Gravel Pack Field Examples of a New Pulsed Neutron Activation Logging Technique
Authors: Watson J.T. , Carpenter W.W. , Carroll J.F. , Smith B.C. 
Published: October 2008

Gravel packs have traditionally been evaluated using gamma gamma density and neutron logging tools.1,2,3 These logging tools, particularly the density tools, do an acceptable job in most logging environments. However, there are some limitations which affect their measurement resolution when attempting to define gravelpack quality. The presence of high-density completion fluids significantly reduces the dynamic range of the conventional measurements. Low-contrast logging resolution is also encountered with the new matched-density gravel pack systems that use matrix materials with densities close to the densities of the completion fluids. This paper presents an alternative measurement of gravel-pack quality that is unaffected by the type of completion fluid present.

Also presented is a series of six field examples demonstrating this new technique. Each example presents a different logging condition; for example, heavy borehole fluid in a conventional gravel pack, gravel pack using sintered bauxite, two completions using Isopac? gravel with different screen sizes, and multiple gravel-pack logs recorded before and after wireline repair work. Several of these examples show that neutron activation can be a useful method for gravel-pack analysis in some logging environments.

INTRODUCTION

Neutron activation logging is an effective technique for gravel-pack quality control in situations where the contrast between fluid density and gravel-pack material density is not sufficient to evaluate with density or neutron measuring devices.4 Additionally, reduced density gravel-pack material in a matched-density gravelpack system (such as the Isopac system) can be used when packing highly deviated intervals or when minimizing the contrast to maintain gravel suspension during the pumping operation. Heavy packing materials such as sintered bauxite have been used because of their physical properties. These environments cause the reduction in log resolution as the fluid density is increased. Each of these situations has proved difficult to evaluate using density and conventional neutron logging devices.

Neutron activation logging is a process in which the elements in the packing material are detected and gravel-pack quality is subsequently determined from the amount of those elements present. The primary elements of interest are silicon and aluminum, which are found in abundance in gravel-packing material. Silicon is found in standard gravel. Aluminum is found in sintered bauxite. Both silicon and aluminum are found in matched density Isopac gravel. Silicon and aluminum are activated by fast neutrons, and they decay slowly with a 2.24-minute half-life. Gamma rays are emitted as the elements return to their stable states. The number of these gamma rays is proportional to the amount of silicon or aluminum being activated. The neutron activation logging technique uses a pulsed-neutron logging tool modified to allow a gamma ray device below the pulsed-neutron tool as shown in Fig. 1. The high energy pulsed-neutron source provides the neutrons to activate the elements in the gravel pack. As the tool is pulled through the gravel pack, the lower gamma ray detects gamma rays resulting from the activation process. Logging speed is controlled such that no gamma rays from fast decaying elements such as the oxygen (7.2-second half-life) in water are detected.