Performance of Gravity Driven and Centrifugal Down-Hole Gas Separators for Continuous and Intermittent Flow

Gas entering the pump is one of the most common causes of decreased efficiency in a sucker rod pumped well or any pumping system. A way to combat this problem is the installation of a down-hole gas separator, also known as a gas anchor at the bottom of the pump. Unfortunately inefficient gas anchors are common and an acceptable guide for optimum gas anchor design does not exist. A gas anchor is composed of an outer barrel (known as the mud anchor) and an inner small diameter tube (known as the dip tube) that is connected to the pump inlet. The produced liquid and some of the gas enter the separator through entry ports on the mud anchor. Inside the separator the produced liquid flows down to the dip tube suction. The gas bubbles in the separator, depending on their size, either flow up and leave through vent holes or flow down into the dip tube suction. Four different gas anchor designs were tested in a laboratory well at the University of Texas production engineering facility. Three of the gas anchor designs are gravity driven gas separators (where gravity separates the gas from the liquid), and thefourth design is a static centrifugal gas separator named the Twister (where centrifugal forces separate the gas from the liquid). Testing was conducted in continuous flow, as that corresponding to a progressing cavity pumped well, and in intermittent flow, as in a sucker rod pumped well.Intermittent flow was obtained by opening and closing an automated valve. The intermittent flow conditions during testing simulated a pumping speed of 10 strokes per minute. The effect of four variables: separator entry port geometry, annular space inside the separator, liquid production rate, and gas rate entering the well, were analyzed. Previous laboratory studies7, 9 undertaken within this project confirm that when the pump inlet is below the perforations all the gas will be separated in the casing and no gas anchor is needed. Also if the gas anchor inlets are in front of the perforations theseparation efficiency is significantly better than when the gas anchor inlets are above the perforations. The new results for intermittent flow, show, that for the conditions in the laboratory well, the entry port geometry did not have a significant impact on the separator efficiency. Also the efficiency of all gravity driven separators was limited by the liquid velocity inside the separator annulus. When the liquid velocity inside the separator averaged approximately 6 in/s or less, complete gas separation was observed. On the other hand, the centrifugal separator had a significantly larger capacity. The most efficient gravity separator achieved complete gas separation at liquid production rates as high as 250 BPD, while the Twister did so up to 390 BPD.