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Not all drilling base fluids have equal environmental performance.
Differences in physical/chemical characteristics result in differences in key environmental performance criteria such as aerobic biodegradability, anaerobic biodegradability, water column toxicity and sediment toxicity.
Non-aqueous fluids have been used for many years in drilling for oil and gas. Typical synthetic base fluids have been internal olefins, alpha olefins, polyalphaolefins, paraffins, esters and blends of these materials. These fluids offer improved lubricity, thermal stability, and well-bore integrity.
In addition to delivering high drilling performance, these fluids can be an important component of an environmentally sound drilling operation. For some of these fluids, drilled cuttings removed by the solids control equipment can be safely discharged into the marine environment because of their enhanced environmental properties.
The environmental properties of base fluids depend on the physical and chemical characteristics of the material. Classification (eg whether a product is a "synthetic") or point of origin does not necessarily guarantee specific environmental performance. Field environmental fate and effects data assessment is generally viewed as the best practice to understand the potential environmental impact of a base fluid.
How are synthetic drilling fluids defined?
In the absence of field data, laboratory biodegradation and toxicity test data is recommended to assess environmental fate and effect. An overall assessment of potential environmental impact can only be completed when a full environmental data set is available. This includes both anaerobic (absence of oxygen) and aerobic (presence of oxygen) biodegradation test data as well as water column and sediment toxicity test data.
The following table illustrates typical toxicity test data for selected water column species.
Typical water column aquatic toxicity of some drilling base fluids.
|Mysid SPP⊃1; 96-h LC50 (mg/L)||Fathead Minnow⊃2; 96-h LC50 (mg/L)||Daphnia magna⊃2; 48-h EC50 (mg/L)|
|Internal olefins||> 1000|
|Alpha olefins||> 1000||> 1000|
|Synthetic paraffin 1||> 1000||> 1000|
|Synthetic paraffin 2||> 1000|
|Synthetic paraffin 3||> 1000|
|Diesel||100 - 300|
¹ Seawater test
² Freshwater test
Water column toxicity test results generally show that olefin and paraffin base fluids are non-toxic to water column organisms, but diesel base fluids do exhibit toxicity.
When toxicity to sediment-dwelling organisms is considered, internal olefin and some alpha olefin products have significantly lower toxicity compared to most paraffinic materials. The table that follows illustrates typical relative sediment toxicity data for several types of base fluids.
Typical sediment toxicity of some drilling base fluids.
|Compound||Relative sediment toxicity ⊃1;|
|1618 Internal olefin||1.0|
|Internal olefin 2||0.6|
|Alpha olefin 1||28|
|Alpha olefin 2||1.0|
|Synthetic paraffin 1||15|
|Synthetic paraffin 2||16|
|Synthetic paraffin 3||19|
|Synthetic paraffin 4||6.0|
|Synthetic paraffin 5||1.0|
The type of olefin or paraffin makes a difference. Most paraffins and some olefins that can be used as base fluids have significant sediment toxicity, similar to that of diesel.
Biodegradation test data for both aerobic and anaerobic conditions are summarised in the table that follows.
Typical aerobic and anaerobic biodegradation of base fluids.
|Aerobic biodegradation (%) ⊃1;||Anaerobic biodegradation (%) ⊃2;|
|Internal olefin 1||50 - 55|
|Internal olefin 2||60 - 80||55 - 60|
|Alpha olefin 1||60 - 75||60 - 80|
|Alpha olefin 2||60 - 75||50 - 60|
|Synthetic paraffin 1||55 - 60||17|
|Synthetic paraffin 2||10|
|Synthetic paraffin 5||63|
|Synthetic paraffin 6||> 90|
|Diesel||60 - 75||3|
¹ Aerobic biodegradation assessed by OECD 301 or 306.
² Anaerobic biodegradation assessed by modified ISO 11734.
Which drilling base fluid is best for the environment?
Unfortunately, there's no simple answer to this question. The full potential environmental impact of a base fluid can only be assessed with a complete environmental data set that includes water column and sediment toxicity, aerobic and anaerobic biodegradation, and deposition of the cuttings and base fluid concentration in the sediment.
All olefin and paraffin base fluids and diesel will biodegrade aerobically. However, under anaerobic conditions, alpha olefin and internal olefin base fluids biodegrade more extensively (> 50%) than paraffins and diesel (< 5 - 20%). As a result, paraffin base fluids may persist in the environment for longer periods of time if they are not exposed to aerobic conditions.
The concentration of base fluid in sediments may decrease with time after discharge by re-suspension, bed transport, mixing, and biodegradation. In many cases, sediment-dwelling microorganisms are able to use base fluids as a source of nutrition. However, biodegradation of base fluids in sediments may result in a decrease in sediment oxygen concentration. If the initial base fluid concentration is sufficiently high, the sediments could become anoxic (oxygen depleted). Ideally, base fluids should be biodegradable under both aerobic and anaerobic conditions.