Coagulant blend in SAGD water treatment

Abstract

Described herein is a coagulant blend for use in SAGD water treatment systems. Specifically, a blend of high charge density polyamine and low charge density poly(diallylmethyl ammonium chloride (poly-DADMAC) is used in the warm lime softening treatment process to coagulate and flocculate solids.

Claims

1. A method for treating water, comprising the steps of: a) obtaining water from an underground hydrocarbon-containing formation during SAGD oil recovery performed on said underground formation, wherein said water contains hardness, and optionally de-oiling said water; b) combining a high charge density polyamine and a low charge density poly(diallylmethyl ammonium chloride) (poly-DADMAC) to form a coagulant, wherein said low charge density is less than 1 meq/g and said high charge density is about or greater than 2.5 meq/g; c) injecting said water, coagulant, and a lime solution into a solids-contact gravity clarifier; d) performing a warm lime softening reaction in said solids-contact gravity clarifier to treat said water and to form precipitated solids; e) injecting a flocculant into said solids-contact gravity clarifier and flocculating said precipitated solids; f) filtering said precipitated solids to form a treated water; and, g) injecting said treated water into a boiler to make steam for said SAGD oil recovery.

2. The method of claim 1, step f) further comprising the step of treating said water with a weak acid cation exchanger.

3. The method of claim 1, wherein said poly-DADMAC has a molecular weight that is ten times the molecular weight of said polyamine.

4. The method of claim 1, said coagulant comprising two-thirds poly-DADMAC and one-third polyamine.

5. The method of claim 1, wherein said injection step c) comprises injecting 50 ppm of said poly-DADMAC and 10-20 ppm of said polyamine.

6. A method for treating produced water for SAGD operations, comprising: a) mixing a produced water stream with an optional makeup water, an optional steam generator blowdown stream, and an optional a warm lime softener unit regeneration stream in a first vessel at a known and constant ratio to form a water mixture; b) combining a high charge density polyamine and a low charge density poly(diallylmethyl ammonium chloride) (poly-DADMAC) in a second vessel to form a coagulant, wherein said low charge density is less than 1 meq/g and said high charge density is about or greater than 2.5 meq/g; c) introducing said water mixture and said coagulant into a warm lime softener unit, said unit comprising a warm lime softener, an ion exchanger, and at least one filter in fluid communication, wherein said warm lime softener unit has an outlet for a warm lime softener unit regeneration stream, an inlet for said water mixture and coagulant, and an inlet for softening chemicals, wherein said warm lime softener unit is fluidly connected to said first and second vessel; d) mixing said water mixture and said coagulant with said softening chemicals in said warm lime softener to form a softened mixture; e) treating the softened mixture with an ion exchanger and at least one filter to form a treated saline water stream; and, f) feeding said treated saline water stream into a steam generator.

7. The method of claim 6, wherein said poly-DADMAC has a molecular weight that is ten times the molecular weight of said polyamine.

8. The method of claim 6, said softened mixture comprises about 50 ppm of said poly-DADMAC and about 10-20 ppm of said polyamine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1. Simplified schematic of a water treatment operation for SAGD according to one embodiment.

(2) FIG. 2A displays the improvement in the WLS outlet turbidity using an optimized blend of coagulants.

(3) FIG. 2B displays a single trend that shows the dosage changes of the epi-DMA and the DADMAC coagulants over the time period when the polyamine is first transitioned.

DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

(4) The invention provides a novel coagulant blend for use in SAGD water treatment operations and methods of treating water. In addition to being a cheaper option, the blend improves the quality of the sludge bed in the treatment system, which leads to improvements in the quality of the treated water quality and reductions in fouling of the steam generators.

(5) The present methods includes any of the following embodiments in any combination(s) of one or more thereof: A coagulant composition for warm lime softener treatments comprising a blend of a high charge density polyamine and a low charge density poly(diallylmethyl ammonium chloride) (poly-DADMAC).

(6) Preferably, the high charge density is at least 2.5 meq/g and higher, most preferably at least 4 meq/g, whereas the low charge density is preferably less than 1 meq/g, most preferably less than 0.5 meq/g. A method for treating water, comprising the steps of obtaining water from an underground hydrocarbon-containing formation during SAGD oil recovery performed on said underground formation, wherein said water contains hardness, and optionally de-oiling the water; combining a high charge density polyamine and a low charge density poly(diallylmethyl ammonium chloride) (poly-DADMAC) to form a coagulant; injecting the water, coagulant, and a lime solution into a solids-contact gravity clarifier; performing a warm lime softening reaction in the solids-contact gravity clarifier to treat the water and to form precipitated solids; injecting a flocculant into the solids-contact gravity clarifier and flocculating the precipitated solids; filtering the precipitated solids to form a treated water; and, injecting the treated water into a boiler to make steam for SAGD oil recovery. The water can optionally be de-oiled prior to combination with the coagulants. The water can also optionally be treated with a weak acid cation exchanger after filtering the solids. A method for treating produced water for SAGD operations, comprising mixing a produced water stream, which is optionally de-oiled, with an optional makeup water, an optional steam generator blowdown stream, and an optional a warm lime softener unit regeneration stream in a first vessel at a known and constant ratio to form a water mixture; combining a high charge density polyamine and a low charge density poly(diallylmethyl ammonium chloride) (poly-DADMAC) in a second vessel to form a coagulant; introducing the water mixture and the coagulant into a warm lime softener unit, the unit comprising a warm lime softener, an ion exchanger, and at least one filter all in fluid communication, wherein the warm lime softener unit has an outlet for a warm lime softener unit regeneration stream, an inlet for the water mixture and coagulant, and an inlet for softening chemicals, wherein the warm lime softener unit is fluidly connected to the first and second vessel; mixing the water mixture and the coagulant with the softening chemicals in said warm lime softener to form a softened mixture; treating the softened mixture with an ion exchanger and at least one filter to form a treated saline water stream; feeding the treated saline water stream into a steam generator. The generated steam can then be injected into a reservoir for hydrocarbon recovery operations, such as SAGD. A coagulant composition for warm lime softener treatments comprising a blend of a high charge density epichlorhydrin-dimethylamine (epi-DMA) and a low charge density poly(diallylmethyl ammonium chloride) (poly-DADMAC), wherein the molecular weight of said poly-DADMAC is ten times the molecular weight of said epi-DMA, and said composition is 70-90% poly-DADMAC and 10-30% epi-DMA. In any of the above, the produced water can be deoiled before being mixed with the coagulant. In any of the above, the poly-DADMAC has a molecular weight that is 5-15 times larger than the molecular weight of polyamine. Preferably, 8-12, or 10 times larger. Any of the above composition can have 70-90% poly-DADMAC by volume and 10-30% of polyamine by volume. Or, 50 ppmv of polyamine and 10-20 ppmv of poly-DADMAC. Or two-thirds poly-DADMAC and one-third polyamine. In any of the above, the polyamine can be epichlorhydrin-dimethylamine (epi-DMA).

(7) Produced water is different than typical wastewater in that it has dissolved organic compositions from petroleum in the water. These dissolved organics interfere with many water treatments processes and chemicals. SAGD produced water is especially difficult to deal with because the amount of dissolved organics is much greater than other produced water because of the steam and heated water interacting with bitumen during SAGD operations.

(8) The standard coagulant for water treatment systems in the oil and gas industry has been a polyamine, particularly epiDMA. However, treatment of produced water, particularly SAGD produced water, with polyamine coagulants lead to unsatisfactory results. To reduce treatment costs and improve the quality of treated water, Applicant began testing different coagulants, including new poly-DADMACs, which coagulant target compounds through different mechanisms than polyamines such as epiDMA. These coagulants did not work, and in most applications, treated water quality decreased exponential. As such, Applicant began testing blends of polyamines and poly-DADMAC. The blends was not expected to work well as there would be negative interactions between the two coagulants due to their different coagulating mechanisms, including fouling of the treatment plant. However, Applicant found that this blend did work. This was unexpected because one component, poly-DADMAC, was known to be unsuccessful for treating oil and gas related water.

(9) FIG. 1 displays an exemplary configuration of a water treatment operation used for SAGD operations for one embodiment of the presently disclosed blend. The operation comprises a deoiled water tank (1001), a WLS unit (1003), a recycle tank for mixing various recycled streams (1002), two coagulant tanks (1005a and 1005b), a flocculant tank (1006) and a steam generator (1004) for heating treated water.

(10) In most SAGD operations, produced water and an optional makeup water are treated using warm lime softening (WLS). However, other water sources can also be treated in a WLS unit. As shown in FIG. 1, the AF and WAC Regeneration Streams (3a) from the WLS water treatment unit (1003) are all combined in a recycle tank (1002) to form a combined recycle stream (1c). The combined recycle stream (1c) can then be mixed with the produced water and optional makeup water in the de-oiled water tank (1001) before being sent to the WLS unit (1003). Optionally, the combined recycle stream (1c) can be injected directly into the WLS unit (1003), similar to the recycle blowdown (4a) from the steam generator (1004) in FIG. 1. Alternatively, the recycle blowdown (4a) stream from the steam generator (1004) can also be introduced into the recycle tank.

(11) The combined recycle stream (1c) is then sent to the deoiled water tank (1001) for treatment. This recycled stream is optional for the deoiled water tank (1001) and may be treated by other treatment processes such as cold or hot lime softening or sent directly to the WLS unit (1003). In some embodiments, the produced water and an optional make up water using e.g. freshwater, is the only water being treated in the deoiled water tank (1001). In other embodiments, the produced water and other untreated water (brackish, saline, etc.) are treated using the disclosed coagulant blend. However, these streams were shown in FIG. 1 as an example of other types of water that can be treated using the presently disclosed coagulant blend.

(12) The coagulants are stored in separate vessels (1005a,b) and their individual streams (5a,b) are combined into stream (5c), but not mixed. The combined stream (5c) is then injected into the piping connecting the deoiling water treatment unit and the warm lime softening unit.

(13) In more detail, untreated, deoiled water (10a) is combined with the coagulants (5c), and is introduced into a warm lime softening unit for treatment (10b). Lime softening is preferably performed in a solids-contact gravity clarifier to optimize the efficiency of the lime softening reaction. Solids-contact clarifiers combine chemical mixing, coagulation, and clarification in a single vessel and use a high concentration of solids to form a bed or blanket of sludge. The WLS (1003) unit also contains a weak acid ion exchange process and filters for further softening of the water and separation of precipitated solids. A vessel containing flocculant (1006) is also attached to the WLS (1003) such that a stream of flocculant (6) can be added when needed in the softening process.

(14) The coagulants (5a,b) are combined at a pre-determined ratio of polyamine to poly-DADMAC and injected into the untreated, deoiled water stream (10a). From there, the coagulant/untreated water stream (10b) is pumped into a solids-contact gravity clarifier in the WLS unit (1003) using the same injection location as the original untreated water stream. As shown here, fresh water (1b), produced water (1a) and recycled water (1c) are deoiled and combined with the coagulants in the WLS unit (1003), treated using warm lime softening and ion exchange mechanism before the treated stream (3b) is sent to the steam generator (1004) to generate steam for downhole operations (4b). As mentioned above, the recycle stream (4a) from the steam generator (1004) is sent to the recycle tank (1002). However, a small fraction (4c) of steam to be recycled is intentionally purged to avoid concentration of impurities during continuing evaporation of steam and is disposed of inside a deep well.

(15) In other embodiments, the two coagulants can be blended together in a mixing vessel before being introduced to the deoiled water stream. Alternatively, the two blended coagulants can also be injected directly into the WLS unit for mixing with the untreated water stream. However, the simplest option is to combine the piping of the two streams to form the coagulant blend used in the WLS unit.

(16) As mentioned above, the higher viscosity coagulants required some modifications to the system. Applicants found that changes to the pumping system and retrofitting valves were all that were needed to account for the changes in the coagulant's properties. However, it is possible that the pumping system in place is capable of handling higher viscosities and retrofitting valves is all that is needed to accommodate the blended coagulant. Once modified, the coagulant blend can be introduced into the warm lime softener unit and used to treat the untreated water (produced, recycled, fresh, and the like).

(17) The present invention is exemplified with respect to the following examples for an Oil Sands reservoir already in operation. However, this is exemplary only, and the invention can be broadly applied to any SAGD reservoir or non-SAGD specific water treatment operations, either in place or being developed. The following examples are intended to be illustrative only, and not unduly limit the scope of the appended claims.

Optimizing Coagulent Blend

(18) Oil Sands 1 is a SAGD bitumen recovery facility, and a multi-decade commercial production project using SAGD recovery began in 2007. The incumbent coagulant used in the WLS reaction mix zone was a commercially available epi-DMA polyamine. The polyamine is typically the most expensive chemical added in the water treatment plant and one of the most expensive chemicals in the entire facility. The incumbent, commercially available epi-DMA polyamine had a low molecular weight, high charge density cationic polyamine coagulant designed to neutralize the surface charge of lime sludge particles so they can collide, agglomerate and settle.

(19) In addition to being costly, use of the incumbent epi-DMA polyamine was problematic. For example, establishing an optimum dosage was challenging. In order to determine coagulant dosage, WLS effluent water quality parameters such as effluent turbidity and particulate hardness were used in conjunction with other tools such as a zeta potential analyzer and jar tests. There are numerous factors that can impact WLS effluent turbidity in addition to coagulant dosage, such as WLS temperature, flow, pH and water composition. Thus, changes to the commercially available epi-DMA polyamine dosage based on WLS effluent turbidity were not an efficient approach. Further, improvements to boiler feedwater (BFW) and WLS sludge bed stability were desired. As such, this system was chosen to implement the disclosed coagulant system.

(20) As disclosed above, the novel coagulant composition was a combination of a commercially available polyamine, here epi-DMA, and a commercially available poly-DADMAC coagulant. A trial was performed at Oil Sands 1 to find the best ratio blend of the chosen epi-DMA and poly-DADMAC with the understanding that a successful alternate coagulant trial could reduce the Oil Sand 1's WTP chemical OPEX by approximately $ 1 MM/year.

(21) The proposed poly-DADMAC coagulant for the proof of concept experiments was a commercially available poly-DADMAC with a high molecular weight, low charge density cationic coagulant designed to increase the settling rate of lime sludge particulate. The selected poly-DADMAC does not function by neutralizing particulate surface charge like the incumbent polyamine, but does increase the settling rate through a sweep mechanism wherein larger particles settling at a faster velocity than smaller particles sweep some of the smaller particles from the suspension.

(22) For comparison, the molecular weight of poly-DADMAC was about 10× greater than the molecular weight of commercially available epi-DMA polyamine. As such, the optimum dosage for selected poly-DADMAC was experimentally determined by monitoring the settling rate in the WLS rapid and slow mix zones.

(23) Table 1 displays a comparison of the viscosity of each coagulant at various temperatures that may be used during the WLS process. Their difference in viscosity over the range of possible WLS operation temperatures necessitated a change in pumping equipment at Oil Sands 1. Ultimately, ProMinent Orlita DR series pumps were installed as they handled viscosity ranges from 100 cp to 1 million cp, which covered the ranges for both the incumbent and alternate coagulants.

(24) TABLE-US-00002 TABLE 1 Coagulant Viscosity Comparison Coagulant Viscosity (cP) Incumbent Alternate Commercially Commercially available available Coagulant Temperature epi-DMA polyamine poly-DADMAC Min 10° C. 200 700 Norm 20° C. 125 550 Max 35° C. 100 480

(25) Once the Oil Sands 1 water treatment system was configured to allow for mixing and pumping of the chosen coagulant blend into the WLS, preferred ratios and other parameters of the proposed polyamine/poly-DADMAC blend were determined through a series of trials.

(26) Prior to trial initiation, the sludge bed height in the solids-contact gravity clarifier was purposely increased from 2.4 meters (normal operating condition) to 2.7 meters because the poly-DADMAC coagulant was expected to increase the settling rate. Increasing the sludge bed to 2.7 meters provides additional time to respond to changes in the WLS performance during the coagulant transition phase.

(27) To find the best mixture of the two coagulants, the current WLS unit was first switched to a pure poly-DADMAC system and then optimized by small additions (˜10% maximum) of the original epi-DMA over a period of time. Operations were gradually transitioned from the commercially available epi-DMA polyamine to the poly-DADMAC according to the transition program shown in Table 2.

(28) WLS sludge bed characteristics and effluent water quality were closely monitored and changes to turbine speed, sludge wasting and sludge recirculation were made as the coagulant transitioned to the poly-DADMAC. Once the system was completely moved to poly-DADMAC, a noticeable drop in performance of the system was determined. This was expected based on Applicant's previous tests of various coagulants. The commercially available epi-DMA polyamine was slowly added back and the water quality was monitored for improvements.

(29) TABLE-US-00003 TABLE 2 Coagulant Transition Program Commercially available Commercially available Day Time epi-DMA polyamine (ppm) poly-DADMAC (ppm) Initial n/a 90 0 0 ~7:00 am 75 10 1 ~7:00 am 50 20 2 ~7:00 am 25 30 3 ~7:00 am 0 40

(30) After slowly adding the commercially available epi-DMA polyamine, Applicant found that a blend of 40 ppm per coagulant improved water quality and stabilized the bed. This blend maintained a nice sludge bed height and characteristics. Further, the water quality obtained when using this blend was exceptional. This improved water quality was first seen during the initial transition period from polyamine to 100% poly-DADMAC and then replicated when commercially available epi-DMA polyamine was reintroduced to the system during the period of WLS instability.

(31) FIGS. 2A-B display the changes in key performance indicators and key operating parameters during the reintroduction of the commercially available epi-DMA polyamine. FIG. 2A shows the dramatic improvement in the WLS outlet turbidity with the blend of coagulants. The turbidity remained low (single digits) and the sludge bed was stable at 2.4 meters. FIG. 2B displays a single trend that shows the dosage changes of the epi-DMA and the poly-DADMAC coagulants over the time period when the polyamine is first transitioned.

(32) All key water quality parameters (turbidity, dissolved hardness and total hardness) were at levels not previously maintained in the Oil Sands 1 water plant before this trial. Given the water quality results, the potential exists that the benefits of this improved water quality could outweigh cost savings from a full transition to the poly-DADMAC and this benefit would be driven by reduced OTSG fouling and increased pigging intervals.

(33) Small adjustments to the rate of blend feed can be made based on estimated coagulant need, bed conditions and other factors; however, the drive to maintain a coagulant blend should be maintained.

(34) After the coagulant ratio is set, the remaining steps in the treatment process can proceed. Typically, coagulants are followed by injections of flocculants. The softened water then undergoes an ion exchange process before the precipitated solids are filtered using an after filter (AF). While these treatment steps are not affected by the choice of coagulants, Applicant did see some effects on the chemicals needed for the remaining treatment steps, particularly in the flocculant.

Effect on Flocculant

(35) A side benefit of the present coagulant blend was a slight reduction in the amount of required flocculants. The optimal flocculant dosage was reduced from 3.5 ppm to 1.5 ppm, a reduction of over 50%.

(36) Applicant conducted another trial using a different commercially available poly-DADMAC, to study the effect on the flocculants. As before, the system was slowly switched to 100% poly-DADMAC and the commercially available epi-DMA polyamine was reintroduced.

(37) For both blends tested, the water quality of the exiting or effluent stream was much improved. This had a significantly positive impact on the steam production, as it was more reliable and less costly when compared to polyamine or poly-DADMAC alone.

(38) Thus, the novel blend of polyamine and poly-DADMAC improved water quality and sludge bed conditions over that experienced with either polyamine or poly-DADMAC alone.

(39) The following references are incorporated by reference in their entirety

(40) US20110147306

(41) US20140166586