Proactive Approach Minimizes Production Losses Due to Slug Flow

Abstract

When wells producing up casing in the Marcellus Shale dip into the slug flow regime, their production begins to drop off significantly. In some instances, wells that begin slugging dramatically can defer the majority of the production that they were delivering just months before slugging, resulting in significant loss in present value. To remedy this complication, engineers can install tubing in these wells to help lift fluids through the vertical section. Because slugging causes large fluctuations in production, most slugging is identified and addressed only after a production engineer notices these fluctuations. This can be months after the decrease in production rate. To challenge this reactive approach to identifying wells ready for tubing installation, I created a workflow—implemented with software—to create a tubing installation schedule which requires little time or effort by the production engineer. While it is unlikely this program can be replicated exactly, the logic I used to create the program can certainly be adopted and applied elsewhere. I used Coleman's model for critical rate to determine quantitatively when a given well is likely to begin to slug. Critical rate is defined as the minimum gas rate needed to maintain steady flow up a well to the surface. When the gas rate in a well dips below this critical rate, the well will dip into the slug flow regime. Bottomhole pressure (BHP) is a necessary input for the critical rate calculation. Because BHP data is not readily available for wells producing up casing, I had to create an alternative approach to determining BHP. After investigation, I determined that using data from different combinations of water-gas ratio (WGR), wellhead pressure (WHP), and gas rate allowed me to create a correlation to approximate the BHP of any given well on any given producing day. The resulting correlation could be incorporated in my workflow and—after combined with other input data—my software could determine, with sufficient accuracy, the critical rate of any given well on any given producing day. An engineer can use my software to create a graph displaying both critical rate and gas rate with time for every well in a data set. Engineers can summarize the pertinent information from those plots in a data table which can assist them with creating a tubing installation schedule. This workflow will help engineers to determine more readily whether any of their wells are on the verge of slugging, allowing them to be more proactive in installing tubing on their wells and preventing costly deferred production.