Thermomechanical fracture for recovery system in oil shale deposits

#In-situ processes are governed by the subsurface structure, the process fluid flows, and their interactions. A major problem with in-situ processes has been to establish intimate contact between the process fluids and the deposits, and to establish sufficient porosity or fractures to permit process fluid circulation. It is important that the fracture of the deposit is sufficiently fine so that the process proceeds at a sufficient rate with a high enough efficiency to be economic, but not too fine because fluid flow pressure drop in a packed bed increases by an order of magnitude for a bed particle size reduction of 2, thereby increasing fluid pump or compressor and piping capital and operating costs by a similar amount. There are numerous fracture techniques which are currently in use, among which are explosions, hydrofracture, leaching a soluble phase, electrofracture, abrasive cutting, and acids, to name a few.

#Where these fracture techniques were applied to oil shale deposits, explosives were the most commonly used. In this potentially dangerous approach, underground tunnels were carved into the oil shale deposits in a predetermined pattern for the purpose of blasting and rubblizing the deposit. In performing the blasting process, care was required to leave sufficient support so that the entire overburden of the deposit was not collapsed into the tunnel voids. Considerable difficulty was experienced in rubblizing the oil shale deposit to produce rubble of the appropriate size which would support a reasonably uniform flame front for the retorting of the hydrocarbon values in the shale. If the rubble was not reasonably uniform and of proper size, a substantially uniform flame front was not maintained, and process and product gases mixed and reacted which contributed to the quenching of the desired retort flame front and reduced product recovery. Thus, much time and consideration was given to the blasting patterns which were used to rubblize the oil shale deposits, and even then the fracturing patterns produced were frequently by chance.

#The present invention relates to an in-place method for the recovery of mineral values from subsurface deposits. More particularly, the present process relates to a method for producing a fracturing pattern in subsurface oil shale deposits.

#Non-steady state thermal stresses arise as a result of transient temperature gradients in a body and the corresponding differential thermal expansion which cannot be accommodated by geometrically compatible displacement within the body. These stresses continually adjust themselves in such a way that the internal forces in the body are self-equilibrating and the displacements are compatible. If, in the process, either the stresses or the strains reach some critical value, failure may occur.

#In general, either fracture or excessive deformation may be taken as the critical failure mode. The expression "thermal shock" has been defined by materials investigators as catastrophic brittle fracture which occurs as a result of high tensile stresses which are generated at the cooler side of transiently heated bodies. These same tensile forces might instead produce excessive deformation in a body if the material were strong enough to resist fracture, or if it were ductile rather than brittle. Even if the deformation were not excessive during a single heating and cooling cycle, multiple cycling can lead to an accumulated deformation which eventually will become excessive.

#One mode of failure for in-situ processing involves both plastic flow and fracture near the heated surface of the body. Regions of checking or cracking have been noted near the heated surface where compressive plastic flow has occurred during heating. It has recently been discovered that the reversal of stress at the hot surface, from compressive to tensile, and the reversal of plastic flow from compressive to tensile, occurs not when the body cools but earlier in the cycle, i.e. as soon as the temperature gradient begins to disappear. This will happen even if the overall temperature of the body is still increasing, as might be the case during sustained heating. Thus, the heated surface material might be put into tension, which would be multiaxial tension in most cases, while it is still very hot to the point of approaching melting, and ductile fracture or hot tearing would very easily take place. Cracking of this nature could also lead to loss of material at the hot surface which might be mistaken for compressive spallation in any post-test evaluation.