盖层有效厚度计算方法及应用

Calculation method and application of cap rocks effective thickness

基于盖层非均质性导致盖层内部产生物性封闭能力的强弱差异,提出盖层有效厚度的计算方法。综合利用泥质岩盖层和砂岩储层的薄片、扫描电镜、突破压力、孔隙度和渗透率等分析测试资料确定盖层和储层的分界,包括突破压力界限、临界岩性(粒度)和物性界限,进而建立盖层物性封闭评价标准;利用自然伽马曲线、声波时差曲线和分析测试资料建立测井解释模型,定量识别盖层岩性并计算孔隙度、渗透率和突破压力;根据盖层物性封闭评价标准综合评价盖层中每种岩性的物性封闭等级,剔除不具有封闭性的部分,从而获得有效厚度。应用该方法表明:X气藏盖层和储层的突破压力界限为2 MPa、临界岩性(粒度)是泥质粗粉砂岩、孔隙度界限约6%、渗透率界限约0.02×10-3μm2,并将盖层物性封闭能力划分为Ⅰ~Ⅴ5个等级,剔除不具有封闭能力的Ⅳ和Ⅴ,获得单套盖层的有效厚度为4~10 m。气柱高度和有效厚度的相关关系证明该方法是适用的,继而利用盖层有效厚度对突破压力进行加权平均获得盖层等效突破压力,有效地确定盖层的整体物性封闭能力为中等。

A method of computing cap rock＇s effective thickness is proposed based on that cap rock＇s heterogeneity may result in different capillary sealing ability within cap rock.Using data of thin section,SEM,breakthrough pressure,porosity,permeability and other test data of argillaceous cap rock and sandstone reservoir,the boundary between cap rock and reservoir can be defined,which includes breakthrough pressure limit,critical lithology（grain size） and petrophysics limit.Subsequentlythe evaluation criteria of cap rock＇s capillary sealing ability is established.With gamma ray curve,interval transit time curve and test data,a logging interpretation model is established to quantitatively recognize cap rock＇s lithology and to calculate porosity,permeability and breakthrough pressure.A comprehensive evaluation on capillary sealing ability of cap rock has been made to first remove the cap rock with no capillary sealing ability and then to obtain the effective thickness.Our results show that between cap rock and reservoir of X gas reservoir,the breakthrough pressure limit is 2 MPa; the critical lithology（grain size） is argillaceous coarse siltstone; the porosity limit is about 6%,and the permeability limit is about 0.02×10^（-3）μm^2.Cap rock＇s capillary sealing ability has been divided into five grades from Ⅰ to Ⅴ.By removing grade Ⅳ and Ⅴwhich correspond to no capillary sealing ability,the effective thickness of single cap rock is obtained between 4 and 10 m.Correlation between the height of gas column and the effective thickness suggests the robustness of the method.Based on the effective thickness,a weighted average of breakthrough pressure has been taken to obtain equivalent breakthrough pressure,and the whole capillary sealing ability of cap rock has been evaluated as moderate effectively.