Water management

Advanced Technologies for Produced-Water Treatment and Reuse

This paper presents the results of a laboratory investigation in which treatment processes were evaluated as treatment methods for produced water (PW) from different oil and gas fields.

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Fig. 1—Results of MD tests on NaCl solutions (Membrane E).

Historically, the treatment of produced water (PW) has been limited to free-oil and suspended-solids removal, using physical separation technologies and injection in disposal wells. However, because of new regulations, combined with geological restrictions and local water scarcity, the drive to have a greater fraction of the PW treated more extensively and ultimately to be reused is increasing. This paper presents the results of a laboratory investigation in which treatment processes were evaluated as treatment methods for PW from different oil and gas fields.

Introduction

A combination of factors is putting great pressure on operators to find new ways of treating and managing PW that promote water conservation and sustainability. However, treating PW to produce a good-quality effluent is a challenging task. PW characteristics can vary considerably. In order to treat PW to a water-quality standard that enables it to be reused, advanced water-treatment technologies (AWTTs) have to be applied, alone or in combination.

To investigate the suitability of several AWTTs to treat and reuse PW from Qatari gas fields, the authors, in conjunction with an operator’s research center, carried out a laboratory investigation in which various treatment processes were evaluated. Four treatment methods were selected to target main contaminants identified in PW from a Qatari gas field: Membrane processes were used to target field chemicals, membrane-distillation (MD) methods were used to target salinity, membrane-bioreactors (MBRs) were used to target organics, and ozonation was used to target field chemicals.

Overview of AWTTs

Membrane Processes. The potential for membrane processes to treat PW has been demonstrated successfully in various field studies; moreover, a number of upstream petroleum full-scale facilities have already installed membrane processes to treat and reuse PW. The treatment trains either are a combination of different membrane processes or involve other conventional water-treatment technologies such as media filtration or clarification. For two case studies involving the use of membrane processes, please see the complete paper.

Thermal Evaporators. These entered the PW-treatment market by finding niche opportunities. Moreover, because almost all waste streams are recycled back to the evaporator, the volumes of fresh water required for makeup are dramatically reduced.

With the proliferation of shale-gas wells in the past decade in the US, the demand for treating flowback water with total-dissolved-solids (TDS) concentrations greater than 100 000 mg/L has expanded the opportunities for thermal systems. Another niche market in which the application of evaporators has been very successful is the steam-assisted-gravity-drainage enhanced-recovery process.

MBRs. MBRs are now considered by the downstream petroleum industry as an excellent solution to treat various wastewater streams. Although MBRs have not yet been applied to treat PW at upstream oil and gas facilities, a number of bench-scale and pilot studies report that PW is biodegradable, achieving chemical-oxygen-demand (COD) and oil-and-grease (O&G) removals greater than 95%. Moreover, full-scale MBRs are also operating successfully in other industries, treating highly contaminated organic streams (COD greater than 15 000 mg/L) and achieving COD removals greater than 95%.

Advanced Oxidation Processes (AOPs). As is the case with MBRs, there is limited information on AOPs being applied to treat PW in full-scale facilities. One of the few studies assessed a combination of ozone/ultraviolet/titanium dioxide to treat PW with 38‑g/L salinity; COD and O&G removals of 74 and 95%, respectively, were achieved after a 30-minute contact period. After a 60-minute contact period, COD removal increased to 89%. Published research investigating refinery-effluent treatment with AOPs appears to be more common.

Some of these studies assessed a number of different AOPs and concluded that the Fenton reaction was able to achieve excellent organic removals. Additionally, some companies have developed AOP-based patented technologies with successful applications in the chemical, petrochemical, and pharmaceutical industries; COD removals of greater than 90% were obtained for systems with CODs of 1 000 to 10 000 mg/L.

Despite the evidence that AOPs may be successful in treating PW, the upstream petroleum industry has yet to benefit from the many opportunities these processes offer.

Investigation of Various Methods To Treat and Reuse PW

Membrane Processes. The effectiveness of ultrafiltration (UF), nanofiltration (NF), and reverse-osmosis (RO) membranes in rejecting kinetic hydrate inhibitor (KHI) was investigated by use of a test cell. The membranes were tested with a solution of 1.5-wt% KHI prepared in a synthetic-brine solution with a TDS concentration of approximately 6 000 mg/L. The objective of the experiment was to analyze KHI rejection for different membranes.

MD. The PW collected from the Qatari fields was high in salinity. The ability of MD to treat high-salinity feed waters can make it suitable for reducing the salinity of PWs. To investigate further, the research team initiated an extensive testing program in which various solutions with salinities comparable to those of PWs were tested with an MD direct-­contact-configuration test cell.

Various saline streams [synthetic sodium chloride (NaCl) solutions, seawater, and brine from a thermal-­desalination plant] were tested under different operating conditions and using different membranes. Salt rejection was assessed by measuring conductivity and TDS in the feed and product water.

MBRs. PW also contains soluble organics in various forms, including hydrocarbons, organic acids, and KHI (during the winter season). To assess the potential of the MBR process for removing organics, tests were conducted both in batch reactors and in an MBR, each of 1-L working volume. The effect of salinity on biotreatability of PW was assessed over the range of salinities from 6 000 to 31 750 mg/L. Biotreatability was calculated on the basis of total-organic-carbon (TOC) mass balances and is reported as percent TOC removal. To assess the biotreatability of KHI, the batch bioreactor was fed with 0.25% KHI (initial tests) or 1.5% KHI (later tests) added to a standardized brine solution mimicking the inorganic content of PW. In this way, the biotreatability of KHI could be measured directly. Biotreatability was calculated on the basis of both COD and TOC mass balances and is reported as percent removal.

In a third investigation, an MBR was fed with PW collected from a different location in the plant. This PW was lower in both salinity and organics than that used for the salinity experiments. After acclimation, for 5 weeks under steady-state conditions, biotreatability was measured through COD mass balances and reported as percent COD removal. The MBR effluent was also treated with a downstream RO process in order to generate an effluent that can be reused for multiple applications.

Ozonation. The removal of KHI in a synthetic-brine solution with a TDS concentration of approximately 6000 mg/L was tested by use of an ozone generator that bubbled ozone into a 1-L column. The ozone concentrations at the inlet and outlet were monitored continuously with an ozone detector. The KHI solution was ozonated for 0.5, 1.5, 3, and 4 hours to evaluate ozonation as a function of time. The removal of KHI was assessed by calculating the percent of KHI oxidized and by measuring cloud point.

Results

Membrane Processes. A Toray brackish-water RO membrane was used in the tests. Initially, the membrane was compacted with 10 g/L of NaCl at 500 psi, achieving a stable flux of 10 L/m2/h (LMH) after 14 hours. The synthetic brine solution containing 1.5% KHI was tested for 22 hours, with the operating pressure kept at 590 psi. Throughout the test, the flux decreased from 15 to 10.5 LMH. At the end of the test, the solution of NaCl at 10 g/L was introduced through the system at 500 psi and the permeate flux was 9 LMH, which confirmed that the membrane can be restored to its original compaction condition (10 LMH). The KHI polymer was removed completely by the RO membrane (99.9% rejection). For a discussion of similar test results seen with NF and UF membranes, please see the complete paper.

MD. The initial tests were performed with NaCl solutions prepared in deionized water, using a single membrane (Membrane E). Fig. 1 above shows that flux remained relatively constant at approximately 25 LMH when testing NaCl solutions of 0.1–35 g/L. However, when the salt concentration was increased above 70 g/L, a slight drop in flux was observed (20 LMH), which may be related to vapor-pressure variations and difference in water viscosity that could impact the thermal conditions at the membrane boundary layer.

Tests on seawater collected from the Arabian Gulf offshore Qatar were conducted with five different MD membranes. Although the initial flux was similar, all membranes showed a drop in flux with time, with the exception of Membrane B (Fig. 2). This flux decline was not observed when the membranes were tested on NaCl solutions. It should be noted that no antiscalant was added to the seawater and the pH was not adjusted before testing. The TDS rejection by all membranes was greater than 99.99%.

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Fig. 2—Results of MD tests on seawater from the Arabian Gulf (Membranes A, B, C, D, and E).

For a discussion of experimental results using brine collected from a local full-scale thermal-desalination plant operating in Qatar, please see the complete paper.

MBRs. Salinity. The salinity of the bioreactor was gradually increased from 6400 to 31 750 mg/L over an 8-week testing period. The daily percent TOC removals fluctuated between 40 and 80%, averaging 65%, over the entire range of salinities. There was no significant effect of salinity on percent TOC removal over this range of salinities.

KHI. Because all the organics present in the feed were attributed to KHI, the percent COD-removal results directly reflect the biotreatability of KHI. The results indicate that, over the 7-week test period, 50–60% of the COD associated with KHI is removed through biotreatment. The percent removal was similar at low and high KHI concentrations.

MBR. The percent COD removal over the 5 weeks of steady-state testing indicates that 63% of the COD could be removed through biotreatment. The UF flux remained excellent over the entire testing period, and no intermediate “maintenance” or “soak” cleaning was required. In the post-treatment experiments with RO, as expected, the effluent produced was excellent, with greater than 98% removal of both the COD and inorganics. The effluent would be suitable for recycle or reuse within the gas-processing facility. Although the RO flux on MBR effluent was lower than the pretest benchmark-water flux, upon completion of the RO tests, a simple flush of the system with tap water restored the flux to the pretest level, indicating that there was no irreversible fouling of the RO membrane.

Ozonation. The KHI solution was ozonated for 0.5, 1.5, 3, and 4 hours to evaluate ozonation as a function of time. KHI removal of 11, 60, and 75% was obtained with ozonation times of 1.5, 3, and 4 hours, respectively. An ozonation time of 0.5 hours did not remove any of the KHI in the synthetic-brine solution. The results clearly show that the effectiveness of ozonation in oxidizing KHI is dependent upon the contact time; significant removals (75%) can be achieved given the necessary contact time. The cloud point also varied with contact time. Visual examination of the treated effluent indicated that the treated effluent was turbid after ozonation. However, after filtration through a 0.45-μm filter, the treated effluent solution was clear.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 17394, “Advanced Technologies for Produced-Water Treatment and Reuse,” by A. Hussain, J. Minier-Matar, A. Janson, S. Gharfeh, and S. Adham, ConocoPhillips, prepared for the 2014 International Petroleum Technology Conference, Doha, Qatar, 20–22 January. The paper has not been peer reviewed.