PROCESS FOR TREATING PRODUCED WATER WITH MAGNESIUM OXIDE
The present invention relates to a process that uses one or more evaporators to treat produced water containing silica. To address silica scaling, a crystallizing reagent is mixed with the produced water upstream of the evaporator. The crystallizing reagent is designed to precipitate a silica adsorbing compound such as magnesium hydroxide. The feedwater with the adsorbed silica is directed to an evaporator that produces a distillate and a concentrate containing the adsorbed silica. At least a portion of the concentrate having the silica adsorbing compound is directed to a separator that separates the silica adsorbing compound from the concentrate and recycles it back to where it is mixed with the produced water.
- 1-24. -24. (canceled)
- 25. A method of recovering oil from an oil-bearing formation comprising:
recovering an oil-water mixture from the oil-bearing formation; separating oil from the oil-water mixture to produce an oil product and produced water containing dissolved silica therein; directing the produced water through a deaerator and removing noncondensible gas from the produced water; directing a magnesium-based reagent into the dearator and mixing the magnesium-based reagent with the produced water in the deaerator and causing magnesium hydroxide and silica to co-precipitate from the produced water and adsorbing the silica onto the magnesium hydroxide; after mixing the magnesium-based reagent with the produced water in the deaerator, directing the produced water having the magnesium hydroxide and adsorbed silica to an evaporator and evaporating the produced water to produce a distillate and a concentrate containing the magnesium hydroxide and adsorbed silica; separating the magnesium hydroxide from the concentrate by directing at least a portion of the concentrate to a hydrocyclone; in the hydrocyclone, producing an underflow that includes the magnesium hydroxide; the hydrocyclone further producing an overflow; directing the overflow from the hydrocyclone to a purge line and purging the overflow; directing the underflow that includes the magnesium hydroxide from the hydrocyclone to the deaerator; and mixing the underflow from the hydrocyclone with the produced water in the deaerator where the magnesium hydroxide contained in the underflow functions to adsorb silica from the produced water.
The present invention relates to recovering oil from oil-bearing formations and more specifically to a method of treating produced water to remove silica therefrom prior to reaching downstream equipment that is prone to silica scaling.
Enhanced oil recovery (EOR) processes employ thermal energy to facilitate the recovery of oil, particularly heavy oil, from oil-bearing geologic formations. One particular process for recovering heavy oil is referred to as steam-assisted gravity drainage (SAGD). In the SAGD process, steam is injected into the oil-bearing formation to supply thermal energy to mobilize the heavy oil. Generally, several tons of steam is required for each ton of oil recovered by the process. Injected steam heats the oil bound in the formation, and this heating lowers the viscosity of the oil. Heat from the steam comes from sensible heat as the steam cools and latent heat as the steam condenses into water. The lowered viscosity of the oil enables the oil to mix with the water, producing an oil-water mixture which may flow to collection areas and ultimately be pumped to the surface. The oil is recovered by substantially removing it from the oil-water mixture leaving a so-called produced water.
The produced water must be treated. Evaporation technology is an accepted method of treating produced water from SAGD processes. This thermal process produces high quality distillate as feedwater for steam generation and allows for the flexibility of employing either traditional once-through steam generators or drum-type boilers. To be sure, treating the produced water to form a relatively pure feedwater for steam generation is challenging. One of the most challenging parts of treating produced water is retarding or preventing silica scaling in the evaporators. Various approaches have addressed scaling. First generation evaporative processes use large amounts of chemicals such as caustic, scalants, disperants, etc. to keep silica soluble. The use of these chemicals is costly and does not always provide scale-free operation which in turn requires additional chemicals or mechanical cleaning. For example, high pH processes mix sodium hydroxide with the produced water to raise the pH of the produced water sufficient to maintain silica soluble. This is costly because a continuous and substantial amount of sodium hydroxide is required. Moreover, this solution does not guarantee scale-free operation. Further, it is known to use a crystallization processes to adsorb silica. These processes too are costly. This is because a continuous supply of fresh crystallizing reagent is required.
Therefore, there is a need for a produced water or feedwater treatment process that utilizes chemical treatment to remove silica but one which is more cost effective than has been realized in the prior art.
The present invention relates to a process that uses one or more evaporators to treat a feedwater stream where the feedwater includes silica. To address silica scaling, a crystallizing reagent is mixed with the feedwater upstream of the evaporator. The crystallizing reagent is designed to precipitate a silica adsorbing compound. That is, the crystallizing reagent causes co-precipitation of silica and a precipitant that adsorbs silica. The feedwater with the adsorbed silica is directed into an evaporator that produces a distillate and a concentrate where the concentrate includes the adsorbed silica. At least a portion of the concentrate having the crystallized precipitant is directed to a separator such as a hydrocyclone. The separator separates the precipitant from the concentrate and recycles it back to where the separated precipitant is mixed with the feedwater. This process gives rise to the crystallization of the precipitant and the formed crystals are recycled and form seed material to adsorb silica.
In one embodiment, the present invention relates to an evaporator process for treating produced water that includes silica. Here again to address silica scaling, a crystallizing reagent is mixed with the produced water which results in the formation of crystals and the co-precipitation of silica which is adsorbed onto the crystals. The crystals and adsorbed silica are directed to the evaporator and end up in the evaporator concentrate. The process entails directing the concentrate from the evaporator to the separator that separates the crystals from the concentrate and recycles the separated crystals back to be mixed with the incoming produced water. This reduces the consumption of the crystallizing reagent and enables the resulting crystals to be reused to adsorb silica from the produced water, thereby substantially reducing the chemical cost incurred for addressing silica scaling.
In one particular embodiment, the crystallizing reagent is magnesium oxide that is mixed with the produced water in a deaerator located upstream of the evaporator. The magnesium oxide, when mixed with the produced water, yields magnesium hydroxide which precipitates to form magnesium hydroxide crystals. Silica co-preciptates with the magnesium hydroxide and adsorbs to the magnesium hydroxide crystals. These magnesium hydroxide crystals having adsorbed silica end up in the concentrate of the evaporator. The concentrate in the evaporator is directed to a separator, such as a hydrocyclone, and the hydrocyclone separates the magnesium hydroxide precipitants or crystals from the concentrate and recycles them back to the deaerator where they are mixed with the incoming produced water.
Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of such invention.
With further reference to the drawing, there is shown therein a system and process for treating a feedwater stream. As will be discussed later, the feedwater may be a produced water stream or other wastewater stream which typically includes suspended solids, hardness, alkalinity, oil and various other dissolved solids including silica. As shown in
The system and process shown in
Now turning to a specific application of the process shown in
In one embodiment, the crystal forming reagent is magnesium oxide. Adding magnesium oxide to the produced water results in the formation of magnesium hydroxide that precipitates from the produced water and forms crystals that adsorb silica. Various forms of magnesium can be added. In some processes, magnesium may be added in the form of magnesium chloride. In any event, the magnesium compound, as noted above, forms magnesium hydroxide crystals that sorb the silica in the produced water, effectively resulting in the conversion of silica from a soluble form to an insoluble form.
Although the magnesium crystallizing reagent may be added at various places upstream of the evaporator 20, in the embodiment illustrated herein, the magnesium compound, which in this case is magnesium oxide, is injected through line 17 into the deaerator 16. From the deaerator 16, the produced water is directed through line 18 to the evaporator 20. Because the silica is sorbed onto the precipitated magnesium hydroxide, then it follows that the silica present in the produced water cannot significantly scale the heat transfer tubes of the evaporator 20. It is appreciated that the magnesium hydroxide crystals and the silica sorbed thereon will become a part of the evaporator concentrate and will be continuously recirculated through the evaporator 20. A portion of the evaporator concentrate will be directed from the evaporator via line 22. It follows that the evaporator concentrate in line 22 will include precipitated magnesium hydroxide or magnesium hydroxide crystals and wherein some of the magnesium hydroxide or magnesium crystals will include adsorbed silica.
The process of the present invention intends to separate the magnesium hydroxide precipitants or crystals from the evaporator concentrate and recycle it to the deaerator 16 in order to be mixed with the produced water. In the process and embodiment shown in
Thus, the present process produces a cost effective and efficient way of removing silica from feedwater and produced water streams. In particular, this avoids the cost disadvantage of a “once through” reagent by incorporating an effective means of recovering silica adsorbing precipitants and growing them into crystals that are used over and over again to adsorb silica from the feedwater stream or produced water stream.
The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.