Formate brines and drilling fluids were primarily developed for high pressure drilling and completion applications. This completion fluid system is now being employed in more benign pressure environments and also to drill and complete formations with specific properties. This was the case where a potassium formate brine with 1.25 specific gravity (S.G.) was specified as the base fluid for the drill-in fluid (DIF) as well as completion brine for several openhole completions requiring gravel packing in offshore Brunei. The DIF used to drill the open hole deposits a protective filtercake on the borehole wall that is typically removed by means of a suitable removal system, enabling undamaged production.
Openhole filtercake breaker systems that are typically employed range from mineral and organic acids, which aggressively attack the filtercake and result in an incomplete cake removal, to ortho-esters, which hydrolyze in the brine and lead to the formation of a delayed acid, which has proven more efficient. The ortho-esters are mixed in the base brine and placed inside the open hole or gravel pack and allowed to react over time.
Formate brines, however, have proven to be poor base fluids for the ortho-esters because of their inherently high pH. A laboratory evaluation was conducted to address this problem. The tests showed that these ortho-ester-based systems, when mixed in a potassium chloride or sodium-bromide brine system, can be employed to break the formate-based DIF filtercake. A chelant-based breaker system, however, provides the most efficient cleanup with the necessary delay. This paper addresses the challenges and details of the laboratory testing performed to arrive at an optimized chelant-based delayed-breaker system in formate brine for removal of formate-based filtercakes. The paper also presents results of the performance of the ortho-ester system.