Abstract

In steam based thermal recovery operations. during production, a knowledge of reservoir temperature and its trend is very important. The current temperature monitoring techniques involve either a temperature sensing element in the well, which is costly and difficult to maintain, or periodic temperature logs, which are expensive and also necessitate suspension and/or interruption of production.

Produced water chemistry was successfully used to monitor the reservoir temperature in several steam stimulation tests and pilot operations in Alberta and offers a very cost effective and reliable method after the initial 30 - 45 days of production.

This method is outlined and its validation by field data is demonstrated.

Introduction

In steam based thermal recovery operations, during production, a knowledge of reservoir temperature and its trend is very important. Because, not only it shows if the injected steam slug has been effective, but also it helps to determine if the production is suffering from lack of mobility due to decreasing temperatures or from some other factors. As such, in the cases where the steam is not confined to the pay zone it may confirm a need for remedial measures; or in huff and puff operations it may be the deciding factor for "turnaround" decisions. The current temperature monitoring techniques involve either the installation of some temperature sensing element in the well, which is costly and difficult to maintain, or periodic temperature logs, which are expensive and also necessitate suspension and/or interruption of production. Produced water chemistry (concentration ratio of two easily determinable ions) was successfully used to monitor the reservoir temperature in several steam stimulation tests and pilot operations and offers a very cost effective and reliable method for use after the initial 30 - 45 days of production.

This idea of using produced water chemistry for determining reservoir temperature in steam based thermal recovery operations came from the practice in Geothermal Reservoir Engineering.

In geothermal reservoir engineering work, geothermometers, based on the concentrations of various ions or silica in the produced hot water, have been in use for about two decades now for determining reservoir temperature(1,2,3,4).

These geothermometers invariably depend on the chemical equilibrium resulting from rock/water interaction at the reservoir temperature. As such, they require a certain residence time of water for the equilibrium to take place. Since, during production the actual travel time through the well is a lot shorter than the residence time during which the equilibrium takes place, the produced water chemistry still carries the equilibrium relationship which directly reflects the temperature of equilibration.

In geothermal reservoir engineering, the three most tested and commonly used geothermometers are:

  • Na/K

  • Na-K-Ca

  • SiO2

The SiO2 geothermometer is based on the solubility of quartz at different temperatures. Although, it is quite reliable, sampling and analysis of SiO2 require certain precautions, and, in comparison to the other two, this geothermometer appears less desirable for monitoring just the reservoir temperature in heavy oil thermal recovery applications.

Both Na/K and Na-K-Ca geothermometers were initially based on empirical correlations, although some theoretical background recently has been accounted for SiO(2.3,4,5).

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