Locating Channels in Multiple Tubingless Wells with Routine Radioactivity Logs
Authors: Pennebaker E.S. 
Published: December 2007

Neutron depth correlation logs used in conjunction with open-hole acoustic logs can help spot poorly placed cement in multiple tubingless completions; and oriented density logs can be used to define channels in the cement and guide the placement of shots for repair work. Thus we have a new use for some old, familiar tools.

Introduction
When two or more parallel strings of casing are cemented in a common wellbore, complete displacement of the mud by cement is very difficult to achieve. McLean et al. point out that in an eccentric annulus, cement may favor the wider side and bypass slower-moving mud in the narrower side. They demonstrated this bypassing phenomenon by means of laboratory models of single-string cementing. Childers added that the only major difference between single- and multiple-casing cementing is the annular geometry. Channels may develop at the narrowest point of separation between one or both strings of casing and the wellbore.
An example of such a channel and its possible consequences is illustrated in the left side of Fig. 1. Two strings of 2 7/8-in. OD casing are shown cemented in a 7 7/8-in. wellbore. A channel between String 2 and the borehole wall extends vertically downward from the oil zone to a salt-water sand. Pressure drawdown in the oil sand immediately adjacent to the wellbore may allow salt water to break through the channel, flood the oil sand around the wellbore, and be produced with the oil as shown in the right side of produced with the oil as shown in the right side of Fig. 1.
If the cement bond around String 1 and the borehole wall on that side is good, repeated attempts to squeeze cement through the perforations oriented as shown could fail to eliminate the salt water. Cement slurry unable to flow through the pores of the rock to the channel would fracture the formation and be pumped away. Without direct access to the channel, pumped away. Without direct access to the channel, repair can be difficult. It can be accomplished only through new perforations directed as shown in Fig. 2, either from String 1 across the hole towards the channel behind String 2, or from String 2 directly into the channel. It is evident, however, that proper diagnosis can be made only if the location of the channel is known.
The following discussion explains how this information can be obtained through the help of (1) neutron perforating depth control logs and (2) oriented density logs.

Neutron Depth Correlation Logs
An uncalibrated neutron log is used in many operating areas to control perforating depth. This tool responds primarily to hydrogen, and therefore measures changes in the amount of pore fluids (water or hydrocarbons) or in the porosity of the formation. Because of the limited depth of investigation of the neutron logging tool, a relatively high percent of its response comes from material in the borehole, particularly in tubingless completions (Fig. 3). particularly in tubingless completions (Fig. 3). Cement Effect
With uniform placement of cement in the annulus, logs run in each string of a multiple-cased well should be identical. Each is influenced equally by the same amount of cement. However, if cement placement is not uniform, its effect cannot be placement is not uniform, its effect cannot be ignored. Variations in response between logs appear correlatable with channels in the cement.
JPT
P. 375