A Quantitative Approach to Gravel Pack Evaluation
Authors: Neal M.R. , Carroll J.F. 
Published: December 2007

Summary
Gravel pack logging has been used during the past few years to evaluate internal gravel packs qualitatively. This paper describes a study that was made under laboratory paper describes a study that was made under laboratory conditions to develop a quantitative evaluation procedure. Field logs are shown that confirm the validity of the laboratory measurements.

Introduction
Previous test well experiments confirmed the application Previous test well experiments confirmed the application of wireline logging techniques to provide a qualitative evaluation of gravel packs. It soon became apparent that a reliable method of quantitative evaluation was desirable. Laboratory measurements have been made to quantify the log response to percent void space in the gravel pack. Measurements were made in 7-in., 32-lbm 117.8-cm, 14.5-kg] casing; 7%-in., 29.7-lbm [19.4-cm, 13.5-kg] casing; and 9%-in., 47-lbm [24.4-cm, 21 -kg] casing. Five standard screens of different sizes and five tubing blank sections of different sizes were used. Table 1 summarizes the hardware used in the laboratory experiments. To establish tool response data to varying percentages of gravel, series of fins were fitted to each size tubing and screen. This enabled data to be collected for 0, 33, 66, and 100% gravel packs. Fig. 1 shows a top view of the laboratory model. Four different types and weights of fluids and high-specification 40/60 resieved gravel were used in the experiments. The measurements taken clearly demonstrate that the gravel pack logging tool does respond to changes in percent pack and that a quantitative approach to the percent pack and that a quantitative approach to the interpretation of the field logs can be made. This paper describes the procedures used and the conditions under which the laboratory measurements were made. Comparisons between laboratory data and actual field logs are given.

Theory

Gravel Pack Logging Tool
The gravel pack logging tool uses a gamma ray source and a single gamma ray detector. The source emits gamma rays radially into the borehole and the surrounding area and is focused upward toward the detector. Fig. 2 shows a schematic of the tool configuration. Some of the gamma rays are scattered back to the detector. The number of gamma rays returning to the detector is an indication of the density of the material through which they traveled. High-density materials cause more gamma rays to be slowed down and absorbed, and low-density materials allow more gamma rays to be returned to the detector. In a gravel-packed well, everything remains constant except the annular space between the casing and the screen or tubing. This space can be totally filled with gravel, partially filled with gravel, or have void spaces containing no gravel. In each of these cases, the volume that is not filled with gravel is filled with some type of fluid of a known density. Since the density of the gravel is different from the density of the fluid, it is possible to correlate gamma ray count rates to percent pack. percent pack. Description of Laboratory Models
The laboratory models consisted of 7-in., 32-lbm [17.8-cm, 14.5-kg] casing; 7 5/8-in., 29.7-lbm [19.4-cm, 13.5-kg] casing; and 9 5/8-in., 47-lbm [24.4-cm, 21-kg] casing. Each was cemented into steel cylinders 22 in. [56 cm] in diameter and 80 in. [2 m] long. Centralizers were placed on the casing to ensure that they were centered placed on the casing to ensure that they were centered in the cylinders before they were cemented into place. Fig. 3 shows how the models were constructed. Rubber fins were attached to the screen and tubing to divide the annular space into three equal parts. This enabled data to be collected for four different percentages of gravel pack. Four different fluids were used in making the measurements. They were 6.8-lbm [3-kg] diesel, 9.5-lbm [4.3-kg] salt water, 11-lbm [5-kg] calcium chloride, and 14.2-lbm [6.4-kg] calcium bromide.

Log Response Measurements
The first laboratory measurements were made with 9 5/8-in., 47-lbm [24.4-cm, 21-kg] casing, 31/2-in., 12.37-lbm 18.9-cm, 5.6-kg] screen, and 11-lbm [5-kg] calcium chloride. The pack was varied from 0 to 100% and the corresponding change in count rate was noted. Fig. 4 shows that the change in count rate as the percent pack is varied is a linear function. Fig. 4 also shows that this linear response was verified by making measurements in 4-in., 13.23-lbm [10-cm, 6-kg] screen and in 4-in., 9.5-lbm [10-cm, 4.3-kg] tubing. Since the response had been verified as being linear, further measurements on other hardware were made only at 0 and 100% pack. Further measurements were made in 9 5/8-in. [24-cm] casing with 41/2- and 5-in. [11-and 13-cm] screens and 4 1/2- and 5-in. [11- and 13-cm] tubing. They were also made in 7 5/8-in. [19-cm] casing with 2 7/8,-, 4 1/2-, and4 1/2-in. [7.3-, 8.9-, and 11-cm] screens and tubing and 4-in.
JPT
P. 1035