Injection – Falloff Tests

Sigra conducts two types of injection – fall off tests. Both utilise Sigra’s drill stem test tools to inject rather than to produce.

The basic procedure is the same in either case. Namely the DST tool is run to isolate the test zone and set. The formation pressure is then allowed to come to near equilibrium. This is followed by a flow period, which in this case involves flow into the formation, followed by a recovery period.

Injection Test

The first test type involves using either the drill rig pump or the DST trailer mounted piston pump to inject fluid into the string. The trailer mounted pump will deliver up to 6 litres per minute beyond which the rig pump is used through a control system that will deliver either constant pressure or controlled flow rate injection. The usual mode of operation is that injection is carried out for a period and then the test zone is shut in to allow pressure to decay.

Falling Head Test

The second test type is typically used when the formation pressure is too low to produce a useful DST (inflow test). In this case the drill string is filled with water and when equilibrium is reached the valve is opened to permit flow from the drill string into the formation. After a period of inflow the valve is closed to permit recovery. The volume of fluid that has flowed is measured by the change in head within the drill pipe.

Analysis

The analysis process is identical to that used for drill stem tests except that inflow rather than outflow is taking place.

Practicalities

While the system may appear to have advantages, particularly in avoiding the desorption of gas from coals, it has been found to have practical complications. The most serious of these is due to well bore damage associated with injection. This comes from two sources. The first is the presence of small clay particles in the injection fluid. These may have come in the water that is brought to site or from the well bore itself. The second is associated with the injection of a fluid that is not the formation fluid, thus causing a disassociation of the clays in the formation which blocks its fractures or cleats. The other problem that has to be overcome is that of being sure of the viscosity of the fluid being injected, as it may be contaminated with drilling fluids.

If any injection test is to be used it must be carried out with a fluid that is as close to the reservoir fluid as possible. In the case of coal seam methane wells this preferably means produced water. This then has to be deflocculated and carted to site in clean containers. The well must then be flushed with this fluid to remove any drilling mud. This is preferably done whilst rotating and reciprocating the drill string so as to break up any filter cake. Then the test may be performed using this fluid for injection.

However,our practical experience is that in all but the most permeable coals, problems are generally experienced with gradually increasing well bore damage throughout the test.

Leave a Reply