Zero-Landing Treatment of Waste Drilling Mud: Process, Advantages and a Field Case
What Is Waste Drilling Fluid?
Drilling fluid is the working fluid of oil-well drilling. Depending on the drilling programme, a range of chemical additives is required. Waste drilling fluid consists of the cuttings discharged during drilling together with small amounts of drilling fluid, the fluid returned from the wellbore during completion, and the heavy mud needed for well control. It is a multiphase, stable colloidal suspension of clay, weighting materials, various chemical additives, wastewater, waste oil and drill cuttings, characterised by a complex composition, high COD, high mineralisation, deep colour and high suspended solids.
Waste drilling fluid arises from several main sources:
Drilling-fluid wastewater: discarded drilling fluid and fluid spilled on site.
Mechanical wastewater: diesel engine cooling water, wash water from drilling-pump rods and water discharged from water brakes.
Washing wastewater: water used to wash shaker screens, the drill floor and drilling tools, and to clean equipment.
Cuttings: large volumes of cuttings are generated during drilling and carry a substantial amount of mud; washing of the cuttings and rain washing over the cuttings pile carry that mud into the drilling wastewater system.
Acidising and cementing operations, which generate large volumes of wastewater.
Drilling accidents, especially blowouts, which also produce large volumes of wastewater.
Oil spilled from storage tanks and mechanical equipment.
Additional wastewater volume from natural rainfall ingress.
Because waste drilling fluid contains large amounts of clay, cuttings and additives, its solids content is high. Earlier studies show that the most environmentally harmful constituents are high-salinity solutions and exchangeable sodium ions, followed by oils, soluble heavy metal ions (Cr3+, Hg2+, Cd2+, Zn2+, Pb2+, Al3+ and Ba2+), organic pollutants (polycyclic aromatic hydrocarbons, phenols, halogenated hydrocarbons and organic sulfides), high-pH additives (NaOH and Na2CO3 solutions) and high-molecular-weight organics, especially the small molecules produced when they degrade.
Harmless Treatment: Three Modules
Harmless disposal of waste drilling fluid consists of three modules - solids-control circulation, waste-solids treatment and waste-liquid treatment - which, combined with skid-mounted equipment, can effectively achieve water reuse and compliant solid-waste disposal. The solids-control system uses mechanical separation to remove rock, sand and cuttings from the waste mud and collects them for the waste-solids system. The waste-solids system solidifies or biochemically treats the cuttings; once the pH, colour and COD of the leachate from the treated solids meet the Class 1 limits of GB8978-1996, the Integrated Wastewater Discharge Standard, the solids can be used for brick making and road construction. The waste-liquid system applies chemical oxidation, adsorption and other advanced treatment to the drilling wastewater produced by dewatering. The treated water can be used to wash drilling tools, to re-mix drilling fluid or to be reinjected into the formation.
Process flow for zero-landing treatment of waste drilling mud
Advantages of Zero-Landing Treatment
No excavation of waste storage pits or solidification/landfill cells is needed; zero-landing equipment minimises the footprint and saves drilling cost.
Waste mud and cuttings are treated harmlessly and recovered as resources: waste is collected without landing and sorted for treatment, and the treated mud is recycled.
Prevention comes first, reducing environmental impact at source to a minimum. Storage time and treatment cycles are shortened as far as possible by processing waste mud at the wellsite as drilling proceeds; the clear liquid from wastewater treatment is used to wash drilling equipment or to prepare mud, while solids such as cuttings are used for brick making.
SINOKLE Combined Process and a Field Case
To address drilling-fluid treatment, SINOKLE has combined its in-house core technologies, including CDFU and CDOF, into integrated processes that achieve effective recovery of drilling-fluid wastewater and compliant solid-waste disposal across different oilfields. One example is a large oilfield mixed-wastewater project in the Republic of the Congo, where the influent is a mixture of central processing facility wastewater, mud, flowback fluid and drilling waste liquid. It has high mineralisation, high suspended solids and a high content of emulsified oil, emulsifies easily and is complex in composition, all of which make treatment more difficult.
Simplified flow diagram of drilling waste liquid treatment
To achieve compliant discharge of oilfield wastewater, the project adopted an integrated technical solution built around a coalescing oil separator, CDOF (integrated ozone catalytic oxidation and flotation unit), CDFU (cyclonic dissolved gas flotation) and KFM (active filter media filter). After treatment by the combined process the wastewater met the standards for discharge to sea, solving the oilfield's wastewater problem economically and efficiently.
Photograph of the project site
