Lecture 13 - Artificial Lift (GasLift) - PE3413 SP2013

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PE 3413 Subsurface Production Engineering Lecture 13: Artificial Lift Gas Lift

Maysam Pournik Assistant Professor Spring 2013

Continuous Vs. Intermittent Flow

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Gas Lift Decision Making

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Optimized Gas Lift GLR

Above or below optimized GLR, production rate is lower

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Optimum GLR

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Optimum Gas Injection Rate q g ,inj = GLRinj ql

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Generation of Gas Lift Curves

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Optimum GLR: Effect of Production Rate

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Optimum GLR of 4,000 SCF/STB for ql = 800 STB/d

Optimum GLR of 7,000 SCF/STB for ql = 500 STB/d

Optimum GLR: Effect of Tubing Determine required gas injection rate: H = 8,000 ft; ql = 2000 STB/d; ptf = 100 psi; pwf = 1610 psi; GLR = 300 SCF/STB ID = 2.5 inch

qg = 1,000,000 SCF/d GLRopt = 800

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ID = 3.5 inch

qg = 100,000 SCF/d GLRopt = 350

ID = 4.5 inch

qg = 0 GLRopt = 300

Optimum GLR: Effect of Injection Depth

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•  Deeper injection point, lower pwf can be achieved •  Limited on reduction in pwf

Gas Lift Design Process •  Gas injection rate and GLR •  Gas injection pressure •  Gas valves selection (injection locations)

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Gas Lift Performance •  Below injection point: reservoir fluid •  Above injection point: reservoir fluid and injected gas •  Operational point of gas injection node: §  IPR curve: IPR of reservoir minus pressure drop from bottom-hole to injection point §  VLP curve: VLP of tubing with total GLR being sum of formation GLR and injected GLR

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Gas Lift Injection Rate & GLR •  If unlimited gas, injection to yield optimum GLR in tubing and maximize production rate:

GLRinj = GLRopt − GLR fm qg ,inj = GLRinj ql The optimum GLR determined from VLP for multiphase flow for given tubing pressure and liquid flow rate •  If amount of gas is limited, estimate well potential:

GLR = GLR fm + 13

q g ,inj ql

Gas Lift Injection Pressure •  Based on mechanical energy balance, similar to pressure drop in wellbore, pressure of injected gas:

Pinj = Psurf e

0.01875γH inj ZT

H inj ⎞ ⎛ ⎟⎟ Pinj ≈ Psurf ⎜⎜1 + ⎝ 40,000 ⎠ •  Two zones of different pressure gradients above and below injection point:

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⎛ dp ⎞ ⎛ dp ⎞ Pwf = Ptf + H inj ⎜ ⎟ + (H − H inj )⎜ ⎟ ⎝ dz ⎠ above ⎝ dz ⎠below ⎛ dp ⎞ Pwf = pinj − Δpvalve + (H − H inj )⎜ ⎟ ⎝ dz ⎠below

Example: Gas Lift GLR, Rate, and Pressure Well is at the depth of 8,000 ft in a reservoir with GLR = 300 SCF/STB, qo = 400 STB/d, qw = 400 STB/d and pwf = 1500 psi (from IPR) and ptf = 100 psi: (a) Determine required GLR and gas injection rate with injection depth at 8,000 ft? (b) Would it be possible to inject at 4,000 ft at same liquid rates? (c) If pwf = 1,000 psi and pressure drop across gas lift valve is 100 psi, determine surface gas injection pressure for depth of 8,000 ft? (d) Determine depth of gas injection and gas injection rate for a production rate of 500 STB/d and pwf = 1770 psi with above (c) surface gas injection pressure?

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Example: Wellbore Gradient Curves

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Solution

⎛ dp ⎞ ⎛ dp ⎞ Pwf = Ptf + H inj ⎜ ⎟ + (H − H inj )⎜ ⎟ ⎝ dz ⎠ above ⎝ dz ⎠below ⎛ dp ⎞ 1500 = 100 + 4000⎜ ⎟ + (8000 − 4000)(0.3) ⎝ dz ⎠ above ⎛ dp ⎞ = 0.05 psi/ft ⎜ ⎟ dz ⎝ ⎠ above

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No GLR can reduce pressure drop to 0.05 psi/ft

Solution •  H inj ⎞ ⎛ 8,000 ⎞ ⎛ ⎟⎟; 1100 ≈ Psurf ⎜1 + Pinj ≈ Psurf ⎜⎜1 + ⎟ ⎝ 40,000 ⎠ ⎝ 40,000 ⎠

Psur = 915 psi

H inj ⎞ ⎛ ⎛ dp ⎞ ⎛ dp ⎞ ⎟⎟ Pwf = Ptf + H inj ⎜ ⎟ + (H − H inj )⎜ ⎟ & Pinj ≈ Psurf ⎜⎜1 + ⎝ dz ⎠ above ⎝ dz ⎠below ⎝ 40,000 ⎠ H inj ⎞ ⎛ ⎟⎟ − 100 = 1770 − 0.33(8000 − Hinj); Hinj = 5490 ft & pinj = 1040 psi 915⎜⎜1 + ⎝ 40,000 ⎠

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Gas Lift Unloading – Valve Locations

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