Carbonate formations around the world and specifically in a Middle East are prone to have total losses while drilling. And the nature of the losses often related to the highly fractured formations of the pay zone. When such fracture(s) is crossed by the wellbore the lost circulation initiated and led to a drilling without a return to a surface.

To avoid undesired well control event or wellbore instability and to maintain the constant bottom hole pressure the mud cap drilling strategy often used as a preventative measure. The mud cap can be either the continuous or based on some volume or time interval, depends on the local practices or the policy of an operator. The mud cap flow rate as well as mud cap mud weight are often based on the best practices, not supported by an engineering study.

To understand the behavior of the drilling fluid level in the annulus while drilling with total losses the drilling bottom hole assembly equipped with annular pressure while drilling tool was used. As the drilling required to use the continuous mud cap, then the specific guideline was developed on measurement of the bottom hole pressure and further conversion of it to the fluid level.

The study was performed across pay zone with one or several loss circulation zones identified. As the result it was confirmed that the used mud cap flow rate had minor to none effect on the fluid level position in the annulus and that the bottom hole pressure remained the same. It showed as well that different loss zones are behaving in a different way, what can be considered as a factor affecting their ability to be sealed.

The obtained knowledge and the information should help to understand better the loss circulation behavior as well be an important step toward development of the product which may cure the losses in high fractured carbonate formations. The results of the study can be implemented in any other project or a field.

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