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平落坝气田须二段是该气田的主要产气层段，产层埋深３４００￣３９００ｍ，以细粒，细一中粒或中粒长石石英砂岩和长石岩屑砂岩组成的裂缝－孔隙型致密储层。粘土矿物含量为５％￣１５％，其中伊利石占８８％￣９４％，绿泥石为５％￣１０％，多样样品中还发现了数量不等的伊蒙混层矿物。

A RESEARCH ON THE DAMAGE OF DRILLING FLUID TO CORE OF T3x2 GAS RESERVOIR IN PINGLUOBA GAS FIELD

??The second member of Xujiahe formation is the main gas2producing interval of Pingluoba gas field. The pay is a tight fracture2pore2type reservoir composed of fine2grained, fine2to2medium2grained or medium2grained feldespathic quartz sandstone and feldspathic lithic sandstone,with a burial depth of 3 400??3 900 m. The content of clay minerals varies from 5% to15%,of which,illite occupies 88%??94% and chlorite 5%?? 10%. In majority of the samples,the mixed2layer minerals of illite and montmorillonite varied in amount are found. The average porosity of the reservoir is 3. 55%,permeability 0. 2 ×10 ^-3μm² and average water saturation 54%. All wells yield slurry in the initial stage after the reservoir was put into production and the production was very unstable. It was necessary to eliminate blokage before wells were put into production in general, and after which, the output could be raised by more than three times. Through experiment,it is proved that the T₃x² gas reservoir of this gas field is damaged by drilling fluid,the mean rate of damage to permeability reaching 50%. The damage mechanism is as follows: First,the filtrate of drilling fluid entered into formation makes the water membrane thickened,and second, the particulates in the drilling fluid invaded in core plug up the percolation flow passage. In order to alleviate the damage,it is proposed that the density of drilling fluid should be improved and the pay should be drilled at a balanced pressure or at a pressure slightly higher than that of the formation.