裂缝型储层流体识别方法

The method for identification of fluid in fractured reservoirs

裂缝型储层的描述包括预测裂缝分布特征和识别裂隙充填物.依据等效介质理论计算的纵波速度随裂缝密度的增大而减小.正演地震记录显示,裂缝介质含气时反射振幅最大,且变化程度比含油或含水时大.叠前方位AVO反演所得的各向异性梯度Bani与裂缝密度成正比,可用于描述有效裂缝发育强度.对于不同的裂缝密度,各向异性梯度Bani与各向同性梯度Biso的比值I(1)fluid近似为常数,且对流体敏感.经裂缝纵横比和背景介质拉梅常数修正后,流体因子Ifluid既不随纵横比变化,又不受背景介质的影响,是裂缝型储层敏感的流体识别因子.在塔里木盆地塔北哈拉哈塘地区热瓦普区块碳酸盐岩储层裂缝发育区域,运用该参数在井点处的流体识别效果与钻井结果一致.

The prediction of fractured reservoirs includes the detection of cracks and the identification of fluid filled.We usually use combination of elastic parameters and AVO attributes as the fluid factor for isotropic media.Now,we should replace it with anisotropic parameters that do not change with the azimuth of fractured reservoirs.Based on Hodson′s effective elastic medium theory,the elastic coefficient matrix of an anisotropic medium depends on four variables：fracture density,filling fluid,crack aspect ratio and isotropic background rocks.One can calculate wave velocity（VP,VS）and Thomson anisotropy parameters（ε,δ,γ）,and then compute the AVO gradient Biso and anisotropic AVO gradient Bani according to the Ruger reflection coefficient formula.We collect lots of Biso and Bani with many crack models,and analyze their relationship using the scatter plot.We fit them as a function of four variables and calculate the fitting error.Obviously,the two gradients change with fracture density and fluid filled.They are also decided by crack aspect ratio and background medium.Based on anisotropic equivalent medium theory,vertical P-wave velocity decreases with increasing fracture density.It is the maximum with water filled,intermediate with oil filled,and the minimum with gas filled.Accordingly,the amplitude of filling gas has the largest reflection,and its change is much greater than the amplitude of oil or water.Through numerical experiments and formula deduction,AVO gradient Biso and anisotropic gradient Bani are approximately linearfunction of fracture density.Their slopes,respectively kiso and kani,depend on the background media,the crack aspect ratio and fluid filling.With the background medium or clastic carbonate rocks,the relative error of approximate formulas is less than 5% for either carbonate rocks or clasolite.The ratio between gradient parameters Bani and Biso is constant approximately for different fracture density values,which is sensitive to fluid.So it is defined as the first fluid（factor I1）fluid,which is inversely and linearly related with the aspect ratio.One can obtain the（second fluid factor I2）fluidthrough correcting the first fluid factor by the aspect ratio.Because the second fluid factor is related with the background medium,we obtain the fluid identification factor Ifluidthrough correcting the second fluid factor by the Lame constant of the background medium.The fluid identification factor Ifluidis approximately equal to the first Lame constant of fluid filled.The fluid factor Ifluidis affected neither by the crack aspect ratio nor by background medium,and its value approximates the first Lame constant of filling fluid,so it is sensitive to the fractured reservoir fluid identification factor.In the Ordovician carbonate fractured reservoir of western China,the result of fluid identification using this parameter is consistent with interpretation of drilling data.