Surface Equipment – Cassing Pressure
Ideally, there should not be any gas accumulation inside the casing of a single completion well. If gas does infiltrate the annular space between the casing and the tubing, excessive pressure will build-up inside the casing. If the casing pressure greatly exceeds the tubing pressure, the tubing will collapse. If you observe the operation of a well that has a collapsed tubing string, the only signs will be low wellhead pressure and diminished gas production. Unfortunately, there are a host of other illnesses that beset gas wells that have identical systems:
• Well loaded up with fluids.
• Perforations covered with sand.
• Low bottom hole pressure.
• Production tubing bridged with sand.
To prevent the collapse of the tubing string, the well operator’s duties include venting off pressure from the casing. A cost effective method to accomplish such venting is shown in figure 3-2. Instead of depressurizing the casing to the atmosphere, the excess gas in the casing is vented into the production tubing downstream of the wellhead choke. This saves money. For example, venting a 1000 psig casing from a well 10,000 feet deep into a production line operating at 550 psig saves about $60 per venting incident.