The Numerical Simulation Of Response Of Dual Laterolog In Fracture-cavity Reservoirs

Complex reservoirs, such as carbonate rock, volcanic rock, have become the main target of petroleum exploration. And it is very important for recognize and predict storage space of fractures and caves commonly existing in these reservoirs. The application of electric imaging has improved the accuracy of fracture interpretation greatly. However, due to their limited investigation depth, facing large scale caves, the micro resistivity logging can not achieve images of good quality. Under this condition, the dual laterolog with large investigation depth and good sensitivity to fractures and caves becomes a convenient and practical tool. By the finite element numerical method, this paper systematically studies the rules and characteristics of the laterolog responses for isolated caves and fracture-caves, expecting that the laterolog can be better applied in evaluation of reservoirs of complex formations, such as isolated caves and fracture-caves.According to the logging principles of dual laterolog, the forward responses of dual laterolog for pore-cave model and fracture-cave model are simulated numerically. Then the variation principle of the 3-D finite element is deduced. Based on a nonuniform gridding, the mesh for the pore-cave model is further locally refined, in order to recognize the discrete elements of the caves. Then the discrete elements are used to displace the caves. The installation and solution of the stiffness matrix are realized by the frontal solution. We optimize installation sequence and carry out the installation along with elimination to reduce the storage of the matrix and further improve the computation speed.The forward computation of the dual laterolog responses for the 3-D pore-cave model matches well with the results of 2-D dual laterolog when the resistivity of the caves is equal to block rock resistivity, simply validating the reliability of the algorithm. Using this algorithm, the dual laterolog responses for the pore-cave model under different borehole, mud resistivity and block resistivity conditions are simulated. The results are compared with those when there are no caves. Then the relationship between the responses of the dual laterolog and the distance of the center of the main electrode to the cave in depth, the racial distance of the cave and the borehole, the cave diameter and the fluid resistivity within the cave is researched, making a primary judgement on the racial and vertical impact scope of the cave.As to the fracture-cave models, the different effects of caves and fractures on dual laterolog are analyzed respectively. The horizontal and vertical fracture-cave models are mainly used to simulated the responses of the dual laterolog, in order to study the influencing factors of the pore-cave model. It is analyzed for the effect of the connectivity of the fractures on isolated caves. In dipping fracture-cave system, the effect due to the existence of caves on a single fracture is also analyzed.