The purpose of a gas buster is to remove gas mixed with the drilling fluid before the drilling fluid goes over the shale shaker. A gas buster works well in fluid with large bubbles of free gas. (Often the gas is starting to break free in the flowline.) A problem with the basic gas buster is that the heavier gases will not rise and be dissipated in the air but settle around the rig.
An old-fashioned but effective gas buster is made from a piece of 9- or 11-inch (228.6- or 279.4-mm) casing (Figure 9.1). An inlet, tangent to the side but tilted up about 5°, is welded into a 6-ft (2-m) length of casing about one third of the distance from the bottom. The mud entering the inlet spins, and the centrifugal force allows the gas to go to the center and out while the mud goes to the sides and down. A pipe on the top carries the gas away, and the bottom of the casing is open to the shale tank on the shaker.
The tangential intake is used on many land rigs where the gas buster is installed in the possum belly (back tank) of the shaker. The tangential
intake balances the force of the drilling fluid and expanding gas so that the gas-buster tank does not need heavy bracing.
Another version of the pipe gas buster uses the same casing size as the standard gas buster but directs the mud and gas mixture onto a blast plate or "baffle," which breaks up the flow pattern and separates gas and drilling fluid. This system is unbalanced, and the pipe needs to be restrained. In heavy mud and with higher than normal initial gel strength, the baffle system may cause some entrainment of the gas, which appears as gas cutting.
The offshore version of the gas buster uses an 11-inch casing up to 20 feet tall ahead of the shale shaker. The offshore gas buster is closed and usually has a U tube on the line to the shaker to build backpressure and force gas to the discharge line.
9.7 SEPARATORS 9.7.1 Atmospheric Separators
The purpose of a separator is to separate free gas or mixed gas and oil from the drilling fluid and convey the gas or oil to a flare or holding tank.
The simplest atmospheric separator is a mud tank that uses gravity separation. With gravity separation, gas rises and escapes; the oil separates from the drilling fluid and rises to the top of the tank. Oil/water separation can then be carried out with a simple siphon. Several extra mud tanks are commonly used when there is significant amount of free oil to be separated from a water-base drilling fluid. The drilling-fluid mixture may be first passed through a West Texas type of separator (see the section to follow) to remove the free gas, and then it is sent to the first of several mud tanks. In the tank sections, the oil is successively skimmed off with siphons or skimmers, and concentrated oil is pumped to a shale tank. The drilling fluid is taken from the bottom and sent to the solids-control system.
Mud tank separators, often used in the southern end of the Austin Chalk trend in Texas, allow the limited hydrocarbon vapor to escape and settle many of the solids in the bottom of the skimmer tanks. They are easy to set up and inexpensive to operate.
A simplified tank separator system is used with some workover and small drilling rigs. The drilling-fluid/gas/oil mixture is sent directly to an open tank, normally a frac tank, with the flowline in line with the long axis of the tank. The open tank then acts as a separator, with the gas escaping to the atmosphere, the oil and drilling fluid separating, and the solids settling out on the bottom. Drilling fluid is taken from the bottom of the far end of the tank, and during connections oil is pumped to a shale tank.
The basic West Texas separator was developed in the 1950s to drill the Permian Red Beds in West Texas and is still the ''gold standard'' atmospheric separator. It is simple, inexpensive, and efficient. It works best with heavy gas cutting, or with gas/liquid (gaseated) mud. It is classified as an atmospheric, nonpressurized tank, but some of the tanks develop from 2 to 5 psi of internal pressure to force the gas to the flare stack (Figure 9.2).
There are many variations of the West Texas separator. One system uses centrifugal separation with a collection tank. Another basic version uses an impact baffle and a collection tank. One method of discharge is via a U tube to hold a liquid seal in the tank. Another version uses a float and valve. Pressure can be held in the tank as a result of the height of the flare stack, or with a balancing valve on the flare line.
The basic design is so old that little math was ever applied to it, but engineering simulations/solutions have been applied to confirm some of
its actions. Tank sizes vary from 4 to 7 feet in diameter and are generally about 12 feet high, mounted in a frame so that the discharge can be gravity-fed to the shaker. Tank or throughput capacity is a function more of tank and discharge line size than anything else. The original design seems to be have been developed for drilling with 350 gpm of drilling fluid in an 8 3A inch hole while the hole produced 1 to 2 MMscf/day (600 to 1200 scf/m) of gas. Present manufacturers appear to feel confident with values twice as high.
The Super Mud Gas Separator (SWACO SMGStm), 22 feet high x 6 feet wide, was first used for underbalance drilling in the northeastern section of the Austin Chalk trend, where there are very high flow rates. In size, the SMGS may be the largest separator of this type. Capacity is quoted as 65 MMscf/day of gas or 38,000 bpd of liquid.
9.8 PRESSURIZED SEPARATORS 9.8.1 Commercial Separator/Flare Systems
There are a number of portable rental flare systems that use a closed separator or free water knockout (FWK) ahead of the flare stack.
Pressure Relief Valve w
Courtesy Welco Energy Services (Calgary, Canada)
The systems range from a trailer-mounted flare stack with a minimal FWK to skids with a 6 x 10-feet, 1.5-atmosphere (7 psig) cylindrical separator (Figure 9.3). The key to using the portable flare is not to overload the liquid end of the separation system, since the primary purpose is flaring.
The separator/flare systems are commonly used with recompletion and workover operations, or where there are strict regulations about safety and flare stack heights.
9.8.2 Pressurized, or Closed, Separators: Modified Production Separators
The major advantages of a closed pressurized system are that it (1) controls gas from the well and sends it to a flare line under pressure and (2) is serviced by a special crew.
The separator is usually operated under 3 to 5 atmospheres of pressure (45 to 75 psig). Horizontal units are typically about 9 feet in diameter and 50 feet long, with a throughput of 5 MMcf or 500 bbl fluid. These are typical numbers, and sizes and pressure vary according to special jobs.
In areas where H2S is common, where there are strict flare regulations, or where gas could accumulate in a closed area on an offshore platform, keeping a separator operating under pressure is a critical factor in drilling safety. Further, the separator system is generally instrumented to give a history of annular pressure and measurements of gas, water, and oil volumes.
Canada began using modified production test separators as closed drilling separators in the 1990s. Production separators already had the capability to separate and measure the amount of gas, oil, water, and BS&W (bottom sediment and water), so they were easily adapted for use in underbalance drilling projects where H2S was to be expected or where the drilling fluid was an oil/nitrogen mixture.
Closed separators are gravity separators, often with an involute spiral feed (a type of centrifugal input). Baffles separate areas inside the tank and start to isolate cuttings, oil, and water. Gas rises and under separator pressure is forced to the flare line. The oil rises out of the drilling fluid and is pumped to an oil tank. Cuttings that start to settle out in the separator are recycled through the tank, and the drilling fluid is returned with the cuttings to the shale shaker (Figure 9.4).
Closed separator systems typically have measurement systems for water, oil, and gas; fluid height; and tank pressure. There is a separate upstream trapping system to collect cutting samples. Horizontal closed separators are the most common, but vertical separators are available and are typically used on offshore rigs.
Cuttings removal has always been a problem in closed separators. Most of the cuttings are trapped in the first compartment. In some early
Diagram of Closed Pressurized Separator
Gas Meter o
Vessel Pressure o
Cuttings & Drilling Fluid
Re-cycle line o
Oil I Water • Drilling Fluid
Cuttings & Drilling Fluid
Oil Out Water Out Drilling Fluid Out
Figure 9.4. Diagram of Closed Pressurized Separator.
designs, a screw, or Moyno, pump automatically pumped the cuttings out. However, generally cuttings are so sticky that they have a high angle of repose and will not fall into the pump suction, and so build up in the front of the tank. Later systems recirculated a part of the mud from the separator back through the cuttings compartment and circulated solids through the system to the rig shale shaker.
The closed separator is operated by a special crew. As a result of the large equipment and extra crew, the closed separator system is expensive to mobilize and operate but provides a degree of safety not found with other types of separator systems.
9.8.3 Combination System: Separator and Degasser
The West Texas separator has been combined with the vacuum tank degasser as the TOGAtm system (the SWACO Total Gas Containment System, or Tri-Flow Separator System). The mud from the flowline enters the separator, where the vacuum degasser controls the fluid flow, so that the degasser is always in balance and can never be overloaded. A "T" in the separator/degasser line allows mud from the pit to enter the degasser if there is surging and periods of no flow from the flowline. Gas from the degasser is pumped to the flare line by the discharge of the vacuum pump.
The extra volume in the base of the separator tank acts as a buffer against surges in the system. The separator tank is pressured to 15-50 psi by a backpressure valve at the base of the exit to the flare line. The vacuum breaker line on the degasser is connected to the output of the separator. No free or entrained gas is released except to the flare line. This system has seen extensive use in urban areas and with H2S gas and tends to be used in the United States in place of the closed (pressurized) modified production separator used in Canada. International operations vary with the vendor or local rules (Figure 9.5).
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