鄂尔多斯盆地上古生界致密砂岩气藏成藏物性界限研究

At present, petrophysical limit of tight reservoir is always analyzed by the empirical method. However, the result cannot reasonably explain and define the characteristics of tight reservoir in terms of real accumulating conditions. In order to study the petrophysical limit of gas accumulation in Paleozoic tight sandstone reservoir, Ordos Basin, on the basis of force analysis of fluid in microscope pore-throat of sandstone, correlation between petrophysical properties (porosity, permeability) and force balance (buoyancy, capillary force, abnormal pressure, hydrodynamic force) can be built, which gives a new idea to investigate the petrophysical limit of gas accumulation in tight reservoir, i.e. with geological conditions of gas accumulation in Paleozoic tight sandstone gas reservoir of Ordos basin, critical pore-throat radius of gas accumulating in tight sandstone reservoir can be induced based on the real geological parameters. After assuming the thickness of irreducible water film as the lower limit of radius for gas injecting, and critical radius, where capillary force is equal to buoyancy, acts as the upper limit, than it can be established the fitting relationship between pore-throat radius and petrophysical properties. At last, according to the limit of pore-throat radius, limit of petrophysical properties can be calculated. Thus, with accurate reconstruction of geological parameters of gas accumulating, tighten degree of Paleozoic sandstone in Ordos Basin can be evaluated reasonably.

目前国内外通常以经验统计法对致密砂岩气储层进行物性界限分析，但是该界限不能从实际成藏状态角度对储层致密特性给予合理解释和界定。为了研究鄂尔多斯盆地上古生界致密砂岩储层天然气充注成藏的物性界限，针对砂岩微观孔喉内流体实际受力状态入手，建立微观孔喉受力平衡（地层温压条件下的浮力、毛细管阻力、异常压力和水动力）与物性（孔隙度、渗透率）之间的科学关联分析，为分析致密砂岩气成藏物性界限提供新思路：从鄂尔多斯盆地上古生界致密砂岩气藏地质条件出发，依据实际地质状态参数推算砂岩储层天然气充注的临界孔喉半径，设定束缚水膜的厚度为天然气充注的孔喉半径致密下限，同时以毛细管阻力等于浮力的临界孔喉半径为致密上限；然后建立孔喉半径与物性参数之间的拟合关系，最终依靠孔喉半径界限来推算相应物性参数界限数值。通过推算成藏过程中地质参数的准确恢复，从而达到对鄂尔多斯盆地上古生界砂岩致密状态给予客观评价的研究目的。

对鄂尔多斯盆地上古生界主要含气砂体致密界限的分析，从致密砂岩微观孔喉内天然气的受力状态入手，综合考虑古今构造形态及储层温压等实际地质条件的独特性，推算结果显示上古生界含气砂体古今成藏物性致密界限高于传统经验统计数值，同一盆地不同地区不同层段致密界限也存在一定差异，其中榆林地区山2段整体相对较低，苏里格地区盒8段次之，榆林地区盒8段数值相对较高。鄂尔多斯盆地上古生界主要含气层段致密上限显示为一定的变化范围，其数值大于对致密砂岩气储层渗透率上限的普遍看法，所以对非常规致密砂岩气的科学界定需更多考虑地质因素而不应一概而论。.首次尝试分别从宏观孔隙度角度和微观孔喉半径角度，以粒径、硅质、高岭石、水云母和方解石含量等成岩参数，建立孔隙度、孔喉半径与上述成岩因素之间的效应函数，效应拟合结果表明影响储层孔-喉体系演化的主要因素为粒径大小及硅质胶结物含量。同时针对不同粒径范围分段建立孔隙度、孔喉中值半径与主要成岩因素之间的效应模拟，综合考虑粒径、硅质、高岭石、水云母和方解石等参数对孔喉演化的影响，效应模型能够更加精细地恢复储层孔隙度及孔喉半径，并通过实例证明拟合结果具有实用价值。