Successful Rigless Water Shutoff in Slim Offshore Horizontal Wells in High CO2-H2S Environment Using Horizontal Production Logging and Fiber-Optic Coiled Tubing
Authors: Musrati Firas , Elmozwagi Ali , Masaud Muhaned , Sherik Ayoub , Abdalla M. Ben , Sadeg S. 
Published: February 2024

In a field offshore Libya, five naturally flowing offshore horizontal wells had been at risk due to increasing water cut. During the last emergency shutdown, two of them died and remained shut-in for almost 1 year; it took several weeks of continuous N2 lifting to bring them back online. Therefore, it was decided to perform a water shutoff (WSO) campaign in the highly corrosive offshore environment to reduce water cut and avoid any production losses following any emergency shutdown.

The risk of killing the well during rig up while performing shut-in passes or during WSO intervention led to the decision to use coiled tubing (CT) rather than a wireline tractor to convey the horizontal production logging tool and CT inflatable plug. Moreover, N2 tanks were available at surface during the entire operation to kick off the wells in case they died during rigging up or during logging shut-in passes. Two types of CT were used: electrical CT (e-coil) to allow long duration logging and fiber-optic telemetry-enabled CT. The telemetry-enabled CT would be used to acquire accurate downhole measurements during CT plug setting while performing the WSO in flowing condition.

Four horizontal wells and one slanted well were successfully logged with the horizontal production logging tool conveyed on e-coil. Water entry points were confidently identified along the lengthy horizontal sections. High H2S and CO2 content in the crude oil required special procedures to finish the logging campaign safely and smoothly.

High flow rates in a slim casing with inside diameter (ID) of approximately 3.95 in. created a high-velocity environment; however, the stratified flow could still be clearly observed, and the five spinners were showing distinctive velocities corresponding to the phase in which the spinner is immersed. Electrical and optical probes clearly showed the fluid distribution along the vertical section of the pipe. A multiphase meter was used during the logging campaign to capture all the changes in flow rates while running in hole the 1.75-in. CT inside the 3.95-in. ID monobore casing.

Two wells out of the five were candidates for WSO as they exhibited bad water being produced from the toe, while the other three wells exhibited good water behavior, which did not make them good candidates for WSO. Two permanent plugs were set in two horizontal wells based on production logging results, which reduced water production by 1,700 BWPD.

The prejob design included building transient multiphase models to predict the changes in flow rates and flow regimes to meet the logging objectives in high flow rates and small casing ID. The integrated approach that combined multidisciplinary services and technologies such as telemetry-enabled CT, horizontal production logging tools, and multiphase meters led to a smooth operation in a hostile offshore environment and minimized the risk of killing the wells during the operations.