Environmental risk assessment of enhanced oil recovery solutions

Abstract

The overall objective of the research presented in this thesis is to contribute new knowledge about the environmental risk related to shortlisted products and processes developed at the National Improved Oil Recovery (IOR) Centre of Norway and about how to assess such risk. According to the World Energy Outlook report presented by the International Energy Agency in 2021, oil and natural gas will continue to be important contributors to the energy mix over the next 20 years. Implementing enhanced oil recovery (EOR) solutions is important to maintain oil production from existing fields, as it is becoming increasingly difficult to discover new oil and gas reserves. An EOR screening study conducted across 53 reservoirs in 27 of the largest fields on the Norwegian Continental Shelf (NCS) found significant potential for additional oil recovery through EOR solutions. The (IOR) Centre of Norway has been developing new products and processes as part of EOR solutions to improve oil recovery on the NCS. Using these products and processes offshore poses an environmental risk to the marine environment and atmosphere, which needs to be assessed and managed. This thesis explores existing environmental risk assessment (ERA) approaches for offshore oil production and identifies knowledge gaps related to assessing the environmental risk of EOR solutions. The knowledge gaps are filled by using laboratory studies to generate new data, using this data in models to generate key insights, and by developing new methods for ERA of EOR solutions and proposing improvements to existing methods. The research conducted in this thesis has resulted in five scientific papers that are summarized below. Paper I presents a literature review on ERA guidelines relevant to offshore oil production. A review of the primary sources of environmental impacts and key environmental stressors resulting from offshore oil and gas production is also conducted. The main sources of environmental impacts from offshore oil production include operational discharges of produced water (PW), drilling waste to the marine environment, and air emissions from energy production using fossil fuels. The literature review indicates that current ERA practices may form a basis for ERA of EOR solutions; however, there are also knowledge gaps related to the ERA of new products and processes planned to be used as a part of EOR solutions. Based on the review, a generalized ERA framework for PW and drilling waste into the sea and for air emissions is proposed in Paper I. Several products and processes are developed at the IOR Centre to quantify and increase oil recovery as a part of EOR solutions. Using these new products and processes results in their back-production with PW, which is typically discharged into the marine environment. As a result, the main focus of this thesis is on the ERA of PW discharges caused by the implementation of EOR solutions. Quantifying residual oil saturation is important for the successful implementation of EOR solutions. The IOR Centre has proposed a group of seven chemicals (tracers) for potential use in quantifying residual oil saturation in oil reservoirs. Using these tracers in offshore oil fields results in their operational discharges (e.g., with PW) into the marine environment. Once released into the sea, marine organisms may become exposed to the tracers, thereby posing an environmental risk to the ecosystem. Paper II first reports on laboratory experiments conducted to measure the biodegradability and toxicity of seven tracer compounds. A hypothetical case of using tracer compounds on the NCS is then assumed. Discharge of PW containing tracers, along with other production chemicals from the Brage field (used as a proxy case), is simulated using the dynamic risk and effects assessment model (DREAM), which estimates the contribution to the environmental impact factor (EIF) values from each tracer. In addition, the seven tracer compounds are ranked from low to high in terms of their environmental impact. This ranking of the tracers can be used to shortlist the tracer(s) with minimum environmental impact for offshore applications. Polymer flooding is a process in which high molecular weight synthetic polymers are injected into an oil reservoir to increase oil recovery. Injected polymers are usually back-produced with the PW, which is typically discharged into the sea. These synthetic polymers have slow microbial degradation rates under aerobic conditions, unless the molecular weight is reduced to less than 3 kilodaltons. Photocatalytic depolymerization rates for several different synthetic EOR polymers have been measured as a part of another project at the IOR Centre. In Paper III, a novel method is proposed to estimate the residual lifetime of synthetic polymers in the marine environment. Residual lifetime is the amount of time the discharged synthetic polymer takes to reach a molecular weight, below which it becomes biodegradable in the sea. The proposed method uses the DREAM model to estimate the concentration distribution of polymers in the sea. Subsequently, the concentration distribution is linked with the depolymerization rate equations to estimate the residual lifetime of synthetic polymers in the sea. The applicability of this developed procedure is demonstrated by estimating the residual lifetime of synthetic polymers discharged from single and multiple oil fields on the NCS. Paper IV assesses the exposure and effects of discharging synthetic EOR polymers into the sea. Two main approaches are used: The first is based on estimating the EIF values of discharging PW-containing polymers using near-field simulations (where the discharge point is placed within a 50*50-kilometer grid). The estimated contribution to EIF values from synthetic polymers suggests negligible environmental impact when no assessment factor (AF) is used and low/moderate impact when an AF of 50 is used. The AF is a simple way to account for uncertainty in the assessment. The second approach, based on far-field simulations (where the discharge point is placed within a 1200*1800-kilometer grid), is primarily studied to assess polymer build-up in the sea, as synthetic EOR polymers show resistance to microbial degradability. In one of the farfield simulations, polymers are repeatedly released annually over a 10-year period from seven arbitrarily chosen oil fields on the NCS. The highest concentration values (based on the 75 percentiles) during the first and tenth years of discharge are used in a regression analysis against the amount of polymer discharged each year. The regression analysis indicates that polymers will not build up within the simulation area at the expected amounts of polymers discharged each year. Moreover, there is a considerable margin of safety between the highest concentration values calculated by the model and the concentration at which harmful effects in aquatic species are predicted. Paper V focuses on the use of species sensitivity distributions (SSDs) in ERA. An SSD is used to determine the threshold effect levels of stressors, below which unacceptable effects on a group of species are not expected. A literature review is performed to understand how risk is currently defined and how uncertainties are addressed when using SSDs in ERA. It is found that current ways of handling uncertainties while using SSDs are not based on unified guidance provided by the field of risk science. In Paper V, a risk-oriented framework is proposed that addresses uncertainties in a systematic manner while using SSDs. The proposed framework addresses uncertainties due to both lack of knowledge and variability. Furthermore, a scheme for assessing bias in theoretical and practical assumptions underlying SSDs is included in the framework. Lastly, a qualitative method is proposed to characterize the strength of knowledge underlying the SSDs.