Abstract

Two main development options were identified for a carbonate stringer reservoir being waterflood and Miscible Gas Injection (MGI). The waterflood project economics were more favourable; however potential showstoppers were identified. The key risks associated with waterflooding this reservoir are halite precipitation, anhydrite swelling, injectivity loss and the quality of produced water.

Faced with this challenge the study team started dedicated studies on rock integrity, water compatibility and scaling mitigation in order to assess and moderate these risks. Reactive Transport Modelling (RTM) work was initiated. The Reactive Transport Model is a reservoir simulator coupled with geochemical simulation. The objective of the model is to quantify reactions between rock and fluid, and forecast pH and salinity over time at the producer. This is dependent on data obtained from core flood experiments, and H2S and CO2 solubility data obtained from literature review and thermodynamic modelling. In order to calibrate the models the following studies and experiments were initiated:

  • Literature reviews (i.e analogues for waterflooding in sour fields, anhydrite swelling, and H2S (and CO2) solubility in brine);

  • Core flood experiments for temperature and salinity range and halite dissolution with the objective to get dissolution rates as a function of surface area, temperature and salinity;

  • In addition a field injectivity trial was executed to proof injectivity and calibrate the models

Significant work has been done on the subsurface risks associated with waterflooding. This integrated and dedicated study has contributed at large to the project’s key decisions. A key conclusion from the study was that anhydrite to gypsum conversion is unlikely to pose a risk to well integrity or injectivity hence there is no need to heat the injected water. This resulted in a saving of $160 mln on operating costs. For halite scaling, the formation water bank poses a higher risk than mineral (halite) dissolution in the reservoir. Hence jet pumping was selected as a lifting mechanism providing also flow assurance.

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