A One-Dimensional Formation Damage Simulator for Damage Due to Fines Migration

Abstract

One of the major causes of formation damage is the movement and trapping of fine particles in pores. These fines may be generated in-situ by the interaction of injected fluids with the formation or they may be injected along with the fluids. A model is presented that simulates the permeability impairment caused by fines migration or injection in a radial geometry. The physical basis for the equations are the same as for a linear model that has been presented earlier. The equations are quite general and can be used to model the permeability reduction for any given pore or particle size distribution. The fines migration phenomenon is a moving boundary type problem. The position of the boundary is determined by assuming that it propagates as an ideal sharp front. The set of coupled, non-linear partial differential equations for the model are solved by an implicit finite-differencing technique. The validity of the simulator is tested against analytical equations that have been developed for some special cases. The agreement between the simulator and the analytical solutions is found to be excellent. The simulator is also used to investigate the importance of various parameters. For example it is found that the pore and particle size distributions, as well as their concentrations have a significant effect on the permeability reduction. Other factors of importance are the rate of release of attached particles and the geometry of the pores. Methods of estimating these parameters are suggested. The use of the simulator to calculate the extent and depth of damage of damage, when fluids are injected into the formation, is demonstrated. It is expected that the simulator will prove useful in the design of such operations. Introduction The water sensitivity of sandstones is of considerable importance in the production of oil and gas since almost all oil bearing sandstones contain clays. The injection of a fluid whose solution chemistry is incompatible with the resident brine or the clays chemistry presents a clear danger to successful oil recovery. This is a potential problem in many field operations such as well workover operations, drilling, acidization, polymer flooding, and in particular water flooding. The quality of water chosen for injection may be such as to trigger the release of fines. Alternatively the water may contain particles that were transported from the water source or were entrained during flow through tubing. These particles can cause significant damage due to blocking of the pore throats. The permeability impairment caused by fines migration or injection shows up as a positive skin around the wellbore. Well testing procedures can be used to measure the skin factor which may be written as (1) It is not possible, however, to obtain independent estimates of both r and k. These parameters are crucial to the design and implementation of well treatment procedures such as acidizing or fracturing. The simulator developed here provides a method for independently determining rd and kd from a given set of wellbore and formation parameters. P. 29^

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