Title:

A Method of Analyzing Tracer Data to Calculate Swept Pore Volume and Thermal Breakthrough in Fractured Geothermal Reservoirs Under Two-Phase Flow Conditions

Authors:

Xingru Wu, Gary A. Pope, G. Michael Shook, and Sanjay Srinivasan

Key Words:

Tracer application, Two-phase flow, The first temporal moment method, Fractured Reservoirs

Conference:

Stanford Geothermal Workshop

Year:

2005

Session:

Tracers

Language:

English

Paper Number:

Wu

File Size:

207KB

View File:

Abstract:

One of the goals of using tracers in geothermal reservoirs is to predict the thermal breakthrough as soon as possible from the tracer data. Shook (2001) has shown how this goal can be accomplished for the case of single-phase flow. In this paper, we show how reasonable estimates of swept pore volume can be obtained in a very simple, practical way for the two-phase flow case under certain conditions.

In particular, we present a method to estimate the swept pore volume of fractures in a hot dry rock geothermal reservoir using partitioning tracers with a high volatility. The method uses the first temporal moment of the tracer concentration distribution recorded at the producer to calculate the pore volume contacted by the injected tracer (the swept pore volume). This method has a rigorous theoretical basis and has been widely used in both groundwater and oil field applications. It can be used without the need for detailed reservoir characterization data or flow and transport models since only a very simple, fast and easy integration of the production data is needed to yield the mean residence time. When a tracer with a high partition coefficient (concentration in the vapor divided by the concentration in the liquid) is injected in spent water, it will partition into the vapor phase and subsequently transport in the vapor phase (steam) toward the production wells.

Suitable tracers are available that will partition almost entirely into the vapor, so even though they are injected in the liquid water phase, they will transport almost entirely in the vapor phase. Since the flow in many geothermal reservoirs is dominated by flow in the fractures, we have tested the method for fractured reservoirs. A geothermal reservoir simulator is used to calculate synthetic tracer concentration data (no simulation is required if experimental data are used).

The produced tracer concentration data are then integrated to yield the mean residence time and swept pore volume and this pore volume is compared with the known pore volume of the fractures in the part of the reservoir connected by tracer injection and production wells. Accurate calculations of swept pore volume are possible for reservoirs with a high permeability contrast between the fracture and the matrix. Even when the porosity and permeability of the rock matrix are relatively high and a significant fraction of the tracer transports in the matrix, integrating only the early tracer response data, but not the long tracer tail that is produced over a much longer period of time, yields a useful estimate of the pore volume of the connected fractures.

Although a more complete investigation is needed before the practical limits of the method will be sufficiently understood, these preliminary results indicate that this remarkably simple and robust method of treating geothermal tracer data shows good potential for yielding useful early information to predict energy production from geothermal reservoirs.


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