Use of Monitor Observation Wells in the Monitoring and Evaluation of Oil Recovery Projects
Authors: Widmyer R.H. 
Published: December 2007

SPE Member

Monitor or observation wells have been used for a considerable time to provide early and/or discrete information about fluid movement within a formation. Most such wells have been used in conjunction with experimental enhanced oil recovery pilot projects. The inherently expensive and complex nature of such projects behooves the operator to obtain the necessary amount and type of information needed to evaluate and justify expansion of the recovery method.

Monitor wells can provide specific quantitative data not usually obtainable from conventional injection or production wells and may allow midstream corrections to the recovery process. However, the cost and quality of such data must be weighed since monitor wells, per se, are not vital to the mechanics of the recovery process.

Monitor wells have been used to obtain a variety of information in many different applications. However, they generally fall into one of two categories:when the wellbore is in communication with the formation for the purpose of obtaining fluid samples, measuring pressure, etc., andwhen the wellbore is isolated from the formation by electrically conductive or non-conductive casing, or by cement alone, for the purpose of traversing the zone of interest with conventional logs or making measurements at fixed depth locations.

This paper describes various types of monitor wells that have been used and their selection for specific applications. Also discussed are well completion procedures including dual or multiple completions, the selection of well locations within the area of interest, and the frequency of sampling, logging or data collection. Numerous references are included.

Introduction

The first significant use of monitor wells probably coincides with the first appreciable field testing of enhanced oil recovery methods. Such recovery methods usually have a substantial time period between the initial injection of the recovery agent and a performance response at a producing well. The monitor well can provide an early indication of whether or not the recovery mechanism is working as planned. If different than expected, it is possible that, depending upon the nature of a detected occurrence, corrective procedural modifications could be made. Monitor well data can also be used to guide ongoing performance adjustments.

RESERVOIR DEFINITION

As is now well established, a detailed reservoir description is necessary to properly design an enhanced oil recovery project. As noted earlier, monitor wells can be used to provide information and guidance early in the life of a project, and also provide performance data at intermediate reservoir positions. The utility of such information will be maximized if reliable reservoir data exists for monitor well locations.

Ideally, each well should be cored using coring procedures to minimize core flushing or alteration, including minimizing drilling fluid/formation pressure differentials. Whole core analyses should be specified where clay or shale laminae are present. Vertical permeability measurements should be included so that the likelihood of vertical fluid movement in the monitor well vicinity can be assessed. Special core analyses for determining cementation factors and saturation exponents should be considered for log derived saturation determinations.

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