Oilfield Wastewater: Sources, Components and Core Characteristics
1. Oil Production Wastewater
Most oilfields in China are developed by water injection. Approximately 2–3 tons of water are injected for every 1 ton of crude oil produced; especially in the later stages of oilfield production, the water content of crude oil can exceed 90%.
Oil production wastewater enters dehydration and desalination stations of the crude oil gathering and transportation system along with crude oil for dehydration and desalination. The "separated" wastewater is then sent to wastewater treatment stations, forming oilfield-specific oily wastewater, also known as "produced water".
(1) High oil content: Generally, oil production wastewater contains 1,000–2,000 mg/L of crude oil, and some contains up to 5,000 mg/L. The oil in oil production wastewater includes free oil, dispersed oil, emulsified oil, and dissolved oil. About 90% of the oil exists as free oil (particle size > 100 μm) and dispersed oil (particle size 10–100 μm), while the remaining 10% is mainly emulsified oil (particle size 0.1–10 μm); the content of dissolved oil (particle size < 0.1 μm) is very low.
(2) Suspended solid particles: The particle size is generally 1–100 μm, mainly including clay particles, silt, and fine sand.
(3) High salt content: The inorganic salt content of oil production wastewater ranges from several thousand to over 100,000 mg/L, varying significantly between oilfields and blocks.
(4) Bacterial content: Oil production wastewater mainly contains saprophytic bacteria and sulfate-reducing bacteria.
(5) High temperature and high pH value: Oil production wastewater also features high temperature (40–80℃) and high pH value.
2. Drilling Wastewater
Drilling wastewater is generated during drilling operations due to mud loss during tripping, leakage in the mud circulation system, and washing of mud and oil stains from ground equipment and drilling tools.
The mud commonly used in drilling engineering is prepared by mixing clay, water, and treatment agents in a certain ratio. Treatment agents act through clay hydrolysis to significantly improve mud performance, ensuring drilling speed and wellbore quality. For shallow drilling, low-solid or solid-free mud is often used, containing fewer harmful substances and causing lower pollution. The deeper the drilling, the higher the requirements for mud; more types and quantities of chemical treatment agents are added, and even a certain proportion of crude oil or waste oil may be mixed, increasing pollution levels. Thus, drilling wastewater from pollution can be regarded as a product of high-dilution mud.
Since drilling wastewater is closely related to the use of drilling mud, the properties of wastewater generated during drilling vary between different oil and gas fields, drilling areas, and well depths. Generally, for shallow water drilling, only the oil content in drilling wastewater exceeds standards; when PAM mud is used, suspended solids, phenol, chromium, and oil in wastewater exceed standards; when ordinary mud is used, oil content exceeds standards, and suspended solids, phenol, and chromium exceed standards individually; for deep drilling, the over-standard rate of oil, phenol, chromium, and suspended solids increases. Therefore, the main harmful substances in drilling wastewater are suspended solids, oil, chromium, and phenol.
3. Well Flushing Wastewater
Water injection wells are dedicated wells for injecting water into oil reservoirs. To prevent suspended solids in injected water from clogging formations, a water distributor filter screen is installed at the end of the water injection pipe. After a period of operation, the pipeline pressure gradually increases and the injected water volume decreases due to the accumulation of suspended solids intercepted by the filter screen. When the planned water injection volume is not achieved, the water injection well undergoes backwashing to remove solids and biofilms deposited on the filter screen, thereby generating well flushing wastewater.
4. Gas Production Wastewater
Gas production wastewater refers to formation water or gas field water produced alongside natural gas. It mainly contains condensate oil, salts, suspended solids, hydrogen sulfide, and some additives (organic substances). The Cr⁻ content in gas production wastewater can reach tens of thousands of mg/L; in addition, it contains sulfur and metal elements such as lithium, potassium, bromine, zinc, cadmium, and arsenic.
