In a method to degrade filtercake in a hydrocarbon reservoir using downhole in-situ heat generation, a filtercake breaker, configured to degrade filtercake at a temperature, is mixed with a drilling fluid. The drilling fluid mixed with the filtercake breaker is flowed through a hydrocarbon reservoir in which a wellbore is being drilled. A filtercake forms in a portion of the hydrocarbon reservoir responsive to flowing the drilling fluid. A temperature of the portion of the hydrocarbon reservoir is less than the temperature at which the filtercake breaker is configured to degrade filtercake. Multiple filtercake chemicals are flowed to the portion of the hydrocarbon reservoir. The multiple filtercake chemicals, when mixed, are configured to react in an exothermic reaction to release heat to increase the temperature of the portion of the hydrocarbon reservoir to at least the temperature at which the filtercake breaker is configured to degrade filtercake.