NOVELTY - The method involves directing a thermal rock cutting system comprising a distal portion placed at a lower side of a borehole. Fluid is isolated within a proximal portion of an annulus formed between the rock cutting system and a wall of the borehole by generating a fluid exclusion barrier within the annulus upstream of the distal portion of the rock cutting system while preventing fluid flow through the barrier toward a distal end (120) of the borehole. Material i.e. hard rock, from the distal end of the borehole is directed to pass through the barrier toward a proximal end of the borehole.    USE - Method for providing a fluid exclusion barrier between fluid regions in a borehole during spalling, cutting and penetrating a material such as rock, in a geothermal industry e.g. resource mining industry, oil industry and gas industry.    ADVANTAGE - The method enables providing the fluid exclusion barrier between the fluid regions in the borehole in a time saving and cost effective manner, thus preventing quenching of a thermal spalling process resulting from undesirable introduction of cooler fluid into a rock cutting region. The method enables preventing individual shims from extending beyond an outer diameter of an outer wall of a drill string to avoid damage to a barrier support structure when a spallation rock cutting system is moved to a desired position.    DETAILED DESCRIPTION - The fluid is selected from a group consisting of drilling mud, aerated foam, water, air, steam, chemical additive, a thermally-sensitive foaming agent and a base. The thermal rock cutting system is selected from a group consisting of a thermo-mechanical rock cutting system, a thermo-chemical rock cutting system, a borehole shaping system, a reaming system, a slotting system, a perforating system and a side-tracking system. An INDEPENDENT CLAIM is also included for an apparatus for providing a fluid exclusion barrier between fluid regions in a borehole, comprising a rock cutting system.    DESCRIPTION OF DRAWING(S) - The drawing shows a side sectional view of a thermal spallation cutting system comprising a spall filter.    Reaction chamber (112)    Distal end of borehole (120)    Cold fluid portion (150)    Flexible physical barrier (306)    Coolant flow jets (307)