EOR: Environmental impact of offshore polymer flooding. Literature review and coming research.

Opsahl. E

The national IOR centre of Norway,

University of Stavanger, eystein.opsahl@uis.no, +47 90 200 597

26th of April 2016

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Figure 3: Acrylamide (AM) Figure 4: Acrylic acid (AA) Figure 5: Vinylpyrrolidone (NVP) Figure 6: Acrylamido Tert-butyl Sulfonate (ATBS) Figure 7: Biopolymer polysaccharide pullulan, other biopolymers have similar structures (Biopolymer)

Abstract

It is projected that within the next few years, copious amounts of water soluble polymers, known as polyelectrolytes, will be used for enhanced oil recovery (EOR) purposes in Norwegian offshore fields. However, polymer flooding (PF) pose a great environmental risk even in the light of very low toxicity and a zero emission goal. Polymers in produced water (PW) adversely affects traditional phase separa-tion and water treatment processes commonly applied offshore and also problematic in produced wa-ter re-injection (PWRI). Per today, synthetic polymers by and large do not qualify for offshore use in Norway due to low degradability, arguably an artifact of current regulations. Furthermore there is lack-ing knowledge about the ultimate biogeochemical fate and/of metabolites, degradation intermediates, and end products, and whether they are toxic, recalcitrant or not. This project aims to establish under-standing of polymers behavior in the environment and propose degradation mechanisms and potential adverse outcome pathways.

Knowledge gaps

Marine environmental fate, analytical techniques for environmental monitoring and sampling Till date, knowledge about marine environmental fate is limited due to several analytical challenges at environmental level concentrations. Some investiga-tions use C14 labeled polymers, while accurate, is very expensive and requires special precautions. Titration agents and reagents are sensitive to contami-nants which are usually abundant in environmental samples and do not provide information on molecular weight distribution (MWD). Light scattering (LS) provides a good measure for concentration and MWD, however it is a physical measure that is not able to distinguish between chemical species, and as such is sensitive to any similarly sized contamination. Further development of LS, up-concentration and separation techniques is a central part of achieving the stated goal. Degradation pathways, metabolites, intermediates, recalcitrance and derived toxicity. In line with modern eco-toxicological developments there is a call for disclosure of chemical pathways and mechanisms. This in stead of the traditional view where net dose response effects was sufficient, put together with analytical challenges there is a lot to discover in this area and is central to this project. This will be achieved through use of fish cell lines, activated sludge/physical/chemical degradation, respirometry, LS and capillary MS. Production, application and stability of biopolymers In spite of being an optimal “green” solution, there are several roadblocks for biopolymers. Production of biopolymers is inherently more complicated than direct synthesis of polyacrylamides from propylene for one, molecular weight (MW) control, purification and microbial control are all major hurdles. However, this is currently being investigated heavily by the industry and is not central to this project. Biopolymers are nevertheless included in the study as “positive controls”. Mitigation and Produced water treatment (PWT) It is clear at the time that it is not feasible to completely reduce overboard discharge of polymer to zero with the current best available techniques (BAT) for PWT and PWRI. As polymer partitions to water, it follows water through the separation stages, traditional PWT is therefore insuffi-cient for clearing polymer before discharge. Manifested by the great number of recent and ongoing research efforts focusing on mitigation, exploiting polymers susceptibility to chemical and physical attack. PPW severely reduces efficacy of phase separation stages and can chelate multiva-lent cations further increasing the need for additional PWT. At the present, Norwegian fields are discharging approximately 70 % of treated PW (TPW) to sea, by the year 2020 this number is expected to decrease to 60 %, with a continuing trend. However, the numbers are uncertain, as old-er predictions have proven to show. Dedicated PWRI facilities are currently able to achieve a mean up-time at around 80 %. PWT and mitigation is not central to this project, but are essential parts of any environmental risk assessment.

Current status

Economical There is wide consensus that enhanced oil recovery (EOR) is a priority area for the Norwegian petroleum industry. Historically, there have numerous more or less successful pioneering projects with chemical EOR (cEOR) using PF since the 1970’s. Currently, there is an increasing number of successful EOR-projects worldwide where PF has become the technology of choice . Recent large scale field experiences show that PF can provide around 5-15 % increase in OOIP recovery. A 1 % worldwide increase in recovery rate amounts to roughly 5 bn. Sm³ oil, or roughly the equivalent to Norway’s estimated remaining oil reserves. Undoubtedly, the economical potential of cEOR is substantial in a past peak oil economy with many aging fields. Legal and political According to national- and EU-regulations and the OSPAR convention, any chemical used offshore on the Norwegian continental shelf (NCS) must undergo a battery of environmental and ecotoxicological testing before usage approval can be given. Most polymers perform poorly in 28 day biodegradation tests. However, if ample evidence of biogeochemical benevolence and efficient mitigation techniques can be provided, approval may still be on the table. Moreo-ver, with the increased environmental awareness of the general public, the resulting scrutiny petroleum industry is subject to, and the ongoing petro-economic turmoil, the need for a thorough investigation on the environmental risk, fate and effects associated with PF, becomes critical at this point. Since polymers does not have one uniform chemical structure and have fundamental physical differences with low molecular weight (LMW) compounds that do, it further convolutes legal and chemical definitions in these matters because the regulations are forged around the latter. Environmental Environmentally, there is limited knowledge about the ultimate biogeochemical fate and long-term consequences in the marine ecosystem for the poly-mers. As a considerable fraction (50-80 %) of structurally modified polymer is back produced some years after injection and that PPW is notoriously diffi-cult to treat makes it plausible that considerable amounts of polymers will end up in the environment, unless new technologies becomes available. Syn-thetic EOR polymers are not very toxic, but are merely biodegradable. Biopolymers on the other hand require generous use of biocides and are not yet op-timal for EOR in a number of ways, they do however degrade fast enzymatically which is desirable. Nonetheless, average polymer concentration in PF is 400 ppm/pore volume and is the main reason for concern. That corresponds to 562 000 metric tons in a field like Johan Sverdrup.1

Figure 1: A rack with a few hundred metric

tons of bulk polymer in store at SNF Floerger’s production facility out-

side of St. Etienne, France.

Source: Authors

picture with permission

Degradation and fate EOR-polymers are prone to physical and chemical degradation and rapidly hydrolyze (deamination) and depolymerize (back bone scission) when exposed to shear, UV light, radicals, and high temperature. An acceptable level of degradation for mitigation purposes is a reduction of MWD to around 3 kDa. However, degradation rates drop off at around 100kDa. Biopolymers are inherently biodegradable enzymatically, whereas synthetic polymers are only biodegradable as oligomers up to ~1 kDa threshold (80 % in 25 days). Polymers that reemerge from the reservoir are significant-ly depolymerized and hydrolyzed. PAM releases ammonia upon hydrolyzation, this contributes to eutrophication and, situationally, oxygen deple-tion. Hydrolyzation occurs rapidly both biotically and abiotically in aerobic conditions or at high temperatures. Biotic depolymerization of HPAM has not been proven, besides the exceptional case of white rot fungi harboring altruistic oxygenases.

PWRI to deposit or production reservoirs the best option for disposal of PPW and completely eliminates any environment risk. However 100 % PWRI rate is not possible, and technical difficulties with water quality ups risk of formation damage. Polishing or degrading residual polymer in the PPW is necessary before discharge, however 100 % removal of polymers is impractical. In the oceanic water column polymers disperse, adheres, flocculates, sediments. Exact numbers are uncertain and are concentration and MW dependent, but the effects are well proven and is commonly employed in general (waste)-water treatment.

Ecotoxicology and ecological impact HPAM show limited or no toxicity at all towards aquatic organisms and have low acute basal cytotoxicy. Acute toxicity of high molecular weight HPAM towards aquatic organisms range from 0,2 g/L no observed effects concentration (NOEC) to more than 10 g/L. Synthetic polymers contain residual monomer that may be toxic, acrylamide which the predominant monomer is a well known human toxicant, however is rapidly non-adversly metabolized by microbial activity.

In alkali surfactant polymer (ASP) flooding PW, surfactant and alkali are the main toxic components. ASP flooding is by any account more severe than PF alone, as surfactants depending on type, is orders of magnitude more toxic and used in similarly high concentrations. pH over 9 is ad-verse for most marine species, one liter of pH 12 PW needs 1000 L of unbuffered water to reduce pH to 9.

Polymers chelate heavy metals and emulsify lipophilic substances, which generally speaking in toxicology, is known for their high toxicity. TPW usually have ecological impact within 5 km range from point of release with toxic effects observed within the innermost 2 km. TPW contains a mixture of inorganics, organics and petrochemical products, presence of polymer influences the composition changing ecological impact, howev-er the extent is case specific and whole effluent toxicity testing will be investigated.

Knowledge

Polymer chemistry

Partly hydrolyzed polyacrylamide is the most common polymer type employed in PF. Of the synthetic linear polymers, the following types are recurrent in literature; polyacrylamide (PAM), polyacrylic acid (PAA), partially hydrolyzed polyacrylamide (HPAM, PAM/PAA copolymer), acrylamido tert-butyl sulfonic acid (ATBS) ATBS/PAM copolymer, N-polyvinylpyrrolidone (PVP) PVP/PAM/ATBS co- and ter-polymers, and a range of hydrophobically modified polymers. Of the biopolymers, prime candidates are; Xanthan (Xa), scleroglucan (SC), schizophyllan (SZ) and pullulan (Pu), cellu-lose derivatives (Cel), and guar (Gu). Ideal molecular weight for optimal inherent viscosity and stability is between 1 000 and 20 000 kilo Daltons (kDa). NVP/ATBS PAM copolymers have greater chemical stability than HPAM alone and must be applied in harsher reservoir conditions, but are more expensive and has lesser environmental information. Lastly, there is differences in chemical stability and inherent viscosity between the biopolymers where mainly SC’s is gaining attention of the industry.

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Figure 2: Holistic lifecycle and polymer fate (Source: Authors creation)

Aggre

gation

and

se

dim

en

tation

Water Polymer Oil

(Compact) Flotation

•Traditional offshore water treatment are impacted by polymer in PW. Flotation can handle polymers, howeveremusions are stronger with polymer or ASP present making it less efficient.

Settling

•At low concentrations, polymers enhance flocculation, at higher concentrations, increased viscosity reducessettling rate. Coagulation with trivalent cations produceample amounts of sludge that need further treatment.

Hydro-cyclone

•Hydrocyclone efficency are severly hampered by polymers in water due to increased viscosity.

Filtering

•Filters foul faster with polymer present. Sand filters arevery sensitive, Walnut shell filters are less sensitive. Varieties of membranes require special treatments, butare viable.

Polishing and degradation

•Oxidation and post degradation of polymers in producedwater is being heavily investigated. Polishing is recommended to remove residual COD before discharge.

Pro

du

ced

Wat

er t

reat

men

tSe

con

dar

yP

rim

ary

Treated PW – Injection to disposal well. Current best available technique. Water

quality is important!

Discharge to sea of treated PW with concomitant polymer.

Water columndistribution? Sorption,

and Diffusion?

Exposure? Absorption? Distribution? Metabolism? Excretion?

Biodegradation?

Onshore PW and wastetreatment, recycling?

Elimination? Retention time? Resuspension?

Degradation? Inactivation?

Organism response to insult? Adaptation? Toxicity?

Behavioral changes? Induction? Stimulation?

Ecosystem response?

Population response?

Human impact?

Polymer supplychain.

Environmental impact? Energy Budget?

Remineralisation?

Eutrophication, Oxygen depletion,

Acidification? Sequestration?

Risk acceptable? Use of biocides?

Biogeochemicalcycle

Physical degradation, Hydrolyzation, Depolymerisation?

Metabolites? Recalcitrants? Intermediates?

Increased Oil output,Environmental

impact?Gas and condensate

output

Decision making, regulations, politics

Global impact?

Reinjection of used polymer is a good alternative. However therisk of formation damage must

be carefully managed.