Solid control equipment mainly includes: drilling fluid shaker, mud cleaner, sand cleaner, desilter and drilling fluid centrifuge. Gas intrusion refers to the process in which gas penetrates into the drilling fluid and deteriorates performance. Gas in drilling fluid will seriously affect solid control equipment. This situation is misinterpreted and understood in most oilfield applications. The main problem caused by gas intrusion in solid control is that the screen of the drilling fluid vibrating screen is "pasted", and the mud output of the sand pump to the sand removal cleaner, desilter and drilling fluid centrifuge is reduced. Gas intrusion often occurs during drilling of gas layers.
Drilling fluid shaker and gas invasion
The steady flow of drilling fluid with cuttings determines the screening of the drilling fluid vibrating screen. The drilling fluid must pass through the screen of the vibrating screen , and the cuttings must either pass through the screen or be removed by the screen. There are three kinds of effects of gas invasion in drilling fluid on the screening of drilling fluid shaker.
a. Gas channeling can cause the volume of drilling fluid to fluctuate, thereby exceeding the drilling fluid's ability to handle drilling fluid. This is mainly because the gas mixed in the drilling fluid rapidly expands at the same time as the surface of the drilling fluid, which makes the drilling fluid fluctuate greatly and is used in overflow pipes. Liquid-gas separator (drilling fluid gas separator) can solve this problem.
b. The presence of tiny air bubbles in the drilling fluid is the main sign of gas invasion. When the air bubbles expand to the gap between the metal wires that fill the screen, the air bubbles may stick to the screen. This problem can be solved by the degassing device (vacuum degasser) to remove the air bubbles mixed in the drilling fluid.
c. Foam associated with gas intrusion will leave a layer of light and wet foam on the drilling fluid shaker screen. These foams are too light to pass through the screen cloth under the effect of gravity, and carry additional mud flow to the bottom end of the drilling fluid shaker. The loss of drilling fluid is usually not obvious and can be easily overlooked. Under normal circumstances, drilling fluid shaker spray or defoamer can be used to break the foam. However, it should be noted that a large amount of cuttings will pass through the screen and cannot be removed by screening.
The fourth problem has nothing to do with gas, because the drilling fluid has increased viscosity due to saltwater or salt intrusion. This may cause the drilling fluid to run away, which prevents the drilling fluid from passing through the screen. If this happens, the contaminated drilling fluid must be bypassed into the mud storage tank.
2. Mud cleaner (sand cleaner, desilter) and gas invasion
The operation of mud cleaners (sand cleaners, desilterers) is usually determined by the hydraulic faucet, and the head pressure is a function of the volume of drilling fluid pumped by the sand pump. Sand pumps are very sensitive to gas intrusion. The gas accumulates in the pressure reduction area of the impeller center, and then quickly reduces the output flow of the sand pump. If the fluid pressure head of the sand pump suction line is very low, a small amount of gas in the drilling fluid may lock the pump airlock, thereby limiting the operation of the drilling fluid solid control system.
The air lock of the sand pump prevents the operation of the mud cleaner (sand cleaner, desilter). As long as there is gas in the overflow pipe, the drilling fluid delivery system (mainly sand pump) in the solid control system should use a degassing device (vacuum degasser).
3. Drilling fluid centrifuge and gas invasion
The feed pump of the drilling fluid centrifuge uses a typical mono pump, that is, a screw pump. Screw pumps are progressive cavity pumps that are not air-locked like sand pumps. However, the displacement of the screw pump is smaller than that of the sand pump. This directly reduces the gas content in the drilling fluid and the input pressure of the centrifuge. After the gas in the drilling fluid passes through the screw pump, its pressure increases to the input pressure, so the gas is compressed to reduce the output of the screw pump. Decreasing the mud input to the drilling fluid centrifuge will inevitably reduce the mud output, thereby changing the point of change of the centrifuge.
Note: The following estimates are minimum values. As the amount of gas intrusion increases, the fluidity of the drilling fluid is better and the head at the suction port is lowered. All these factors will affect the performance of the screw pump and thus the performance of the drilling fluid centrifuge.
The conditions described below apply to screw pumps, as well as to two- or three-cylinder drilling pumps.