Many of the matured fields across the Middle East have formations with below-hydrostatic pressure, when drilling is completed without return to a surface. Most of the losses while drilling across vugular, fractured or weak formations happens suddenly, and a fluid level is not visible at the surface. Further the continuous mud cap is applied while drilling to maintain the constant bottom hole pressure. However, the mud cap flow rate and mud weight (MW) most of the time are based on local best practices and not supported by any engineering study, for example using 1.5 ppg at the rate of 1.5 BPM. To have a proper understanding of the fluid level the bottom hole assembly (BHA) is equipped with an equivalent static density (ESD) sensor was used. As continuous mud cap is required while drilling a special procedure was developed to allow tool application.
This study was performed initially across the surface section (which together with an intermediate section often has losses in the Middle East), where based on offset well analysis a single lost circulation zone present and the chance of the well control is low. Based on the result of the study the fluid level was clearly identified. Further, to understand the effect of the mud cap flow rate the section was drilled by changing this parameter. The data obtained allowed to understand the effect of a flow rate on the fluid level in the annulus. Having this information the bottom hole pressure can be maintained with lower and constant mud cap flow rate, avoiding risk of wellbore instability or well control, as well as positive financial impact as expensive mud was not used at a high rate to top up the annulus.
The obtained information and knowledge explore further the underground behavior of the depleted, fractured and vugular formations across the Middle East. The manuscript can be used by anyone across the world aiming to improve the service quality and reduce the cost related to mud cap as well as have better knowledge to present the well control risk.