致密气藏束缚水分布规律及含气饱和度研究

Study on distribution law of irreducible water andgas saturation of tight sandstone gas reservoir

油气藏储层含气(油)饱和度是储量计算、储层评价及制定合理开发方案的关键参数之一.针对致密砂岩气藏孔喉细微、毛管力和束缚水饱和度较高的特点,经气水离心实验优选认为,2.07 MPa是致密砂岩气藏岩样的合适离心力,传统使用的0.69 MPa及适合于低渗油藏的1.38 MPa离心力均不适用于致密气藏.结合毛管压力方程,计算得到致密砂岩气藏储层有效渗流喉道半径下限为0.07μm.结合低磁场核磁共振弛豫谱技术,发现致密气藏束缚水并不完全分布在微孔隙中,被小喉道所控制的较大孔隙中仍含有约16.39%的束缚水,这与该类储层喉道半径较小、孔喉半径较大有关.与密闭取心岩样核磁共振实验结果对比表明,优选离心力2.07 MPa的离心实验结果与气藏原始状态束缚水分布规律及原始含气饱和度具有很好的一致性.致密气藏束缚水饱和度较高,初始含气饱和度仅为26.76%.

Initial oil or gas saturation is one of the most important parameters to calculate and evaluate the reserves.A set of various centrifugal forces were used in the centrifugal experiment in order to find the suitable force corre-sponding to the tight gas reservoir.The experiment indicated that2.07 MPa was suitable and the classical centrifugalforces such as 0.69 MPa and 1.38 MPa were less appropriate to the tight gas reservoir.Combined with the Laplaceequation,results indicated thatthe lower limitof the effective flowthroatradius of the tightgas reservoir was close to0.07μm.This value laid the foundation for the reservoir evaluation.Lowfield nuclear magnetic resonance（NMR）experimental results also implied that the irreducible water located in micro-pore space and traped in some largepores,which was related to the small throat radius but large pore-to throat ratio.Compared with the results of sealedcores,the centrifugal experimental results such as irreducible distribution feature and gas saturation are consistent with the results obtained from the initial gas reservoir.In summary,the irreducible water saturation of the tight gasreservoir is high and its initial gas saturation is as lowas 26.76%.