Depth Control Issues for Drill Stem Testing
A Drill Stem Test is a temporary completion of a potential zone in an open drill hole, normally with the drilling rig on location and taking place after the well has been drilled to total depth and logged. Typically two packers are used to isolate a zone, one below and one above. The packers are inflated against the well bore and form a seal. The test string is then manipulated to open and close valves that allow the zone to flow into the wellbore, into the test tool and up the drill pipe to surface.
There are a number of issues that arise with regards to setting the packers in the right place in the well bore: The first one comes from the fact that the zone to be tested is picked off the logs. The corresponding depth depends on the measuring wheel for the wireline working properly - no mud or ice build-ups. The counter was zeroed at the rotary table at the correct position of the logging tool stack. That the other secondary systems of depth control are working, such as the magnetic markers on the electric line, etc. The logging crew has had some sleep in the previous 30 hours and is paying attention to their present operation. Most people will just assume that there are no issues with the logger's depth.
Here is the main issue for depth control for Drill Stem testing. All straddle inflate tests are strapped as the drill pipe is run in. This means that the rig crews physically measure each joint of pipe with a tape measure, in all kinds of weather and times of day or night. This is a totally different method than how the depth of the zone was determined from the logs!
Quite often there are really three critical depths for a DST (Drill Stem Test). You want to straddle the zone and you want good in gauge (not oval) hole for the two packers to expand against and form a seal. The packers are one meter in length where they contact the well bore so if you have a wash out, water contact or oval section that you are trying to avoid, depth control becomes very important. Accuracy better than one meter can become a critical issue so as to not blow a packer or hit a water contact.
Everyone who has called out DST's will have several wells, which stick in their minds, doubting whether they were on depth. Of course everyone will swear that they counted stands prior to testing and that the pipe tally and stick up calculations were done absolutely correct. Even if the rig crew double straps coming out of the hole and the pipe tally matches this doesn't address the possibility that the loggers depth could be off.
At an industry cost of $20,000 to $35,000 per test (including rig cost and test invoice) it becomes economical to use Depth Correlation on all DST's. Let's look at a new trend in the industry. With the majority of rigs being outfitted with monitoring systems, people are relying more on the data that is being entered and stored by these systems.
The trend now is for the rig crews to ask the Drill Stem Testing Rep on location if they can use the numbers straight from their system and just copy them over to a pipe tally sheet. In fact some rig crews can't remember the last time they actually strapped the pipe as they ran in for a DST. Unfortunately who knows what was up-dated and when for the rig tracking system. Also, apart from the possibility of errors this still does not address the separate issue of the logger's depth being off!
The only way to ensure that the DST test tool is placed on the formation you want to evaluate is to do a correlation log.