Many articles have been writing about this matter, some with high technical explanation other with complicated terminology; this time we like to simplify and explain it using simple terminology the complex behavior of influxes (gas kicks) in Oil Based Mud (OBM) and Water based mud (WBM). In the video below you can see the differences side-by-side of the same influx (10 barrels of gas) using different drilling fluids.
Let’s refresh some concepts:
Gas migration and migration speed:
A gas influx will tend to migrate upwards in a well, this is due to the low density of the gas compared to the drilling fluid. In WBM the migration speed is higher than in OBM since in the kick is also affected by the gas slip velocity.
Solubility of Gas in Water:
For most cases, the solubility of a gas in the water-based fluid can be neglected because of the small contribution made to the overall solubility of a gas in the drilling fluid.
Solubility of Gas in Oil Based Mud Drilling Fluids:
Experimental results in OBM fluids showed that gas solubility increases with pressure and decreases with temperature, the molecular weight of the oil and the amount of solid, brine and emulsifier in the drilling fluid.
Effect of the pressure of the solubility of a gas.
The solubility of gas molecules in the liquid solvent increases with increased pressure.
If the pressure is reduced a sudden release of gas will occur. This sudden release of gas at a specific temperature and pressure is referred to as the bubble point and would normally occur if the fluid were already saturated with gas at another temperature and pressure.
Bubble point depth:
Bubble point depth is that depth in the annulus/hole at corresponding temperature and pressure at which the first bubble of drilled gas would form from the drilling fluid-gas mixture either as a result of saturation of saturation of the mud by the gas or due to reduced temperature and pressure or vice versa affecting the solubility of the gases in the mud.
Bubble Point Pressure:
The Bubble Point Pressure is defined as the pressure at which the oil is saturated with gas. Above this pressure the oil is undersaturated, and the oil acts as a single phase liquid. At and below this pressure the oil is saturated, and any lowering of the pressure causes gas to be liberated resulting in two-phase flow.
What problems can be found when taking a kick in OBM and WBM?
If a kick is taken in OBM and the well is closed, the gas kick will completely dissolve in the mud and stay at bottom until the well is circulated again. Also undetected kicks can be taken without a significant pit gain and they will not be detected before free gas starts to liberate of the solution when they are close to surface. For WBM the gas kick will be able to migrate up in the well even at closed in conditions.
With gas going into solution in OBM, pressure builds up until all the gas was dissolved and there was no further gas migration. However, gas kick migrates in WBM, and it builds up the pressure until gas reaches the surface.
How different is controlling a kick with WBM and OBM?
When the gas kick enters the wellbore while drilling with a WBM the volume of drilling fluid displaced from the wellbore is proportional to the volume the gas kick occupies at the pressures and temperatures existing in the well. However, if the gas kick is taken in an OBM, the volume of drilling fluid displaced from the well is much less due to gas being more soluble in oil based drilling fluids than in water-based drilling fluids. As the under saturated fluid is circulated up the annulus, both pressure and temperature decline until the resultant decrease in solubility allows any gas to break out of solution and manifest itself as a bubble of gas. This gas is often released in large volumes over a short interval of time and at shallow depths. Unfortunately, these phenomena can lead to inappropriate decisions since the behavior is quite different from situations when gas does not dissolve practically in any WBM.
Pressure controlling a kick with OBM and WBM
This video shows a comparison circulating a kick out in WBM and OBM. Using Drillsoft HDX Plus software.
1. In WBM the gas reach surface quicker due slip velocity and migration.
2. Pressures at top are similar due the final expansion in both cases.
3. Depending on the bubble depth the sudden expansion can occur above or below casing shoe, misleading the well control situation.
4. Solubility can affect the behavior of the well control since pressure can deceive the operator.
5. For HPHT wells there is a risk for taking small undetected kicks in oil based mud, because the influx of gas dissolves totally and hides in the mud.