Practical Use of Drill-Stem Tests

Abstract

Abstract Drill-stem testing has advanced from simple sampling of the formation to complete pressure buildup analysis. This paper reviews the mechanical arrangement of the tool, its running and pressure measurements. Various typical drill-stern test pressure charts are shown as an aid in trouble-shooting. Purpose THE ORIGINAL PURPOSE of drill-stem tests included recovery of a sample of the formation fluids and possibly measurement of the formation pressure. Today, the nature and use of the drill-stem test is more sophisticated and most techniques developed for drawdown and buildup tests have been adapted to the shorter drill-stem test. These might include computation of formation permeability, k, skin, s, flow efficiency or damage ratio, D.R., productivity index, P.I., and radius of investigation, ri. At times, reservoir discontinuities such as faults, small reservoirs, two-layer performance, lenses in the pay and fluid contacts may be recognized. A sample of the formation fluids is usually caught at near formation pressure during the test and brought to the surface without release of pressure. If properly removed and transported, such sample might be analysed in a PVT laboratory. At present, the sample usually is only used to establish fluid-type, gas-oil ratio, viscosity, gravity, etc., as measured in the field. In a drill-stem test the pipe above a shut-off valve is filled with air adjusted by water cushion as the pipe is lowered into the mud-filled well. This valve can be intentionally opened and closed after the pipe has been lowered to the desired depth and packer, set to seal off the mud liquid within the annulus of the well. Thus the well may be produced against a controlled amount of back pressure when the tool is initially opened. Test Procedure-Mechanical Equipment Most drill-stem tests taken today are using equipment and procedures developed by major service companies. Some of the tools involved and typical charts for two such companies are included in this paper as exhibits A and H, respectively, to give the reader an appreciation of complexity. Two or more bottom hole pressure gauges are usually employed. The upper gauge measures the pressure inside the drill pipe or tubing string below the shut-in valve. The lower gauge measures the pressure on the outside of the tool. Both gauges are located near the bottom of the equipment. A section of slotted pipe is used to permit entry of the fluids from the well bore below the set packers into the inside of the string of pipe. To reduce buildup of pressure ahead of the unset packer while it is being lowered into the well, fluids are permitted to flow through these slots and various other mechanisms located below the shut-in tool. Such fluids constantly move from below the unset packer through the pipe to the annulus above the packer while the tools are run into the well. In most cases this technique results in a shorter running time without damage to the tools or plugging of the slots or other restrictions in the flow string below the shut-in tool in the drill pipe.