Successful Implementation of a Modified Carbonate Emulsion Acid System Combined with an Engineered Diversion Approach Delivers Outstanding Results – A Case Study from the Moho Nord Deep Offshore Field in Congo

Abstract

Moho Nord deep offshore field is located 80 kilometers offshore Pointe-Noire in the Republic of the Congo. The wells produce crude from the Albian age reservoir and lithology consists of alternating sequences of carbonates and sandstone layers with high heterogeneity and permeability contrast, including the presence vacuolar layers called "hyperdrains". This paper describes the application of a novel acid system and the methodology successfully applied to effectively acid stimulate the Albian drain. The combination of long perforation intervals with lithology and permeability contrasts, natural fractures, and the potential for asphaltene deposition resulted in adoption of a Modified Carbonate Emulsion Acid (MCEA) fluid system containing a solvent to provide asphaltene deposition prevention. The MCEA stimulation treatments were bullheaded from a stimulation vessel and an engineered diversion process was implemented for effective acid diversion using a combination of mechanical ball sealers and a degradable particle system (DPS). The selection of number of ball sealers and the DPS diverter design depended upon the interpretation of zone permeability profile from the logs, and the final distribution of perforations selected along the drain. A fluid placement simulator indicated low sealing efficiency of the ball sealers would lead to an overstimulation of the highest permeability areas. Subsequent simulations indicated that the DPS would provide better acid coverage with lower skin (S). Results and observations presented indicate that the decision to improve the acid diversion design and combine ball sealers with a DPS diversion technique to improve zonal coverage was validated. During the stimulation treatment execution, the high stimulation treatment efficiency was clearly apparent from the pressure responses to the acid and the diverter system which sealed off perforations and diverted the treatment to other layers with lower permeability. The MCEA also has proven to have self-diverting properties due to its high viscosity and low reaction rate which creates a better coverage of the drain, even with limited pumping rate, allowing live acid penetrating deeper into the formation. The production results reported from the 15 wells stimulation campaign (10 producers, 5 injectors) indicated that the productivity indexes (PI) exceeded expectations and resultant post-stimulation skin values ranged from −2.5 to −4.1. The Moho Nord deep offshore stimulation campaign yielded outstanding production results and showed significant validation for use of the MCEA system and the diversion methodology applied. On the producer wells the use of both chemical and mechanical diversion was valuable, as the DPS proved to complement the Ball Sealers for layers with lower injectivity and also at the high injection rates. High injectivity gain coupled with effective diversion was crucial for enhanced wormholing and good drain coverage.