This paper describes a method of measurement to be used in completing an open-hole frac procedure. Unlike cased hole work, a gamma ray - CCL log is used in correlation with a steel line measurement (SLM) to supply the completion rig crew with correct depths for placement of completion activities. This assures that the dollars spent for stimulation of the reservoir rock will be used to achieve the greatest results.
Proper depth control in open-hole fracing is Proper depth control in open-hole fracing is crucial to the successful stimulation of the reservoir rock. The correct placement of air notches and establishment of the depth of plugback and frac packer all require precise measurements. In wells that include fairly thin reservoir rocks and multiple zones targeted for treatment, this procedure becomes especially critical; a method procedure becomes especially critical; a method must be devised to tie in accurately to information found on open-hole logs.
Open-hole logs may be unusable for accurate depth determination for several reasons. Footage increments in wireline equipment may differ from physical measurements made by the rig crew who work with either a pipe tally, SLM, or combination of both. Additionally, when open-hole logs are run, the zero-point (point from which the log is measured on the surface) is commonly a drill floor or Kelly bushing which may be from two to fourteen feet above the surface casing. However, when the completion rig moves in to start the frac procedure, these reference points are no longer available, and confusion as to whether to add or subtract footage and in what amount may contribute to errors.
To lessen the possibility of such errors, a correlation gamma ray - CCL log is run inside a string of tubing (usually the notch pipe) and correlated to its length, preferably by use of a SLM. These measurements are then the known dimensions by which the rig crew must perform its work.
Open-hole fracing is a completion technique that has been commonly used in upper Devonian oil and gas well completions in the Appalachian Basin. Instead of a string of production casing cemented in the well and perforated to frac, a frac packer on the end of a string of frac pipe is packer on the end of a string of frac pipe is suspended in the hole and utilized to isolate the top of the zone of interest. A fine mesh gravel or coarse sand known as plugback fills the hole beneath the zone and a groove or "notch" is cut directly into the formation at the point that is desired to be fraced. This notch is designed to create a point of weakness in the well bore to initiate or lead the frac. All of these elements aid in the control of the frac, and it is imperative that an accurate measurement system be used to determine their appropriate placement. Unlike a string of cemented casing with perforations placed with the use of a gamma ray - CCL log, after which no other measurements need be made, an openhole procedure requires repeated usage of known depths and the ability to return to them (Figure 1).
Experience has shown that logs run in the same well by different wireline companies may differ as to depth of identifiable features by as much as several feet. Similarly, the logger's total depth (LTD) may differ from the driller's total depth (DTD) which is usually measured from either a pipe tally or SLM.