Waterflood Geochemical Modeling and a Prudhoe Bay Zone 4 Case Study
- Ying-Hsiao Li (Arco E&P Research) | Steven D. Crane (Arco Global Energy Ventures) | E. Mark Scott (Arco E&P Technology) | John C. Braden (Arco Alaska Inc.) | W. Greg McLelland (Arco Alaska Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 1997
- Document Type
- Journal Paper
- 58 - 69
- 1997. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.3.2 Multiphase Flow, 5.2 Reservoir Fluid Dynamics, 4.3.4 Scale, 5.1.1 Exploration, Development, Structural Geology, 5.4.1 Waterflooding, 1.8 Formation Damage
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Scale formation during waterfloods can damage reservoirs far beyond the wellbore region. A comprehensive analysis with geochemical modeling can improve waterflood design in the selection and/or mixing of source waters and thus, mitigate formation damage arising from injecting incompatible fluids. The method also predict the types of scales and their severity at various production stages. This will help optimize treatment schedules and reduce operation costs.
In the case study of Zone 4, Prudhoe Bay Unit (North Slope, Alaska), the geochemical model was validated with the laboratory analyses of Zone 4 produced water samples. Then it was used to evaluate the impact of mixing formation brine and seawater on rock-fluid interactions and scale formation. The prediction is consistent with the observation of calcite formation in early production at Prudhoe Bay. The model also predicts the precipitation of iron carbonate and iron sulfide scales as the seawater flood continues.
In a waterflood, the fluid injected into the reservoir may be incompatible with formation rock and brine thus altering the reservoir geochemistry. The chemistry of the formation rock/fluid interactions is complex and involves numerous chemical species and reaction paths. Transport of reactive species through permeable media further complicates the situation. The interactions cannot be easily evaluated by simple stoichiometric relationships. A geochemical flow model is an ideal tool for watertflood geochemical analyses and for predicting scale formation from dissolution/precipitation processes because it can model multiple reactions simultaneously.
The source water for Prudhoe Bay waterflood is seawater. Because the reservoir gas phase contains up to 13 percent carbon dioxide, concerns have been raised about the reservoir geochemical changes induced by the reaction of seawater, CO2 in the brine and the host reservoir rock. In Prudhoe Bay, there is a continuous change of the total dissolved salts and acidity in the aqueous phase along the injection path, because of the changes of reservoir temperature and pressure and/or dissolved CO2. This can cause scale to form and impair production. The solubility of carbonates decreases as temperature increases. By injecting 80 deg F seawater into the 195 deg F reservoir, the siderite dissolved nw the injector wellbore could re-precipitate as seawater is heated during migration through the reservoir1,2.
A complete model for this study should address the following features: mass transfer, heat transfer, chemical equilibria, kinetics of rate-limited reactions, and the physics of multi-phase flow through porous media. Due to the complexity of the physics involved, the chemical reaction module was built first for field data interpretation. The success of the field data interpretation warrants further enhancement of the model with heat transfer and fluid flow in a pseudo 3-D framework for field simulation.
Prudhoe Bay Waterflood Geochemical Model
The model is a one-dimensional, aqueous-phase, reactive flow model simulating reactions in porous media. The model includes adsorption/desorption phenomena in ion exchange and equilibrium-based mineral precipitation/dissolution. The option of the various cross sections perpendicular to the flow direction approximates a pseudo three-dimensional model. Nonuniform grid spacing is used to simulate water movement from an injector to a producer. A detailed chemical description can be obtained near the wellbore or at any location between injector and producer at any time in the waterflood system's life.
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