A high-power microwave borehole fracturing device for an engineering rock mass includes a high-power microwave generator, a high-power microwave coaxial heater, a high-power low-loss microwave coaxial transmission line, and a microwave power adaptive regulation and control system. The high-power microwave generator includes a continuous wave magnetron, a permanent magnet, a waveguide excitation chamber, a coaxial circulator, a coaxial matching load, a coaxial coupling converter, a waveguide coaxial converter, and an output waveguide. The high-power microwave coaxial heater includes a microwave transmission inner conductor, a microwave transmission outer conductor, a microwave input connector, a microwave short circuit cap, and a conductor supporting cylinder. The high-power low-loss microwave coaxial transmission line includes an input end coaxial line, middle section coaxial lines, and an output end coaxial line. The microwave power adaptive regulation and control system includes an impedance matching regulator, a microwave power controller, and a temperature sensor.

A gas-fired steam-injection boiler has a flue and a water supply pipeline. The flue is provided with a radiation section and a convection section. The water supply pipeline has, in the flue gas flow direction, a first pipe section located at an upstream portion of the convection section, a second pipe section is located at a downstream portion of the convection section, and a third pipe section arranged between the first pipe section and the second pipe section. In a water flow direction of the water supply pipeline, the second pipe section, the first pipe section and the third pipe section are arranged in sequence. By using a flue gas condenser, the temperature of flue gas can be further reduced, and the latent heat of vaporization of steam in the flue gas is absorbed to form condensate water.

A process for in situ thermal recovery of hydrocarbons from a reservoir is provided. The process includes: providing an oxygen-enriched mixture, fuel, feedwater and an additive including at least one of ammonia, urea and a volatile amine to a Direct-Contact Steam Generator (DCSG); operating the DCSG, including contacting the feedwater and the additive with hot combustion gas to obtain a steam-based mixture including steam, CO.sub.2 and the additive; injecting the steam-based mixture or a stream derived from the steam-based mixture into the reservoir to mobilize the hydrocarbons therein; and producing a produced fluid including the hydrocarbons.

A process for in situ thermal recovery of hydrocarbons from a reservoir is provided. The process includes: providing an oxygen-enriched mixture, fuel, feedwater and an additive including at least one of ammonia, urea and a volatile amine to a Direct-Contact Steam Generator (DCSG); operating the DCSG, including contacting the feedwater and the additive with hot combustion gas to obtain a steam-based mixture including steam, CO.sub.2 and the additive; injecting the steam-based mixture or a stream derived from the steam-based mixture into the reservoir to mobilize the hydrocarbons therein; and producing a produced fluid including the hydrocarbons.