Challenging the Status Quo of Filter Cake Removal During New Well Completions

Abstract Filter cake stability in the reservoir section of all wells, especially when completed as open hole is a key part of the drilling process. This ensures that drillers have good well bore stability that enables the successful running of the lower completion, whether it is a liner or sand control system. The challenge with the filter cake is ensuring that it can be removed to enable low completion skin and high productivity index (PI) from the well. Current practices for filter cake management are to remove the filter cake with a slow acting acid system. The work performed in offshore Angola in Block 17 and 32 challenges this process. In block 17 the current drilling practices require mud weight ranging from 1.10sg to 1.35sg to ensure wellbore and filter cake stability once the reservoir section has been drilled. The previous practice was to run the lower completion and then place a slow reactivity mud breaker. This mud breaker would remain in the lower completion and isolated with the formation integrity valve (FIV). After running the upper completion, the well would be brought online with a series of well tests. The alternative method that has been developed is to deploy a highly reactive emulsified hydrochloric acid emulsion to rapidly break the filter cake. In some cases, this is pumped from the toe and circulated to the heel. However, for long drains where there are concerns regarding unspent acid reach the heel of the well, a second treatment is performed at the heel of the well using a wash pipe. This process has been in place since 2022 with 12 wells completed using this method. The results show that 90% of wells treated with this new method reached productivity and skin targets, compared to only 33% of wells treated with slow acting cake breakers. The key conclusion from these observations is that the speed of the reaction between the emulsified system and the filter cake, enables near complete dissolution before pressure equalization below the FIV, which causes the open-hole to collapse around the screen, leading to leak-off of unreacted fluids into the formation and remnants of undissolved filter cake materials. The emulsified hydrochloric acid system can effectively break and dissolve the filter cake and clear any emulsification from the drilling fluid. The improved performance results have been confirmed by well test analysis. This method of completion and filter cake removal has become the default practice in Block 17 and 32 in Angola This work challenges the current practice of a slow reaction filter cake breaker and shows that a more efficient method is available. Using a highly reactive fluid has demonstrated successful results in both well performance and completion and drilling safety.