Reversible Fluids and Mud Breaker Technology Outperforms Productivity and Injectivity in West Africa–Fit to Purpose and Designed for Success

Abstract

Lessons learned in Moho-Nord field Congo improved well productivity and injectivity, from Miocene formation. Due to the type of drill-in fluid used in the initial development phase of the field, the skin factor showed extremely high values, above 150. The values dropped to near 50 after remediation and stimulation operations. Productivity and injectivity rates were significantly poor and did not meet the targets. To mitigate these issues, the most suitable fluids for drilling, completion and removing the filtercake for a horizontal open hole standalone completion were required. Fluids were identified by an extensive laboratory study. The selection was based on the reservoir characteristics and sand control completion type. It was also based on review of past experiences from the original project phase, and continuous discussion between operator and the fluids provider. Changes were made to improve practices during the field implementation. One reservoir drill-in fluid (RDF), a reversible invert emulsion (RIE) non-aqueous base mud (NABM) system and two filtercake breaker systems were selected and implemented in the field. The RIE showed suitable properties required for NABM drilling. It also demonstrated properties of easier filtercake destruction when exposed to lower pH breaker fluids. Its filtercake did not need surfactant/solvent pills to change the wettability of the solids in the filtercake which notably simplified the borehole cleanup operation. The improvement occurred throughout the second and third phases of the field development. The ameliorations were measured through skin value, production and injectivity rates and initial flow initiation pressure. Values were compared against initial targets. As an example, the skin value went from 150 in phase one to less than 5 in phase three. No acid stimulation was required in any of these wells, providing a huge cost saving for the operator. The combination of the fluids selected, and improved drilling and completion practices led to skin values for most between 0 and 3. The productivity and injectivity outperformed and surpassed expectations. Knowledge gained from the study re-established the importance of selecting the suitable fluids systems and using best in class drilling and completion practices. This paper summarizes upfront fluid design requirements and fluids management procedures implemented during drilling, completion and filtercake breaker placement to ensure safe and successful open hole completions.

The Phase Inversion Mechanism of the pH-Sensitive Reversible Invert Emulsion

“…However, during the completion process, the application of oil-based drilling fluids presents several challenges, such as the difficult filter cake removal, the weak bonding strength between cement and the formation, and the handling of oil-contaminated cuttings and waste mud. Therefore, water-based drilling fluids are more effective than oil-based for subsequent completion operations [9,11]. By controlling the acidity and alkalinity of the system, drilling fluids can be converted between oil-based and water-based at different stages of drilling and completion [12], combining the advantages of both to achieve the ideal drilling and completion effects [13][14][15].…”