平落坝气田须二段气藏岩心钻井液损害研究

A RESEARCH ON THE DAMAGE OF DRILLING FLUID TO CORE OF T_3x^2 GAS RESERVOIR IN PINGLUOBA GAS FIELD

平落坝气田须二段是该气田的主要产气层段，产层埋深３ ４００ ～３ ９００ ｍ ，以细粒、细—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 ％ ，多数样品中还发现了数量不等的伊蒙混层矿物。储层平均孔隙度为３．５５ ％ ，渗透率为０．２ ×１０－ ３μｍ２ ，平均水饱和度为５４％ 。气藏各井投产初期均产出泥浆水，生产极不稳定，一般都需要解堵后投产，其产气量可提高３ 倍以上。通过实验证实，钻井液对该气田须二段气藏存在伤害，渗透率平均伤害率为５０ ％ ，其损害的机理一是钻井液滤液进入地层增大了水膜厚度，二是钻井液微粒侵入岩心堵塞了渗流通道而造成。为了减轻损害，建议改进钻井液密度，在平衡压力或略高于地层压力条件下钻开产层。

The second member of Xujiahe formation is the main gas producing interval of Pingluoba gas field.The pay is a tight fracture pore type reservoir composed of fine grained,fine to medium grained or medium grained 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% to 15%,of which,illite occupies 88%～94% and chlorite 5%～10%.In majority of the samples,the mixed layer minerals of illite and montmorillonite varied in amount are found.The average porosity of the reservoir is 3.55%, permeability 0.2×10 -3 μm 2 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 T 3x 2 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.