Ream
To ream is to enlarge an existing wellbore to its intended diameter by passing a specialized cutting tool (a reamer) through a section of hole that has been drilled or has become smaller than the design specification — reaming may be performed for several distinct operational reasons including: (1) restoring nominal hole size in intervals where a worn or undergauge bit drilled below the original bit diameter (commonly discovered when the bit is tripped to surface and the actual cutting structure diameter is found to be smaller than the original outside diameter, indicating that some portion of the hole was drilled at the worn bit's reduced size); (2) enlarging the hole through plastic formations (overpressured shales, reactive evaporites, mobile salt) that have flowed inward into the wellbore over time, creating tight spots that prevent normal drilling progression or pipe handling; (3) removing dogleg or hole irregularities (key seats, ledges, washout boundaries) that interfere with subsequent operations including casing running, completion, or wireline logging; and (4) creating a larger-diameter hole specifically designed to accommodate larger casing, completion equipment, or specialized downhole tools that exceed the original bit diameter; the reaming operation is performed with various tool types including rotating reamers (with extending arms or expandable diameters), under-reamers (for enlarging hole below an existing casing string), bicenter reamers (for combination drilling and reaming in a single trip), or rotary steerable systems with reaming functions integrated into the drilling assembly.
Key Takeaways
- Undergauge hole detection from worn bit diameter measurement is the most common trigger for reaming operations on routine drilling — when the bit is tripped from the hole at the planned bit life endpoint, the cutting structure outside diameter is measured at the rig floor before the next bit is run; if the measured diameter is more than approximately 0.10 to 0.15 inches below the new bit diameter (typical wear tolerance), the hole drilled with the worn bit may be undergauge by a similar amount over some unknown depth interval; the next bit (full gauge) cannot pass through the undergauge interval without difficulty and may experience drilling problems, stuck pipe, or premature wear at the gauge contact points; reaming is performed by rotating the new bit through the undergauge interval at low weight on bit while circulating, allowing the new bit to slowly cut through the undersize hole back to nominal gauge; modern PDC bits with diamond or tungsten carbide gauge protection maintain near-original diameter through their full life and have substantially reduced the frequency of undergauge hole problems compared to roller cone bits, but the inspection and reaming procedure remains standard practice for any bit that shows even modest wear.
- Plastic formation flow into the wellbore creates tight spots that require reaming to maintain hole gauge over the operational life of the well — overpressured shales (where pore fluid pressures approach lithostatic pressure) deform plastically over hours to weeks of exposure, creating gradual hole-size reduction that may not be apparent until the drillstring is tripped and resistance is encountered on the way back to bottom; rock salt and other evaporites (anhydrite, gypsum) creep at all temperatures and pressures, with creep rates accelerating dramatically at temperatures above 100°C; tight spots in plastic formations are typically identified by the driller as locations where the drillstring takes longer to pass through than during the original drilling, with overpull-and-slack-off cycles during tripping providing diagnostic information about the location and severity of tight spots; reaming through these tight spots restores hole gauge but does not prevent re-formation of tight spots in continuing plastic formation flow, requiring repeated reaming over the well's drilling phase or the use of casing/liner to isolate the plastic interval.
- Bicenter reamers and under-reamers enable reaming operations integrated with drilling — bicenter reamers (also called eccentric reamers) have an offset cutting structure that drills a hole larger than the bit diameter, allowing the assembly to drill a section of larger-diameter hole while passing through smaller-diameter hole or casing; under-reamers are run below the casing shoe and enlarge the open hole below an existing casing string while maintaining the in-casing diameter required for the upper section; both tool types are commonly used to drill production hole sections that are larger than the production casing inside diameter (specifically, where the hole must accommodate completions equipment with diameters up to 95 percent of the casing ID); under-reaming is also used to create rathole sections for deep-well plug-and-abandonment operations where the rathole must accommodate plugging tools larger than the deepest casing diameter; the integration of under-reaming with rotary steerable drilling systems has enabled efficient drilling of expanded production hole sections in horizontal wells where tripping the assembly multiple times for separate drilling and reaming operations would be operationally costly.
- Reaming-while-drilling integration with modern drilling automation has reduced the operational time penalty for routine reaming operations — modern rig automation systems track bit wear from wear indicators (vibration patterns, torque trends, ROP decreases) and can automatically initiate reaming sequences when wear indicators suggest that the in-progress hole may be undergauge; similarly, real-time wellbore stability monitoring (using MWD pressure data, ECD trending, and gas-influx detection) can identify formation flow into the wellbore before the situation becomes severe enough to cause stuck pipe; automated reaming sequences integrated with the rig control system allow controlled reaming operations to be performed without dedicated driller attention during the actual reaming phase; the integration of these capabilities has shifted reaming from a reactive operation (initiated when problems are encountered) toward a proactive, preventive operation (scheduled based on predictive indicators) that minimizes total operational time impact while maintaining wellbore quality.
- Reaming for casing-running clearance is a standard pre-casing operation in deviated and horizontal wells where the original drilling bit may have produced a hole that meets nominal gauge specifications but does not provide adequate radial clearance for the casing string with its centralizers and float equipment — the typical requirement is a minimum 0.5-inch radial clearance between casing OD and hole ID, with greater clearance preferred for long deviated runs where running torque and drag accumulate; if the actual clearance is below this threshold (commonly observed where the drilling assembly's integrated stabilizers were aligned with the casing run after the bit), the pre-casing wiper trip includes reaming through any tight spots identified during the trip down; the reaming pass is followed by a final wiper trip to verify clear passage at the planned casing tripping speed before casing is brought to rig floor for running; this pre-casing reaming sequence is now standard practice for deviated and horizontal wells in major basins worldwide and significantly reduces the incidence of casing-stuck or partial casing run events.
Fast Facts
The terminology of reaming dates from manufacturing precision-machining operations where reamers are used to finish drilled holes to precise diameter and surface finish requirements; the oilfield use of "ream" applies the same fundamental concept to the much larger scale of wellbore drilling. Modern oilfield reamers range from simple welded-blade tools (used for routine wellbore conditioning) through hydraulically actuated expandable reamers (used to enlarge hole below restrictions) to sophisticated electrically-controlled rotary steerable systems with reaming integration. The under-reamer market is a substantial niche within the global drilling tools market, with major suppliers including Halliburton (Hughes Christensen division), Schlumberger (Smith Bits / Geo-Logic / Drilling and Measurements), Baker Hughes, and specialty providers including NOV (National Oilwell Varco) and Weatherford. The ability to ream-while-drilling using rotary steerable assemblies with under-reamer modules has enabled the construction of complex deepwater and ultra-deepwater wells where multiple hole sizes must be combined in single trips to minimize the rig time consumed by separate runs.
What Is Reaming?
Drilling a wellbore creates a hole at the bit's outer diameter, but the resulting hole is rarely perfectly cylindrical and exactly at the bit diameter throughout its length. The bit wears during drilling, gradually reducing its cutting diameter; formations may flow into the wellbore after drilling, narrowing the hole; doglegs and hole irregularities create restrictions; and the next operation may require a larger hole than was originally drilled. In all of these cases, the solution is to ream — to pass a cutting tool through the existing hole and enlarge it to the required diameter or restore it to nominal specification.
Reaming is a routine drilling operation rather than a specialized intervention. Every drilling crew performs reaming during normal operations — every bit trip-in pass through previously drilled hole that may have undergone formation flow or hole-size reduction is fundamentally a reaming operation, even when not explicitly labeled as such. Specialized reaming operations (using dedicated reaming tools rather than the next drilling bit) are performed when the magnitude of hole-size correction required exceeds what the next bit can provide, or when specific hole-enlargement geometries are needed (under-reaming below casing, bicenter drilling for production hole expansion). Understanding when and how to ream — and what tools to use — is part of every drilling engineer's basic competency.
Reaming Operations and Tool Types
Routine reaming with drilling bits is performed by tripping the new bit through any tight spots identified during the trip down to bottom, rotating slowly with low weight on bit and circulating to remove debris generated by the reaming. The driller monitors torque and overpull during reaming and adjusts the operation parameters to maintain efficient hole-size restoration without excessive bit wear. Specialty reaming tools are deployed when the routine bit-based approach is inadequate or when specific hole-enlargement geometries are required. Hole openers are basic reaming tools with multiple cutting blades arranged radially around a central body that enlarge an existing hole to a specified diameter; they are deployed when the open-hole diameter must be enlarged above the bit diameter and a single dedicated trip is justified. Bicenter reamers have a primary cutting structure on the central axis (which drills the basic hole size) and an offset secondary cutting structure (which enlarges the hole during drilling); the assembly drills a hole larger than the central bit diameter while still able to pass through restrictions sized for the central bit. Under-reamers are deployed below an existing casing string on the drillstring and enlarge the open hole below the casing shoe; they typically have hydraulically actuated cutting arms that fold inside the assembly during deployment through the casing and extend outward when activated below the casing shoe by drilling fluid flow. Each tool type serves specific operational needs and is selected based on the geometry of the reaming operation required.
Reaming Operations Across International Drilling Practice
Canada (AER / WCSB): AER's drilling regulations include general requirements for wellbore quality but do not specifically prescribe reaming procedures, leaving operational decisions to the operator and drilling contractor; WCSB drilling operations include extensive reaming experience in plastic formations including the Mannville coal-bearing shales, Devonian salt sequences, and the McMurray Formation oil sands shales which can creep substantially during long drilling operations; major Canadian operators (Cenovus, Suncor, Imperial Oil, Tourmaline) have extensive operational experience with reaming protocols that maintain hole gauge through plastic formations during the often-long drilling phases of horizontal SAGD wells and unconventional horizontal wells.
United States (API / BSEE): API RP 7G (Drill Stem Design and Operating Limits) and API RP 13 series for drilling fluids include guidance on reaming operations as part of overall wellbore conditioning; BSEE's offshore drilling regulations require operators to demonstrate adequate wellbore quality through pre-casing logging and pressure testing, with reaming operations being part of the standard preparation; deepwater Gulf of Mexico drilling operations include extensive use of under-reaming for production hole expansion in the deep Lower Tertiary plays, with major operators (Chevron, ExxonMobil, BP, Equinor) integrating under-reaming with rotary steerable drilling for efficient construction of complex wellbore geometries.