Removal of Mercury and Arsenic from Produced Water ABSTRACT

Transcription

1 Removal of Mercury and Arsenic from Produced Water Darrell L. Gallup and James B. Strong, Chevron Corporation ABSTRACT Chevron (formerly Unocal) Thailand has been producing natural gas and crude oil in the Gulf of Thailand since Co-produced water is contaminated with hydrocarbons, mercury and arsenic. Where feasible, Unocal Thailand pursued a philosophy which entails re-injecting produced water back into the rock formations from whence it originated. Currently, over 50,000 bwpd are injected in two fields in legacy Unocal s contract area. Re-injection of all produced water is projected to be implemented by year-end Overboard discharge limits for produced water are 40 mg/l TPH, 10 ppb Hg and 250 ppb As. Characterization of the produced water has shown that hydrocarbons are present as stabilized emulsions, mercury is primarily present in the elemental form, Hg, and arsenic is present as As 3+. A patented process was developed several years ago to remove these contaminants from produced water prior to overboard discharge. The process consists of a three-phase separator to remove gas and condensate. Water leaves the bottom of the separator and passes through desanding and deoiling hydrocyclones. The water then enters the chemical treatment process. In the chemical treatment process, an oxidant (NaOCl), ferric ions and a flocculent are sequentially added to the wastewater to form a floatable sludge consisting of ferric hydroxide, chemi-sorbed mercury, ferri-arsenate, and hydrocarbons. Bleach is added at the inlet to a degasser, ferric chloride is injected into the retention tank, and cationic polymer is added just upstream of the IGF (induced gas flotation) unit. The oxidation-reduction potential of the water is controlled by oxidant addition to allow As 3+ to be oxidized to As 5+ whilst maintaining Hg in elemental form. The water treatment process requires relatively short residence times between chemical additions and provides for large water throughputs (typically up to 20,000 bwpd). In certain Gulf of Thailand fields, the concentration of Hg and As in the produced water is too high for the oxidant Fe 3+ flocculent process to achieve clean water for discharge to the environment. New, improved technology has now been developed to process waters containing high concentrations of Hg and As. It consists of: 1. Installing desanding hydrocyclones to remove elemental Hg, HgS and sand particles to which Hg is attached. 2. Sequentially treating the desanded water with oxidant, ferric ions, a thiol, and the flocculent, or thiol, oxidant, ferric ions and flocculent. The new thiol addition step is especially effective in precipitating excess elemental Hg and any Hg 2+ that cannot be removed using the oxidant Fe 3+ flocculent process.

2 INTRODUCTION The Gulf of Thailand forms a large embayment, over 700 kilometers (km) in length and 600 km in width, on the Sunda Shelf and opens onto the South China Sea (See Figure 1). The Gulf is relatively shallow with a maximum depth of less than 90 meters (m). Water depths in the northern and far eastern portions do not exceed 20 m for distances of 60 to 100 km offshore. Along the western shore, water depths greater than 20 m occur within 10 to 20 km of the coast. Only in the central 10 percent of the Gulf, 150 km from the Thailand coast, do water depths exceed 75 m. Chevron Thailand s Unocal legacy offshore natural gas fields are situated at the northwestern edge of the deeper central portion of the Gulf, in waters ranging from 65 to 75 m (Figure 1). Oil and natural gas deposits in the Gulf of Thailand are contaminated with mercury and arsenic. The mercury and arsenic are thought to originate in coal, carbonaceous shale and tin (Type II) granites in or near high temperature producing reservoirs. Water coproduced with the hydrocarbons is contaminated both with stabilized emulsions and with the heavy metals (i.e., mercury and arsenic). Unocal conducted a number of studies since the mid 1980 s to better understand and quantify the impact of oil and gas operations in the Gulf of Thailand. Using physical chemical characterization data, a treatment Figure 1 - Map Gulf of Thailand process was designed and engineered to remove contaminants from the produced water for discharge to the environment. The treatment process has been operating successfully since early 2000 in legacy Unocal fields. Although the total mass of heavy metals produced along with the natural gas, condensate and oil is not large, the metals concentration in the produced water can exceed Thailand s regulatory discharge standards. Unocal voluntarily committed to reduce the concentration of contaminants in the produced water to the following parts per billion (ppb) and parts per million (ppm) levels: o Mercury (Hg).. < 10 ppb o Arsenic (As).. < 250 ppb o Hydrocarbons.. < 40 ppm TPH (40,000 ppb)

3 CHEVRON THAILAND OPERATIONS Unocal Corporation (now Chevron Corporation) has three decades of successful energy development history in the country. Gross natural gas production averages more than 1.2 billion cubic feet per day (bcfd). The company s cumulative natural gas production since the startup of operations in 1981 surpassed 4 trillion cubic feet (Tcf) (113 billion cubic meters [Bcm]) in May Legacy Unocal Thailand operates >134 platforms in the central Gulf of Thailand, supplying natural gas to generate 30 percent of the nation s total power demand (See Figure 2). Of the natural gas produced, more than 75 percent is used to generate electricity. The remainder is used for industrial fuel, transportation fuel, cooking gas and petrochemical feedstock. Unocal legacy exploration and production covers 13 gas and condensate fields for its co-concessionaires, Mitsui Oil Exploration Co., Ltd., PTT Exploration and Production Public Company Limited (PTTEP), Amerada Hess (Thailand) Ltd., and Moeco Thai Oil Development Co., Ltd. This includes eight blocks in the Gulf of Thailand, covering 2.9 million acres (11,838 square kilometers) and three blocks in the Arthit Project, covering 988,000 acres (4,000 square kilometers). Recently, Unocal legacy started its first crude oil production from the Gulf of Thailand. Gross oil production Yala-Plamuk complex is nearly more than 40,000 barrels per day (Bpd). Just north of the Unocal legacy assets, Chevron operates two oil fields, which are now combined since the merger of Unocal into Chevron. The Bongkot gas field shown in Figure 2 is operated by PTT, the national oil company. Figure 2 Unocal Legacy Thailand Operations GEOCHEMISTRY AND ORIGIN OF MERCURY AND ARSENIC IN GULF OF THAILAND Mercury occurs naturally in several forms in the environment, including elemental mercury (Hg (0)), inorganic mercury (Hg (II)), and methylated forms (mono-methyl mercury and dimethyl mercury). Measurable levels of mercury occur in rocks, oils, condensate and gas in the Gulf of Thailand reservoirs, although concentrations vary widely. Mercury concentrations in rock cuttings from a series of Platong wells, for example, range from less than 100 ppb to

4 greater than 400 ppb, but are less than 600 ppb throughout the drilled section. Bedded coals tend to concentrate mercury with some shales and coals in the Gulf of Thailand containing about 65 times as much mercury as the surrounding shales. Worldwide, mercury-in-coal concentrations drop significantly with increasing rank level and this is expected to be the case in the Gulf of Thailand coals as well. Three major origin hypotheses can be advanced to explain the occurrence and distribution of mercury in geochemical materials of the Gulf of Thailand -- the genetic, coal, and mineral origins. The genetic origin, which suggests a co-occurrence of Hg and total organic carbon (TOC) content in sedimentary rocks, derives from the observation that TOC contents and Hg concentrations in sedimentary rocks show good correlations. The coal hypothesis is related to the genetic hypothesis, and derives from the observation that certain coal-forming land plants act as concentrators of mercury present in the soils upon which they grow. The Hg is subsequently stripped from the coal by passing aqueous or hydrocarbon flows. The mineral origin hypothesis suggests that Hg leaves a mineral matrix and enters hydrocarbon solutions at high temperatures in the absence of sulfide. These three hypotheses depend critically upon the solubility relationships between Hg, water and hydrocarbons. Given the solubility of elemental mercury in both water and hydrocarbons, it is reasonable to expect elemental mercury to co-migrate with petroleum, either in aqueous solution or as a solute in hydrocarbons. Indirect physico-chemical evidence suggests that mercury is produced primarily in the elemental state and that arsenic is produced as an AsH 3 gas. Since arsine is soluble in liquid hydrocarbons, it tends to concentrate in the condensate and pass into the produced water by a hydrolysis reaction. Mercury condenses in the gas as it comes up the well bore and is found to be largely associated with produced solids although some free mercury droplets can be found, as can some soluble Hg o. Table 1 summarizes water filtration data from three processing platforms in the Gulf of Thailand. Note that mercury is largely removable by filtration, while arsenic is not. Table 1. Water filtration data Operations Mercury Concentration (ppb) Arsenic Concentration (ppb) Erawan Produced Water As Received micron filtrate micron filtrate 10 Platong Produced Water As Received micron filtrate micron filtrate <1 Funan Produced Water As Received micron filtrate

5 0.45 micron filtrate PRODUCED WATER TREATMENT PROCESSES Characterization of the produced water has shown that hydrocarbons are present as stabilized emulsions, mercury is primarily present in the elemental form, Hg, and arsenic is present as arsenous acid. A patented treatment process was developed to remove these contaminants from produced water prior to overboard discharge (See Figures 3 and 4). Where feasible, Chevron Thailand pursues a philosophy which entails re-injecting produced water back into rock formations from whence it originated. Currently, over 50,000 barrels of water per day (bwpd) are injected in two fields in legacy Unocal s contract area. Erawan Central Processing Platform (CCP) injects 98% of its produced water and South Pailin CCP has injected 100% since January Unfortunately, the re-injection of all produced water is not a simple task given the large geographic area over which gas production occurs. Funan, North Pailin, Platong and Satun discharge water overboard (5,000 to 20,000 bwpd) from each CCP. Re-injection of all water from central processing platforms is expected to be implemented by late Produced Water Treatment Process Separator Chemical Additions Deoiler Cyclone Desander Cyclone Chemical Treating Process Sludge Sludge To Wellhead Platform for Reinjection Injection Pump Skim Tank Clarified Water to Sea

6 Figure 3. Original water treatment process Water from Deoiler Degasser Retention Tank Bleach FeCl 3 Thiol Chemical Treating Process (Hg/As Removal) GLCC To Sludge Clarifier Induced Gas Flotation (IGF) Unit To Sump Caisson Polymer Figure 4. Chemical treatment portion of water treatment process The process consists of a three-phase separator to remove gas and condensate. Water leaves the bottom of the separator and passes through desanding and deoiling hydrocyclones. The water then enters the chemical treatment process (See Figure 3 Yellow Box). In the chemical treatment process, an oxidant (NaOCl), ferric ions and a flocculant are sequentially added to the wastewater to form a floatable sludge consisting of ferric hydroxide, chemi-sorbed mercury, ferri-arsenate, and hydrocarbons. Bleach is added at the inlet to a Degasser, ferric chloride is injected into the Retention Tank, and cationic polymer is added just upstream of the IGF (Induced Gas Flotation) unit. The oxidation-reduction potential of the water is controlled by oxidant addition to allow arsenous acid to be oxidized to arsenic acid, whilst maintaining Hg in elemental form. The water treatment process requires relatively short residence times between chemical additions and provides for large water throughputs (typically up to 20,000 bwpd). The chemical reactions utilized in the process are: NaOCl + H 3 AsO 3(aq) = H 3 AsO 4(aq) + NaCl H 3 AsO 4(aq) + FeCl 3 = FeAsO 4(s) + 3HCl FeCl 3 + 3H 2 O = Fe(OH) 3(s) + 3HCl

7 Fe(OH) 3(s) + Hg = chemisorbed Fe-O-Hg (s) In certain Gulf of Thailand fields, the concentration of Hg and As in the produced water is too high for the oxidant Fe 3+ flocculent process to achieve clean water for discharge to the environment. New, improved technology has now been developed to process waters containing high concentrations of Hg and As. The new technology consists of: 1. Installing desanding hydro-cyclones to remove elemental Hg, HgS and sand particles to which Hg is attached. 2. Sequentially treating the desanded water with oxidant, ferric ions, a thiol, and the flocculent. The thiol may also be added just upstream or downstream of the desander, followed by the other chemicals. The new thiol addition step is especially effective in precipitating excess elemental Hg and any Hg 2+ that cannot be removed using the oxidant Fe 3+ flocculent process. We have recently identified a variety of thiols that are effective in decreasing Hg concentrations to below discharge limits. One of these thiols has also proven to precipitate a little additional As that is not precipitated by oxidation and Fe co-precipitation. Thiol + Hg or Hg 2+ = Hg-S precipitate where Thiol = dithiocarbonate; trithiocarbonate; sulfide, and the like 3. Re-injecting the Fe-As-Hg sludge back into rock formations from whence the water and petroleum were produced. To ensure that As and Hg will not be leached in the rock formations, the sludge may be fixed/stabilized with proprietary cement formulations or placed into steel containers for disposal in deep wells. Produced water treatment analyses for Funan CCP (Table 2) and North Pailin CCP (Table 3) are shown below. Date TPH (ppm) Hg (ppb) As (ppb) Comments May Sep * Weak Bleach Injection Jan May Sep Jan May Jul * Outside established limits Table 2. Funan central processing platform (CCP) discharge water analyses Date TPH (ppm) Hg (ppb) As (ppb) Comments May ,228 * Insufficient NaOCl Sep * 87 Excess NaOCl Jan * Start thiol May Sep * Insufficient Fe 3+

8 Jan May * Insufficient Fe 3+ Jul * 126 Excess NaOCl * Outside established limits Table 3. North Pailin central processing platform (CCP) discharge water analyses Legacy Chevron oilfields in the Gulf of Thailand are required to meet produced water overboard discharge limits of 40 mg/l TPH and 10 ppb Hg, but no limit has been established for As. At one field, water is passed through a walnut shell filter, beds of Hg-A (sulfur impregnated activated carbon) and polishing cartridge filters. At the other field, water is discharged overboard from FPSO storage tanks after treatment with dithiocarbonate mercury precipitant and water clarifier. These treatment processes successfully remove TPH and Hg to the discharge limits. These fields will also switch to 100% injection next year. SAMPLING PROTOCOLS Overboard discharge limits for produced water are 40 mg/l TPH, 10 ppb Hg and 250 ppb As. Water samples are monitored daily for TPH on respective platforms. Closed drain sump caisson water samples are collected the 6 th of each month and analyzed at Chevron s Songkhla central laboratory. Thailand Department of Minerals Resources personnel may also collect replicate samples for confirmation analyses. If produced water samples exceed any of the limits, a second sampling is conducted later in the month after adjustments to the water treatment process are made in an effort to comply with discharge limits. SUMMARY AND DISCUSSION Physical and chemical characterization data on the hydrocarbon emulsions, mercury species and arsenic species present in the Gulf of Thailand produced fluids were used to develop a produced water treatment process. The process has allowed legacy Unocal Thailand fields to comply with established discharge standards for produced water for total petroleum hydrocarbons (<40 ppm TPH), mercury (< 10 ppb) and arsenic (< 250 ppb). The new desander - oxidant Fe 3+ thiol - flocculent water treatment process has been operating relatively successfully at two fields since Further process improvements are being tested. We have attempted to develop continuous on-line Hg and As monitors to determine the concentrations of these metals entering the degasser. The monitors have been problematic and are not yet installed commercially. These monitors are needed to allow adjustment of chemical treatment rates to achieve desired metal discharge concentrations. In some fields, continuous, on-line TPH monitors are used to ensure that hydrocarbons are floated out of the IGF sufficient to meet sump caisson discharge limits. Chevron Thailand is confident of their capability to meet the overboard discharge water quality specifications for the Gulf of Thailand. REFERENCES Gallup, Darrell L. Summary of Water treatment Systems Used in Gulf of Thailand Operations. PERF Mercury Cooperative Project (PERF 03-01). Unocal Corporation. 2005

9 Lambert, Charles A., August Summary of Analytical Methods Used to Measure Mercury in Produced Water from Gulf of Thailand Oil and Gas Operations. McDaniel Lambert, Inc. Frankiewicz, Theodore C., Tussaneyakul, Sutus, Curiale, Joseph A. The Geochemistry and Environmental Control of Mercury and Arsenic in Gas, Condensate and Water Produced in the Gulf of Thailand. Abstract for AAPG Paper