Identifying, Bypassing, and Avoiding Gas Traps in Horizontal Wells

Abstract When dealing with horizontal wells in gas reservoirs with low bottomhole pressure, it is important to avoid traps –– that is, S-shaped portions of the wellbore. These wellbore traps look and act similarly to the traps under a sink or toilet, which hold water in the dip to prevent low-pressure, odorous gases in the sewer from rising into a house. When analyzing survey data from a horizontal well, it is sometimes difficult to identify traps just by looking at the numbers. There are a few different techniques that can be employed to solve this problem. One technique is to import the survey data into a spreadsheet and solve for the slope of the well. Once the slope is found, the traps can be determined from multiple changes of slope orientation in row. Another, more visually friendly method of identifying traps is the use of 3-D graphs. By importing the survey data into a 3-D graph, the traps can easily be identified by the "S" shaped curves in the wellbore. Once located, traps can be bypassed; depending on the length and severity of the trap, a line can be run through the trap to provide an exit for the trapped gas. Periodic treatments of N2 can be used to blow trapped water out of traps, allowing gas to migrate to surface. This paper will discuss the concept of traps in horizontal gas wells and different methods of identifying traps, including spreadsheets and 3-D graphs. The paper will conclude with useful means of bypassing existing traps and preventing traps in future horizontal drilling.