Quantification of the Loss Conduit Aperture During Lost Circulation in Fractured Carbonates

Abstract

Fractured carbonate formations around the world are prone to lost circulation that not only affects the well construction process but creating a longtime effect on the wellbore integrity. Despite multiple attempts to cure them the success rate is usually low. This manuscript is aiming to provide a new vision on the reason of lost circulation across carbonates. To have better understanding of the complete losses across the fractured carbonates the series of studies were initiated. At first to understand the strength of the loss zone the fracture closing pressure was evaluated via study of the fluid level in the annulus and back-calculation of the drilling fluid density effect on it. Secondary, the rock properties across the loss circulation zones were studied by using the microresistivity images, dip data, and imaging of fluid-saturated porous media. At last, the trial tests with different treatment materials were performed to evaluate the effect of it on curing the losses. The results of the studies brought new information and explained some previous unknowns. The formation strength across lost circulation zone was measured and it was confirmed to remain constant despite other changes of the well conduction parameters. It was also confirmed that the carbonates are naturally highly fractured having over 900 fractures along the wellbore. The lost circulation zone was characterized, and it was confirmed that the losses were not related to the fractures but rather to the karst, dissolution and to mega-fractures. The size and dip of the fractures were identified, and it was proven the possibility to treat them with conventional materials. However, the size of identified mega-fractures and karst zones exceed the fractures by 100 times in true vertical depth, and in horizontal wells the difference is thousands times due to measured depth. This new information explains the previous unsuccessful attempts with the conventional lost circulation materials. Further based on the newly available information the mathematic description of the lost circulation zones was provided.