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

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

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: 20-200 MMcf/d Full-pressure BU between rates * Simulation model adapted from the paper: IPTC 10692 Khuff Gas Condensate Development (Whitson & Kuntadi). Sampling Issues in Khuff-like Gas Condensate Reservoirs

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

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

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

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 30714 Modeling Gas Condensate Well Deliverability Sampling Issues in Khuff-like Gas Condensate Reservoirs

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

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

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

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.

Sampling Issues in Khuff-like Gas Condensate Reservoirs

Sampling Issues in Khuff-like Gas Condensate Reservoirs

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

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

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