Multi-Species Mixing Models Used to Determine the Amount of Awibengkok Recharge Liquids

Abstract

While the initial fluid produced from a liquid-dominated geothermal reservoir is usually pristine reservoir brine, later production often includes a mixture of reservoir and other fluids (e.g., marginal recharge, injected brine and steam condensate). Instead of utilizing conventional mixing models using a single chemical species (i.e., Cl-) to characterize reservoir mixing processes, a seven-element mixing model was developed and applied to assess the changing composition of the Salak production liquids. The results of these mixing models help constrain the Awibengkok numerical simulations used for resource management. The measured concentrations of seven chemical species (i.e., Cl, B, SO4, HCO3, NH4, Ca and Mg) in the produced fluids of each liquid-producing well can be matched by using up to four end-member fluids in each mixing model. The mixing components (end-members and the liquid composition to be matched) are constrained by the historical produced brine and measured injected brine/condensate compositions and, where available, analyses of downhole samples. As with any mixing calculation, the mixing models rely heavily on the strength of underlying assumptions. For instance, the liquid chemical flash-corrections are dependent on estimated reservoir liquid enthalpies.

These mixing models yield approximate matches of the historical data with varying degrees of accuracy. The semiquantitative nature of this technique still represents a geochemical interpretation. The matching of produced liquids through 2012 is encouraging and has led to new insights into the flow of fluids in the Awibengkok reservoir. For example, changing ratios of injected brine through time is observed in numerous producers, and several marginal recharge fluids of different origins are required to match the production chemistry. Several eastern Awibengkok wells also have enrichments in some species that require a rapid boiling process as an explanation.