The evaluation of permeability in reservoir assessment is a complex problem. Thus, it is difficult to perform direct evaluation permeability with conventional well-logging methods. Considering that reservoir permeability significantly affects mud invasion during drilling, we derive a mathematical model to assess the reservoir permeability based on mud invasion. A numerical model is first used to simulate the process of mud invasion and mud cake growth. Then, based on Darcy＇s law, an approximation is derived to associate the depth of mud invasion with reservoir permeability. A mathematical model is constructed to evaluate the reservoir permeability as a function of the mud invasion depth in time-lapse logging. Sensitivity analyses of the reservoir porosity, permeability, and water saturation are performed, and the results suggest that the proposed model and method are well suited for oil layers or oil-water layers of low porosity and low permeability. Numerical simulations using field logging and coring data suggest that the evaluated and assumed permeability data agree, validating the proposed model and method.
We conducted a study on the numerical calculation and response analysis of a transient electromagnetic field generated by a ground source in geological media. One solution method, the traditional discrete image method, involves complex operation, and its digital filtering algorithm requires a large number of calculations. To solve these problems, we proposed an improved discrete image method, where the following are realized： the real number of the electromagnetic field solution based on the Gaver-Stehfest algorithm for approximate inversion, the exponential approximation of the objective kernel function using the Prony method, the transient electromagnetic field according to discrete image theory, and closed-form solution of the approximate coefficients. To verify the method, we tentatively calculated the transient electromagnetic field in a homogeneous model and compared it with the results obtained from the Hankel transform digital filtering method. The results show that the method has considerable accuracy and good applicability. We then used this method to calculate the transient electromagnetic field generated by a ground magnetic dipole source in a typical geoelectric model and analyzed the horizontal component response of the induced magnetic field obtained from the ＂ground excitation-stratum measurement method. We reached the conclusion that the horizontal component response of a transient field is related to the geoelectric structure, observation time, spatial location, and others. The horizontal component response of the induced magnetic field reflects the eddy current field distribution and its vertical gradient variation. During the detection of abnormal objects, positions with a zero or comparatively large offset were selected for the drill- hole measurements or a comparatively long observation delay was adopted to reduce the influence of the ambient field on the survey results. The discrete image method and forward calculation results in this paper can be used as references for relevant researc
