CONTINUOUSVACUUME CLEANOUT TECHNOLOGY:Coal particlesregularly flow into coalbed methane (CBM) wells with the formation water. Oncethey settle down in the bottom of CBM wells, the pump will get stuck and thereservoir will be buried. In conventional oilwells, sand cleanout is usuallyoperated by circulating the cleaning fluid into the wellbore to bring sandparticles to the surface. However, when applied in CBM wells, this traditionalhydraulic particles hoisting technology would leak the working fluid into theformation, destroy the coalbed formation structure and jam the formed channelof gas because the bottom-hole pressure (BHP) is low.
The CBM wells belong tothe 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 coalreservoir fracture characteristics are highly developed. In principle, coalparticles are dragged by the formation fluids, which are water and gas, carriedout 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 zonebecomes plugged and the down-hole pump gets stuck. Workover activities have tobe carried out to bring the well back into production. Frequently, removal ofwellbore fill is considered inadequate, leaving large quantities of coalparticles in the well, which often requires repeating well cleanouts in arelatively short time interval.
On average, the period between two cleanoutoperations is 5 months for CBM wells. In addition, wellbore cleanouts areextremely time consuming and costly, preventing timely return of wells toproduction and increasing the cost of well maintenance. Structure and Principle:According to theworking area, the coal particles cleanout technology system is divided into twosubsystems: the surface subsystem and the Downhole subsystem. The surfacesubsystem consists of a plunger pump, a separation tank, a gas recoveryfacility, and flow meters etc.
, as shown in Fig. 1 The downhole subsystemis composed of the Concentric Tubing String (CTS), the modified jet pump with aflow diverter and a down nozzle, etc. as shown in Fig. 2. In principle, theworking 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 thendivided into two parts by the flow diverter, as shown in Fig. 2.
The jet pumpnozzles are used to convert the high pressure head of the working fluid intothe high velocity head. One part of the working fluid flows through the downnozzle to impact the coal particles, form the coal particles carrier fluid. Thehigh velocity of the carrier fluid will accelerate to stir up the coalparticles at the bottom of the wellbore and carry them upwards to the throat ofthe jet pump via the suction chamber. The other part of the working fluid flowsthrough the up nozzle of the pump, generates high velocity and lowers thepressure at the bottomhole to suck the carrier fluid together with the coalparticles into the pump. Then, the above mentioned fluid–solid stream and thepower fluid are mixed in the throat of the jet pump and enhanced pressure inthe 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 returnsare routed to the recovery facility. The fluids in the separation tank arefiltered and to be recirculated to the fluid pumps.