Abstract

Reactive flow in a permeable medium is a combination of reactions between the fluids and minerals in the host formation and reactions within the same fluid. Among these reactions, solid dissolution and precipitation are important reactions since precipitated reactions products, in particular, can plug flow channels and cause flow impairment. The subject of this work is the simulation of these aqueous phase reactions during flow.

Simulation of these geochemical reactions is based on solubility calculations, which depend on pressure and temperature among other parameters. Pressure dependency becomes important where large pressure changes occur as in near wellbore flow. Similarly, injecting a fluid that is at a different temperature from the reservoir (as in hot water injection) can cause geochemical reactions even if the injected fluid and resident fluid have the same composition. Modeling these geochemical reactions can get even more complicated during production after hot water injection (push-pull cycles) since the inlet fluid composition may change in each cycle. Neglecting these effects can lead to an unrealistic estimate of the predicted precipitates, since solubility depends on pressure and temperature.

This work presents a simplified approach, based on experimental literature data as well as thermodynamic properties to account for solubility changes as a function of pressure and temperature in geochemical modeling. Reactive flow is based on nonequilibrium aqueous-solid reactions. The approach is novel because it allows for the effects of pressure and temperature changes on solubility calculations during the production and injection period as well as the change in the inlet fluid composition for push-pull cycles.

We illustrate the model results on two cases, one production and the other an injection-production combination:

  1. We model flow from a producer in West Texas carbonate reservoir in which there may be substantial well impairment due to gypsum precipitation. Pressure changes have more effect on mineral precipitation in producer than injectors, since most of the sulfates tend to become less soluble as pressure decreases, however, water must flow to cause significant precipitation.

  2. We also study the effect on the productivity of a well after hot water has been injected and the well returned to production. We use data from a sandstone reservoir that initially contained kaolinite. K-feldspar and quartz are possible secondary minerals. Results show that during the production cycle kaolinite re-precipitation is likely to occur; quartz and K-feldspar precipitate during the injection cycle.

The damage during production is more severe that during injection.

Introduction

Modeling rock-fluid reactions with transport (transport is only by fluid convection here) has been widely studied in the last 25 years. Among the variables that might affect the modeling are the solubility products and equilibrium constants. Both depend on pressure and temperature. Tabulated experimental data are always referred to 1 atm and 25°C, which are very different from common operational conditions. As a consequence, these "constants" should be corrected for pressure and temperature while performing these simulations.

Previous works1,2,3 have developed expressions based on experimental data to correct solubility products for pressure or temperature at constant temperature or constant pressure, respectively, for specific minerals. For most cases experimental data is not available at a wide range of pressures and temperatures to develop these expressions; therefore, there is a need for a general expression to perform temperature and pressure corrections.

The purpose of this work is to simulate reactions involving mineral dissolution and precipitation under non-isothermal, non-isobaric and non-equilibrium conditions as part of the reactive flow occurring through a permeable medium. This paper describes a simplified approach to account for the effects of pressure and temperature changes on solubility calculations as well as the change in the injected fluid composition during injection-production cycles.

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