SP-1 · PVT & Phase Classification: GK-22

Sub-Problem 1 · The Question

What Type of Fluid Is GK-22: and Does That Change the Flow Model?

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Engineering Trigger

A calculation error at this step propagates through all five sub-problems

A colleague has run a first-pass AOF estimate for GK-22 using the Vogel (1968) two-phase IPR equation, citing that "it works for both oil and gas wells." Before accepting this result or building on it, your task is to determine whether the Vogel model is physically appropriate for GK-22 at its initial reservoir conditions. This requires you to:

Classify the GK-22 reservoir fluid type using the five-fluid-type scheme from Topic 1.1.
Locate the initial reservoir state (p̅, T_res) on the p-T phase envelope relative to the dew-point locus and the cricondentherm.
Determine whether the reservoir is in single-phase gas or two-phase gas+condensate at p̅ = 7,350 psia.
Identify the specific pressure threshold at which retrograde condensate will begin to accumulate in the reservoir during production, and explain what that means for long-term deliverability.
Select and justify the correct inflow model, and explicitly state why the Vogel equation is or is not appropriate here.

Your conclusion from SP-1 determines which deliverability equation you use in SP-5. Getting it wrong here does not simply introduce an error, it invalidates the entire calculation chain.

Stop and Think First

Before scrolling to the background section: write down your initial hypothesis about the fluid type and flow model. What does the combination of p̅ = 7,350 psia, p_d = 6,200 psia, and T_cric = 320°F tell you immediately, before any calculation? Commit this to your KWL W column.

SP-1 Data Slice

Data Relevant to This Sub-Problem

The rows highlighted below are those you need for SP-1. All other data will be used in later sub-problems. Resist the temptation to read ahead into the non-highlighted rows. Part of the PBL discipline is working with the subset the problem gives you at each step.

Parameter	Symbol	Value	Units	Relevance to SP-1
Initial average reservoir pressure	p̅	7,350	psia	Compare to p_d to determine phase state
Reservoir temperature	T_res	285	°F	Compare to T_cric to determine retrograde possibility
Initial dew-point pressure	p_d	6,200	psia	The phase boundary, crossing this triggers retrograde
Cricondentherm	T_cric	320	°F	Maximum temperature of the two-phase envelope
Condensate yield (CVD)	R_v	112	STB/MMscf	Primary fluid classification diagnostic
Condensate API gravity	°API_c	52	°API	Light condensate: confirms gas-condensate system
Gas specific gravity	γ_g	0.65	—	Used in SP-2 (not needed for phase classification)

Background Knowledge

Key Concepts for SP-1: Phase Classification & Inflow Model Selection

This section provides the minimum background needed to complete SP-1. For full treatment, consult Topic 1.1, PVT Behaviour & Phase Envelopes, Sections 2–5.

The Five Reservoir Fluid Types

Fluid Type	GOR (scf/STB)	API°	R_v (STB/MMscf)	Phase envelope characteristic	Correct flow model
Black oil	< 2,000	< 40	—	Bubble point; reservoir T well below cricond.	Vogel IPR or Darcy with B_o
Volatile oil	2,000–3,300	40–50	—	Bubble point; T near cricond.	Compositional or modified Vogel
Rich gas-condensate	3,300–100,000+	50–70	50–300+	Dew point; T_res between cricondenbar and cricondentherm	Single-phase gas deliverability (while p > p_d)
Wet gas	> 50,000	> 55	10–50	Dew point; T_res > T_cric	Single-phase gas deliverability
Dry gas	—	—	< 10	No two-phase region at any surface conditions	Single-phase gas deliverability

Phase Envelope Interpretation: Where Is GK-22?

A p-T phase envelope for a gas-condensate has a dew-point locus on its right flank and a cricondentherm, the maximum temperature of the two-phase region at the far right. The reservoir state point (p̅, T_res) can fall in three zones:

Zone	Condition	Phase state	Flow model
Above dew-point locus, T < T_cric	p > p_d(T) and T < T_cric	Single-phase gas: all components in vapour phase	Single-phase gas deliverability; Darcy linear IPR in p²
Inside two-phase envelope	p < p_d(T) and T < T_cric	Retrograde: gas + condensate both present in reservoir	Two-phase (compositional or simplified with k_rg(p) tables)
Above cricondentherm	T > T_cric	Gas at all pressures: no two-phase region possible	Single-phase gas deliverability

Why Vogel Is Wrong for GK-22 Above p_d

The Vogel (1968) equation was derived from reservoir simulation of solution-gas-drive oil reservoirs where free gas evolves from liquid oil below the bubble point. It has the form q/q_max = 1 − 0.2(p_wf/p̅) − 0.8(p_wf/p̅)². This parabolic relationship arises from the specific two-phase (oil+gas) flow dynamics of a solution-gas-drive system. It is NOT valid for:

Single-phase gas above the dew-point: where the IPR is linear in (p̅² − p_wf²) and the Vogel curvature does not exist
Gas-condensate systems in general: which require either the real gas pseudo-pressure m(p) formulation or a compositional simulator when below dew-point

For GK-22 at initial conditions (p̅ = 7,350 > p_d = 6,200 psia): the correct IPR is the pseudo-steady-state real gas deliverability equation:

Correct Flow Model: Single-Phase Gas Deliverability (PSS) q_g [Mscfd] = k_eff · h · (p̅² − p_wf²) ────────────────────────────────────── 1,424 · μ_g · Z · T · G_total Where G_total = ln(r_e/r_w) − ¾ + S + D·q_g This is valid while p̅ > p_d = 6,200 psia (single-phase gas in reservoir). Below p_d, retrograde condensate accumulates and a modified model is needed.

The pressure-squared form (p² − p_wf²) is an approximation valid when μ_gZ is roughly constant over the pressure range. For high-pressure gas like GK-22, the real gas pseudo-pressure m(p) formulation is more rigorous but requires a computed m(p) table. The p² form is used here as a first-order field estimate, adequate for FEED-gate scoping.

Retrograde Condensate Banking: The Long-Term Risk

As GK-22 is produced and p̅ declines from 7,350 psia toward the dew-point of 6,200 psia, the reservoir remains in single-phase gas and the above equation applies. Once p̅ crosses 6,200 psia, liquid condensate begins to condense from the gas phase in the reservoir, retrograde behaviour. This condensate:

Is initially immobile (saturation below critical condensate saturation S_cc ≈ 0.10–0.15)
Accumulates preferentially near the wellbore where pressure drawdown is greatest
Severely impairs k_rg once S_c exceeds S_cc, field cases show k_rg dropping from 0.90 to 0.25–0.40
This condensate loss represents permanent loss of valuable C5+ liquids from the reservoir

SP-3 quantifies the k_rg impairment; SP-5 flags this risk in the recommendation memo. Your SP-1 task is simply to identify the threshold pressure at which this risk activates: p_d = 6,200 psia.

Step-by-Step Tasks

SP-1 Tasks: Work Through in Order

Each task should take 3–6 minutes. Show your reasoning, not just a final answer. Tasks build on each other. Do not skip ahead.

Classify the fluid type using R_v and API°
Using the five-fluid-type table in the Background section: GK-22 has condensate yield R_v = 112 STB/MMscf and condensate gravity 52°API. State clearly which of the five fluid types applies. Write a one-sentence justification from the R_v value alone, then a second sentence referencing the temperature criteria (T_res vs. T_cric). Do the two criteria agree?
Sketch the p-T phase envelope and locate the GK-22 reservoir state
On a rough hand-drawn sketch (or annotate the schematic in Topic 1.1), plot the following points: (a) dew-point at reservoir temperature, (6,200 psia, 285°F); (b) cricondentherm vertex, (~5,500 psia estimated, 320°F); (c) initial reservoir state, (7,350 psia, 285°F). Draw the dew-point locus connecting these. Shade the two-phase region. Label which zone the initial reservoir state occupies. Add an arrow showing the direction of depletion (decreasing p at constant T_res = 285°F) and mark the pressure at which it crosses into the two-phase region.
Confirm phase state at p̅ = 7,350 psia and T = 285°F
Calculate the margin above the dew-point: Δp = p̅ − p_d = 7,350 − 6,200 = _____ psia. Express this as a percentage of p_d. State clearly: is the reservoir in (a) single-phase gas, (b) two-phase gas+condensate, or (c) single-phase liquid at initial conditions? Explain in one sentence why T_res = 285°F < T_cric = 320°F is relevant to the answer.
Identify the retrograde banking threshold pressure
State the exact pressure at which retrograde condensate will begin to accumulate in the GK-22 reservoir during production. Explain why this happens specifically at this pressure and not at a lower pressure. Then estimate — very roughly, using the margin calculated in Task 3 and assuming the reservoir pressure declines at ~150 psia/month — how many months of production at DST rate before the banking risk activates. (This is a scoping estimate only: the real answer depends on IGIP, which you will estimate in SP-5.)
Select and justify the correct inflow model, and correct your colleague's error
Write a three-sentence technical justification for your chosen flow model: (1) State the model name and equation form. (2) Explain why it is physically appropriate for GK-22 at initial conditions. (3) Explain explicitly why the Vogel equation is not appropriate. Your answer to this task is your SP-1 carry-forward: it determines which equation you assemble in SP-5.
Think-Pair-Share: The Retrograde Problem
Discuss: if GK-22 is produced at high rate without pressure maintenance, the reservoir will cross the dew-point in a few years. What are the options? List at least three engineering responses to the retrograde banking problem (e.g., cycling, rate management, stimulation). For each, state in one sentence whether it is feasible given GK-22's subsea, deepwater context. Your answer to this task feeds directly into the recommendation section of SP-5.

Guided Calculations

SP-1 Worked Answers

Attempt all tasks before reading the guided answers. Use this section to check your work or diagnose where your reasoning diverged.

Task 1: Fluid Classification

Primary diagnostic — R_v: R_v = 112 STB/MMscf falls in the range 50–300+ STB/MMscf that defines a rich gas-condensate. A dry gas has R_v < 10, a lean wet gas has R_v = 10–50, and a near-critical condensate has R_v > 300. GK-22 at 112 is a rich gas-condensate, moderate yield, significant banking risk below dew-point.

Temperature criterion: T_res = 285°F < T_cric = 320°F. This confirms the reservoir temperature is inside the temperature range where a two-phase envelope exists: retrograde condensation is physically possible during depletion. If T_res were above 320°F, the system would be a wet gas (always single-phase gas at all pressures below some surface dew-point, but no retrograde in the reservoir).

Conclusion: Rich gas-condensate. Both criteria agree.

Task 3: Phase State Verification

Phase State at p̅ = 7,350 psia Dew-point pressure at T = 285°F: p_d = 6,200 psia Initial reservoir pressure: p̅ = 7,350 psia Margin above dew-point: Δp = p̅ − p_d = 7,350 − 6,200 = 1,150 psia Δp/p_d = 1,150 / 6,200 = 0.185 → 18.5% above dew-point Phase state: SINGLE-PHASE GAS ✓ p̅ > p_d → all hydrocarbon components exist in a single gas phase No free condensate in the reservoir at initial conditions T_res < T_cric (285 < 320°F) → retrograde condensation IS possible during production once p̅ falls below p_d = 6,200 psia

Task 4: Banking Threshold

The retrograde banking threshold is exactly p_d = 6,200 psia, the dew-point pressure at reservoir temperature. This is not a lower pressure because the dew-point is the thermodynamic boundary of the two-phase region at 285°F. As soon as reservoir pressure (anywhere in the drainage area) crosses 6,200 psia, condensate begins to nucleate from the gas phase.

Rough timing estimate: Margin = 1,150 psia. Decline rate ≈ 150 psia/month (rough scoping). Time to banking = 1,150 / 150 ≈ 7–8 months at maximum rate. This is very early in field life: pressure maintenance above p_d is worth evaluating.

Task 5: Flow Model Justification

Correct model: Pseudo-steady-state real gas deliverability equation: q_g = k_eff·h·(p̅² − p_wf²) / [1,424·μ_g·Z·T·G_total]. This is physically appropriate because at p̅ = 7,350 > p_d = 6,200 psia, the reservoir is in single-phase gas, only gas flows, and the linear-in-p² Darcy relationship holds. The Vogel equation is not appropriate because it was derived from simulation of solution-gas-drive oil reservoirs where gas evolves from a liquid oil phase below bubble point: the physical mechanism (oil degassing) does not exist in this single-phase gas reservoir.

KWL Reflection

Update Your KWL Table: SP-1

Return to your Hub KWL table. Fill in the L-column entry for SP-1 now, before proceeding to SP-2. Use the prompts below.

K: Confirmed

Gas-condensates have dew-point pressures
Phase state depends on (p̅, T_res) position relative to the phase envelope
T_res < T_cric → retrograde banking is possible during depletion

W: Questions Answered

GK-22 is a rich gas-condensate (R_v = 112 STB/MMscf)
p̅ = 7,350 > p_d = 6,200 → single-phase gas initially
Banking threshold = 6,200 psia
Vogel is wrong: single-phase gas deliverability equation applies

L: New Learning

Write your own L-column entry here
What surprised you? What did you think before that was wrong?
Example: "I assumed Z = 1 was a safe approximation for gas — SP-2 will test this"

Just-in-Time Resources

Reach for these the moment you get stuck on this sub-problem. They map one-to-one onto the physics and the calculation you are doing here.

Study

Module 01 · Topic 1: PVT Behaviour & Fluid Classification

Watch

Lecture 1.1 — Expanding the Phase Envelope: From Gibbs to Reservoir Fluid Dynamics
Lecture 1.1B: Fluid Density, Viscosity & Practical PVT Data Sources
Lecture 1.1C: Reading and Interpreting a PVT Report

Self-check

pvt_correlations.py, verified calculator; reproduce your numbers.

Knowledge Check

SP-1: Five MCQ

Answer each question independently. Select your answer, you get one attempt. Correct answers reveal an explanation.

Q1: Fluid Classification

GK-22 has condensate yield R_v = 112 STB/MMscf and reservoir temperature 285°F, below the cricondentherm of 320°F. Which fluid type is correct?

A. Black oil: the high pressure (7,350 psia) compresses all components into a liquid phase

B. Wet gas: R_v = 112 is too high to be a wet gas but the temperature is below the cricondentherm, making this impossible to classify without EOS

C. Rich gas-condensate: R_v = 112 STB/MMscf sits in the 50–300 range that defines rich condensates; T_res < T_cric confirms retrograde condensation is possible during depletion

D. Dry gas: gas specific gravity 0.65 indicates a lean system with negligible condensate

Explanation

R_v = 112 STB/MMscf uniquely places GK-22 in the rich gas-condensate category (50–300+ STB/MMscf). Gas gravity alone says nothing about condensate yield. Black oil has a bubble point; GK-22 has a dew point: these are fundamentally different phase behaviours. T_res < T_cric confirms that as p declines during production, the reservoir will enter the two-phase region, the definition of retrograde behaviour.

Q2: Phase State at Initial Conditions

At p̅ = 7,350 psia and T = 285°F, what is the phase state of the GK-22 reservoir fluid?

A. Two-phase gas+condensate: some retrograde condensate is always present in a rich gas-condensate reservoir

B. Single-phase gas: p̅ = 7,350 psia is 1,150 psia above the dew-point of 6,200 psia at this temperature, placing the state point outside the two-phase envelope

C. Single-phase liquid: high pressure of 7,350 psia forces all components into a compressed liquid state

D. Cannot be determined without a full compositional EOS simulation

Explanation

The dew-point at 285°F is 6,200 psia. The initial reservoir pressure is 7,350 psia, 1,150 psia above the dew-point. Therefore the state point (7,350 psia, 285°F) lies above the dew-point locus on the p-T phase diagram (outside the two-phase envelope) in the single-phase gas region. No retrograde condensate exists in the reservoir at initial conditions. This does not require EOS simulation: it follows directly from comparing p̅ to p_d measured by CCE laboratory experiment.

Q3: Retrograde Banking Threshold

During production, at what reservoir pressure will retrograde condensate begin to accumulate in the GK-22 reservoir?

A. 7,350 psia: condensate banking begins immediately at first production as drawdown creates local pressure below dew-point

B. 3,500 psia, the minimum flowing wellbore pressure, because condensate forms at surface conditions

C. 6,200 psia, the dew-point pressure at reservoir temperature 285°F; this is the thermodynamic boundary below which two-phase conditions exist in the reservoir

D. Banking only occurs at surface separator conditions, not in the reservoir itself

Explanation

The dew-point is the phase boundary: it is precisely p_d = 6,200 psia at 285°F. When reservoir pressure (average or local) falls below this value, liquid condensate nucleates and accumulates. Option A is partially true near the wellbore at high drawdown, but the average reservoir banking threshold is p_d. Option B confuses minimum flowing BHP with the dew-point. Option D is incorrect: retrograde condensation is a reservoir phenomenon unique to gas-condensate systems.

Q4: Why Vogel Is Wrong

A colleague uses the Vogel IPR equation to estimate AOF for GK-22 at initial conditions. This is wrong because:

A. Vogel requires knowing B_o, which is not available for a gas well

B. Vogel was derived for solution-gas-drive oil reservoirs below bubble point; GK-22 is a single-phase gas reservoir above the dew-point where the IPR is linear in p², not the parabolic Vogel relationship

C. Vogel is only valid for shallow reservoirs below 3,000 psia, not for HPHT systems like GK-22

D. Vogel can be used with a correction factor of (1 − 0.2 × R_v/1000) applied to the result

Explanation

Vogel's empirical equation describes the curved (parabolic) IPR of a solution-gas-drive reservoir where oil and gas flow simultaneously below the bubble point. The physical mechanism (gas evolving from liquid oil) does not exist in GK-22 at initial conditions. For single-phase gas above the dew-point, Darcy's law gives a linear relationship between q_g and (p̅² − p_wf²). The Vogel curvature is physically absent. There is no correction factor that makes Vogel valid here: the model is simply inapplicable.

Q5: Inflow Model Below Dew-Point

Once average reservoir pressure p̅ drops below the dew-point of 6,200 psia, the single-phase gas deliverability equation becomes:

A. Still valid: the equation uses average reservoir p and k_eff, which automatically account for two-phase conditions

B. Invalid: and should be replaced with the Vogel equation as the reservoir transitions to two-phase flow

C. An approximation only: a more rigorous treatment requires a pressure-dependent k_rg(p) to account for condensate accumulation, or a compositional simulator; the single-phase equation still gives AOF in the gas phase but understates the deliverability penalty from condensate banking

D. Invalid: and cannot be used at any pressure below the original dew-point because the gas phase no longer exists

Explanation

Below the dew-point, gas still flows — there is still a gas phase — but its relative permeability k_rg is now a function of condensate saturation, which in turn depends on pressure. The single-phase gas deliverability equation can be adapted by using k_rg(p) from a CVD experiment or SCAL data at each pressure step, making it a pressure-dependent calculation. A full compositional simulator handles this most rigorously. Vogel is still wrong: it models oil-gas two-phase flow from a liquid-dominated system, not gas-condensate two-phase flow from a gas-dominated system.

SP-1 Complete

Carry-Forward to SP-5

SP-1 Carry-Forward Values: Record These Now

CF-1a — Fluid classification: Rich gas-condensate (R_v = 112 STB/MMscf; T_res < T_cric)
CF-1b — Phase state at p̅: Single-phase gas (p̅ = 7,350 > p_d = 6,200 psia; Δp = +1,150 psia)
CF-1c — Correct flow model: PSS real gas deliverability: q_g = k_eff·h·(p̅² − p_wf²) / [1,424·μ_g·Z·T·G_total]
CF-1d — Banking threshold: p_d = 6,200 psia — model remains valid while p̅ > 6,200 psia
CF-1e — Vogel assessment: Incorrect for GK-22, physically inappropriate model; single-phase gas deliverability equation replaces it