coarse

Coarse solids in drilling fluid technology refers to solid particles larger than 74 micrometres (retained on a No. 200 US standard mesh sieve), which in a water-base or oil-base mud system include the drilled formation cuttings generated by the drill bit, coarse barite weighting agent particles, and coarse sand or gravel contaminants introduced from the formation or surface mixing water, distinguished from the colloidal and ultra-fine solids (below 2 micrometres) that provide rheological and filtration-control properties to the mud; the removal of coarse drilled solids from the circulating mud system is the primary objective of the surface solids control equipment train at WCSB drilling locations, because coarse drilled solids have no beneficial function in the mud, increase mud density without contributing to weighting agent effectiveness, abrade pump liners and bit bearings, and accumulate in the annulus during low-flow periods causing stuck pipe incidents and wellbore bridging in WCSB horizontal sections where gravity causes coarse cuttings to settle on the low side of the borehole. In Western Canada Sedimentary Basin drilling operations, the coarse solids removal system at a typical intermediate or production hole drilling location consists of a primary shale shaker (linear motion or balanced elliptical motion screens with API mesh size #80 to #200, corresponding to 177 to 74 micrometre openings) that removes the bulk of drilled cuttings as they return from the wellbore, followed by a degasser to remove entrained gas, a desander (hydrocyclone with 6-inch cone, removing particles above 45 to 75 micrometres) to remove coarser solids that passed the shaker screens, a desilter (hydrocyclone with 4-inch cone, removing particles above 15 to 45 micrometres) to remove finer coarse-range solids, and a centrifuge (high-speed, 1,800 to 3,200 RPM bowl speed) that polishes the undersize fraction leaving the hydrocyclones and recovers barite for re-use while discarding fine drilled solids. The distinction between coarse and fine solids is operationally critical in WCSB weighted mud programs (density above 1.30 SG using barite, density above 1.50 SG using barite-hematite blends) because the solids control system must discriminate between coarse barite (useful weighting agent, to be returned to the active mud system) and coarse drilled solids (harmful contaminants, to be discarded), a discrimination accomplished by the centrifuge decanting sequence in which the centrate (overflow) containing fine barite and colloidal solids is returned to the mud pit while the solids cake (underflow) containing coarse drilled solids and some degraded barite is discarded.

• Coarse solids control equipment selection and API screen sizing for WCSB intermediate hole drilling programs: API Recommended Practice 13C standardizes the measurement and reporting of drilling fluid solids content and the mesh size designation for shale shaker screens used in coarse solids removal; the API mesh number corresponds to the number of openings per linear inch, so a #120 screen has 120 openings per inch (approximately 125 micrometre opening) and removes particles above this size as the primary coarse solids separation step. In WCSB intermediate hole programs drilling through Mannville sands and shales at 800 to 2,500 m with water-base mud at 1.20 to 1.40 SG, the primary shaker typically runs #120 to #150 API screens (100 to 120 micrometre opening) balancing solids removal efficiency against mud loss through the screen; finer screens (#200, 74 micrometre) capture more fine drilled solids but also lose more liquid mud through the screen, increasing mud consumption cost. WCSB production hole programs drilling through tighter Cardium, Viking, and Spirit River sandstones at 1,500 to 3,000 m depth generate finer cuttings (average particle size 200 to 500 micrometres from PDC bits versus 1,000 to 3,000 micrometres from roller cone bits on the same formation), justifying finer primary shaker screens (#150 to #200) to capture PDC-ground fines that would pass a coarser screen and accumulate in the active mud system.
• Coarse drilled solids accumulation and equivalent circulating density effects in WCSB horizontal wells: In WCSB horizontal wells drilled at inclinations of 85 to 92 degrees (near-horizontal laterals in Montney, Duvernay, and Cardium), coarse cuttings (500 to 3,000 micrometre) settle gravitationally to the low side of the borehole when annular velocity drops below the critical transport velocity of 0.6 to 1.0 m/s, forming a cuttings bed that restricts the effective annular cross-section and increases equivalent circulating density (ECD) by 0.02 to 0.08 SG above the static mud density. ECD increases from cuttings bed buildup in WCSB horizontal sections are additive to the already-elevated ECD from friction in the long horizontal annulus (1,500 to 3,000 m laterals); if total ECD exceeds the fracture gradient of the weakest formation in the open hole section (typically 0.015 to 0.020 SG/m for WCSB Cardium at 1,800 m depth), lost circulation occurs. WCSB engineers manage horizontal cuttings transport by maintaining annular velocity above 0.8 m/s at all points in the lateral (requiring pump rates of 25 to 45 L/s depending on bit size and casing annular gap), running wiper trips (pulling the drill string to move cuttings beds updip), and optimizing mud rheology (yield point 12 to 20 Pa and plastic viscosity 15 to 25 mPas) to maximize cuttings-carrying capacity without excessive ECD contribution from mud viscosity.
• Coarse solids content measurement and mud retort analysis in WCSB drilling fluid quality control: The coarse solids content of a WCSB active drilling mud is measured by retort analysis, in which a 20 mL mud sample is heated in a calibrated retort to drive off liquids (water and oil), and the remaining solids volume is measured as a percent by volume of the original sample; low gravity solids (LGS, density 2.2 to 2.6 g/cm3, primarily drilled formation cuttings and clays) are distinguished from high gravity solids (HGS, density 4.2 to 4.5 g/cm3 for barite) by combining retort data with mud density measurement. For a WCSB weighted WBM at 1.40 SG, the API standard maximum LGS limit is 6 percent by volume; LGS above 8 percent indicates inadequate coarse solids removal and signals the need to reduce flow rate to improve shaker efficiency, cut the active mud volume with dilution water, or increase centrifuge processing rate to remove fine LGS that pass the shaker. In practice, WCSB mud engineers target LGS below 5 percent by volume in weighted mud systems by running the full solids control train (shakers, desander, desilter, centrifuge) continuously and discarding centrifuge underflow rather than returning it to the active system when LGS is elevated.
• Coarse solids in WCSB oil sands drilling and completion: sand influx management in Athabasca thermal programs: WCSB oil sands (Athabasca McMurray Formation, Cold Lake Clearwater Formation) present a unique coarse solids challenge because the unconsolidated sand grains that constitute the reservoir matrix flow into the wellbore as a naturally occurring coarse solid during drilling, well testing, and thermal production; in SAGD vertical observation wells and horizontal producer-injector pairs, sand influx rates of 0.5 to 5 kg/m of wellbore per day create a continuous coarse solids management problem distinct from drilled cuttings removal. SAGD steam circulation in the injector string generates steam-condensate flow that can carry coarse sand grains (200 to 2,000 micrometre) along the wellbore and deposit them in production equipment; sand filters with 100 to 500 micrometre retention rating are installed in SAGD production wellheads and ESP strings to intercept these coarse sand grains before they reach pump impellers, with filter differential pressure monitored as an indicator of sand accumulation rate and filter change-out frequency determined by pressure buildup rate in CNRL and Cenovus Athabasca SAGD operations.
• Coarse weighting agent particle size and barite sag in WCSB high-angle wells: Barite used as a weighting agent in WCSB drilling muds has a specified particle size distribution under API 13A: 97 percent must pass a 75 micrometre screen (coarser particles cause excessive sag) and no more than 30 percent may pass a 6 micrometre screen (finer particles impair filtration control). In WCSB high-angle and horizontal wells with mud densities above 1.60 SG, coarse barite particles (above 25 micrometres) settle preferentially relative to fine particles under the combined effects of gravity and rotation of the drill string during drilling; this dynamic sag (density variation along the wellbore during circulation) can create density windows of 0.03 to 0.10 SG between the upper and lower sides of the horizontal wellbore, causing pressure spikes when circulation is re-established after a connection (sag surge) that can exceed the fracture gradient at shallow shoe depths. WCSB operators drilling high-density (above 1.70 SG) horizontal sections through Devonian evaporites or high-pressure Duvernay specify micronized barite (d50 below 8 micrometres) or barite-hematite blends with narrow particle size distribution to reduce dynamic sag below 0.02 SG across the horizontal section.

Coarse Solids Buildup Triggering Lost Circulation in WCSB Montney Horizontal Section

A northwest Alberta Montney horizontal lateral (2,400 m lateral at 88 degrees inclination, 1.25 SG WBM) experienced a sudden partial lost circulation event (10 to 15 m3/hour losses) at 1,850 m into the lateral that was attributed to ECD exceedance rather than natural fracturing. Retort analysis of the active mud at the time showed LGS at 9.2 percent by volume (above the 6 percent API limit), confirmed by a rising trend over the previous 36 drilling hours from 5.8 percent at lateral entry. The centrifuge had been bypassed 18 hours earlier due to a mechanical fault. ECD calculation including the elevated LGS viscosity contribution and estimated cuttings bed restriction showed total ECD of 1.39 SG against a Montney fracture gradient of 1.37 SG at the lateral section depth. The centrifuge was repaired and returned to service; a 2 m3/hour sweep of 1.04 SG freshwater displaced 15 m3 of active mud to reduce LGS to 5.1 percent; a wiper trip moved the cuttings bed to surface. Circulation was re-established without further losses, and the lateral was completed to total depth 550 m deeper without recurrence after maintaining LGS below 5.5 percent.

Related Terms

Solids control is the systematic process of removing coarse drilled solids from circulating WCSB drilling mud using shale shakers, hydrocyclones, and centrifuges; maintaining low gravity solids below 6 percent by volume in weighted mud systems is the primary performance target for the solids control equipment train. Shale shaker is the primary coarse solids removal device at WCSB drilling locations; linear motion or balanced elliptical motion shakers with #80 to #200 API screens intercept particles above 74 to 177 micrometres as the first stage of the solids control equipment train. Barite (barium sulfate, density 4.2 SG) is the standard weighting agent in WCSB WBM and OBM; API 13A requires 97 percent of barite to pass 75 micrometre screen to prevent coarse-particle sag in high-angle WCSB horizontal wells drilled with mud densities above 1.60 SG. Equivalent circulating density (ECD) in WCSB horizontal wells is elevated by coarse cuttings bed accumulation on the borehole low side; LGS above 8 percent combined with cuttings bed restriction can increase ECD by 0.05 to 0.12 SG above static mud density, risking fracture gradient exceedance in shallow shoe WCSB intermediate hole programs. Centrifuge is the fine-solids polishing stage in the WCSB solids control train; operating at 1,800 to 3,200 RPM bowl speed, the centrifuge recovers fine barite in the centrate overflow while discarding coarse drilled solids in the underflow cake, maintaining weighted mud LGS below the 6 percent API limit.