Drill cuttings
Drill cuttings are solid products removed from the well while drilling. How they are drilled and what chemicals are used in the process is discussed below, in a brief overview of the oil and gas extraction process.Drilling procedure
Drilling is carried out by using a rotating drill bit under the weight of the drill string (metal pipes) connecting the bit to the drilling rig. As the drill bit cuts into the rock the pieces of rock are removed to the rig by the drilling mud pumped down the well. Aside from removing the solid material, the drilling mud also lubricates the drilling process and cools the drill bit, countering the heat build up from friction with the rock and is designed to prevent a blow-out by being heavier than the pressure in the well.
Drilling of a standard exploration well takes about 30- 60 days. The well is normally drilled in the following sections: 36”, 26”, 17 ½”, 12 ¾” and 8 ½”. The wells might be drilled vertically or horizontally and the length varies from several hundred meters up till more than 10 km.
The well is lined with concrete and/or metal as it deepens, to maintain the structural integrity of well and to isolate the hole from the surrounding rock layers.
Photo: SINTEF
Drilling Discharges
The largest-volume solid wastes generated during drilling of oil and gas wells offshore are drilling muds (also called drilling fluids) and drill cuttings.
Drill cuttings are particles of crushed sedimentary rock produced during the drilling operation. Drilling muds are usually reprocessed and recycled as much as possible during drilling.
Dry drill cuttings. Photo: Statoil
There are three types of drilling fluids: water based (WBM), synthetic based (SBM) and oil based (OBM). Most offshore oil and gas wells are drilled with WBM, this is due to strict regulations on discharges of OBM and SBM. Discharges of OBM as contamination on cuttings have been prohibited in Norway since 1993 (and 1996 within the OSPAR area).
The drilling fluid varies with the composition of the rocks through which the well is being drilled.The most significant additive used in European offshore drilling is the weighting material barium sulphate (barite). The return to the surface consists of the drilling fluid pumped into the well plus the drill cuttings from the drilling process. As much of the drilling mud as possible is reused, with the mud being removed from the rock chippings on a 'shale shaker'. The shaker separates the drilling mud from the drill cuttings , that may be discharged to the environment (if permitted by local regulations), reinjected into a well, sent to shore for reprocessing or disposal in a land fill, or applied to agricultural land as a soil amendment (WBM only).
'Shale Shaker' on deck of rig. Photo: Statoil
Composition and discharges of drilling fluids
Drilling fluids consist of a wide range of chemicals with different functions in well drilling and maintenance; weighting agents and inorganic salts are the ingredients used and discharges in largest amounts.
Discharges from the drilling activities represent the majority of the total discharges from the offshore petroleum activity. In 2004 discharges of these chemicals represented 82% of the total discharges on the Norwegian Continental Shelf (NCS). Most of the drilling chemicals are so called green or PLONOR (OSPAR List of Substances/Preparations Used and Discharged Offshore which Are Considered to Pose Little or No Risk to the Environment). These substances are typically salts, cellulose and weak acids . The table below summarises the functional groups of chemicals with use greater than 1000 tonnes and discharge greater than 100 tonnes on the NCS in 2004.
- Table 1.1 Functional groups with use > 1000 tonnes and discharge > 100 tonnes on the Norwegian Continental Shelf in 2004.
Function |
Use (tonnes) |
Discharge (tonnes) |
Lost circulation chemicals |
12141 |
1193 |
Scale inhibitor |
2778 |
893 |
Completion chemicals/fluids |
19644 |
3273 |
Clay stabiliser |
6080 |
4089 |
Cementing chemicals |
26175 |
1845 |
pH regulating chemicals |
2617 |
340 |
Weighting agents and inorganic chemicals |
178226 |
53976 |
Viscosity reducing chemicals (incl. lignosulfonates, lignite) |
6554 |
3714 |
Distribution of the discharged drill cuttings
While coarse cuttings particles and flocculated clays from the drilling operation usually settle near the discharge point, the finer-grained particles in the drilling discharge, such as unflocculated clay and fine barite particles are often transported in the water column over larger distances before settlement. One of the aims of the CORAMM project is to determine to what extent discharges of drill cuttings with WBM might impact on the health of cold water coral communities and thereby to what extent might drilling commence in regions close to cold water coral reefs. The findings of the project will help develop future rig location and discharge strategies, with the maintenance of the 'zero harmful emissions' requirement of Norwegian law being of paramount consideration.
Required testing of chemicals in the drilling fluid
Within the OSPAR area a testing regime is required on substance level for all chemicals added to the drilling fluid . In addition to the required toxicity testing on 3-4 different organisms, the potential for bioaccumulation and biodegradation of each substance also have to be fulfilled before the chemicals are allowed to be used in the drilling operation.
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