Quantitative evaluation of fracture porosity from dual laterlog based on deep learning method

Fracture porosity is one of the key parameters for characterizing fractured reservoirs.However,fracture porosity calculation is difficult with conventional logging data due to severe anisotropy of the reservoirs.To deal with the problem,the equivalent macroscopic anisotropic formation model based on dual laterolog(DLL)data is adopted to cyclically assign such parameters as bedrock resistivity(RB),fluid resistivity in fractures(RFL),fracture dip angle(FDA)and fracture thickness as well as fracture spacing,and to produce massive data for formation modeling.A large number of training data obtained through three dimensional finite element forward modeling and the functional relationship between DLL responses and fracture parameters that are trained and summarized by deep neural network,are combined to establish a new fast forward model for calculating DLL responses in fractured formations.A new fracture porosity inversion model for fractured reservoirs based on gradient optimization inversion algorithm combined with multi-initial inversion strategy is then proposed.While running the model,formation is divided into eight intervals according to bedrock resistivity and fracture dip angle from 0°to 90°is divided every 0.5°to improve the operation speed and efficiency.The results of numerical verification show that when bedrock resistivity is greater than 1000Ωm,the mean absolute error(MAE)of fracture porosity inversion is 0.001658%for horizontal fractures,0.00413%for intermediate fractures and 0.0027%for quasi-vertical fractures.When bedrock resistivity is between 100Ωm and 1000Ωm,MAE of fracture porosity inversion is 0.003%for horizontal fractures,0.0034%for intermediate fractures and 0.00348%for quasi-vertical fractures.Fracture parameters determined by the fracture porosity inversion model with actual data are in good agreement with the results of micro resistivity imaging logging.

Interpreting Dual Laterolog Fracture Data in Fractured Carbonate Formation

The estimation of fractures is key to evaluating fractured carbonate reservoirs. It is difficult to evaluate this kind of reservoir because of its heterogeneously distributed fractures and anisotropy, A three-dimensional numerical model was used to simulate the responses of the dual laterolog （DLL） in a fractured formation based on a macro-isotropic anisotropic model, Accordingly, a fast fracture computing method was developed. First, the apparent conductivity of the DLL is linearly related to the porosity of the fracture and the conductivity of pore fluid. Second, the amplitude difference of the deep and shallow apparent resistivity logs is mainly dependent on the dip angle of the fracture. Then the response of the DLL to a formation with dip angle fractures is approximately depicted as a function of the bulk resistivity of the rock, the porosity of the fractures and the conductivity of fracture fluid. This function can be used to compute the porosity of fracture quickly. The actual data show that the fracture parameters determined by the DLL closely coincide with the formation micro imager log.