The Numerical Stimulation And Experimental Verification Of Electrical Imaging Logging In Fractured Formations

For the low porosity and permeability and the complicated fracture, the study of fractured reservoirs was limited in qualitative evaluation. Quantitative evaluations of fractured formations are still difficult problems today. electrical Imaging logging method has a high vertical resolution and visual image display and shows a remarkable advantage in fractured formation evaluation. So a good understanding of the electrical Imaging response can lead to the deep application of electrical Imaging in fracture evaluation.The paper introduces the basic principles of the finite element method, and derives the differential equations, interface conditions and boundary conditions of the constant current field equations basing on the electromagnetic theory. Depending on the instrument structure and logging principle of FMI, the n umerical simulation method of electrical Imaging response is built and the three dimensional finite elemental variational method derived of electrical Imaging potential field is derived. Then we study the boundary conditions and the mesh method of finite element method. According to the field situation of fractured formations, we chose the tetrahedral elements for meshing, and increase the mesh density in the large current change area, such as button electrodes, cracks, and plates. Finally we used conjugate gradient method to solve the finite element equations.We used the three dimensional finite element numerical stimulation method to study the logging response of fracture in the formations. Some conclusions are reached as following:(1) Openness of fracture is a key factor of electrical Imaging logging response for fracture. The greater the openness, the larger the stimulated value of conductivity.(2) Mud resistivity is a important factor in electrical Imaging response of fracture. The smaller the resistivity of mud, the larger the stimulated value of conductivity in fractured area.(3) The radial extension of fracture is influential to the electrical Imaging logging response. In a certain range of depth, the conductivity value in the area in front of the button electrodes increases as the increase of radial extension of fracture. But for the limited detected area of electrical Imaging logging tool, the conductivity becomes stable when the radial extension reached a certain depth.(4) In a condition of fracture strike parallels to the plate strike, the detected outliers’area of conductivity value maintains symmetry when the fracture dip is low, and the conductivity curve becomes asymmetry when the dip is larger than60°. According to this phenomenon, the dip of fracture can be evaluated.(5) Two parallel horizontal fractures are stimulated in situations of different distance. The stimulated results reveals that the conductivity curve of two parallel horizontal fractures has a remarkable bimodal feature when the distance of the two fractures are larger than5mm, and the bimodal feature is not clear when the distance of the two fractures are smaller than5mm. So the fractures with a high density are hard to recognize in the electrical Imaging logging images.By the comparison with resistivity experiment of outcrop rock samples, we find the numerical stimulation results are consistent with the experimental results.