倾斜储层压力拱比计算方法

Calculation method for stress arching ratio in inclined reservoir

压力拱效应影响储层应力、有效应力的计算,而有效应力又影响孔隙度和渗透率的应力敏感性分析结果。通常用压力拱比来表征压力拱效应的强弱,而常规压力拱比计算模型未考虑地层倾角和方位角的影响。以复合材料细观力学中的夹杂理论为基础,建立倾斜储层压力拱比计算模型,利用张量运算法则进行求解,分析地层倾角、方位角、泊松比和岩石固结系数对压力拱效应的影响;在此基础上理论推导和分析压力拱效应对孔隙度和渗透率的影响。研究结果表明:当地层倾角、泊松比较小,纵横比较大,岩石固结系数较小时,垂向压力拱效应越强;地层倾角较大,纵横比、泊松比较小,岩石固结系数较小时,水平压力拱效应越强;平均压力拱比仅与岩石固结系数和储层泊松比有关。压力拱比一般为正值,但受应力集中的影响,倾斜储层主地应力方向的压力拱比可能为负值。有效应力的计算与压力拱比有关,而有效应力影响储层的应力敏感性。对于垂向、最大水平、最小水平和平均压力拱比分别为0.107 2,0.674 7,0.718 1和0.5的储层,当地层压力降低30 MPa时,利用夹杂理论计算的孔隙度和渗透率较常规变形理论的计算结果分别提高32%和76.7%。压力拱效应的存在降低了储层孔隙度和渗透率的应力敏感性,因此在低渗透致密油气藏开发过程中须考虑压力拱效应的影响。

Stress arch effect has influence on the calculation of stress and effective stress in reservoirs,and the effectivestress has influence on the analysis result of stress sensitivity of porosity and permeability. The stress arch ratio is usuallyused to characterize the degree of stress arch effect. The influence of reservoir dip and reservoir azimuth wasn’t consideredin general model of calculating the stress arch ratio. A new model of calculating stress arching ratio of inclined reservoirwas established using the inclusion theory which is used to solve the problem in composite material mechanics. This modelcan be solved by tensor algorithm. Correlated influence factors were analyzed,such as reservoir dip,strike direction,Pos?sion’s ratio and rock consolidation coefficient. Then the stress arch effect on porosity and permeability change were also an?alyzed. The result shows that the stress arch effect is obvious in vertical direction for the reservoir with small reservoir dipangle,large aspect ratio,small Possion’s ratio and small rock consolidation coefficient. While,it is significant in horizontaldirection for the reservoir with large reservoir dip angle,small aspect ratio,small Possion’s ratio and small rock consolida?tion coefficient. Average stress arching ratio are only related to rock consolidation coefficient and reservoir Possion’s ratio.The stress arch ratio is usually positive,while it is negative in principal crustal stress direction due to stress concentrationespecially for inclined reservoir. The effective stress is relative to the stress arching ratio,while stress sensitivity is influ?enced by the effective stress. When vertical,minimum horizontal,maximum horizontal and average arching ratios of the res?ervoir are 0.107 2,0.674 7,0.718 1 and 0.5 respectively,the porosity and permeability calculated by the inclusion theoryincrease by 32% and 76.7% respectively compared with the values calculated by conventional deformation theory for thepressure drop of 30 MPa. The stress arch effect reduces the degree of stress sensitivity of the permeability and porosity inactual reservoir,which cannot be neglected during the development of low permeability tight oil reservoir.