The method for identification of fluid in fractured reservoirs

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(e,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.