The term ‘waste’ has been loosely used by various authors. For the purpose of this article, waste is defined using the system approach to any industrial process, i.e. activity such as manufacturing, construction (well drilling and completion) or extraction (petroleum production). As a system, the industrial process is functionally cut off from the market by the market boundary and is physically isolated from the environment by an environmental boundary, as shown in Figure 1. Any process including oilfield operations can be visualised as such an entity having both market and environmental boundaries.
Of course, manufacturing processes are best fitted to this schematic because their boundaries are visible and clearly defined. By contrast, oilfield processes do not have readily perceived environmental boundaries, particularly in the subsurface environment. However, they may generate subsurface pollution, which implies a flow of pollutants across a subsurface environmental boundary. The presence of such a boundary is implicit in the issues of borehole integrity and migration across confining (sealing) zones into underground sources of drinking water.
The surface boundary of the oilfield process is somewhat easier to define, but the decision is still based on subjective judgement rather than scientific definition. In drilling operations, for example, reserve pits were initially included in the drilling fluid circulation systems (hence the name ‘reserve’) and considered part of the drilling process. Later, the pits were often used as a waste dump that belonged to the environment. After well completion, reserve pits were either abandoned or opened and spread on the surrounding land. Today, on modern rig sites, reserve pits during drilling are carefully isolated from the surrounding environment and are closed promptly after well completion using various environmental techniques, described below. In this modern approach, reserve pits are considered part of the drilling process rather than part of the environment; they reside within the environmental boundary that surrounds the whole rig site and underlays the bottoms of the pits.
It also follows that no waste exists inside the process – just material streams. On leaving the process (i.e. crossing the process boundaries) a stream of material becomes either a product (including by-products) or waste. The difference stems from the market value of the material. Having a positive market value, the material becomes a product. Material with zero value becomes waste. When the value is negative, the material becomes regulated waste, i.e. it involves a disposal cost.
Classification of Oilfield Waste
In the US, the most general classification of oilfield waste defines primary and associated waste categories. The classification considers the origin and volume of waste generated.
Wastes with a large volume and low toxicity constitute primary waste. The category of primary drilling waste comprises drilling mud and drill cuttings. The volume of primary drilling waste can be from five- to 10-fold larger than the volume of the wellbore and mud system as a result of a volumetric increase in the mud system while drilling – a phenomenon inherent in the drilling process. Oilfield primary production waste is the produced water and its volume is extremely large – exceeding the oil volume by three-fold (worldwide average) and by up to 50-fold in some areas. These very large volumes result from various mechanisms by which water invades wells – phenomena inherent in the oilfield processes of production and recovery.
Associated oilfield waste is considered as having a small volume and (possibly) high toxicity. In the drilling process, associated waste may include rigwash and service company wastes such as empty drums, drum rancid, spilled chemicals, spent acids and workover, swabbing, unloading or completion fluids. In production operations, most of the materials used and discarded fall into the associated waste category. A list of associated production waste is given in Table 1.
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