Abstract: A subterranean formation containing oil shale is prepared for in situ retorting by forming a fragmented permeable mass of formation particles containing oil shale in an in situ retort site. The retort is formed by excavating a lower level drift adjacent a lower portion of the retort site and excavating an upper level void adjacent an upper portion of the retort site. An undercut is excavated below a zone of unfragmented formation remaining within the retort site above the lower level drift. The undercut can be free of roof-supporting pillars. Explosive is loaded in the zone of formation from access provided by the upper void, and the zone of formation is blasted downwardly toward the undercut for forming the fragmented mass. The volume of the void space within the undercut prior to explosive expansion is similar to the void volume within the principal portion of the fragmented mass being formed.

Abstract: A method for forming an in situ oil shale retort in a subterranean formation containing oil shale is provided. The in situ retort contains a fragmented permeable mass of formation particles within top, bottom, and generally vertically extending side boundaries of unfragmented formation. A lower portion of the fragmented permeable mass of formation particles having a nonlevel top surface is initially formed in the retort. A void space is left within the retort boundaries extending between the nonlevel top surface of the fragmented mass lower portion and a generally horizontally extending free face of an overlying layer of unfragmented formation. Thereafter, the overlying layer of unfragmented formation is explosively expanded into the void space to thereby form the remaining portion of the fragmented mass in the retort. The overlying layer is expanded in a plurality of separate horizontally spaced regions with a time delay between explosive expansion of each successive region.

Abstract: An in situ oil shale retort is formed in a retort site in a subterranean formation containing oil shale. The in situ retort contains a fragmented permeable mass of formation particles containing oil shale within top, bottom and generally vertically extending side boundaries of unfragmented formation. Formation is excavated to form at least one void in the subterranean formation while leaving zones of unfragmented formation above and below such a void and at least one support pillar of unfragmented formation in the void. Such a pillar of unfragmented formation extends between the zones of unfragmented formation above and below the void and is explosively expanded by use of techniques which minimize shock damage to the zones of unfragmented formation. The techniques, which can be used individually or in various combinations, include placing explosive charges into the top, bottom and middle portions of the pillar.

Abstract: An in situ oil shale retort containing a fragmented permeable mass of formation particles having a generally uniform permeability across horizontal cross-sections of the retort is provided. To form the retort, a void is excavated in a subterranean formation within the boundaries of the retort site, while at least one zone of unfragmented formation is left adjacent the void. A plurality of explosive charges are placed in the zone of unfragmented formation in a selected pattern; an array of central explosive charges and a plurality of perimeter charges arranged in rows surrounding the central charges. The spacing distance between each row of perimeter charges and an adjacent outer row of central charges is about equal. The combination of spacing distance and powder factor of the central charges is selected to produce a center portion of the fragmented permeable mass having a first average particle size (d.sub.1) and a first average void fraction (e.sub.1).

Abstract: In situ oil shale retorts are formed in spaced-apart rows, with adjacent rows of such retorts being separated by load-bearing inter-retort pillars of unfragmented formation sufficiently strong for preventing substantial subsidence. Each retort contains a fragmented permeable mass of formation particles containing oil shale. An air level drift is excavated in formation directly above the inter-retort pillar so that the roof and/or floor of the air level drift is spaced above the upper boundaries of the retorts in such adjacent rows. This causes the roof of the air level drift to be in compression, rather than in tension, which stabilizes the roof and avoids dangerous rock falls. During retorting operations, air is introduced at the upper edge of each retort through lateral air inlet passages sloping downwardly from the air level drift. Off gas and liquid products are withdrawn from each retort through a production level passage at the bottom of each retort at the edge opposite the air inlet.

Abstract: A method for loading explosive charges into blastholes in a subterranean formation for forming a fragmented permeable mass of formation particles in an underground cavity in the formation is provided. Upper and lower voids spaced apart vertically from each other by unfragmented formation are excavated into the subterranean formation. A generally vertical blasthole is formed in the unfragmented formation between the voids. Upper and lower explosive charges, separated from one another by stemming, are formed in the blasthole. Each such explosive charge includes at least one primer operationally connected to an explosive initiating lead that extends from the charge. The explosive initiating lead of the upper explosive charge extends from the top of the blasthole into the upper void and the explosive initiating lead of the lower explosive charge extends from the bottom of the blasthole into the lower void.

Abstract: A method for controlling the average particle size of a fragmented permeable mass of formation particles formed in an in situ retort in a subterranean formation containing oil shale is provided. At least one void is excavated in the subterranean formation, leaving zones of unfragmented formation above and below the void. Such a zone of unfragmented formation has naturally occurring cleavage planes and a substantially horizontal free face adjoining the void. An array of a plurality of substantially vertical blastholes is formed in at least one of the zones of unfragmented formation and each such blasthole is loaded with explosive for forming a substantially horizontal array of explosive charges. The spacing distance between adjacent blastholes or explosive charges in the array is from about 10 to about 15 times the average distance between cleavage planes when the oil shale formation has an average grade of more than about 20 gallons per ton.

Abstract: A method for forming an in situ oil shale retort in a retort site in a subterranean formation containing oil shale and having at least one set of naturally occurring cleavage planes is provided. The in situ oil shale retort contains a fragmented permeable mass of formation particles formed within top, bottom, and side boundaries of unfragmented formation. A void is excavated in the subterranean formation within the boundaries of the retort site, while a zone of unfragmented formation is left within the retort boundaries adjacent the void. A plurality of rows of horizontally spaced apart explosive charges is formed in the zone of unfragmented formation where each such row is in a line about perpendicular to the strike of the major cleavage plane set in the formation.

Abstract: There is provided a method for forming an in situ retort containing a fragmented permeable mass of formation particles in a retort site within a subterranean formation. A void is excavated into the formation and a zone of unfragmented formation is left adjacent the void. A plurality of explosive charges are formed in the zone of unfragmented formation. At least one central explosive charge is in a central portion of the zone of unfragmented formation, and a plurality of outer explosive charges are in the zone of unfragmented formation nearer the side walls of the void than the central explosive charge. The distance from each such outer explosive charge to an adjacent side wall of the void is about equal to the crater radius of the outer explosive charge. The central and outer explosive charges are detonated for explosively expanding the zone of unfragmented formation toward the void for forming a fragmented permeable mass of formation particles in the in situ retort.

Abstract: An in situ oil shale retort is formed in a subterranean formation containing oil shale by excavating one or more horizontally extending voids across a retort site, leaving a zone of unfragmented formation having a horizontal free face adjacent such a horizontal void. In one embodiment, such a horizontal void is excavated across less than the entire width of the retort site, leaving "side pillars" of unfragmented formation spaced inwardly from adjacent side boundaries of the retort site at opposite sides of such a horizontal void. This reduces the maximum span of the horizontal void, when compared with supporting overburden above the void with one or more interior isolated pillars spaced inwardly from the side boundaries of the retort. The side pillars are explosively expanded. Then such a zone of unfragmented formation is explosively expanded toward such a horizontal void for forming a fragmented permeable mass of formation particles containing oil shale in the retort.

Abstract: A method for forming an in situ oil shale retort in a subterranean formation containing oil shale is provided. The in situ oil shale retort has top, bottom, and generally vertically extending side boundaries of unfragmented formation and contains a body of expanded oil shale formation that completely fills the retort to its top boundary. The retort is bulked full by explosively expanding a layer above a fragmented permeable mass of formation particles forming part of the body of expanded formation in the retort. The layer is expanded with an available void fraction of no more than about ten percent.

Abstract: An in situ oil shale retort is formed in a subterranean formation containing oil shale. The formation comprises at least one region of relatively richer oil shale and another region of relatively leaner oil shale. According to one embodiment, formation is excavated from within a retort site for forming at least one void extending horizontally across the retort site, leaving a portion of unfragmented formation including the regions of richer and leaner oil shale adjacent such a void space. A first array of vertical blast holes are drilled in the regions of richer and leaner oil shale, and a second array of blast holes are drilled at least in the region of richer oil shale. Explosive charges are placed in portions of the blast holes in the first and second arrays which extend into the richer oil shale, and separate explosive charges are placed in portions of the blast holes in the first array which extend into the leaner oil shale.

Abstract: An in situ oil shale retort is formed by excavating a lower level void adjacent the bottom boundary of a retort site in a subterranean formation, leaving a zone of unfragmented formation within side boundaries of the retort overlying the void. At least a portion of the overlying zone is explosively expanded downwardly towards the void in a plurality of lifts. Vertical columnar explosive charges are placed in each lift and detonated in a single round with a time delay between successive lifts sufficient for formation of the first lift to move out of the way and not interfere with initial explosive expansion of formation in the next successive lift. Preferably, the explosive charges in a second lift are detonated after detonation of explosive charges in a first lift with a time delay in the range of from about five to thirty milliseconds per foot of burden distance of the charges in the second lift.

Abstract: A method for forming an in situ oil shale retort in a retort site in a subterranean formation is provided. The in situ oil shale retort contains a fragmented permeable mass of oil shale particles formed within top, bottom, and side boundaries of the retort site. At least one void is excavated in the subterranean formation within the boundaries of the retort site, while a zone of unfragmented formation is left within the boundaries of the retort site adjacent such a void. An inlet is formed in a zone of the retort adjacent the intersection of a first side boundary of the retort site and the top boundary of the retort site and an outlet is formed in a zone of the retort adjacent the intersection of a second side boundary of the retort site and the bottom boundary of the retort site. The second side boundary is on the opposite side of the retort site from the first side boundary.

Abstract: In situ oil shale retorts are formed in spaced apart rows, with adjacent rows of such retorts being separated by load-bearing barrier pillars of unfragmented formation sufficiently strong for preventing substantial subsidence at the ground surface. Each retort contains a fragmented permeable mass of formation particles containing oil shale. Separate air level drifts are excavated on an upper level of the retorts within alternating barrier pillars, and separate production level drifts are excavated at a lower production level of the retorts within intervening barrier pillars between the barrier pillars having the air level drifts. Each air level drift extends between a pair of adjacent rows of retorts adjacent upper edges of the retorts in the adjacent rows, and each production level drift extends between a pair of adjacent rows of retorts adjacent lower edges of the retorts on sides of the retorts opposite the air level drifts.

Abstract: Liquid and gaseous products are recovered from an in situ oil shale retort formed in a retort site in a subterranean formation containing oil shale. A void is excavated in the subterranean formation within the boundaries of the retort site and a zone of unfragmented formation is left in the retort site adjacent the void. A retort inlet is at one upper edge of the retort site and a retort outlet is at the lower edge of the retort site opposite the retort inlet.Explosive charges are placed in the zone of unfragmented formation and detonated in an asymmetrical time delay sequence for forming a fragmented permeable mass of formation particles which substantially fills the retort to the top boundary in the region of the retort inlet.

Abstract: A fragmented permeable mass of formation particles containing oil shale and having a substantially uniformly distributed void fraction is formed in an in situ oil shale retort in a subterranean formation containing oil shale. The fragmented mass is formed by explosive expansion towards a horizontal free face. At least one generally horizontally extending void is formed in the subterranean formation and zones of unfragmented formation are left above and below such a void. An array of horizontally spaced apart substantially vertical columnar explosive charges is formed in at least one of the zones of unfragmented formation. The array of explosive charges includes a plurality of central explosive charges which are spaced apart from side boundaries of the retort and a plurality of outer explosive charges surrounding the central charges.

Abstract: An in situ oil shale retort is formed in a subterranean formation by excavating voids adjacent the top and bottom boundaries of the retort, leaving an intermediate zone of unfragmented formation between the voids. The lower level void is substantially larger than the upper level void. A lower portion of the intermediate zone is explosively expanded downwardly towards the lower level void for forming a first moiety of a fragmented mass of formation particles in the retort and leaving a void space over the top of the first moiety having about the same volume as the upper level void. Thereafter an upper portion of the intermediate zone is explosively expanded upwardly towards the upper level void and downwardly towards the void space for forming a second moiety of the fragmented mass in the retort. The fragmented mass has an average void fraction up to about 25% and no substantial part has a void fraction less than about 20%.

Abstract: A method for forming a fragmented permeable mass of oil shale particles in an in situ oil shale retort in a subterranean formation containing oil shale is provided. At least one void is excavated within the retort site in the subterranean formation while zones of unfragmented formation are left adjacent such a void. The zones of unfragmented formation are then explosively expanded toward the void for forming the fragmented permeable mass of formation particles in the retort. To provide a fragmented mass with a flat top surface and reasonably uniform void fraction distribution, at least one of the zones of unfragmented formation is explosively expanded toward the void in a plurality of contiguous regions. Each such contiguous region has a free face adjacent the void and is expanded by first explosively expanding a first portion of that region toward the void. The free face of the first portion expanded is near the center of its respective region.

Abstract: An array of explosive charges is formed in a retort site in a subterranean formation containing oil shale. The explosive charges that are located around the perimeter of the retort site are smaller than the explosive charges located more remote from the perimeter. Formation within the retort site is explosively expanded toward a void formed in the site by detonating the explosive charges. This explosive expansion of formation results in a fragmented permeable mass of formation particles in the retort. Damage to objects near the retort site, which is caused by seismic shock from the detonations, is minimized by using smaller explosive charges around the perimeter than in the center of the retort site.