Many high-permeability, soft sandstone formations are completed with horizontal wellbores. In these formations, completions are typically openhole with screens or screens combined with gravel packs. The drill-in fluid filter cake is left in place until the completion operations are finished. At that time, a cleanup solution is pumped to remove the filter cake from the wellbore face. These cleanup solutions can be acids, chelating agents, oxidizers or enzyme treatments, or combinations of these materials. A common disadvantage of these treatments is that they are highly reactive and may remove the filter cake at the point of circulation before the treatment can be placed over the entire openhole interval. This is especially true for the cleanup solution used to address the bridging particles in drill-in fluids. A need exists for cleanup solutions that have a delayed effect on filter-cake integrity, allowing the cleanup solution to be circulated across the interval before leakoff to the formation becomes a problem. Ideally, the treatment's reaction with the filter cake would commence when the entire solution is in place and continue until the filter cake is uniformly removed. If a sufficient delay is possible, the breaker solution can be placed with a gravel pack. Break times for this situation would range from 4 to 24 hours, depending on the need for fluid-loss control during subsequent pipe trips.

Conducting breaker tests manually for this type of delay could be extremely time-consuming and prone to variations between tests. Repeatability of the filter-cake deposition becomes even more important during sensitivity testing of the various components of a proposed cleanup solution. Clearly, automating the filter-cake construction and breaker solution testing would be advantageous. Fortunately, commercially available dynamic mud-filtration testing equipment can provide much of the needed automation capability. This paper discusses the additional modifications necessary for converting the instrument into the automated, delayed-breaker testing device required. Example test results are given for long-delayed breaks of water-based drill-in fluid filter cakes showing the sensitivity of the break time as a function of various formulation and testing parameters.

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