Numerical simulation of array laterolog responses in anisotropic formation with mud invasion

Mud invasion and formation anisotropy can cause the separation of array laterolog curves with different investigation depths.The curve separation is the key to the inversion of formation parameters.Based on 3D finite element method(3D-FEM), the array laterolog responses in anisotropic formation with mud invasion were simulated and the affection of anisotropy, mud invasion, wellbore deviation and surrounding rock on the curve separation was studied.The following conclusions were drawn.①In vertical wells, array laterolog curves are mainly affected by the resistivity along bedding direction.With the increment of stratigraphic dip angle, the contribution of vertical resistity increases.In infinite thick anisotropic formation, there is a negative separation among the curves (RLA1~RLA5) with different investigation depths, when the angle is small.When the angle is large, the separation is positive.The critical angle is around 60°.②The influence of mud invasion on curve separation is obviously greater than that of anisotropy.③For anisotropic reservoirs with mud invasion, the difference characteristics among the curves with different investigation depths are messy, which makes it difficult to identify reservoir fluids (oil or water) using the differences among the curves.④For isotropic formations with layered mud invasion, the curve separation degree decreases with the increment of well deviation angle, affected by low-resistivity surrounding rocks.When the mud invasion is not deep (0.1~0.4m), the curve separation degree increases with the increment of well deviation angle.The achievements of the study on array laterolog responses in anisotropic formation with mud invasion have referential significance for data processing and reservoir evaluation. © 2020, Editorial Department OIL GEOPHYSICAL PROSPECTING. All right reserved.