Production Performance of Perforated Wells of Tight Gas Reservoirs
Engineering Honours 2007
University of Adelaide
The objective of this project is to investigate the different factors that can affect the production performance of perforated wells in a tight gas reservoir. The key parameters that can impair the production efficiency of a perforated well were identified in this study through parametric study based on field representative tight gas field data .The factors considered for this study were related to both perforation and reservoir point of view, such as, perforation density, orientation, length, reservoir thickness, reservoir permeability anisotropy and degree of damage (radius of damage) in case of both vertical and horizontal well.
The analyses have been conducted using commercial software called NE Tools for different cases. As a part of this study, comprehensive literature survey has been done to acquire a fundamental understanding of the theory, basic mechanisms and fundamental basis of mathematical equations used for predicting the well performances of perforated wells, especially for tight gas reservoirs. From analysis, it was observed that:
The maximum productivity can be achieved for certain number of perforation per unit length of total perforation interval (what is called shot density) for a given reservoir condition for both horizontal and vertical well.
Increase in positive skin, the production decrease. The decrease in productivity, however, is greater for horizontal wells with increase in skin. Therefore, a proper economic analysis is required to determine what type of well needs to be drilled (either horizontal or vertical).
Horizontal wells seem to work well in most cases and provide a great boost in production (nearly 10 times greater than vertical wells) for thin reservoir. This trend is not substantial for relatively thick reservoir.
The greater the difference in the vertical and horizontal permeability, lower would be the production rates. This is more commonly observed in horizontal wells, where huge permeability anisotropy could lead to a significant production loss. A possible recommendation to improve the vertical permeability would be gas injection or hydraulic fracturing.
During completion practices there is a fair amount of damage near the well bore, this damage was represented by radius of damage in this study. It was observed that there was a critical point after which the productivity loss really shot up. This was seen in both horizontal and vertical wells. Permeability anisotropy further reduces this production, so it is important to keep the completion damage under control. Possible recommendations to keep the damage radius under check are under balanced drilling and coiled tubing clean out.
From this study, it can be clearly seen that how the above mentioned factors tremendously affect productivity of perforated wells in tight reservoirs. A proper analysis of these factors was done and the results were displayed.
The results, conclusion, inferences and recommendations are made to provide a better insight to production performances of perforated wells in tight gas reservoirs and how different alternatives could be looked to maintain substantial rates of production.
There are a few other factors which affect productivity, but it is difficult to control these factors, but it is important to identify these factors so that alternatives can be looked at to maintain the production rates.
These factors include fluid retention effects, extremely poor quality reservoirs, fluid- rock interactions etc.