Drilling-in producing formations at negative pressure in borehole-formation system is one of the most perspective technologies, increasing efficiency of drilling. However effectiveness of operations frequently happens below expected. The purpose of work is to establish the reasons of this condition and development of recommendations on their removal.
Technological process analysis of opening-up carbonaceous reservoirs with depression on the basis of actual materials on 66 wells has shown, that one of the reasons of producing possibilities reduction of wells is improper object selection for application of the given technology.
Other factors which can lead to efficiency reduction of drilling technology with depression are shown. One of them is formation mudding by fine-dispersed cuttings, which completely did not separate from a solution in mobile separator. Calculations and following field observations shown that cuttings particles remain in aerated oil weighted and do not remove from drilling mud while opening productive stratum process. Difficult particles precipitation from oil assisted highly viscous emulsion constitution from gas and water, dispersed in oils, and strengthen water-gas-oil emulsion by formation «armours» from fine-dyspersate cuttings at surfaces of water or gas particles.
Offers on configuration of drilling equipment and technology of open-up producer for formation mudding prevention are formulated and experimental works on wells are executed.
Technology of drilling-in producing layers at negative pressure in borehole-formation system upon well completion is actively enough applied by the Perm branch of "Eurasia" drilling company at Russian oilfields, especially in the Perm region, Tatarstan and some other regions. However operations efficiency is insufficiently tall. For example, at one Tatarstan republic deposit were drilled underbalanced more than 25 wells which flush productions exceeded production rates of neighbors, drilled wtih traditional technology, in 1,2 - 2,7 times . However these indexes are lower, than on world experience, moreover, average specific production rates on wells have got a trend to decrease in a time: 2002 - 1,07 t/day·m; 2003y. - 0,61 t/day·m; 2005y. - 0,63 t/day·m; 2006y. - 0,45 t/day·m.
In the paper outlined the analysis of underbalanced drilling-in formation with the purpose of an establishment of the possible reasons reducing potential expected results.
The analysis we shall carry out on an instance of one compact operation of a deposit where was drilled the greatest number of wells with underbalanced drilling-in foration. Here petroleum gas aerated by nitrogen used as the flushing fluid. Operations were executed by the packaged drill rig including: individual blowout prevention system (BOP), manifold binding, mobile separator installation (MSI), a power-pump facilities, automatics system, azotic compressor installations, technological tanks, etc.
Drill string included drill bit, positive displacement downhole motor, float valves, heavyweight and drilling pipes. For monitoring process under heavyweight installed containers with autonomous probes which wrote pressure and temperature at bottomhole.
After opening formation and trip-out drill string probes data are decoded. The example of the diagram which have been written by bottomhole manometer on one well given on fig. 1.