A downhole chemical reactor can be placed in a downhole environment to generate gas in-situ. This gas can pressurize an inner pressure chamber of the downhole chemical reactor to create a pressure-on-demand source or to maintain a constant pressure reservoir, depending on the configuration of a reactor controller coupled to an inlet of the inner pressure chamber. The inner pressure chamber contains one or more desired chemical reactants and the reactor controller operates to permit well fluid to flow from the wellbore and into the inner pressure chamber. The well fluid reacts with the desired chemical reactants and generates one or more gases such as hydrogen or carbon dioxide. The generated gases pressurize the inner pressure chamber and a pressure regulator coupled to the inner pressure chamber maintains a maximum pressurization of the inner pressure chamber. For a constant pressure reservoir, the reactor controller repeats this cycle indefinitely.