Mud/Gas Separator Sizing and Evaluation
- G.R. MacDougall (Chevron Canada Resources Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1991
- Document Type
- Journal Paper
- 279 - 284
- 1991. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 1.6 Drilling Operations, 1.10 Drilling Equipment, 4.6 Natural Gas, 4.1.9 Heavy Oil Upgrading, 4.1.5 Processing Equipment, 1.7.5 Well Control, 4.1.2 Separation and Treating, 4.1.9 Tanks and storage systems, 5.1.1 Exploration, Development, Structural Geology
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Summary. Recent wellsite disasters have led to an increased emphasis on properly sized mud/gas separators. This paper reviews and analyzes existing mud/gas separator technology and recommends separator configuration, components, design considerations, and a sizing procedure. A simple method of evaluating mud/gas separation within the separator vessel has been developed as a basis for the sizing procedure. A mud/gas separator sizing worksheet will assist drilling personnel with the sizing calculations. The worksheet provides a quick and easy evaluation of most mud/gas separators for a specific well application. A brief discussion of other mud/gas separator considerations is provided, including separator components, testing, materials, and oil-based-mud considerations.
The mud/gas separator is designed to provide effective separation of the mud and gas circulated from the well by venting the gas and returning the mud to the mud pits. Small amounts of entrained gas can then be handled by a vacuum-type degasser located in the mud pits. The mud/gas separator controls gas cutting during kick situations, during drilling with significant drilled gas in the mud returns, or when trip gas is circulated up.
This paper discusses design considerations for mud/gas separators. The purpose of this paper is to allow drilling rig supervisors to evaluate mud/gas separators properly and to upgrade (if required) the separator economically to meet the design criteria outlined in this paper, and to provide office drilling personnel with guidelines for designing mud/gas separators before delivery at the drillsite.
Principle of Operation
The operating principle of a mud/gas separator is relatively simple. The device is essentially a vertical steel cylindrical body with openings on the top, bottom, and side, as shown in Fig. 1. The mud and gas mixture is fed into the separator inlet and directed at a flat steel plate perpendicular to the flow. This impingement plate minimizes the erosional wear on the separator's internal walls and assists with mud/gas separation. Separation is further assisted as the mud/gas mixture falls over a series of baffles designed to increase the turbulence within the upper section of the vessel. The free gas is then vented through the gas vent line, and mud is returned to the mud tanks.
Operating pressure within the separator is equal to the friction pressure of the free gas venting through the vent line. Fluid is maintained at a specific level (mud leg) within the separator at all times. If the friction pressure of the gas venting through the vent line exceeds the mud-leg hydrostatic pressure within the separator, a blow-through condition will result sending a mud/gas mixture to the mud tanks. As one can readily see, the critical point for separator blow-through exists when peak gas flow rates are experienced in the separator. Peak gas flow rates should theoretically be experienced when gas initially reaches the separator.
Types of Mud/Gas Separators
Three types of mud/gas separators commonly are used today: closed bottom, open bottom, and float type. The principle of mud/gas separation within each type of vessel is identical. Differences can be found in the method of maintaining the mud leg, as discussed below.
The closed-bottom separator, as the name implies, is closed at the vessel bottom with the mud return line directed back to the mud tanks, as shown in Fig. 1. Mud leg is maintained in the separator by installation of an inverted U-shaped bend in the mud return line. Fluid level can be adjusted by increasing/decreasing the length of the U-shaped bend.
Commonly called the poor boy, the open-bottom mud/gas separator is typically mounted on a mud tank or trip tank with the bottom of the separator body submerged in the mud, as shown in Fig. 2. The fluid level (mud leg) in the separator is controlled by adjusting the fluid level in the mud tank or by moving the separator up or down within the tank. Mud-tank height can restrict the maximum mud leg obtainable for open-bottom mud/gas separators.
Fluid level (mud leg) is maintained in a float-type mud/gas separator by a float/valve configuration, as shown in Fig. 3. The float opens and closes a valve on the mud return line to maintain the mud-leg level. Valves can be operated by a manual linkage system connected from the float to the valve, or the valve can be air-operated with rig air. Mud-leg height can be controlled by adjusting the float assembly.
There are some inherent problems in the use of float-type mud/gas separators. The manual linkage separator has experienced problems with linkage failure resulting in improper opening or closing of the mud-return-line valve. Air-operated valves fail to function if rig air is lost, resulting in no control of fluid level within the separator. Mud-return-line valves are prone to plug with solids, preventing mud flowback to the mud pits.
Because of these problems, float-type mud/gas separators are not recommended and a closed-bottom separator is preferred. Open-bottom separators are acceptable; however, one should be aware that they are restricted to a maximum mud leg, somewhat lower than the mud-tank height. Although float-type mud/gas separators are strongly discouraged, these separators can be modified easily for disconnection of the float, removal of the valve, and installation of a mud leg in the mud return line.
For the purpose of this paper, a closed-bottom mud/gas separator will be considered for all separator designs.
Sizing the Mud/Gas Separator
Table 1 shows a mud/gas separator worksheet to assist with the sizing calculation. The mud/gas separator illustrated in Fig. 4 will be evaluated for sufficient sizing in this paper.
Peak Gas Flow Rate. As discussed previously, the critical time for separator blow-through exists when peak gas flow rates are experienced. Mud/gas separator blow-through is defined as inefficient separator operation resulting in a mud/gas mixture returning to the mud tanks through the mud return line.
Two situations can cause separator blow-through.
1. Friction pressure of the gas venting through the vent line exceeds the mud-leg hydrostatic pressure, resulting in evacuation of fluid from the separator. Friction pressure of the mud through the mud return line is considered negligible because of its short length.
2. Vessel ID is too small, causing insufficient retention time for the gas to separate efficiently from the mud. This situation is commonly called insufficient separator cut.
To estimate a peak gas flow rate properly, we must consider a "typical" kick. The typical kick will depend on the well location, depth, type size, and component ratios of influx. Kick data should be based on previous offset well data and should be a realistic worst-case gas kick. The well and kick data in Fig. 5 will be used in this paper.
The volume and pressure of the gas upstream of the choke must first be calculated.
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