THE EVOLUTION OF A CLAY CRUST IN A BOREHOLE

Based on the previously proposed quasi-stationary model of clay crust formation when drilling wells in depth intervals with permeable rocks, software was created and computational experiments were performed, which made it possible to clarify the contribution of various physical parameters to the process of its growth.Based on the analysis of the results of computational experiments, the contribution of the physical parameters of the formation and clay crust to the dependence of its size on time has been clarified. The model assumes that the filtration pressure fields are described by stationary piezo conductivity equations. This made it possible to construct explicit dependences of pressure, thickness of the clay crust, radius of the zone of pressure disturbances in the reservoir during drilling and other parameters on time. Simple approximate formulas have been found for the dependence of the thickness of the clay crust, the filtration rate and the radius of the pressure disturbance zone in the reservoir on time, which describe their evolution with high accuracy.Based on the analysis of the calculation results, it was found that the thickness of the clay crust significantly depends on the permeability of the reservoir and repression, and these parameters determine the process of its formation. The porosity of the collector makes a smaller contribution, and the filtration and capacitance parameters of the clay crust have little effect on its thickness.New possibilities for estimating the physical parameters of reservoirs based on cavernometry have been identified. The solution of this problem is especially important for horizontal wells, in which the clay crust intervals have a significant length, and its formation occurs under conditions of a wide range of reservoir properties of the formation. The considered problem is of practical importance, since the zone of penetration of drilling mud filtrate into the formation has a screening effect on the use of geophysical methods to determine reservoir saturation. The results obtained can be used both to improve the interpretation of geophysical methods and to develop those.When conducting computational experiments using an exact formula, the stochastic behavior of the calculated curves was found. It was found that the noted effect is associated with rounding error, and in this sense is a manifestation of the features of the processor. An approach is proposed to eliminate the detected stochastic errors.