1. Gas Expansion and Circulation in a Deep HPHT Well

Written By Shrikant Tiwari

Case Study of a deep re-entry drilling campaign:

The driller felt a subtle but unmistakable increase in flow at 13,780 ft true vertical depth while circulating. The rig took an 8 bbl (≈1,272 L) gas kick from the reservoir. At that time, the mud on the string was 1.85 SG, the surface-integrated differential pressure probe (SIDPP) read 420 psi, and downhole conditions were estimated as 300°F (≈149°C) with a downhole gas compressibility factor Z = 1.55. The team elected a cautious circulation rate of 3.0 bbl/min (≈477 L/min) while they set up the choke. The crew asked: How much will that gas expand on the way to the surface? How much gas reaches the surface per day at that circulation rate? And how long before the gas already at the choke is driven out? (Also — how should the choke be managed and what immediate rig/flare actions are required?)

  • Calculations (step-by-step)

    1) Bottomhole pressure (psig → psia) and temperature (°R)

    • Hydrostatic head = SG×0.433×TVD = 1.85×0.433×13 ,780 ft = 11,038.469psi

    • Bottomhole pressure (psig) = SIDPP + hydrostatic pressure = 420+11,038.469=11,458.469 psig

    • Convert to absolute: Pb (psia)=11 458.469+14.7=11 473.169 psia

    • Bottomhole absolute temperature: Tb=300∘F+460=760∘R

    2) Overall gas expansion ratio (Std volume ÷ reservoir volume)

    We use the same formulation as in the description https://www.drillopedia.com/wc-gas-behavior to calculate the gas volume when it reaches the surface (visit the link to understand the gas behavior): 

    Vs / Vb = (35.37 x Pb) / (Zb Tb), where Pb, Vb, Tb and Zb are the bottom hole conditions at the point of gas entry and Ps, Vs, Ts and Zs as the respective surface conditions. 

    Vs / Vb = (35.37 x 11,473.169 x 8) / (1.55 x 760) 

    Expansion Ratio = Vs / Vb ≈ 344.5 : 1

    3) Surface gas production rate at 3.0 bbl/min circulation

    If 1 reservoir bbl of gas expands to 344.5 standard bbl, then with 3.0 bbl/min circulation: 

    Std bbl/min = 344.5×3.0≈1,033.5 Std bbl/min

    Convert to SCF/day (1 Std bbl ≈ 5.615 SCF; 1 day = 1,440 min) 

    SCF/day = 1,033.5×5.615×1 440 = 8.356 MMscf/day (0.236 MMscm/day)

    4) Time to exhaust the gas already at the choke

    If the expansion up to the choke is 5.8 : 1 (given for this scenario) and the well took an 8 bbl kick: 

    The expanded volume at choke = 5.8×8=46.4 bbl = 46.4 bbl 

    Time to drive that volume through the choke at 3.0 bbl/min = 46.4/3.0 ≈ 15.5 minutes Description text goes here

  • A well control operation is only as safe as the team’s understanding of the surface system limits and response strategy during gas expansion. Always plan, verify, and adjust before initiating the kill operation. 

    The analysis and operational insights for handling this scenario are summarized below: 

    1. Understanding the Practical Impact

    • Every well control team must know what information to ask for when planning safe and efficient well-killing operations. 

    • The real field case analyzed in Drillopedia’s Wellbore Wisdom Series #1 revealed a gas expansion ratio of approximately 344:1, resulting in an estimated surface gas flow of ~1,033 standard bbl/min when the kick is circulated out at a kill rate of 3 bbl/min. 

    • This shows how a small downhole gas influx can multiply into a very large surface gas flow, stressing the Mud-Gas Separator (MGS), vent lines and the flare system. 

    • In this example, calculations showed that circulating the kick at 3 bbl/min would produce roughly 8.36 MMscf/day of gas flow to the surface. 

      2. Key Practical Considerations

    • Know the MGS Capacity – What is the actual gas-handling capacity of the MGS on your rig? Even if the MGS can technically handle the maximum flow, it’s not advisable to operate at full capacity. 

    • Operational best practice is to limit surface gas flow to 70–80% of MGS capacity. 
      For example, if the MGS capacity is 8 MMscf/day, the safe operating limit should be 5.6–6.4 MMscf/day. 

    • In that case, the 3 bbl/min kill rate exceeds the safe range. Reducing the circulation rate to 2 bbl/min brings the surface flow down to about 5.6 MMscf/day, within the recommended limit. 

    • This highlights the importance of evaluating MGS and flare capacity before starting circulation. 

    • Without such planning, surface equipment could be overloaded, creating unsafe and potentially hazardous conditions. 

      3. Choke Management and Pressure Control

    • Maintain steady annular pressure using controlled choke adjustments. 

    • Apply small, incremental choke changes and observe the pressure response. 

    • Avoid large or rapid choke movements. 

    • Remember: there’s always a short response lag between choke movement and the visible pressure changes. Hence, allow time for stabilization before further adjustment. 

    • Automated choke systems provide smoother and more consistent control, particularly under high gas expansion conditions. 

      4. Monitoring and Safety

    • If available, use downhole telemetry or calculated BHP trends to confirm stable bottomhole pressure. 

    • Perform continuous pit gain monitoring, flow-in/flow-out checks, and gas detection in mud returns. 

    • Ensure surface gas handling is within safe limits, no overloading of the MGS and no wet gas carryover to flare Description text goes here

Shrikant Tiwari