Oilfield Waste Control
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Life-cycle assessment (LCA) is another method of analysis for economic production strategies that combines the productivity and pollution aspects of the production process at hand. In petroleum production, the LCA approach qualifies for macro-analysis of reservoir development projects in environmentally sensitive areas, economic impact analysis of environmental regulations or, on a smaller scale, design of environmental management of a single drilling well or production site. To date, LCA has also been used for the analysis of new drilling fluid systems and well stimulation projects.
Process Modification with the Addition of Environmental Control Components
Conceptually, process modification through the addition of environmental- control components requires a systematic approach that can be summarised in the following steps:
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define the environmental boundary of the process; identify the inherent mechanisms of environmental impact; consider ECT methods and create options for process modification;
evaluate the technical performance (upstream and downstream) of each ECT option;
calculate the ECT cost–benefit; and • select the best variant for modification with minimum net ECT cost.
A general formula defines the net cost of ECT in terms of a relative change in the present process economics as: Net ECT cost = ∑Cupstream – ∑Cdownstream + CECT
where ∑Cupstream = impact of the modification on well/project productivity: (-) increase and (+) decrease) in US$, ∑Cdownstream = savings in compliance cost (US$) and CECT = cost (fixed and variable) of the modification (US$).
It is instructive trying to describe a specific problem using the concept of the equation above. Also, for short-term projects (drilling a well, for example) the equation can be formulated using a static (break-even analysis) approach, while for long-term projects (development drilling, well production strategy or reservoir management planning), the formulation might employ time–value- of-money analysis methods.
Preventative methods for oilfield waste control comprise a large spectrum of technologies, such as closed-loop drilling systems, subsurface injection, borehole integrity control, fluid toxicity testing, downhole reduction of produced water and use of land for onsite storage and disposal of oilfield waste. The methods may proactively reduce waste volume and toxicity but cannot altogether eliminate the waste that has to be disposed of to land, to water or in the subsurface.
Waste Disposal
In onshore operations, oilfield waste is temporarily stored in earthen impoundments (pits) before its ultimate disposal either in land or at the subsurface; disposal to the surface waters is prohibited. In offshore operations, on the other hand, discharge of primary oilfield waste into seawater is possible but is strictly controlled by regulations. The three disposal methods are briefly discussed below.
EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
There are two potential problems with waste disposal on land that may limit future applications. First, land treatment provides little control over migration of the mobile (leachable) fractions that may eventually enter the food chain of animals or humans. Second, spreading of oily wastes results in emissions of volatile organic compounds resulting in the violation of some local laws and regulations controlling air pollution.
Heavy metals in soil can become incorporated and accumulate in the food chain or contaminate local sources of drinking water if leaching and migration occur from oilfield pits. Table 2 gives a comparison of soil loading factors for metals recommended by the American Petroleum Institute guidelines with those from Louisiana State Wide Order 29-B and the Canadian Interim Soil Remediation Criteria for Agriculture. Maximum concentrations detected by independent evaluations by the American Petroleum Institute and US Environmental Protection Agency are also included.
Injection into the Subsurface
Injection into the subsurface is the most widely used method for the disposal of petroleum industry wastes. Liquids are usually injected into permeable formations through injection wells. Solids are grinded and slurrified before being injected into the petroleum well’s annulus or into a designated slurry-injection well. During the injection, the disposal strata are fractured by the slurry. The solids are then filtered out at the fracture face and permanently stored inside the fracture.
Initially, it was thought that slurry-injection technology disposed the slurry into a single long, continuous fracture. Later, however, Amoco studies showed that a series of periodic injections into the same rock would create multiple (or branched) fractures rather than one continuous fracture. Periodic slurry injection has become the established technology for subsurface disposal of waste in a local volume of fractured rock around the disposal well. It is termed the ‘disposal domain’. Slurry injection has proved that large volumes of waste can be stored in a well’s vicinity, the injection cycles are repeatable and the disposal process is permanent.
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Land Disposal
Land disposal of oilfield waste, also known as ‘pit closure by land treatment’, may be performed using land-spreading or land-farming. Land-spreading involves spreading the waste over the surface of the ground and tilling it into the soil. After this initial tilling, no further action is needed. In land-farming, the soil is commonly processed for several seasons after the initial application of the waste. This additional processing may include adding fertilisers and tilling repeatedly to increase oxygen uptake in the soil.
The preventative approach to waste control, often called ‘waste minimisation’ or ‘environmental control’ technology, addresses the material streams inside the process.
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