Mineralogy of Cuttings
Drill cuttings are particles of crushed rock produced by the grinding action of the drill bit as it penetrates into the earth. Drill cuttings range in size from clay-sized particles (~ 2 μm) to coarse gravel (> 30 mm) and have an angular configuration. Their chemistry and mineralogy reflect that of the sedimentary strata being penetrated by the drill.
The solids control equipment (notably shale shaker) discharge (mainly drill cuttings) from an exploratory rig on the Mid-Atlantic outer continental shelf contained 21 % montmorillonite, 11 % each of illite and chlorite (all three are clays), and 22 % quartz, with smaller amounts of dolomite and siderite (iron carbonate).
Cuttings from wells drilled in the North Sea typically are composed primarily of sandstone and shale. Westerlund et al. identified quartz and barite as the two most abundant minerals in cuttings from the Beryl A and Ekofisk 2/4A platforms. The quartz probably is from the sandstone in the cuttings and the barite is primarily from the drilling mud solids adhering to the cuttings. Pyrite (iron sulfide) also is abundant. Illite and kaolinite are the dominant clay minerals present in North Sea drill cuttings and may have come, in part, from the drilling mud solids adhering to the cuttings particles. These also are the dominant clays in continental shelf sediments of the North Atlantic.
Drilling muds containing cuttings are circulated through several separation devices on the platform to separate the drill cuttings particles from the drilling mud, which is recirculated down the hole (Figure 1). The coarser, sand/gravel-sized cuttings particles are removed by the shale shakers. The solids that pass through the shale shaker screens may be passed to hydrocyclones (mud cleaner) and, occasionally, decanting centrifuges, where finer particles are removed. However, hydrocyclones (mud cleaner) and decanting centrifuges are not efficient in selectively removing cuttings solids from bentonite-based WBM, because the target cuttings for these devices are in the size range of the barite and clay fractions of the WBM. The solids control system on a platform usually removes more 75 percent of the cuttings from a WBM, unless the cuttings are composed mainly of silt-clay sized particles. The remaining 25 percent, mostly clay-sized particles, tends to increase the viscosity of the mud.
Frequently, when drilling a clay-rich formation, mud viscosity is controlled by dilution (addition of make-up water) of the return from the shale shaker to maintain an optimum bentonite concentration and viscosity. This WBM management strategy requires frequent bulk discharges of drilling mud.
Chemical Composition of Drill Cuttings
Drill cuttings contain, in addition to formation solids, small amounts of liquid and solid drilling mud components. The amounts of drilling fluid solids that remain attached to cuttings vary, depending on the grain size of the crushed rock from the strata being drilled. Clay sized cuttings are more difficult than larger cuttings to separate from drilling mud. A typical cuttings discharge during drilling with WBM usually contains 5 to 25 percent drilling fluid solids after passage through the solids control equipment on the platform. Cuttings from the fossil fuel-bearing intervals in a well also may contain crude oil or gas condensate.
|Table 1. Concentrations of several metals in WBM and drill cuttings from two offshore platforms in southern California. Concentrations are mg/kg dry wt (ppm). From Phillips et al.|
|Metal||Platform 1||Platform 2|
|Drilling Mud||Cuttings||Drilling Mud||Cuttings|
The chemical composition of drill cuttings reflects the geochemistry of the formation being drilled and the amount of drilling mud ingredients adhering to the cuttings at the time of disposal. For example, the metals concentrations in drill cuttings discharged to offshore waters of California are similar to those of the drilling muds used to drill the wells (Table 1). Barium is more abundant in drilling mud than cuttings, as expected because of its abundance in drilling muds. Lead in one sample and zinc in both samples are more abundant in cuttings than in the drilling muds, indicating high concentrations in the formation rocks being drilled, or contamination of the cuttings with pipe thread compound. Most of the metals associated with cuttings are in immobile forms in minerals from the geologic formations.
Cuttings produced during drilling with WBM may contain small amounts of petroleum hydrocarbons. The hydrocarbons in cuttings generated with WBM may come from spotting fluids and lubricants added to the mud, or from the geologic strata being penetrated by the drill. Drilling mud and cuttings from all depths in a well off California contained petroleum hydrocarbons (Table 2). At all depths the drilling muds contained higher concentrations of total petroleum hydrocarbons and lower concentrations of total and individual PAH than the cuttings, suggesting that the PAH were derived primarily from the formations being drilled.
|Table 2. Concentrations of hydrocarbons in composite samples of WBM and drill cuttings (in parentheses) from three drilling depths in a well on a platform in the Point Arguello Field, California. Concentrations are μg/g dry wt (ppm).|
|Total Petroleum Hydrocarbons(1)||159 (600)||137 (95)||988 (526)||390 (407)|
|Total PAH(2)||0.87 (2.3)||8.0 (12)||51 (121)||25 (45)|
|Naphthalenes(3)||0.27 (1.2)||5.4 (8.9)||39 (96)||18 (35)|
|Fluorenes(3)||ND (ND)||0.38 (0.35)||4.1 (8.2)||2.8|
|Phenanthrenes(3)||0.34 (0.79)||0.94 (0.64)||4.5 (9.3)||2.8 (3.6)|
|Dibenzothiophenes(3)||0.03 (ND)||0.71 (0.40)||3.9 (8.1)||1.9 (2.8)|
|(1)Total resolved + unresolved hydrocarbons. (2) Total 2- through 5-Ringed PAHs plus alkyl homologues. (3) Includes parent PAH and alkyl homologues.|
The source of the high TPH in the cuttings from near the surface is unknown. Total and individual PAH concentrations increased with depth in the well. TPH and PAHs in the cuttings from the bottom of the well closely resembled the crude oil in the reservoir and probably were from the hydrocarbon-bearing formation.