Gas-Cut Mud

Key Takeaways

• The density reduction caused by gas-cut mud is the primary well control concern, because the hydrostatic pressure of the mud column must exceed the formation pore pressure throughout the open hole section to prevent a kick from developing into a blowout: the hydrostatic pressure of a gas-cut mud column is lower than that of the uncontaminated mud because the dispersed gas bubbles occupy volume within the mud column without contributing proportionally to its weight (gas density is approximately 0.002 g/cc at surface conditions, compared to 1.5-2.0 g/cc for weighted drilling mud); as the gas-cut mud rises in the annulus toward the surface, the pressure decreases and the gas bubbles expand (by Boyle's Law), further reducing the effective mud weight and accelerating the hydrostatic pressure reduction; the critical point is whether the reduction in bottomhole pressure caused by the gas-cut mud column is sufficient to allow additional formation gas to enter the wellbore — if the bottomhole pressure remains above the formation pressure despite the gas contamination, the well remains in overbalanced conditions and the gas can be circulated out safely; if the gas cutting is severe enough that the bottomhole pressure falls below the formation pressure, additional gas influx begins (the kick is self-reinforcing) and the well must be shut in immediately and killed with increased mud weight before returning to circulation; the mud logger and driller monitor both the gas level in the returns and the pit volume simultaneously to distinguish gas cutting without volume gain (formation gas dissolved in the mud at depth that expands and comes out of solution near the surface without significant wellbore influx) from a true kick with volume gain (formation fluid physically flowing into the wellbore and displacing mud from the annulus).
• Trip gas and connection gas as sources of gas-cut mud must be distinguished from a true formation gas influx (kick) because they have different causes and different remediation requirements: trip gas is gas that accumulates in the wellbore annulus and near the bit face when circulation is stopped and the drill string is pulled out of hole for a bit change, casing run, or other trip operation, and arrives at the surface as a single slug of gas-cut mud when circulation resumes after running back in the hole and re-establishing flow; trip gas is identified by its timing (arriving as a single high-gas-concentration slug shortly after circulation is re-established following a trip, rather than as a continuous background level) and by the absence of a corresponding pit gain during the trip (no new formation fluid entered the wellbore, only dissolved gas migrated from the formation into the stagnant mud column); connection gas is a smaller version of trip gas that arrives at the surface after each drill pipe connection (a brief pump-off period that allows some formation gas to enter the near-bit mud) and appears as periodic small gas spikes in the mud log at the drill pipe length intervals; neither trip gas nor connection gas typically represents an active kick or formation fluid influx, but increasing magnitude of trip gas or connection gas (which indicates that the overbalance margin has been reduced to the point where significant gas enters the wellbore during even brief circulation stops) is an early warning of inadequate mud weight that should prompt consideration of increasing the mud density before continuing drilling.
• Mud weight increase to control gas cutting is the primary engineering response to moderate gas cutting that causes concern about overbalance adequacy: the mud weight increase required to restore a specified overbalance margin (typically 0.2-0.5 ppg or 200-400 psi above the estimated formation pressure) is calculated from the formation pressure estimate (derived from pore pressure prediction from seismic velocity, regional pressure data, or offset well experience) and the current mud weight; the mud weight is increased by adding barite (barium sulfate, density 4.2 g/cc, the standard mud weighting material) to the water-based mud or by adding hematite or barite to oil-based mud; the addition of weighting material also increases the plastic viscosity and yield point of the mud (which must be managed by adding water and thinners to maintain pumpability), and the resulting mud must be thoroughly mixed (by recirculating through the mix pump system) before the heavier mud reaches the bit depth and restores overbalance at the formation; the time required for the heavier mud to reach the bit from the surface (the lag time divided by the pump rate) is a critical operational window during which the gas-cut mud of the previous mud weight continues to provide the overbalance — if the gas cutting is severe and the pit gain continues to increase during the mud weight increase mixing and pumping, the well must be shut in on the BOP rather than continuing to circulate with the original gas-cut mud.
• Degassing equipment at the surface removes dissolved and entrained gas from gas-cut mud before the mud is recirculated into the active pit and pumped back down the well, preventing the reduced-density gas-cut mud from being returned to the wellbore and continuing to undermine the hydrostatic pressure: the primary degasser is a vacuum degasser or atmospheric degasser installed downstream of the shakers and upstream of the active pits, which passes the gas-cut mud through a thin film across a low-pressure chamber (either by impeller agitation or by vacuum suction) that allows the dissolved and dispersed gas to flash out of the mud and be recovered in a vent line that discharges the gas to a safe location (typically a flare or vent stack at a safe distance from the rig); the secondary degassing function is performed by the mud agitators in the active pits (low-speed impellers that circulate the mud within each pit compartment) and the mud guns (high-velocity fluid jets that agitate the mud surface), which facilitate natural gas bubble coalescence and surface release in the pits before the mud is pumped downhole; effective surface degassing is essential for maintaining the correct mud weight in the circulating system, because gas-cut mud that bypasses the degasser and is returned to the active system dilutes the effective mud weight below its specified value, undermining the overbalance calculation that was used to justify the current mud weight as adequate for the formation being drilled.
• Gas-cut mud in oil-based drilling fluid (OBM) presents additional challenges compared to water-based mud because gas is highly soluble in the oil continuous phase at bottomhole temperature and pressure, remaining dissolved in the mud at depth and only coming out of solution as the mud rises and the pressure decreases near the surface, creating a delayed gas release that can produce a large surface gas flow from relatively modest gas influx at the bit: in OBM, the gas-oil partition coefficient at typical reservoir conditions means that significant volumes of gas can be dissolved in the oil phase without producing the visible bubble formation and pit gain that would indicate gas influx in water-based mud; the mud engineer must rely on the mud log gas readings (not just the visual observation of the returns) to detect gas cutting in OBM, and the rate of gas extraction from OBM samples at the gas trap may be lower than from WBM samples at the same gas concentration (because OBM retains dissolved gas more tenaciously); the well control response to gas-cut OBM follows the same principles as for WBM (shut in if pit gain develops, kill with heavier mud weight if required), but the calculation of the equivalent gas influx volume from the surface gas measurement must account for the dissolved gas fraction that comes out of solution as the OBM rises through the riser in deepwater applications, potentially creating a riser gas unloading event that must be managed by diverting the gas-cut returns to the mud-gas separator before allowing them to flow into the active pits.