Relaunch Coiled Tubing Foam Descaling in Depleted Sour Gas Well – Lessons Learned from Operation Failures
Authors: Al-Dossary Modhesh , Al-Driweesh Saad , Al-Sagr Abdulaziz Mutlag , Bolarinwa Simeon , Haekal Muhammad , Noguera Jose Antonio 
Published: May 2016

Scale buildup in sour gas is increasing in Saudi Arabia. Clean out with foam, especially for a depleted reservoir, is known and proven as an effective method to solve the problem. With more complex well completion and trajectory, heavier and harder scale materials and high hydrogen sulfide (H2S) content, the foam descaling job encounters more challenging conditions, which apparently create unexpected problems during operation. After several failures, mainly caused by lost circulation and uncontrolled gas (H2S), due to dynamic bottom-hole changes during operation and alternation of foam properties, coiled tubing (CT) foam clean out was suspended for further investigation for almost 3 years.

The well subject discussion was selected to relaunch the foam descaling after the suspension period. Completed horizontally with 7" liner, 4½" tubing and 5?" open hole, the well was selected to be sidetracked to enhance the productivity on the lower carbonate reservoir. Several gauge cutter runs were performed to check the well accessibility prior to drifting the well with the dummy whipstock and tag above the target depth. The obstruction was suspected to be a thin layer of scale. As the well condition is depleted, descaling using foam is proposed.

This paper will discuss the lessons learned from the previous failure and the correction while preparing the foam descaling design and flexibility to overcome dynamic operation on the field. This operation result opens the window to bring back foam clean out as the most efficient option to descaling or clean out in a depleted well.

Introduction

Over the last several years, scale buildup in the Ghawar gas field has become a serious issue, which requires prompt scale removal work to restore well productivity and regain wellbore accessibility, to perform surveillance activities and well intervention work. The observed scales in this area are mainly composed of various iron sulfide species, iron carbonate and calcium carbonate (CaCO3) as well.

Several scale removal methods were applied to the field to remove the scale. As the best result observed, since 2007, rigless foam descaling coiled tubing (CT) was deployed and applied on several wells, especially for the wells with low reservoir pressure. Due to some operational safety related issues, such as CT stuck and dangerous uncontrolled gas release in 2010, rigless descaling operations were suspended and performed under a workover rig, which has more capabilities to handle the issues that may be encountered during the descaling process.

On the rig operation, the reservoir was initially plugged with lost circulation material, such as CaCO3 chips; therefore, the risk of stuck CT due to lost circulation during the descaling process could be minimized. With the continuous circulation, foam is no longer required in the fluid system and reduces the complexity of the operation. Following the successful descaling operation on the rig, to be more cost-effective, the descaling work was decided to be performed rigless with a similar approach. Since then, this descaling work process has become best practice to perform any descaling job with CT in the Ghawar gas field.