WIRELINE DEPTH CONTROL
Depth control for perforating is almost universally obtained through radioactivity instruments run in the cased hole in conjunction with the Casing Collar Locator (CCL). The Gamma Ray Log is generally used (Figure B28) though, in some cases, the Neutron Log or both Gamma Ray and Neutron are run. Accurate correlation of radioactivity logs with open hole logs establishes the position of casing collars with respect to the formation to be perforated. A short sub in the casing string is highly desirable to eliminate ambiguities with CCL identification, particularly when all joints of casing are about the same length. If the depth control log is made on a separate trip in the well, the proper shooting depth is determined on the perforating run by recording a second collar log with the collar locator attached to the perforator.
2.6.1.1. Gun-Gamma Ray Tool
If the combination Gun-Gamma Ray Tool is used, the entire equipment for depth control and perforating is run on a single trip in the well. The Gun-Gamma Ray Tool includes a rugged, shock proof gamma ray detector. A casing collar locator and a perforating gun can all run together. This offers greater assurance of accuracy and considerable saving of rig time. Depth control should always be used to accurately position TCP guns. A reference radioactive collar is run in the work string and its distance from the top shot is measured. Once on bottom, a through-tubing GR/CCL log is run and compared to open hole logs to establish how guns should be moved for exact positioning opposite the target formation. A variation of this procedure has been used from floating vessels in sand control completions. A sump packer is positioned and set with a wireline and becomes the locating device. The TCP gun string then is run with a locator and collet assembly on bottom. The distance from bottom gun shot to the collet latch is selected to place guns on depth. A radioactive collar should still be run to allow adjustment by logging in case of pipe tally discrepancies or slippage of the sump packer downhole.
2.6.2.2. Precision Identified Perforations
P.I.P. tags are used to provide a record of the position of perforations with respect to casing collars and/or formation boundaries. Special shaped charges fired at top and bottom of the perforated section leave traces of radioactive material within the perforations. The top and bottom perforations are then identified by sharp peaks on a Gamma Ray curve after perforating. Small size, low activity and short half-life of radioactive material used in the special charges prevent significant contamination of produced fluid. When run with Gun-Gamma Ray tool and Hollow Carrier perforators, no additional rig time is required other than that needed to log through the perforated interval.
2.6.2. TCP DEPTH CONTROL
Four main techniques are used to ensure that the guns are at the correct perforating depth:
- Running a through-tubing gamma ray collar locator to locate a reference point in the string and tie into openhole logs.
- Setting the packer on wireline at a known depth, and stinging through the guns and completion string.
- Setting the packer and guns on wireline at a known depth, and stabbing the completion string in the packer.
- Tagging a fixed and accurate reference point such as a bridge plug. The first method is the most accurate. It relies on a radioactive marker sub in the string, and the distance from the radioactive marker sub to the top shot being precisely measured at surface. The string is run in the hole to approximately the correct depth and a short section of GRCCL (Gamma Ray-Casing Collar Locator) log is run over the zone where the sub is located. The gamma ray log indicates the position of the sub (a short radioactive peak anomaly) relative to the formation gamma ray as shown in Figure B30. As the distance from the sub to the top shot is known, the position of the guns can be calculated, and corrected if necessary by spacing out the string at surface. After the packer is set, the gamma ray may be rerun to ensure that the guns are at the correct depth. Fig. B30: TCP Depth Control Log.
As the cased hole gamma ray log can be considerably attenuated, a low logging speed will achieve better correlation results between the cased hole and the open hole gamma ray logs. If the formation gamma ray curve does not show much activity, a radioactive pip tag may be placed
As the cased hole gamma ray log can be considerably attenuated, a low logging speed will achieve better correlation results between the cased hole and the open hole gamma ray logs. If the formation gamma ray curve does not show much activity, a radioactive pip tag may be placed in or below one casing joint. (Placement of the pip tag must be included in the casing setting program.) Alternatively, a TDT or a neutron log can be run in place of the gamma ray log.
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