Abstract

Many operators have included observation wells in pilot testing patterns for thermal recovery methods. Information gained from these wells, in the form of temperature and pressure, has been used to attempt to conceptualize recovery mechanisms and calculate heat balances. Assumptions must be made to distinguish hot water from steam to qualitatively and quantitatively analyze observation well data. Often, completion methods used to obtain pressure information effectively mask some temperature data, and, as pressure and temperature data are required to ascertain the existence of steam, some information is Sacrificed to gain steam information.

The paper describes the principles and algorithms for distinguishing live steam zones from hot water zones at observation wells using temperature gradients and independent of pressure information. If sufficient data exists, the analysis can be represented in real time video display to yield qualitative analysis of steam zone conformity. Knowledge of the growth and shape of live steam zones can be useful in conceptual project design for well spacing, pattern orientation, aspect ratios and completion Intervals. Distinction of hot water zones from live steam zones can provide insight into changing recovery mechanisms over the life of a project.

Necessary conditions for confident analysis and discussion of the shortcomings of the procedure are outlined.

Introduction

Observation wells are often includes in in-situ steam pilot projects with hopes of gaining information useful for design of larger projects. Objectives usually used to justify drilling observatlon wells include:

  1. determine preferential flow directions

  2. Determine vertical flow patterns reservoir fluids

  3. observe changes of fluid flows over time

  4. gain understanding of the major recovery mechanism

  5. help calculate overall recovery factors

  6. help define well spacing for commercial operations

  7. observe test parameters for special tests

  8. Provide a back-up wellbore should a "working" well fail

Quite often the objectives used to justify drilling these wells are difficult to fulfill and this failure has prompted some operators to abandon the use of observat1on wells. Poor data quality, lack of diligence in data collection, and "drilled in the wrong location" are the most common complaints. Another, more significant reason, usually not quoted, is the lack of effective analysis techniques.

This paper address two problems; lack of analysis techniques and poor data quality.

Observation Wells

Typically, observation wells are equipped to measure temperatures across the zone of 1nterest, usually by means of fixed multi-point or traversing thermocouples. Temperatures measured at specific depths are recorded periodically and saved for future analysis. See figures l and 2. <Figure 1 Available In Full Paper>

If a well is completed to measure only temperatures, data collected by multi-point thermocouple Strings is considered to have high integrity. When using 3 traversing thermocouple, profiling procedures must allow enough time for temperature transients to dissipate. Again, this type of data is generally considered to leave high integrity.

In doing heat balance calculations, one must make major assumption about the existence of steam and its quality if temperature data alone is available.

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