Balanced activity oil mud is a kind of oil-base mud in which shale hydration is prevented by a balance in vapor pressure between the oil-base mud and the formations being drilled. This is achieved by adjusting the salinity of the water in the oil base mud to the water found in the shale, thereby balancing their activity (exchange of ions and water vapor between the salt water and the formation).
Reactive shale swelling in contact with water to create a bit ball problem
By ensuring that water is not adsorbed to the shale surface when drilling with a balanced activity oil mud, shale hydration, swelling and its resultant borehole instability problems can be avoided.
How Balanced Activity Oil Mud Works
For balanced activity oil mud, the base fluid (continuous phase) is oil and the dispersed phase is salt water. Salt water is used instead of fresh water because salt water has lower activity than fresh water and so the salt water will not invade or be adsorbed onto the shale.
In terms of activity, the activity of fresh water is 1.0, the activity of an oil base mud saturated with sodium chloride brine is 0.75 and the activity of an oil base mud saturated with calcium chloride brine is 0.30. Oil base mud with an activity of 0.75 is to be chosen ahead of one with an activity of 1.0. The lower the drilling mud activity the more suitable it will be to overcome shale swelling problems.
Wellbore instability caused by different kinds of reactive shale
This is why it is not advisable to make use of fresh water as the dispersed phase in oil base mud when reactive shale is expected to be encountered since there will be a transfer of ions and water vapor from the water in the drilling mud into/onto the shale leading to shale hydration, swelling and disintegration. It is best to make use of water containing salts (brine) like sodium chloride (NaCl) or calcium chloride (CaCl) having lower activity.
Caution when Preparing a Balanced Activity Mud
Actually, we may not need to saturate the oil base mud with the selected salt to achieve our goal of minimizing or preventing shale swelling problems. This is because a mud system that is already saturated with salt has no space to dissolve any other drilling fluid additive like viscosifiers or weighting materials. The higher the salt concentration in the mud system, the lower the activity will be.
Generally, an activity of 0.60 will satisfy most shales encountered so CaCl brine with an activity of 0.50 will do just fine, instead of fully saturating the oil base mud with Cacl brine until it gets to 0.30. Excessively adding salt to the oil base mud system, beyond that which will satisfy the clay, only makes the mud more expensive and difficult to maintain.
Dry shale cuttings sample
To test the effectiveness of the balanced activity oil base mud, we can simply check the hardness of the shale cuttings circulated out of the hole. Hard shale should remain hard unless hydrated which is when they absorb water, swell and disintegrate. Apart from physically observing the hardness of shale cuttings at the shale shaker, a hygrometer can also be used in certain cases to test for the effectiveness of a balanced activity oil base mud.