Drill Stem Test (DST)

Sigra carries out testing to measure permeability and reservoir pressure in formations such as coal seams to determine their in-situ conditions. The tests are typically undertaken in HQ size (96 mm) exploration holes but may be undertaken in larger holes too. The most successful testing involves drawing fluid from the formation to be tested rather than injecting.

The reason for this is that the testing is being undertaken using the reservoir fluids of known viscosity and temperature. Injection testing can be performed but frequently leads to sharply increasing well bore losses during the test as clays in the drilling fluid tend to plug cleats near the well bore. The form of testing most frequently undertaken is a drill stem test. This involves emptying part of the fluid from the drill string, sealing the test zone, waiting for pressure stabilization and then opening a valve between the test zone and the drill string to induce flow. After a period of inflow the valve is closed to induce pressure build up. This procedure may in some cases be repeated to confirm behaviour.

The extreme range of characteristics of coal seam reservoirs prevents the use of an identical test for each seam. It is essential to be able to monitor at surface the formation response to the test process and to adapt testing to meet in-hole behaviour. Sigra surface readouts permit viewing of the well test progress in numerical or graphical form. Typically the plots produced are of the total test, the derivative plot with respect to Agarwal time, and the Horner build up plot.

Having the ability to view these plots in real time ensures that all tests that Sigra controls and undertakes with its own equipment produce a valid result, provided that the packers can be seated on a sealing formation. The DST system developed by Sigra comprises a trailer containing data logging equipment, control equipment, and down hole tools. The down hole tools are in two forms.

Sigra DST Trailer
Sigra DST Trailer

The first tool is designed to be run through the HQ or HRQ drill string on the drill rig’s wire line. It may be run as a straddle or bottom test tool, depending on whether a bottom packer and extension rods are used. Therefore it can be run to depth and be used without pulling the drill string. It may be used in holes up to 105 mm diameter.

The advantage of this tool is that it may be run quickly without pulling the drill string and therefore tests may be conducted with unstable sections of borehole above the test zone. The second tool is an end of string tool and may also be used in straddle or bottom test configuration. It utilizes a bottom assembly that is designed to be screwed on to the bottom of the drill string. Packer inflation and communication lines are lowered down inside the drill pipe on the drill rig’s wire line to connect with the tool. The tool can be used to test multiple times without pulling the drill string, provided that the test zone straddle spacing remains the same. Both tools use either compressed air or nitrogen to push down the fluid in the drill string prior to a test.

Each system contains a zero volume change valve that controls connection between the test zone and the inside of the drill string that is operated by raising or lowering the string once the packers are set. Pressure transducers monitor pressure in the test zone, above the valve in the drill string, and packer pressure.Packer inflation is from either a water pump at surface or in shallower holes by compressed air. The communication and inflation lines are taped to the drill rig’s wire line for raising and lowering. At the top of the drill string is a snubber to shut in around the cables should a blow out occur during running of the tool or cables.

Above this is a sealing head which seals around the cables during operation. The surface equipment contains a regulator to control the pressure of air from a compressor that is used to push the water level down in the drill string. It also contains high and low flow gas meters to measure the flow rate of gas from the drill string. In addition it contains a water regulation system to permit water injection into the string at a controlled pressure or rate should that be required. When the through the bit tool is used then the drill string is set at the correct level and the tool is lowered through the seal, snubber and string to protrude through the bottom of the core barrel. If the end of drill string tool is used then the tool is attached to the bottom of the string and the string lowered to position. Regulated air pressure is used to displace water from the drill string. A pressure is used that will not blow air out of the bottom of the string into the hole.

The packers are set and the drill string is then lowered to close the valve and the compressed air is bled off. The valve may then be opened to induce flow from the test section into the string. The water inflow can be monitored by a change in head on the in-string pressure transducer whilst the flow of displaced air and any gas produced can be measured on the gas flow meters. Any gas flow can be detected as the difference between water inflow to the string and gas flow out of the string. Following the test procedure the drill string is filled, the pressures inside and outside the tool are equalized, the packers are deflated and the top seal around the cable and inflation line is opened. The through the bit tool may then be withdrawn through the string or the case of the end of string tool the electronics module may be pulled. In each case blowout control is afforded during this operation by the snubber.

Test Time

Permeability tests take as long as the reservoir demands to provide a solution. Failure to take tests to completion leads to their being unanalysable. In high permeability cases the tests may be completed in a few hours. In medium to low permeability cases the recovery part of the test is frequently conducted overnight. In very low permeability reservoirs the tests may need to be run over two days. It is not normal practice to conduct DST tests for longer than this.

Analysis

The basic analysis of results is based on the inflow rate and the slope of the Horner build up plot when this has stabilised. The stabilisation is defined by a constant value of the plot of the derivative of pressure with respect to Agarwal time versus Agarwaltime. Analysis is generally on the basis of a single phase which may generally be a liquid (water) but may also be gas. Analysis of mixed phase flows can also be undertaken in terms of the permeability for each individual phase. In addition to the permeability, the reservoir pressure is assessed and the well bore loss terms described. The well bore loss terms are evaluated in terms of skin and effective well bore radius. The mean effective radius of investigation is also calculated. It is based on the actual times of the points used in determining the slope of the Horner plot and represents the mean radius at which the pressure changes between these times may be considered to act. Sigra may at times conduct more detailed analysis of early shut in behaviour to determine effective stress – permeability characteristics of the formation – in particular coals.

The DST equipment is conveyed to site in a trailer or in a pod. This houses the electronics, reels, pumps and flowmeters required for an operation. A snubber is also carried to shut in the test string in the event of packer failure. The pods are suitable for a helicopter lift.

Sigra Drill Stem Test (DST) Full Test Plot
Sigra Drill Stem Test (DST) Full Test Plot
Plot of Derivative Pressure with Respect to Agarwal Time Plot
Plot of Derivative Pressure with Respect to Agarwal Time Plot
Horner Plot
Horner Plot

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