Mud Loss

Certain North Carolina Coastal Plain aquifers are known for their highly transmissive aquifer material. While this is good from the standpoint of the water user, who can easily obtain large quantities of water from wells in these aquifers, it can wreak havoc on the unprepared driller.

During mud rotary drilling, it is natural to lose some portion of the circulating mud as it infiltrates the aquifer material. As the mud is lost, the driller replaces it by mixing additional mud in the mud pit. However, some aquifer materials in the Coastal Plain are so transmissive that mud is lost at a rate greater than it can be practically replaced. Or, the mud can be lost when the driller stops drilling activities for the day. Examples of aquifer materials that may cause excessive mud loss are highly transmissive zones of the Castle Hayne limestone and channel deposits in the Yorktown Formation.

DWR has found that the first step in combating excessive mud loss is knowing before drilling whether we are in an area in which mud loss may be a problem. The knowledge is obtained primarily from our experience in drilling throughout the Coastal Plain and from talking with drillers familiar with specific areas of the Coastal Plain. If we discover we may have a mud loss problem at a given site, we can be prepared to take the necessary steps to abate the mud loss before we encounter it, thereby saving ourselves down time after drilling has commenced.

Actual measures taken to mitigate excessive mud loss tend to be somewhat site specific, as no two boreholes are identical. However, there are a few common ways to combat mud loss.

24-hour drilling. In some boreholes, mud loss does not become a problem until drilling stops for the day. During the night, the mud infiltrates the aquifer such that there is no mud in the hole when drilling commences the next day. Not only can this add to project costs by requiring more mud to be mixed every day, it can result in collapsed boreholes, which require extra time and expense to redrill. Also, any tools that are in the hole when the collapse occurs may be permanently lost.

One solution to this problem is 24-hour drilling. This involves having two drill crews working on the site such that drilling proceeds 24 hours a day until the project is completed or until the problem zone can be cased off. While this is more expensive than drilling during standard work hours, it save a lot of time, money, and headaches for drilling projects where mud loss is a problem.
Using higher viscosity mud. Sometimes the answer to serious mud loss can be as simple as using a more viscous mud. Higher viscosity muds will penetrate aquifer materials less readily, thereby reducing the amount of mud loss.