluids for reuse.

The fluid incorporates dissolved oxygen from the air with normal pumping and handling, which converts the iron to Fe2O3 in the form of a 0.5 micron colloidal suspension, but the quantity of oxygen dissolved in this manner is seldom enough.

Such small colloidal suspensions are very difficult to filter.

Leaving 0.5 micron solids downhole is a problem since the formation is essentially a porous medium that cannot be backwashed.

Everyone knows about iron, but until now no one has developed a practical solution for iron removal.

One can add oxygen scavengers to try to keep the iron in solution, but that masks the problem and is never a permanent solution.

One cannot add enough oxygen scavenger to prevent the iron from precipitating in the formation.

There is simply too much oxygen.

In addition, iron oxidation is a relatively slow process.

Thus, the problem has been that the ubiquitous iron is usually in solution in a used clear completion fluid, but it will damage the formation if it is not removed; removal without diminishing the other components of the fluid, or undertaking an enormous expense, has been elusive.

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