Drillstem test (DST)

Parameters to be determined by the DST:reservoir productivity

reservoir permeability

reservoir pressure

reservoir size=> the DST is a conventional method of formation testing

Steps to perform a DST:run DST assembly in hole to the zone to be tested

isolate the zone to be tested from the drilling mud=> set inflatable rubber packers above and below the zone to be tested=> packers seal the annular space between test string and formation

open the valves inserted into the test string between the two packers=> any fluids contained in the zone to be tested can now flow into the test string=> recorders chart static pressures=> a sampling chamber traps formation fluids at the end of the test

kill the well, close the valves, remove the packers and pull the DST assembly out of hole

analyze data obtained from the DST

Basic hardware for well test operations:open hole (uncased) DST=> conventional DST string (with mechanically set packer system)=> modern DST string (with inflatable dual packer system)

cased hole DST=> RCI straddle packer=> wireline formation tester

intelligent well systems=> vertical wells=> multilateral wells

Shut-in pressure:pressure exerted at the top of the wellbore when it is closed at either the X-mas tree or the BOP stack

the pressure may be from the formation or from pressure maintenance operations (water injection)

if the shut-in pressure is zero, any open formations are effectively balanced by the hydrostatic column of fluid in the well => the well is considered to be dead and can normally be opened safely to the atmosphere

Pre-flow period:cleaning the well

removal of any supercharge given to the formation due to mud infiltration while drilling

Initial shut-in period:allows the formation to recover from pressure surges caused during the pre-flow period

Main flow period:long production period designed to test the formation flow properties in a more detailed way

flowing pressures and temperatures are recorded

fluid samples are gathered

Final shut-in period:formation pressure is recorded

permeability, degree of formation damage and reservoir size can be derived from the pressure build-up curve

pre-flow period, initial shut-in period, main flow period and final shut-in period can also bedistinguished from the following diagram

the irregular lines at the beginning and the end represent indistinct data due to flow not having stabilized yet

Modern DST string (with inflatable dual packer system):