Gas Tracers: A Decade of Learning and Experience
Authors: Carpenter Chris 
Published: August 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 183413, “A to Z for Gas Tracers: A Decade of Learning and Experience,” by M.N. Khan, H. Iwama, A. Al-Neaimi, and O. Al-Shehhi, Abu Dhabi Marine Operating Company, and M. Chatterjee, Tracerco, prepared for the 2016 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 7–10 November. The paper has not been peer reviewed.

In interwell tracer studies, reservoir complexities often make tracer-breakthrough time difficult to assess. Although reservoir-simulation studies serve as an effective tool in predicting a breakthrough, the tracer behavior in the real porous media sometimes presents intriguing surprises. This paper discusses a crestal gas-injection project that was carried out in a supergiant heterogeneous-carbonate oil field.

Introduction

A tracer-survey study can provide a variety of information about the heterogeneity of a formation. While transient tests can provide information about reservoir continuity, thief zones are difficult, if not impossible, to detect. This is because pressure-transient tests provide an arithmetic average for reservoir total transmissibility over the tested formation thickness, while a tracer survey provides a direct evaluation of the flow field between the injection and production wells.

Apart from heterogeneity, tracer studies also serve as an informative tool to yield fluid-flow paths inside the reservoir, popularly known as reservoir streamlines. A tracer survey not only can precisely identify these preferential paths that are detrimental to the sweep of the reservoir in the enhanced-oil-recovery stage but also can provide information about the time that fluid takes to move from one point (injection well) to another point in the reservoir. This time is termed the mean residence time. In addition, any flow barrier or directional thief zones such as faults can be identified by delayed tracer recovery. The mass-balance technique can be used to calculate the amount of tracer recovered to distinguish between the existence of a fault, a flow barrier, or a low-permeability zone.

Tracer-Project Design

Interwell tracer testing consists of injecting chemical tracers into injection wells at the beginning of a flood project or after the reservoir has reached its fill-up condition, depending on the objective of the project, and subsequent sampling of production wells for a prescribed period of time, which also depends on the objective of the project. Samples are analyzed for tracer content, which will delineate communication between the injection and production wells. The time during which samples are collected and analyzed depends greatly on the objective of the project. If the objective is to identify thief zones, deduce mean residence time, and determine other factors that could lead to full-field reservoir characterization that could be fed directly into a simulation model, sample collection and analysis must continue for a considerable period of time after tracer is first detected in order to establish a more-defined elution curve.