Methods Of Formation Resistivity Correction For Salty Mud Invasion

Salty mud has much higher salinity and stronger conductivity, when drilling with high salinity mud, compared with formation water, mud invasion made conventional resistivity logging response to be seriously affected, leading to the actual formation resistivity much less than the true resistivity. It is very difficult to accurately compute the formation oil(or gas) saturation, and the logging evaluation results are affected.The mud filtrate invasion process is influenced by the formation pressure, permeability, porosity, water saturation, water (oil) viscosity, time of mud soaked formation, and other factors. The mud invasion of correction is studied by many experts, and a large number of new methods and techniques have been put forward, however, it is difficult to correct the mud invasion, because the mud invasion is influenced by many factors, it is one of the important subjects in logging data processing until now.In this paper, based on the mechanism of mud filtrate invasion, methods of formation resistivity correction for salty mud invasion have been explored and studied, the following work had been done:Firstly, the process of mud filtrate dynamic invasion is simulated, the formation water saturation and the formation water-salinity of the dynamic relations are calculated, then the corresponding resistivity response is calculated, both the mud invasion profile and the resistivity response laws over the time have been studied in detail. This process contain mainly three parts,the first the mud filtrate dynamic model of invasion is established, the mud filtrate replace the original formation is simulated by the two-phase (oil/water) flow equation and the water-salinity interface is tracked by the convective transport model. The second the mud filtrate dynamic model of invasion is solved, the finite difference method was used to solve the equation, the"implicit pressure and explicit saturation"approach to the two-phase (oil/water) flow equation, the formation pressure and water saturation over the invasion time is obtained, the convective-diffusive equation is directly solved by the finite difference method, the formation water-salinity is obtained. The third the rules of changes for the dynamic response model of resitivity logging is studied, the impact of mud filtrate invasion for the single parameter is analyzed, and the impact of the porosity, permeability and mud cake permeability to mud filtrate invasion analyzed, then, integrated in the formation parameters, studing the formation pressure, water saturation, formation water salinity and the formation resistivity laws of changes.Secondly, Based on the research of the laws of formation resistivity over time, using lateral geometry theory of the induction logging, the radial response of the induction logging equipment over the time is studied in this paper, the deep/ shallow investigate induction logging characteristics over the invasion time is obtained. Up on the double lateral resistivity logging, using other's research about the infinite formation geometry, combined the relationship between the invasion time and the invasion depth, the salty mud invasion resistivity correction is completed by chart interpolation.The last, the resistivity logging correction methods on the saline mud invasion circumstances is studied, and mainly research on four salty mud invasion correction methods. Firstly, the spontaneous potential logging curve characteristics is used to complete the resistivity correction in the permeable formation, the spontaneous potential logging curve will appear abnormal when the mud salinity is greater than the formation water salinity, according to this conclusion, the resistivity correction on the permeability formation is completed. Secondly, based the measure principle of the double lateral logging, combined the electrode mode of the double lateral logging, the mud invasion correction formula for resistivity correction is derived, however, when the diameter of mud penetrated depth over the deep lateral logging exploration depth, this method will be failed. Thirdly, using the actual resistivity time-lapse logging data, the impact of time-lapse resisitivity logging for the invasion time and the invasion depth were analyzed, then using statistical method to identify different time logging data, the experience formula is obtained of correction. Fourthly, electricity logging chart is used, through interpolation for the chart,according to the invasion profile information, the deep investigate resistivity logging is corrected, the invasion profile is confirmed by the middle/shallow resistivity logging data, then using image grid sampling and the analysis of weight, the program is developed to complete the invasion correction for the actual logging data.In this paper, processing software for all of formation resistivity correction methods for the salty mud invasion are developed, and applied in Banqiao-BeiDagang area, Dagang oil field which has achieved good effect.