When coal particles flow
with formation water then it stuck the CBM well. In conventional oil wells,
sand cleanout is usually operated by circulating the cleaning fluid into the
wellbore to bring sand particles to the surface. However, when applied in CBM
wells, this traditional hydraulic particles hoisting technology would leak the
working fluid into the formation, destroy the coalbed formation structure and
jam the formed channel of gas because the bottom-hole pressure (BHP) is low.
The CBM wells belong to
the low reservoir pressure and absorption wells. The bottom hole pressure (BHP)
of CBM wells is very low, usually ranges from 1 to 5 MPa, and the coal
reservoir fracture characteristics are highly developed. In principle, coal
particles are dragged by the formation fluids, which are water and gas, carried
out through the formation fractures and settle down at the bottom of the well.
By the time the casing is full of the sedimentary solids, the production zone
becomes plugged and the down-hole pump gets stuck. Workover activities have to
be carried out to bring the well back into production. Frequently, removal of
wellbore fill is considered inadequate, leaving large quantities of coal
particles in the well, which often requires repeating well cleanouts in a
relatively short time interval. On average, the period between two cleanout
operations is 5 months for CBM wells. In addition, wellbore cleanouts are
extremely time consuming and costly, preventing timely return of wells to
production and increasing the cost of well maintenance.
Structure and Principle:
According to the
working area, the coal particles cleanout technology system is divided into two
subsystems: the surface subsystem and the Downhole subsystem. The surface
subsystem consists of a plunger pump, a separation tank, a gas recovery
facility, and flow meters etc., as shown in Fig. 1
The downhole subsystem
is composed of the Concentric Tubing String (CTS), the modified jet pump with a
flow diverter and a down nozzle, etc. as shown in Fig. 2.
In principle, the
working fluid, i.e., water, is boosted by the plunger pump on the surface,
injected into the downhole jet pump through the annulus of the CTS, and then
divided into two parts by the flow diverter, as shown in Fig. 2. The jet pump
nozzles are used to convert the high pressure head of the working fluid into
the high velocity head. One part of the working fluid flows through the down
nozzle to impact the coal particles, form the coal particles carrier fluid. The
high velocity of the carrier fluid will accelerate to stir up the coal
particles at the bottom of the wellbore and carry them upwards to the throat of
the jet pump via the suction chamber. The other part of the working fluid flows
through the up nozzle of the pump, generates high velocity and lowers the
pressure at the bottomhole to suck the carrier fluid together with the coal
particles into the pump. Then, the above mentioned fluid–solid stream and the
power fluid are mixed in the throat of the jet pump and enhanced pressure in
the diffuser, thus, lifted upwards to the surface via the inner tubing string.
Return fluids are routed to a gas/water/solids separation tank. Any gas returns
are routed to the recovery facility. The fluids in the separation tank are
filtered and to be recirculated to the fluid pumps.