The productivity of many hydrocarbon-bearing formations appears to be reduced during water flooding operations. The reason for this reduction is mainly attributed to the permeability damage of the formation surrounding the bore hole. A possible cause of this damage is the precipitation of suspensions on the surface or in the interior of the porous medium.
This study investigates experimentally the effect of maximum suspension size in the injected water on the permeability of some Iraqi reservoir rocks. For thirty three core samples taken from one sandstone and two carbonate formations, the results showed that most of the sandstone cores were plugged partially with 10 microns maximum size while the carbonate cores plugged partially with 1.2 - 3.0) microns particle size.
Also the relation between maximum suspension size) pore size distribution and permeability reduction have been studied. The results showed that plugging occurs when the maximum suspension size is nearly half the pore throat size due to the irregularity in pore geometry which is of omplex nature and of varying cross-sectional area.
An important factor affecting the success of a water flooding project is the quality of the water being injected. Permeability damage around the injection wellbore caused by improper water quality arises many technical problems such as using extra high well head injection pressures, complete formation plugging and fractured well bores. Permeability damages are usually of two types:
Chemical damage caused by swelling of clay particles and interaction between the injected water and the formation water (1).
Physical plugging caused by the solid suspensions present in the injected water and movement of formation fines originally exist in the pores (2).
In addition to those, some damage might be caused by the growth of bacteria and other organisms living in the injected media (3.4.5). The process of permeability damage by bacteria and other organic material is similar to the process of plugging by suspensions.
The way suspensions reduce permeability is a very complex phenomena and affected by many factors such as maximum uspension size, total suspension solid content (T.S.S.). their distribution and the pore size distribution of the rock. It is impossible to correlate these factors mathematically.
Considering physical plugging (which is the main purpose of this study), the best water for injection is such a water which contains no solid particles.
Preparation of such water practically is impossible. In many cases watercontaining suspension sizes smaller than 0.45 microns are considered pure and free of suspensions. Injection of such a high quality water is a costly process.
While a formation can accept suspensions sizes of up to 5 microns without a considerable damage in its permeability, it is not necessary toinstall filtration plants to prepare water containing suspension sizes smaller than 5 microns.
Maximum allowable size of suspensions in an injection water depends on the nature of the formation in term of the pore sizes which is possibly linked with the absolute permeability.
The concept of pore size distribution has been introduced as an attempt to characterize the complicated geometry of the flow channels in porous media
