circulate out

Circulate out in drilling engineering is the well control procedure of pumping drilling fluid continuously around the full wellbore circuit (down the drill string, through the bit, and up the annulus to surface) to progressively displace and transport an influx of formation fluid (kick) from the wellbore to the surface choke manifold and degasser where it is safely vented or flared, reducing annular kick volume to zero and restoring the wellbore to a fully overbalanced state with kill-weight mud filling the entire annular column; in Western Canada Sedimentary Basin well control operations, circulate out is the active kill phase that follows detection of a kick, shut-in of the well using the blowout preventer stack, and calculation of kill mud weight from shut-in drill pipe pressure (SIDPP) and shut-in casing pressure (SICP) readings per AER Directive 036 well control requirements. The circulate-out procedure is operationally critical because the influx (kick) fluid in the WCSB wellbore annulus is typically less dense than the drilling mud (gas kick specific gravity 0.1 to 0.3, compared to mud specific gravity 1.3 to 1.8), so as the kick migrates upward during circulating-out it expands according to Boyle's Law as annular pressure decreases, requiring the driller at the choke to continuously adjust the choke opening to maintain constant bottomhole pressure (BHP) equal to formation pressure throughout the circulation; if choke adjustment is too slow or miscalibrated, BHP either underbalances the formation (risk of additional influx) or overbalances it (risk of lost circulation). In WCSB sour gas drilling operations on Devonian targets in the Kaybob, Edson, and Peace River areas where H2S concentrations above 10 mole percent are common, the circulate-out procedure carries additional personnel safety requirements under AER Directive 036 Section 8 and WHMIS H2S regulations: all surface personnel must be upwind with SCBA units donned before choke opening, flare lines must be ignited before gas reaches the flare pit, and H2S detectors must be placed at the choke manifold, degasser vent, and flare pit to monitor for H2S concentrations approaching the 10 ppm TLV-TWA action level. The two principal methods used for circulate-out in WCSB operations are the driller's method (first circulation removes the kick with original mud weight; second circulation replaces original mud with kill-weight mud) and the wait-and-weight method (kill mud is prepared before circulating, kick is removed in a single circulation with kill mud), with the choice between methods determined by well depth, kick size, kick type (gas versus oil versus saltwater), and rig equipment capabilities.

• Driller's method circulate-out procedure in WCSB gas kick situations: In the driller's method applied to a WCSB gas kick, the first circulation proceeds with the original mud weight while maintaining constant bottomhole pressure through choke adjustment; the driller holds constant pump stroke rate (SCR, slow circulating rate, typically 30 to 50 SPM on a WCSB triple rig) established before the kick, and the choke operator adjusts choke opening to hold drill pipe pressure at the pre-calculated constant value (SIDPP plus pre-kick circulating pressure at SCR). As the gas kick circulates up the annulus, it expands and the choke operator progressively closes the choke to maintain constant casing pressure in the lower annulus; the gas arrives at surface as a slug of expanded gas that must be routed through the degasser and flare system before entering the mud pits. After the first circulation removes the kick (confirmed by return flow density returning to original mud weight and gas show at degasser ceasing), the second circulation pumps kill-weight mud from the suction pit down the drill string and up the annulus, displacing the original-weight mud; WCSB kill mud weight is calculated as (SIDPP/TVD) + original mud weight in pressure-equivalent SG units.
• Wait-and-weight method circulate-out for WCSB sour gas kicks: single-circulation kill: The wait-and-weight method for WCSB H2S kicks delays circulation until kill-weight mud is fully prepared in the active pit and suction system, then circulates out the kick and replaces the original mud in a single pump cycle; this approach is preferred for WCSB sour Devonian wells because it minimizes the time H2S gas spends in the annulus and at surface during the kill operation compared to the driller's method's two full circulations. Kill mud preparation time for a WCSB well requires adding barite (barium sulfate, 4.2 SG) to the active mud system to increase mud weight; a typical WCSB Devonian kick requiring a 0.1 SG mud weight increase in a 1,500 m3 active system requires approximately 180 to 220 kg/m3 of barite addition over 60 to 90 minutes of continuous mixing while the rig remains shut in. During wait-and-weight circulation, the choke pressure schedule (plot of expected drill pipe pressure versus strokes pumped) is calculated before circulation begins and posted at the driller's console and choke manifold; the choke operator follows the pressure schedule, maintaining actual drill pipe pressure within plus or minus 0.2 MPa of the schedule value at each stroke count checkpoint as the kill mud front advances from surface to bit and from bit up the annulus.
• Choke management and BHP control during WCSB circulate-out operations: Maintaining constant bottomhole pressure during circulate-out requires continuous manual or automated choke adjustment as the low-density kick fluid migrates up the annulus; in a WCSB 3,000 m gas kick scenario with a 0.5 m3 gas influx at 25 MPa formation pressure, the gas volume at surface conditions will expand to approximately 60 to 90 m3 before it reaches the choke, creating a surge of gas flow through the choke that requires rapid choke closure to prevent BHP drop. The WCSB driller monitors three pressure gauges simultaneously during circulate-out: drill pipe pressure (must stay at constant circulating pressure), annular (casing) pressure (increases as gas migrates upward and decreases as kill mud replaces it), and choke inlet pressure; modern WCSB land rigs use remote-operated hydraulic chokes with digital pressure displays at the driller's console to facilitate single-operator BHP management without requiring separate choke and driller personnel. Automated choke control systems (Hydril Autochoke, Cameron DeltaVal) are standard on WCSB critical sour service wells above 10 mole percent H2S, providing PLC-controlled choke positioning that responds to drill pipe pressure deviation within 0.1 MPa and 2-second control loop time, significantly reducing the human error risk in choke management during H2S kick circulate-out.
• Gas migration and U-tube effects during shut-in before WCSB circulate-out: When a WCSB gas kick is shut in while kill mud is being prepared (wait-and-weight method), the gas in the annulus migrates upward through the kill mud due to buoyancy at a rate of 50 to 200 m/hour depending on mud viscosity and gas bubble size; gas migration causes casing pressure to increase (often 0.5 to 2.0 MPa/hour in WCSB Devonian gas kicks) without any change in bottomhole pressure, because the pressure increase at the casing head equals the reduction in annular hydrostatic pressure as the lower-density gas replaces mud above it. WCSB drillers and company men manage gas migration during shut-in by bleeding small volumes of mud at the choke (lubricate-and-bleed technique): opening the choke to bleed casing pressure by 0.5 to 1.0 MPa, then closing it and allowing pressure to restabilize, effectively keeping casing pressure within BOP rated working pressure limits while the gas migrates. AER Directive 036 and IADC well control guidelines require WCSB operators to record and plot casing pressure versus time during shut-in to detect and quantify gas migration rate before initiating circulate-out, ensuring the kill procedure is designed with the correct kill mud weight accounting for any gas that has migrated above the original kick location.
• Post-circulate-out wellbore verification and well security in WCSB operations: After completing circulate-out and confirming zero pit gain and kill mud returns at surface density, WCSB well control procedure requires a flow check (shut-in the well with BOP closed, observe for 10 to 15 minutes for any casing pressure buildup indicating residual influx) before resuming drilling or running logging tools. If the flow check shows zero pressure buildup (static casing and drill pipe pressures equal, no pit gain), the well is considered killed and drill-in can resume; if residual pressure is detected, a second kill circulation may be required or the kill mud weight must be recalculated. In WCSB H2S wells post-circulate-out, the gas remaining dissolved in the kill mud is a persistent hazard: WCSB operators circulate the mud through the degasser for a minimum of one complete surface-pit cycle after kick removal to strip dissolved H2S to below 10 ppm before mud pit and surface equipment access is restored to personnel without SCBA, with mud degassing confirmed by H2S monitor readings at the degasser vent.

Circulate-Out of H2S Gas Kick in WCSB Devonian Kaybob Nisku Well

A WCSB Devonian Nisku horizontal well at 3,780 m TVD (5,120 m MD) in the Kaybob sour gas area took a 0.8 m3 kick at 28.4 MPa formation pressure while making a connection; H2S content 18 mole percent. SIDPP was 2.1 MPa, SICP 2.8 MPa. Kill mud weight calculated at 1.68 SG (original 1.55 SG). Wait-and-weight method selected. Kill mud prepared in 85 minutes; all surface personnel donned SCBA, flare ignited. Circulate-out at 35 SPM; choke pressure schedule posted. Gas arrived at choke at stroke 1,240 (43 minutes): casing pressure peaked at 6.8 MPa, choke operator held drill pipe pressure within plus or minus 0.15 MPa of schedule throughout. Gas cleared degasser at stroke 2,100; H2S peaked at 85 ppm at degasser vent (within flare zone). Kill complete at stroke 4,800 (2.3 hours total). Flow check: zero casing pressure buildup after 15 minutes. Well killed on first circulation; drilling resumed after 4.5 hours from kick detection.

Related Terms

Kick is the formation fluid influx that initiates the circulate-out procedure; detection by pit gain or flow increase during WCSB drilling triggers immediate BOP shut-in and SIDPP/SICP measurement before kill mud weight calculation. Kill mud is the weighted drilling fluid pumped during WCSB circulate-out to restore overbalance; kill weight in SG is calculated as original mud weight plus SIDPP divided by TVD of the kick zone in pressure-equivalent units. Choke manifold is the surface pressure control equipment through which kick fluid is routed during WCSB circulate-out; choke adjustment maintains constant BHP as expanding gas migrates up the annulus. Driller's method is the two-circulation kill procedure used in WCSB operations when wait-and-weight is impractical; first circulation removes kick with original mud, second circulation replaces mud with kill weight. Blowout preventer (BOP) isolates the WCSB wellbore during shut-in before circulate-out; annular and pipe rams close on the drill string to prevent uncontrolled flow while kill mud is prepared and the kill procedure is initiated.