Modified in situ retorting of oil shale

A process for retorting a pair of discrete raw oil shale beds, each raw shale bed being associated with a spent shale bed having been previously retorted and containing pyrolysis char capable of being combusted by contact with air, a first of said raw shale beds and a first spent shale bed associated therewith being at an upper pressure and the second of said raw shale beds and the second spent shale bed associated therewith being at a lower pressure lower than said upper pressure, said process comprising

• (1) withdrawing gas from said first raw shale oil bed until the pressure therein drops to about said lower pressure,
• (2) simultaneously with step (1) allowing the pressure in said second spent shale bed to increase to about said upper pressure by supplying an essentially oxygen-free gas thereto,
• (3) thereafter continuing to withdraw gas from said first raw shale bed and opening communication between said first raw shale bed and said first spent shale bed so that gas in said first spent shale bed is drawn into said first raw shale bed, step (3) continuing until the pressure in said first spent shale bed drops to about said lower pressure, and
• (4) simultaneously with step (3) opening communication between said second spent shale bed and said second raw shale bed so that gas in said second spent shale oil bed is drawn into said second raw shale bed, step (4) continuing until the pressure in said second raw shale bed reaches about said upper pressure.

A process for recovering shale oil from a plurality of rubblized oil shale beds arranged in sets of four, each of said sets comprising a pair of spent shale beds containing char and a pair of raw shale beds containing kerogen, a first of said spent shale beds associating with a first of said raw shale beds and the second of said spent shale beds associating with the second of said raw shale beds, said process comprising combusting char in said spent shale beds in a combustion mode and pyrolyzing the kerogen in said raw shale bed in a pyrolysis mode, said kerogen being pyrolyzed with heat generated from combustion of char in said combustion mode,

• said combustion mode comprising at least one cycle of first and second combustion phases wherein air is passed into said spent shale beds to cause combustion of the char therein and heating of said spent shale beds,
• said first combustion phase comprising passing air into said first spent shale bed so that the gas pressure therein increases and withdrawing flue gas from said second spent shale bed so that the gas pressure therein decreases,
• said second combustion phase comprising passing air into said second spent shale bed so that the gas pressure therein increases and withdrawing flue gas from said spent shale bed so that the gas pressure therein decreases, said pyrolysis mode comprising at least one cycle of first, second, third and fourth pyrolysis phases carried out in order wherein heat in said spent shale bed is transferred by forced convection to raw shale beds to pyrolyze the kerogen therein and produce an offgas product containing pyrolysis gas and shale oil vapors,
• said first pyrolysis phase comprising passing pyrolysis gas into said first spent shale bed so that the gase pressure therein increases and withdrawing offgas product from said second raw shale bed so that the pressure therein decreases,
• said second pyrolysis phase comprising passing pyrolysis gas in said first spent shale bed to said first raw shale bed so that the pressure therein increases and passing pyrolysis gas from said second spent shale bed to said second raw shale bed so that the pressure in said second spent shale bed decreases,
• said third pyrolysis phase comprising passing pyrolysis gas into said second spent shale bed so that the pressure therein increases and withdrawing offgas product from said first raw shale bed so that the pressure therein decreases, and
• said fourth pyrolysis phase comprising transferring pyrolysis gas from said first spent shale bed to said first raw shale bed so that the pressure in said first spent shale bed decreases and transferring pyrolysis gas from said second spent shale bed to said second raw shale bed so that the pressure in said second raw shale bed increases.