Abstract

Evaluating reservoirs by conventional wireline logging in open hole has become an accepted and necessary technique over the years. In geologically uncertain areas, openhole logs run in a particular well provide data not only for the well being evaluated but also for future offsetting wells. One obvious drawback with these logs is that rig time must be expended to condition the hole to log, to do actual logging, and then to recondition the hole to cement.

The openhole data lose some of their usefulness once the well has been cased. Conventional open hole logs cannot be rerun to re-evaluate reservoir performance once casing has being set. To monitor or re-evaluate these cased reservoirs, the pulsed-neutron capture and inelastic GST* Gamma Spectrometry tool has been successfully utilized.

The paper will discuss how utilizing this cased reservoir monitoring and reevaluation device can lead to innovative interpretation techniques that permit the replacement of openhole logs with cased hole evaluations in heavy oil EOR projects. The resulting optimization of rig time and the potential improvement of cementations in new wells resulting in part from this technique will be discussed. Examples of Canadian logs in conventional heavy oil reservoirs, in EOR heavy oil reservoirs, and in conventional light oil reservoirs will be presented and discussed.

Introduction

Present reservoir evaluations utilizing data obtained prior to running production casing allow accurate estimates of hydrocarbon potential. These evaluations are critical in geologically uncertain areas. Data obtained from cores, logs run in open hole, and data from drillstem tests prior to the casino being set have led to critical decisions, such as whether or not to run the casing, where to complete the well; how to complete the well; and what production mechanisms are needed.

Running openhole logs has become arguably a main form of reservoir evaluation. Cores and tests mayor may not be taken. This form of reservoir evaluation requires hole conditions that permit the openhole logging tools to get down to potential hydrocarbon horizons. Openhole logging is an economical form of evaluation in deeper wells or in wells that are in nondevelopment areas. In shallow development areas, the time dedicated to openhole logging as compared to the total rig time to drill the hole becomes a substantial percentage of the total rig costs. Using the cased hole GST tool for primary formation evaluation on closely spaced new wells, such as those in heavy oil reservoirs, has proved to be more economical.

The openhole log data lose some of their usefulness once casing has been set. Reservoir performance has to be monitored by logs run in casing, oil companies have been uncomfortable with some of the cased hole log evaluation techniques because the raw data have been difficult to relate to raw openhole logging data. The need for evaluating reservoir conditions and performance has not been diminished by this uneasiness.

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