Cuttings

Key Takeaways

• Cuttings generation rate and shape provide the first geological information from a new formation: the rate at which the bit penetrates the formation (rate of penetration, ROP) combined with the mud circulation rate determines the volume of cuttings produced per unit time and their size distribution; tricone roller cone bits generate irregularly fractured rock fragments (typically 1 to 10 mm in maximum dimension, depending on tooth design, WOB, and RPM) by a crushing and scraping mechanism, while PDC (polycrystalline diamond compact) bits generate thinner, more tabular shale cuttings by a shearing mechanism that tends to produce thinner chips; the shape of cuttings provides information about formation type (platy, shale-like cuttings typically indicate clay-rich shales; blocky, angular cuttings indicate well-cemented sandstones or carbonates; curved, conchoidal fragments indicate carbonates with well-developed cleavage or brittle chalk) and about bit condition (very fine, powdery cuttings may indicate bit balling or worn teeth that are grinding rather than cutting); cuttings size also affects their transport efficiency in the annulus, with larger cuttings (above 5 to 6 mm) in high-angle or horizontal wells settling to the low side of the wellbore and forming cuttings beds that reduce drilling efficiency and can cause stuck pipe if not periodically cleaned by high-circulation-rate sweeps or mechanical agitation.
• Mud logging and cuttings description at the wellsite provides a continuous geological record from spud to total depth: mud loggers collect cuttings samples from the shale shaker (the primary solids-control device, a vibrating screen that separates cuttings from the returning drilling fluid) at intervals of 1 to 5 meters (adjusted for the lag time between drilling depth and sample arrival at surface, calculated from the annular volume and the circulation rate); the washed and dried cuttings sample is examined under a binocular microscope (5 to 50x magnification) by the mud logger, who describes the lithology (sandstone, shale, limestone, dolomite, evaporite, or coal), grain size (in sandstones), sorting, cementation, and porosity type (intergranular, vuggy, fracture) using a standardized geological description sheet; fluorescence testing (exposing the sample to ultraviolet light and observing yellow, white, or blue-white fluorescence that indicates the presence of crude oil hydrocarbons) provides the primary field screening test for oil shows, supplemented by cut fluorescence (applying a solvent such as toluene to the sample and observing the color and fluorescence of the solvent extract), total hydrocarbon analysis (the continuous mud gas chromatograph measuring C1 to C5 hydrocarbon gases extracted from the drilling fluid), and the chromatographic ratio of C1/(C2+C3) (the wetness ratio, which distinguishes gas shows from oil-associated gas); the cuttings log and show description is transmitted to the operator's geologist in real time via digital data transmission and constitutes the primary decision-support document for formation evaluation during drilling.
• Cuttings lag time (the elapsed time between the bit cutting a formation and the corresponding cuttings appearing at the surface shale shakers) must be accurately calculated to correctly depth-correlate cuttings samples with the drill depth at which they were generated: lag time = annular volume (from bit to surface) / circulation rate (gallons per minute or liters per minute); the annular volume is calculated from the hole diameter and drillstring OD at each depth interval, accumulated from bottom to surface; in a typical land well at 3,000 meters depth with a 215 mm hole (8-1/2 inch) and 127 mm drillpipe (5 inch), the annular volume is approximately 50 to 80 barrels and the lag time at 600 gpm circulation is 5 to 8 minutes per 1,000 meters of depth; lag calculations must be updated whenever the hole size changes (casing point, bit changes) or when the circulation rate changes; incorrect lag calculation results in cuttings being assigned to the wrong depth, misidentifying formation tops, show zones, and geological contacts by the lag error amount; in high-angle or horizontal wells, cuttings transport lag is further complicated by the tendency of cuttings to settle in the horizontal section and be transported in slugs rather than continuously, causing intermittent arrivals of old cuttings from previously drilled intervals that must be distinguished from fresh cuttings representing the current drilling depth.
• Offshore cuttings disposal is one of the most significant environmental management issues in marine drilling operations: in water-base mud drilling, cuttings can typically be discharged to the sea (overboard discharge) in most jurisdictions, subject to the toxicity and bioaccumulation requirements of the applicable regulatory framework (OSPAR Decision 2000/3 in the North Sea, US EPA National Pollutant Discharge Elimination System permits in the Gulf of Mexico, and equivalent national regulations worldwide); cuttings from oil-base mud (OBM) or synthetic-base mud (SBM) drilling must meet oil-on-cuttings (OOC) discharge limits (6.9 percent by weight in the North Sea, 9.4 percent in the US Gulf of Mexico for synthetic-based fluids meeting the biodegradation requirements) or be collected and either reinjected into a disposal formation (cuttings reinjection, CRI) or transported ashore for treatment and disposal; cuttings reinjection is the preferred disposal method for OBM cuttings in deepwater operations where transport to shore is impractical, involving slurrying the cuttings with seawater or spent base oil, grinding to reduce particle size (to below 1 to 2 mm for injectability), and injecting the slurry into a formation below the seafloor at pressures sufficient to fracture and accept the slurry; the regulatory trend in the past two decades has been toward stricter OOC discharge limits and greater use of CRI and closed-loop drilling systems (which eliminate overboard discharge by containing and reusing all drilling fluid and treating all cuttings onboard).
• Cuttings re-use and recycling as construction materials, soil amendments, or secondary aggregates is an emerging sustainable disposal option that reduces the volume of cuttings requiring costly incineration or landfill disposal: water-base cuttings (after de-watering to below 20 to 25 percent moisture content by centrifuge or pressure filtration) can be used as fill material for road construction, land reclamation, or berm construction if they pass leachate testing requirements for heavy metals (barium from barite, chromium from lignosulfonate muds); OBM cuttings can be processed by thermal desorption (heating to 350 to 500 degrees Celsius to vaporize and recover the base oil) to produce dry solids with OOC below 0.1 percent that are suitable for construction use, while the recovered oil is re-refined and recycled back into the mud system; the economic case for cuttings re-use depends on the proximity of the drilling operation to markets for the recovered materials (particularly road construction aggregate markets in developing regions where demand for construction materials is high and disposal regulations for cuttings are less stringent than in the North Sea or US Gulf of Mexico) and the availability and cost of alternative disposal methods (incineration at $200 to $500 per tonne, landfill at $50 to $200 per tonne, or CRI at $100 to $300 per tonne in deepwater); the oil and gas industry generates approximately 1.5 to 2.0 million tonnes of drill cuttings per year in the North Sea alone, making sustainable disposal and re-use an economically and environmentally significant challenge.