1. SAMPLING ISSUES in KHUFF-LIKE GAS CONDENSATE RESERVOIRS
Sissel Øksnevad Martinsen
Curtis H. Whitson
March 15, 2007

2. Summary of Ongoing Work
Study the variation of condensate-gas ratio (CGR) during short-term sampling tests.
Rate variation, CGR = f(qg)
For a range of well kh products
Relative permeability effect
Layering effect
Rate sequence (Increasing/Decreasing)
Sampling Issues in Khuff-like Gas Condensate Reservoirs

3. Sampling Issues in Khuff-like Gas Condensate Reservoirs
Base Case*
One geologic unit, uniform properties
Reservoir Fluid: K2 & K3, Compositional simulation
h = 1000 ft
k = 5 md
Nr = 200, Nz = 1 (Single well, r-z model)
pi = pd
qg range: MMcf/d
Full-pressure BU between rates
* Simulation model adapted from the paper: IPTC Khuff Gas Condensate Development (Whitson & Kuntadi).
Sampling Issues in Khuff-like Gas Condensate Reservoirs

4. Sampling Issues in Khuff-like Gas Condensate Reservoirs
Sensitivities
Relative Permeability
Layering
A thin high permeable zone (HKLH) in the middle of a low permeable reservoir.
Multi-rate test sequence
Additional sensitivities:
Degree of undersaturation (pi > pd)
Geologic layers with different reservoir fluids
Degree of depletion
Sampling Issues in Khuff-like Gas Condensate Reservoirs

5. Sampling Issues in Khuff-like Gas Condensate Reservoirs
Producing CGR is decreasing
linearly with increasing gas rate.
slope
The slope is approximately inversely
proportional to the kh product
Sampling Issues in Khuff-like Gas Condensate Reservoirs

6. Understanding CGR – Rate Variation
Based on the Fevang-Whitson 3-Region model, our hypothesis is that CGR-rate variation is mainly due to the change in p* with rate.
p* is defined as the dewpoint of the wellstream mixture and equals the pressure at the boundary of Regions 1 and 2.
p* is strongly coupled with the FBHP, pwf.
Sampling Issues in Khuff-like Gas Condensate Reservoirs

7. Sampling Issues in Khuff-like Gas Condensate Reservoirs
Condensate Blockage
Reservoir Pressure
Distance
Dewpoint Pressure
Wellbore
Pressure 2 3 Pd
Pwf P* r* rd 1
Adapted from Fevang & Whitson paper:
SPE Modeling Gas Condensate Well Deliverability
Sampling Issues in Khuff-like Gas Condensate Reservoirs

8. Sampling Issues in Khuff-like Gas Condensate Reservoirs
p*, r*
BHFP
pwf
p*, r*
BHFP
pwf
Sampling Issues in Khuff-like Gas Condensate Reservoirs

9. Sampling Issues in Khuff-like Gas Condensate Reservoirs
CGR Stabilization
qg = 100 MMcf/d
The CGR stabilizes more rapidly in the simulations with:
Low kh product
High production rate
Sampling Issues in Khuff-like Gas Condensate Reservoirs

10. Relative Permeability
The saturation exponents, ng & nog , are varied. Endpoint gas relative permeability is equal at initial conditions.
The higher value of n, the higher drawdown and saturation buildup
Producing CGR’s are lower for higher n at equal rates.
Controlling
Region 2
Controlling
Region 1

11. Flow-After-Flow Test
Multi-rate testing with increasing rates are recommended.
Time needed to stable CGR is shorter.
If stable CGR is reached, the CGR vs. rate trend is equal to the full-pressure BU between rates.

12. Sampling Issues in Khuff-like Gas Condensate Reservoirs

13. Sampling Issues in Khuff-like Gas Condensate Reservoirs

14. Sampling Issues in Khuff-like Gas Condensate Reservoirs
Data from paper by Sage & Olds:
Volumetric Behavior of Oil and Gas from Several San Joaquin Valley Fields
Sampling Issues in Khuff-like Gas Condensate Reservoirs

15. Sampling Issues in Khuff-like Gas Condensate Reservoirs
qg= 180 MMcf
qg= 180 MMcf
qg= 120 MMcf
qg= 120 MMcf
qg= 60 MMcf
qg= 60 MMcf
qg= 20 MMcf
qg= 20 MMcf
Sampling Issues in Khuff-like Gas Condensate Reservoirs

16. Sampling Issues in Khuff-like Gas Condensate Reservoirs
*Figure from Fevang & Whitson paper:
SPE28829 Accurate Insitu Compositions in Petroleum Reservoirs
Sampling Issues in Khuff-like Gas Condensate Reservoirs