Gravel Pack Quality Control by Neutron Activation Logging

ABSTRACT A successful gravel-pack is hard to achieve in wells that have highly deviated boreholes and long completion intervals. The primary difficulty in such wells is placing the pack material in the screen/casing annulus and in the perforations without leaving void areas. Using a low-density particle for packing will help prevent void area in allowing the use of the matched-density concept, where the particle density equals the density of the carrier fluid. Since most gravel pack quality control devices rely on the usually sizable difference of density between borehole fluid and packing material, they may not be suitable for gravel-pack operations involving high-density borehole fluids and/or low-density gravel packing material. This paper introduces a new application of existing logging devices to provide a quantitative evaluation of the packing quality. The method is based on neutron activation of particular elements contained in the gravel pack material and applies to standard gravel as well as to new low-density materials. The technique is relatively insensitive to formation characteristics because the depth of penetration of the activation neutrons is shallow. A base pulsed neutron capture log can be recorded simultaneously with log for gravel-pack quality control. The lack of sensitivity of the formation capture cross-section measurement to the presence of the gravel material, whether conventional or lightweight, has been demonstrated. The logging procedure was developed using a series of laboratory measurements in formations with a simulated gravel-pack completion. Field results obtained with different types of gravel-pack material are compared to a density-based log when the density contrast of the packing material and completion fluid permits the use of the latter logging technique.