Open Hole Formation Testing
Authors: Shack E. 
Published: December 2007

Drill stem testing is a method of determining the potential productivity of a subsurface formation in either open hole or case hole. In order to accomplish this, a section of the well bore must be opened to atmospheric or reduced pressure.

The procedure is to lower the testing string into the hole on drill pipe with the hydraulic valve closed to prevent the entry of well fluids into the drill pipe. The packers then isolate the zone to be tested and the hydraulic valve is opened. The expanded packer supports the hydrostatic weight of the fluid, leaving the zone exposed to the atmospheric pressure in the empty drill pipe. The amount of fluid flow into the drill pipe is a measure of the ability of the well to produce fluid. After a certain flow period the shut-in valve is closed. This closes the drill pipe and allows the formation pressure to build up below the packer. The shut-in valve can be opened and closed as desired for multiple flow tests and shut-in tests. At the end of the test period the hydraulic valve is closed and the pressure equalized across the packer for unseating. By opening the reverse circulating sub the formation fluid captured during the test can be reversed out before pulling the drill pipe. pulling the drill pipe. Pressure reorders record all subsurface pressures during Pressure reorders record all subsurface pressures during the drill stem test.

The recording of multiple flows and shut-ins provides valuable information for more accurately determining the well's productivity, transmissibility, permeability, actual flow capacity, formation damage, etc.

The inflatable straddle drill stem tester combines an inflatable packer element, with a downhole pump to give the industry packer element, with a downhole pump to give the industry a more dependable tool for up-hole drill stem testing.

This testing tool provides many advantages over conventional drill stem testing tools. The inflatable element has greater expansion for setting and sealing in an enlarged hole as well as more run-in clearance. The down hole packer inflation pump sets both packer elements simultaneously by pumping pump sets both packer elements simultaneously by pumping fluid from the well bore into the elements. This is achieved by right hand rotation of the drill pipe with no weight being used. A by-pass from above the top packer to below the bottom packer provides equalization at all times. Seats can be obtained in holes that are too large for conventional packers, primarily because of their greater expansion capability, packers, primarily because of their greater expansion capability, but also because the inflatable element seals over its entire length, and confirms to irregular bore holes.

The test tool is made up and run with the proper spacing between packers. When the desired setting depth is reached, rotary slips are set and the flow line swivel and other accessory surface equipment hooked up. The drill pipe is then rotated to the right about 90 rpm to drive the down hole pump and inflate the packer elements. Rotating - or pumping time, is calculated from hole size, packer size and pump output per revolution. To insure expansion pipe is rotated until 150% of the necessary inflating volume, based on pump capacity, has been displaced. When the required inflation pressure is reached, the pump automatically stops displacing fluid into the packer elements. To further check whether packers are set, the drill pipe is picked up. If the elements are fully expanded, an increase in weight will be shown on the weight indicator.

Then the tool is opened for testing by setting weight on the drill pipe (about 20,000 pounds) which will open the multiple shut-in tool. Several minutes are required for the tool to open once weight has been set on it. Drill pipe can be lowered 6 inches before the tools start taking weight, (a slip joint built into the down-hole pump has this much free travel). This slip joint also acts as a bumper sub. Once the tool is open, well fluid can flow the drill string through the test ports.

To take a shut-in pressure, drill pipe is picked up to c lose the multiple shut-in tool. The tool can be opened and closed for taking pressures as often as desired. Because the packer seal is maintained with fluid pressure, the amount of load set or pulled is not critical, while taking various flow and shut-in readings, packers cannot be unseated due to these loads. At the end of the test period, weight is slacked off on the drilled pipe to equalize the tool. After slacking off and setting the slips, the drill pipe is rotated to the right to obtain one quarter turn at the tool.

This engages a clutch in the downhole pump section which will prevent rotation in either direction. Actuation of the clutch mechanism is usually heard on the surface, and indicated further by a backward spin of the rotary table. Allow several minutes for pressures to equalize. Then pick up the drill pipe slowly, to unset tool. This operation releases the fluid pipe slowly, to unset tool. This operation releases the fluid from the packer elements into the annulus.

Working the pipe during the unsetting procedure does not actuate the hydraulic tool because weight must be applied for several minutes for it to operate. Once the packers are free, the test tool may be repositioned at another point for further testing.

A belly spring device is run on bottom to provide drag and prevent the tool from rotating as drill pipe is rotated. The prevent the tool from rotating as drill pipe is rotated. The lower packer section, with a 52" long seal, is installed immediately above the belly spring device.

Next, sufficient drill collars and collar subs are installed in the string to space out the packers. A recorder sub, which can accommodate three recorders is made up on top of the drill collar section.