After reading this article you will learn about the generation of wastewater by petroleum industry.
Drilling Activity in Petroleum Industry:
In petroleum industry, drilling is normally performed by rotary method which includes a rotary table, drilling bit, drilling pipes and a system for circulating a fluid down through drilling pipe and back to surface.
Specially formulated mud called as drilling mud, either water based mud (WBM) or oil based mud (OBM) is circulated through the hole:
(i) To remove cuttings from around drilling bit,
(ii) To provide lubrication for the drill string,
(iii) To protect the walls of the hole, and
(iv) To control down hole pressure.
The composition of drill mud varies with the type of formation, drilling depth and other operational factors. The mud, with an average composition as given in Table 1, is recycled after removing impurities such as shale’s, silt, sand and earth cuttings by passing the fluid through ‘solids control systems’ comprising of shale-shaker, desander, desilter and mud-cleaner.
Wastewater from Drilling Activity:
The rejected drill cuttings which may be contaminated with hydrocarbons, sand, silt and mud along with impurities form the components of wastewater generated at drill site. The quantity of wastewater generated depends on type of formation, quantity of water used in washings in shale shaker, desander and mud cleaner, drilling depth and the type of drilling activity. Normal water requirements at exploratory drilling site is around 120 m /day of which 50-60 m is generated as wastewater.
Characterisation of wastewater through collection of representative samples, its preservation and analysis is usually done by following the procedures recommended by standard methods for examination of water and wastewater. Here in this case the wastewater is viscous, oily in nature and contains 6-8% solids, high COD, oil and grease and heavy metals.
The major components match with drilling mud characteristics and are mostly inorganic in nature with poor settle ability. All these wastes are generally stored in waste pits at the site for chemical treatment to coagulate emulsified oil, organic content contributing to COD before final disposal.
Crude Oil/Gas Processing Facility:
At processing facility, the first step employed is separating the oil, gas and the formation water (associated water) produced by the well. Water in oil can form an emulsion. This emulsion is broken using heat in heater treated or electric energy in devices such as electrostatic coalesces. Cleaned oil flows from the emulsion breakers to crude oil storage tanks. The block diagram for the production of oil and gas is depicted in Fig. 2.
Wastewater from Processing Facility:
The water that is recovered during emulsion breaking is called as produced water and is often recycled through skimmers to remove excess free oil, is filtered and then is stored in water tanks prior to underground injection to enhance the recovery. It is difficult to generalize the quantity of produced water likely to be generated as it varies with the type of formation and increases with the age of the well.
The produced water contains dissolved solids, chloride, emulsified oil, COD, BOD and heavy metals. Typical analysis of wastewater generated at drill site and produced water is given in Table 2. Crude oil before being transported through pipeline to crude storage tanks or crude oil terminal may need pretreatment in order to meet the specifications laid down by the consumer refinery.
The sources of wastewaters and their quantity depend upon the pretreatment to be given to crude oil. However, irrespective of pretreatment, the wastes are characterised by high mineral content, salinity, oil and grease and hydrocarbons.
Refinery Operations:
The petroleum industry involves numerous processes that convert crude oil into liquefied petroleum gas, gasoline, kerosene, jet fuel, diesel fuel, other fuel oils, lubricating oils and feedstock for the petrochemical industry. Petroleum refining activities include the storage of crude at the refinery, petroleum handling and refining operations, and storage of the refined product prior to shipment.
Refining operations consist of separation processes, conversion processes, treating processes, feedstock and product handling, and associated auxiliary operations. The flow scheme at a particular refinery is determined by the composition of the crude oil and the chosen slate of products. An example of a complex refinery flow scheme is presented in Fig. 3.
Wastewater from Refinery:
Process wastewater containing hydrogen sulphide, ammonia and oil is termed as sour water. It is stripped with steam to remove hydrogen sulphide, ammonia and light gases, which are then recovered or incinerated. Wastewater generation from the refinery is detected by the diversity of refinery unit processes and operations.
The major sources of wastewater in the refinery are process/boiler areas and account for 2/3 of the total wastewater (10,000-15,000 m3/day). The other units like cooling water and domestic activities will also contribute to total wastewaters to the order of 20% and 15% respectively.
The wastewater includes:
1. Water formed or eliminated during reactions.
2. Wash water from cleaning operations, water drawn from storage tanks.
3. Excess stripped sour water from stripping operations and boiler blow down.
4. Spent caustic and spent amine from product treaters and process unit respectively.
5. Cooling tower blow down.
6. Potentially contaminated storm water consisting of surface drainage from refinery process units, utility units, off site paved areas and truck/rail loading areas.
7. Storm water sources consisting of rainwater and surface drainage, from non-hazardous areas.
8. Sanitary wastewater stream.
The common pollutants as identified by MINAS (1981-82) for oil refineries, include oil & grease, hydrocarbons, phenols, sulphides, dissolved solids and organics expressed in terms of COD & BOD. Typical wastewater characteristics of effluent from a refinery are summarised in Table 3.
