Drying towers in natural gas service can become rapidly fouled with drilling mud or formation and frac sand. The sand appears in the wellhead gas when the rate of gas production becomes excessive, and the sand is thus sucked out of the formation and into the well’s tubing. Drilling mud is found in natural gas for two reasons:
1. A new well is not properly circulated and flowed-back to clear the drilling mud out of the production tubing prior to commissioning.
2. During the drilling operation, excessive mud pressures are accidentally applied to the well, and the drilling mud is thus inadvertently forced into the producing formation. Some of this mud must eventually reappear in the downhole production tubing.
Not infrequently, a dehydrator loses its ability to dry gas from a field in which a new well has been put on-line. When this occurs, the culprit is invariably drilling mud plugging the contactor internals. For remote locations, one procedure that has proved to work is as follows:
1. A large water truck equipped with a pump to deliver about 50 psig, is sent to the site.
2. The dehydrator tower is blocked in and depressured. Both the tower inlet and outlet are disconnected from the gas piping. A special flange attachment, designed to mate up with a hose connection, is installed on the gas outlet line.
3. A two-inch hose from the discharge of the truck’s pump is connected to the dehydrator tower gas outlet line.
4. The pump is started and adjusted so that the pressure at the top of the tower—i.e., the water inlet—is about 5 psig. It is important not to apply too great a pressure because the trays could collapse.
5. Once the water draining from the bottom of the contactor tower appears clear, switch the water inlet to the bottom gas inlet. Over-flow the tower until the water is again clear. The water overflow rate must be substantially higher than the normal glycol circulation rate to obtain enough liquid traffic to effectively wash the trays.
Why, you might ask, it is necessary to initially wash a badly fouled tower from the top, down? A tray plugged with mud will severely restrict the flow of water. The resulting pressure drop may be sustained by the tray when it is pressed down onto the tray support ring when applied from the bottom of the tray.
In more accessible locations, it is a good practice to acidize a contactor tower after water washing. Acidizing consists of circulating an inhibited hydrochloric acid solution (typically 5% HCI) to the bottom of the tower with an acid truck. This is an effective method to clean contactors without promoting channeling of the gas flow through the trays. Acidizing is especially effective when iron scale deposits make up a portion of the fouling deposits. Including the acid disposal expense, acidizing a drying tower can cost between $20,000-50,000. When hydrocarbon deposits consisting largely of polymers formed in the glycol reboiler are the major fouling component, a caustic wash, as opposed to acidizing, is in order. In the caustic washing procedure, a degreaser is also employed. A more elaborate, but thorough, procedure is summarized in Table 6-1.