WO2014085065A2

WO2014085065A2 - Method for treating glycol-containing well streams

Info

Publication number: WO2014085065A2
Application number: PCT/US2013/069385
Authority: WO
Inventors: H. Robert Goltz; Aaron M. Johnson
Original assignee: Dow Global Technologies Llc
Priority date: 2012-11-29
Filing date: 2013-11-11
Publication date: 2014-06-05
Also published as: WO2014085065A3

Abstract

A continuous method for treating a well stream from natural gas or petroleum production comprising the steps of: i) recovering the well stream from a natural gas or petroleum formation wherein the well stream comprises mono and divalent salts including scale-forming cations, ii) adding glycol to the well stream to form a glycol mixture, iii) heating the glycol mixture to a temperature of at least 35C, iv) passing the glycol mixture through a packed bed of strong acid ion exchange resin to remove at least a portion of the scale forming cations and to form a softened effluent, v) removing at least a portion of water present in the effluent to precipitate monovalent salts, and vi) repeating steps i) through v) and reusing at least a portion of the glycol from step v) in step ii).

Description

METHOD FOR TREATING GLYCOL-CONTAINING WELL STREAMS

FIELD

The invention is directed toward methods for removing divalent cations from aqueous glycol mixtures including the treatment of produced water.

INTRODUCTION

Many petroleum and natural gas recovery techniques involve the production of water from subterranean geological formations. The resulting "produced water" contains various hydrocarbons, residual production chemicals and mono and divalent salts. Methane present in the produced water can react with water to form methane hydrates. These hydrates restrict and even block fluid flow through production equipment. This problem is exacerbated at low temperatures (e.g. below 10 °C), such as those commonly encountered in offshore production. Hydrate formation can be mitigated by flushing production equipment with a dry hydrophilic solvent such as monoethylene glycol (MEG).

Unfortunately, during recovery of the MEG many of the divalent salts present in the produced water also lead to scaling of the process equipment. While precipitation techniques such as those described in US 2010/0191023 remove at least a portion of such scale forming salts, the technique generally requires large mixing tanks and filtration equipment along with sufficient time for precipitation to occur. Due to space constraints, precipitation is not always a viable approach, particularly for offshore platforms. Moreover, in remote areas divalent-carbonate salts used in precipitation may not be readily available. Where available, disposal of carbonate salts can be expensive. Finally, the solubility of divalent- carbonate salts varies with pH, C0₂ concentration and background water composition. As a consequence, adequate removal of divalent cations can be difficult to achieve.

SUMMARY

The invention includes a continuous method for treating a well stream from natural petroleum production comprising the steps of:

i) recovering the well stream from a natural gas or petroleum formation wherein the well stream comprises mono and divalent salts including scale -forming cations,

ii) adding glycol to the well stream to form a glycol mixture,

iii) heating the glycol mixture to a temperature of at least 35 °C,

iv) passing the glycol mixture through a packed bed of strong acid ion exchange resin to remove at least a portion of the scale forming cations and to form a softened effluent,

v) removing at least a portion of water present in the effluent to precipitate monovalent salts, and

vi) repeating steps i) through v) and reusing at least a portion of the glycol from step v) in step ii).

A variety of embodiments, systems and related methods are also described. BRIEF DESCRIPTION OF THE DRAWINGS

Graphs la-lc and 2a-2b are plots showing calcium concentration present in the effluent of an aqueous glycol mixture passing through a packed bed of strong acid exchange resin. The plots illustrate calcium "leakage" from the bed (breakthrough) as a function the volume of treated glycol mixture (in units of bed- volumes per hour). The specific resin and operating conditions for each experimental run are provided in Table 1. Further details are provided in the Example section below.

Table 1 :

DETAILED DESCRIPTION

The invention includes a method for removing divalent cations from aqueous glycol mixtures. Such mixtures may include produced water (e.g. well streams) generated during the recovery of petroleum or natural gas. Preferred glycol species include monoethylene glycol and Methylene glycol. Various combinations of glycol species may also be present. The aqueous glycol mixture preferably comprises from 10 to 90 wt % produced water and from 90 to 10 wt % glycol. The well stream includes mono (e.g. NaCl) and divalent salts including scale -forming cations such as Ca⁺², Mg⁺², Ba⁺², Sr⁺².

The method includes heating the aqueous glycol mixture to a temperature of at least 35°C, 38°C or even 40°C and then passing the mixture through a packed bed of strong acid ion exchange resin to remove at least a portion of the scale forming cations to form a "softened" effluent. Water may be also removed from the effluent, e.g. by way of evaporation, distillation, etc. At least a portion of the monovalent salts remaining in glycol precipitate out as a solid and can be recovered, e.g. by gravity separation or filtration and reused. For example, the recovered monovalent salt may be used in combination with other sources of brine or water, e.g. as part of a make-up resin regeneration solution.

In a preferred embodiment, the invention includes a continuous method for treating a well stream including the steps of:

i) recovering the well stream from a natural gas or petroleum formation wherein the well stream comprises mono and divalent salts including scale -forming cations (e.g. Ca⁺², Mg⁺², Ba⁺², Sr⁺²), ii) adding glycol to the well stream to form a glycol mixture,

iii) heating the glycol mixture to a temperature of at least 35 °C, iv) passing the glycol mixture through a packed bed of strong acid ion exchange resin to remove at least a portion of the scale forming cations and to form a softened effluent,

vi) repeating steps i) through v) and reusing at least a portion of the glycol from step v) in step ii). The calcium concentration of the glycol from step v) is preferably less than 10 ppm.

The method may optionally include the step of periodically discontinuing step iv) and regenerating the ion exchange resin by passing a brine solution through the packed bed. The brine solution used to regenerate the packed bed may comprise at least a portion of the monovalent salt produced and recovered in step v). The packed bed is preferably regenerated by passing 1 to 3 bed volumes of brine solution through the bed, preferably providing at least 200 g and more preferably 300g NaCl per liter of resin. Resin regeneration is preferably conduced counter current. Representative counter current systems include AMBERPACK™ packed bed systems operating in an upflow production and down flow regeneration mode and DOWEX™ UPCORE™ packed bed systems operating in a down flow production and upflow regeneration mode.

The temperature of the glycol mixture passing through the packed bed is an important operating parameter. Preferred operating temperatures are at least 35°C, 38°C or 40°C. Preferred temperature ranges include: 30-80°C, 35-80°C, 38-80°C and 40-80°C. Preferred flow rates are less than 20 bed- volumes per hour, and more preferably from 10 to 15 bed- volumes per hour.

The selection of cation exchange resin is not particularly limited but strong acid gel type resins are preferred. Uniform particle size resins are also preferred, as are resins having average particle sizes less than 650 μιη, 550 μιη, and 450 μιη. Representative examples include Dowex™ Marathon C Dowex™ (600 μιη) and Marathon C400 (400 μιη) brand resins from The Dow Chemical Company. EXAMPLES

A series of experiments were performed by passing an aqueous glycol solution through a glass jacketed column containing cation exchange resin. The experiments were conducted using two types of ion exchange resins operating under a range of temperatures and flow rates as specified in Table 1.

The ion exchange resins were Dowex™ Marathon C and Marathon C400 strong acid exchange resins (both in Na form). Both resins are uniform particle size, gel-type, cation exchange resins having a styrene-divinylbenzene copolymer matrix. Dowex™ Marathon C resin has an average particle size of 600 μιη whereas Dowex™ Marathon C400 resin has an average particle size of 400 μιη. Both resins are commercially available from The Dow Chemical Company.

During the experiments the column was packed almost full with a freeboard space of less than 5% of the total volume of the column. The aqueous glycol solution consisted of an approximately 50 wt solution of monoethylene glycol along with approximately 450 ppm of sodium and 100 ppm of calcium. The glycol solution was heated and pumped through the column under the conditions specified in Table 1. Fractions of treated effluent were collected and analyzed with a calcium selective electrode. The ion exchange resin was subsequently regenerated at ambient temperature by pumping a 10% NaCl solution through the column. The total dose of NaCl used was 300 g/liter of resin. At the end of the salt regeneration, the resins were rinsed with low salinity water.

The results of the experimental testing are provided in Graphs la, lb, lc, 2a and 2b. As illustrated in Graph la, operating at ambient temperature and 20 bed-volumes/hr resulted in an almost immediate calcium breakthrough (leakage). The impact of heating is illustrated in Graphs lb and lc where the calcium breakthrough point was significantly postponed. Graphs 2a and 2b illustrate the impact of resin particle size.

Claims

1. A continuous method for treating a well stream from natural gas or petroleum production comprising the steps of:

ii) adding glycol to the well stream to form a glycol mixture,

iii) heating the glycol mixture to a temperature of at least 35 °C,

2. The method of claim 1 further include the step of periodically discontinuing step iv), and regenerating the ion exchange resin by passing a brine solution through the packed bed, wherein the brine solution comprises monovalent salt produced in step v).

3. The method of claim 1 wherein the well stream further comprises cationic scale forming compounds which are removed by the strong acid ion exchange resin during step iv).

4. The method of claim 1 wherein the glycol mixture is heated to a temperature of at least

40°C.

5. The method of claim 1 wherein the ion exchange resin has an average particle size of less than 650 μιη.

6. The method of claim 1 wherein the ion exchange resin has an average particle size of less than 450 μιη.

7. The method of claim 1 wherein the flow rate of the glycol mixture through the packed bed is less than 20 bed- volumes per hour.

8. The method of claim 1 wherein the flow rate of the glycol mixture through the packed bed is from 10 to 15 bed- volumes per hour.

9. The method of claim 1 wherein the calcium concentration of the glycol in step v) is less than 10 ppm.