WO2008028255A1

WO2008028255A1 - Recovery of hydrocarbon products from oil shale

Info

Publication number: WO2008028255A1
Application number: PCT/AU2007/001334
Authority: WO
Inventors: Raymond Walter Shaw
Original assignee: Technological Resources Pty. Limited
Priority date: 2006-09-08
Filing date: 2007-09-10
Publication date: 2008-03-13

Abstract

A method and a system for recovering hydrocarbons in liquid form from oil shale is disclosed. The method includes forming a heap (3) of oil shale, controlling a thermal profile of the material within the heap to allow at least partial decomposition of solid hydrocarbons in the oil shale to form a liquid hydrocarbon product and recovering the liquid hydrocarbon product from the heap.

Description

RECOVERY OF HYDROCARBON PRODUCTS FROM OIL SHALE

The present invention relates to recovering hydrocarbons in liquid form from oil shale.

The term "oil shale" is understood herein to mean rocks that include organic material in the form of kerogen in amounts that are sufficient to yield hydrocarbons , such as petroleum, upon treatment such as by distillation.

The present invention provides a heap-based method and system for recovering hydrocarbons in liquid form from oil shale .

Currently, there is some limited production of petroleum from oil shale using a long-established thermal retorting process. In general terms, the process includes heating oil shale to 450-500⁰C in the absence of air, whereby the kerogen is converted chemically to oil.

Potential alternatives to the retorting process are a number of in-situ processes and direct extraction processes . Research and development work to varying extents has been carried out on these processes. However, the processes are not being used commercially at this stage .

In-situ processes are based on heating oil shale underground to release gas and oil. The Shell Oil Company is one company that is developing a particular in-situ process. Chevron and ECL also have active programs. There are difficulties with these processes in obtaining high recovery, the amount of energy and time taken, the need to seal the deposits to avoid leakage of fluids, and difficulties in achieving acceptable heat transfer through the rocks of the deposits. There is also limited permeability of the rocks. Moreover, the processes are only applicable to oil shale which is well below the surface .

Direct extraction processes have been attempted based on the known ability of light hydrocarbons such as toluene to extract kerogen, especially when used under supercritical conditions and with the inclusion of sources of hydrogen. One such process is that described in US patent 4,108,760 in the name of Williams et al . Other processes include a so-called ATS (Australian Thermal Solutions) process invented by John Rendall . The ATS process is described in a series of patent families which include Australian patents 609782 and 779333 and Australian patent application 2003255192, all in the name of John Rendell .

None of these prior art processes are being used commercially as they have not been able to be made economic.

Red Leaf Resources has published information on a capsule-based method referred to as the "EcoShale" method. The method includes heating oil shale in a capsule via a plurality of indirect heating elements positioned to extend into the oil shale within the capsule .

The above discussion of prior art processes is not to be taken as an admission of common general knowledge in Australia (or elsewhere) .

According to the present invention there is provided a method of recovering hydrocarbons in liquid form from oil shale that includes :

(a) forming a heap of oil shale; (b) controlling a thermal profile of the material within the heap to allow at least partial decomposition of solid hydrocarbons in the oil shale to form a liquid hydrocarbon product; and

(c) recovering the liquid hydrocarbon product from the heap.

The above-described method avoids the need to use a conventional retorting apparatus .

In addition, work carried out by the applicant indicates that it may not be necessary to generate temperatures within the heap that are as high as temperatures required in the conventional retorting apparatus to achieve a commercially viable operation.

Preferably step (a) includes forming a sealed heap, i.e. a heap that at least substantially prevents one or more than one of uncontrolled (i) loss of volatiles or other gases, (ii) liquid flow (which term includes slurries), and (iv) heat loss.

The method may include operating two or more than two heaps at different stages of the method with heat transfer between the heaps using suitable carrier fluids .

The use of such a multi-stage heap system allows for heat recovery to further improve the thermal efficiency .

The heap or heaps may be constructed either as fixed bodies or in a race track or similar configuration where the material is removed after treatment and disposed of separately . The heap or heaps may be above ground.

The heap or heaps may be below ground.

The heap or heaps may be built within natural valleys or in a mine pit such that one or more sides of the heap are naturally sealed by the topography.

Preferably step (b) includes controlling the thermal profile within the heap so that the bulk temperature of the material within the heap is at least 200⁰C.

Preferably step (b) includes controlling the thermal profile within the heap so that the bulk temperature of the material within the heap is in the range of 200-400⁰C.

The above-described thermal profile may be self- generated by virtue of the properties of the oil shale supplied to the heap and/or the geometry of the heap and/or other properties of the heap (including particle size distribution and packing density within the heap) .

The thermal profile may also be generated by external heat sources , such as the use of electrical resistance heating positioned within the heap and/or by supplying heated gas to the heap.

Preferably step (b) comprises controlling the thermal profile within the heap by a combination of (a) indirect heating via heating elements extending into the heap and (b) direct heating using a heated fluid, such as a heated liquid, such as water.

The method can be managed to give cost effective and high extraction rates through the use of a combination of indirect heating and direct heating/extraction with fluids , preferably heated liquids .

Preferably the heated fluid is a heated liquid.

The heated fluid may be a heated gas .

Preferably step (b) includes supplying the heated liquid into an upper section of the heap, whereby the heated liquid flows downwardly through the heap.

Preferably step (b) includes supplying the heated liquid so that there is substantially uniform flow of the heated liquid through the heap so that there is heat transfer via the heated liquid to substantially all, i.e. at least 70%, preferably at least 80%, of the material in the heap .

The use of the heated fluid in step (b) facilitates heat transfer through the heap and makes it possible to heat at least substantially all of the material in the heap. Heat transfer is a significant issue for processing oil shale. Indirect heating elements, such as proposed in the EcoShale method, rely on conductive heat transfer to heat material in heaps. Such conductive heat transfer through packed material in heaps is relatively poor. The use of the heated liquid overcomes this problem.

In addition, the use of a liquid, i.e. the heated liquid, in step (b) facilitates removing hydrocarbon products produced in the heap by decomposition of solid hydrocarbons in oil shale from the heap .

The liquid may be any suitable liquid.

Preferably the liquid is selected on the basis that the boiling point of the liquid is above the temperature of the material in the heap, thereby to retain the liquid in a liquid phase and avoid pressure build-up in the heap that would occur if the liquid boils and forms a gas phase .

Preferably the liquid is selected on the basis that the liquid has a sufficiently low viscosity to flow through the heap at the operating temperature of the heap .

Suitable liquids include by way of example water and liquid hydrocarbons .

The liquid hydrocarbons may contain added compounds such as Teralin (Trade Mark) or Decalin (Trade Mark) which are capable of supplying hydrogen through acting as hydrogen donors to improve the extraction of the kerogen analogous to their use in direct liquid extraction .

The heating elements to indirectly heat the material in the heap in step (b) may be heated by way of example by any one or more of electrical resistance heating, radio frequency heating, burning fuel , super- heated steam, and other heated fluids.

Preferably step (c) includes discharging liquid that includes the liquid hydrocarbon product from the heap from a lower part of the heap .

Preferably the method includes discharging volatiles from the heap and thereafter recovering heat from the volatiles and using the heat for example to heat the liquid supplied to the heap in step (b) .

Preferably the method includes crushing or otherwise processing mined oil shale and forming a required particle size distribution for forming the heap in step (a) .

In one embodiment the required particle size distribution includes at least 80% of the ore being within a size range of 10-100 mm. However, the required particl;e size distribution in any given situation will depend on a range of factors including, for example, the required bed permeability.

The method may include processing mined oil shale for forming the heap by processing oil shale through a grinding/crushing apparatus that includes a plurality of grinding/crushing rolls and forming a ground/crushed oil shale product, typically in the form of a cake that at least substantially includes minus 10 mm particles . This method is described and claimed in International application PCT/AU2005/001645 in the name of the applicant . The specification of the International application is incorporated herein by cross-reference..

Preferably the grinding/crushing rolls are arranged in a pair of grinding/crushing rolls with a nip separating the rolls and the rolls rotating in opposite directions, and the method includes supplying oil shale to the nip between the rolls so that the oil shale is thereafter forced through the nip and thereby ground/crushed.

The term "pair of grinding/crushing rolls" is understood herein to include arrangements in which the rolls in the pair of rolls are backed up by back-up rolls.

The crushing/grinding apparatus may include a single pair of rolls or a plurality of pairs of rolls arranged in a series so that the ground/crushed oil shale from an upstream roll pair is a feed material to a downstream roll pair.

Preferably the grinding/crushing rolls are high pressure grinding/crushing rolls .

The method may include exposing oil shale to microwave energy for physically and/or chemically altering the oil shale prior to forming the oil shale into the heap.

In one embodiment the method includes exposing oil shale to microwave energy prior to or after processing oil shale in the grinding/crushing apparatus .

Preferably the grinding/crushing apparatus further includes a means for exposing ground/crushed material flowing between upstream and downstream pairs of grinding/crushing rolls to microwave energy for physically and/or chemically altering the material .

Preferably the upstream pair of grinding/crushing rolls produces ground/crushed material that is in a form that is suitable for microwave energy treatment of the material .

For example, preferably the upstream grinding/crushing rolls produce a uniform stream of crushed material that is well suited to microwave treatment .

By way of further example, preferably the upstream grinding/crushing rolls produce a defined geometry of crushed material that is well suited to microwave treatment . The term "microwave energy" is understood herein to mean electromagnetic radiation that has frequencies in the range of 0.3-300 GHz.

Preferably the microwave energy exposure means is adapted to cause micro-cracking of material that facilitates subsequent breakdown of material .

Preferably the microwave energy exposure means is adapted to expose the material to pulsed high energy microwave energy.

The term "high energy" is understood herein to mean values substantially above those within conventional household microwaves, ie substantially above 1 kW.

Preferably the energy of the microwaves is at least 20 kW.

More preferably the energy of the microwaves is at least 50 kW.

The use of pulsed microwave energy minimises the power requirements of the method and maximises thermal cycling of the oil shale particles.

The time period of the pulses and the time period between pulses of microwave energy may be set as required depending on a number of factors .

According to the present invention there is also provided a system for of recovering hydrocarbons in liquid form from oil shale that includes:

(a) a heap, preferably a sealed heap, of oil shale ; (b) a system for controlling a thermal profile of the material within the heap to allow at least partial decomposition of solid hydrocarbons in the oil shale to form a liquid hydrocarbon product; and

(c) one of more than one outlet for a liquid including the liquid hydrocarbon product.

Preferably the system is adapted to control the thermal profile of the material within the heap by heating the oil shale in the heap by a combination of (i) indirect heating via heating elements extending into the heap and (ii) direct heating using a heated fluid, such as a heated liquid, such as water, to allow the at least partial decomposition of solid hydrocarbons in kerogen in the oil shale to form the liquid hydrocarbon product .

Preferably the system includes an apparatus for recovering the hydrocarbon product from the outlet liquid stream such as the distillation process used for treating hydrocarbons from other sources .

Preferably the system includes one or more than one outlet for volatiles and other gases produced in the heap .

Preferably the system includes an apparatus for recovering heat from the outlet flows of the liquid and/or from the volatiles and other gases and using the heat in the system.

Preferably the apparatus for recovering the hydrocarbon product also produces the heated liquid for direct heating the oil shale in the heap in the system of sub-paragraph (b) above and thereby extracting the hydrocarbon contents . Preferably the system includes heat exchange units separate to the heap in which the fluid used in heating (and cooling) the heap is heated to the desired temperature for use in the system of sub-paragraph (b) above generally by transferring heat between the fluid and other process fluids and/or by indirect contact with heated fluids prepared separately.

The present invention is described further by way of example with reference to the accompanying drawings, of which :

Figure 1 is a diagram that shows (in a very simplified form) an embodiment of a multi-stage heap system of the present invention for carrying out an embodiment of the method of the present invention; and

Figure 2 is another diagram that shows (in a very simplified form) another embodiment of the multi-stage heap system of the present invention for carrying out another embodiment of the method of the present invention .

The heap-based method of the present invention works most efficiently when there are multiple heaps which are at different stages of the method. However, it is emphasised tha the present invention is not confined to such an arrangement.

Figure 1 illustrates a multi-stage system having two such heaps 3 at different stages.

The first step in the embodiment of the method of the present invention described herein is to beneficiate mined oil shale prior to forming heaps .

In cases where the mined oil shale is relatively heterogeneous the oil shale can be beneficiated to separate out different grades of material which may be processed separately.

Mined oil shale may be beneficiated to separate the ore into different grades, whereby lower grade oil shale is discarded and higher grade oil shale is treated in one or more than one treatment method.

The treatment methods may be a combination of (a) the heap-based method of the present invention for medium grade oil shale and (b) a direct extraction method such as thermal retorting or direct liquid extraction for the highest grade oil shale and/or for fine material which is not well suited to direct use in a heap arrangement.

It is preferred that the oil shale be mined with minimum gangue associated with it.

Preferred oil shale is in material that is fairly close to the surface to reduce costs.

Mining oil shale may be via a strip system such that a mined-out area can subsequently be used either as a location for the heap-based method of the present invention and/or as a site to dispose of residue from the method.

The mined oil shale is crushed to a suitable size for treatment within a heap. In any given situation, this size depends upon permeability of the rock and hence rate at which the hydrocarbon component of the oil shale can be extracted. Typically, the majority of the ore is within a size range of 10-100 mm but in some cases "run of mine" oil shale may be used which has a much wider size distribution and includes some much coarser material.

The method includes forming the crushed oil shale ore into sealed below-ground heaps 3 shown in Figure 1 and thereafter thermally treating the oil shale in situ in the heaps 3 to recover a liquid hydrocarbon product from kerogen in the oil shale.

The thermal treatment is (a) indirect heating via heating elements 7 positioned to extend into and through the heaps 3 and (b) direct heating via a heated fluid, preferably a heated liquid supplied to the tops of the heaps 3.

The main issues in designing the heaps 3 include:

(a) avoiding loss of volatiles to the atmosphere;

(b) achieving efficient heat transfer to give a reasonable heat-up rate of the material in the heaps ;

(c) having a simple heat supply system to minimise costs;

(d) having an efficient collection system to recover "oil", i.e. a liquid hydrocarbon product, from kerogen in the oil shale;

(e) incorporating heat recovery to improve overall thermal efficiency; and

(f) "washing" the heap to maximise recovery and minimise residual carbonaceous material within the heaps .

A preferred heap arrangement is one where each heap 3 is virtually sealed to atmosphere and incorporates specific take-off points to collect gas (including volatiles) indicated generally by the numerals 11 and liquid indicated generally by the numerals 13 from the heap as kerogen transforms within the heap into a liquid hydrocarbon product (which term includes a range of liquid hydrocarbon products) .

Preferably the gas and liquid take-off points 11,

13 of each heap 3 are maintained under suction for efficient removal and to minimise leakage.

Preferably each heap 3 includes one or more than one layer (not shown) of an "inert" rubble, preferably with a fairly narrow size range, within the heap to give enhanced permeability in the heap through which the heated liquid and gas can readily pass either to aid supply or to assist in collection of these fluids .

One method of sealing a part or all of the heaps 3 is to use a "cover" system such as is employed in mining operations where a low permeability layer material is placed on exposed surfaces of the heap to minimise fluid ingress and outflow.

This cover system, generally identified by the numeral 15, can most simply be made using low permeability soils such as clay material and may deliberately be kept saturated with water to further decrease the permeability to other fluids .

Another method of sealing a part or all of the heaps 3 is to use an impermeable barrier such as provided by plastic membranes or by an engineered material such as concrete or more probably the class of materials known as geopolymers .

The constructed heaps 3 may incorporate one or more systems for sealing the different surfaces depending upon the characteristics of the specific location where the heaps are being formed. In some cases the heap construction may also include having one or more reactive barriers placed to stop unwanted hydrocarbon movement either within the heap 3 or from the heap to the surrounds, with these barriers typically being of the type commonly used in remediation of contaminated soils and to stop flow of hydrocarbon spillages .

In one embodiment the top of each heap 3 is arranged such that there is (a) a high permeability layer to allow liquid supply and volatiles collection, (b) a low permeability layer above it to act as a barrier to fluid outflow, and (c) another higher permeability layer on the top to reduce evaporation from the heap and allow the low permeability layer to be kept saturated with water.

This layered system on top of each heap 3 is also advantageous in that it can provide a suitable formation for final closure either as is or with the addition of suitable soil allowing revegetation if desired.

The heating elements 7 can be any suitable elements . Examples of suitable heating elements include (a) electric heaters such as proposed for in situ work based on resistance and/or RF heating, (b) tubes in which fuel such as natural gas and/or volatiles from the heap itself are burnt to generate heat, and (c) tubes that are heated by passing hot liquids or gases heated in external heating systems through the tubes.

The heating elements 7 in the embodiment shown in Figure 1 are in the form of a plurality of tubes 7 extending horizontally through the sides of the heaps 3. Hot liquid that has been heated in an external heating system 9 is pumped through the tubes 7 successively through the heaps 3 and provides heat indirectly to the oil shale in the heaps . The liquid flowing from outlet ends of the tubes 7 is reheated and recirculated through the tubes 9 via a system (not shown) .

The heating elements 7 may also function as

"cooling elements". Specifically, in the case of the embodiment shown in Figure 1, there may also be heat transfer from the heaps 3 to liquid passing through the tubes 7 after the heating stage of the method has been completed and all of the readily extractable kerogen has been extracted from the oil shale in the heaps 3. In this situation, the liquid passing through the tubes is a means of extracting heat and thereby cooling the heaps . The resultant heated liquid can be used to heat another heap or heaps.

Vertically extending heating elements (not shown) may be used where there is restricted access to the sides of the heaped ore that makes it difficult to insert heating elements through the sides or in combination with horizontally extending heating elements.

Different sources of heat for the heating elements 7 can be used depending upon cost and availability. One option is to primarily use electric heating with electricity from an external source such as a national grid or alternatively generated locally using products such as volatiles from the kerogen itself. A preferred alternative in some locations is to use natural gas for heating and as volatiles are emitted from the kerogen add them to the fuel mix to supplement or possibly even replace the natural gas . In cases where there is a ready supply of coal , the coal can be used to generate heat for use in the heap, largely through fluids and hot gases, and in a cogeneration system also supply electricity for use and possible sale. The heaps 3 shown in Figure 1 each include a system to supply (for example by injection) a heated liquid (and/or a suitable heated gas) to the top of the heap 3 which is able to transfer heat directly to the material in the heap and also provide added heat transfer in sections of the heap that are remote from the heating elements 7.

The liquid may also be selected to assist in the removal of the liquid hydrocarbon product produced from kerogen in the oil shale and to avoid problems of not having good product flow because of high viscosity of some of the oil fractions generated from the kerogen.

The liquid is preferably chosen to have a boiling point that is above the temperature of the heap (at the pressure within the heap) to reduce the likelihood of pressure build-up disrupting the heaps 3 and the barrier systems incorporated in the heaps .

In one embodiment the method includes initially supplying hot water to the heaps 3 until the heap temperature approaches 100°C and thereafter switching liquids and supplying a light fraction hydrocarbon and then longer carbon chain higher boiling point hydrocarbons as the temperature increases towards a maximum operating temperature in a range of 350-400⁰C. The maximum operating temperature is within the range achievable using relatively simple hydrocarbons with carbon chain lengths from C15 to C30 such as in diesel, lubricating and fuel oils .

Once the kerogen in the oil shale is substantially extracted form the oil shale the sequence of supplying liquids to the heap 3 described in the preceding paragraph is reversed. In the reverse sequence, the hydrocarbons wash out any residual hydrocarbon product and a final stage involving a "hot water wash" leaves the heap capable of being closed and/or the remaining material in the heap in a form that is suitable for removal from the heap and separate disposal .

A preferred arrangement used to achieve a

"sealed" heap 3 with the required heating system (indirect heating elements and direct heating by heated fluid) and the required liquid hydrocarbon product removal system may vary depending upon the location of the operation and the physical properties of the oil shale ore being processed.

In one embodiment the oil shale-containing ore is mined, crushed to size, and then deposited on an impervious pad using a conveyor stacker arrangement. The heating elements and barrier layers are then added to the stacked heap to give the desired system. After completion of the kerogen extraction and subsequent cooling of the residual ore, the heating and fluid system is then disconnected and the ore is removed using a reclaimer - conveyor system and transported to a separate site for final disposal and rehabilitation.

In one embodiment the overall layout is in an oval shape referred to as a race track configuration in copper heap leaching. This system has a disadvantage of double handling of the ore but compensates for that in being able to reuse both the barrier placed underneath the ore and the infrastructure in place for heating and fluid handling. The oval is quite large such that there are several distinct heaps in operation around it each at a different stage of the heating and cooling cycle such that heat can be transferred readily between them.

In another embodiment shown in Figure 2 separate above-ground heaps 3 are constructed, typically of the order of 10 m high, as stand-alone units. The oil shale within the heaps is enclosed by barriers 15 with requisite inlets and outlets to enable heating fluid supply and liquid hydrocarbon product removal . As is the case with the below ground heaps shown in Figure 1 , several heaps 3 are in operation at once in the system and heat is transferred between the heaps. Upon completion, the heaps 3 are "closed" and rehabilitated in situ and/or can be used as the base for a subsequent heap whereby more oil shale is stacked on top of the initial heap and the operation repeated. Several heaps are placed on top of an initial heap to minimise the overall area covered by the heaps and also to reduce the costs associated with having impermeable barriers under each one .

The ability to have multiple heaps 3, and the height of individual heaps, is dependent upon the strength of the host rock which holds the kerogen. In cases where the rock is weak after removal of the kerogen and tends to break down with any force applied then the height of any individual heap may be restricted to less than 10 m and multiple heaps may not be feasible. In this case the race track arrangement may be favoured.

In another embodiment a heap 3 is placed into a valley such that one or more than one side is enclosed by a natural terrain, thereby leaving only the surface and one exposed side to be sealed. The valley may be a natural feature. Typically, the heap 3 is placed in a pit left from mining the ore. This approach is particularly useful where ore has been strip-mined and is reasonably close to the surface. In this case a series of heaps are constructed as the mining progresses with probably the first one or more being on the surface and then the mined out pit being used once it has sufficient volume to be usable.

An alternative arrangement is to use a pit as a site for disposing of the residual ore from a "race track" arrangement after which it can be rehabilitated.

The physical arrangement of the heating elements 7 and the fluid supply system and the liquid removal system can vary depending upon the physical properties of the ore and the arrangement chosen for a heap . In a simple system these components are included in a heap in a horizontal configuration spaced to give appropriate fluid and heat flow distances. In any given situation, the optimum spacing depends upon balancing the costs of the equipment against the loss of efficiency in recovering and injecting fluids, and in achieving heat transfer to give desired heating rates .

A minimum system includes a single fluid supply inlet for supplying hot fluid (liquid or gas) to a sealed heap and a corresponding single collection point to recover the fluid and a liquid hydrocarbon product extracted from kerogen discharged from the heap. The system also includes one or more heating elements to heat the fluid within the heap.

More complex systems include multiple hot fluid supply points and collection points. The fluid may be heated or cooled external to the heap either through dedicated heat exchange systems and/or by passage through other heaps at different stages of the cycle . The system can be arranged to have the fluid travel either upwards or downwards through having appropriate pumping systems to control the pressure gradient.

Many modifications may be made to the embodiments of the method and the system of present invention described above without departing from the spirit and scope of the invention .

Claims

1. A method of recovering hydrocarbons in liquid form from oil shale that includes :

(a) forming a heap of oil shale;

(b) controlling a thermal profile of the material within the heap to allow at least partial decomposition of solid hydrocarbons in the oil shale to form a liquid hydrocarbon product; and

(c) recovering the liquid hydrocarbon product from the heap .

2. The method defined in claim 1 wherein step (a) includes forming a sealed heap, i.e. a heap that at least substantially prevents one or more than one of uncontrolled (i) loss of volatiles or other gases, (ii) liquid flow (which term includes slurries) , and (iv) heat loss .

3. The method defined in claim 1 or claim 2 wherein step (a) includes operating two or more than two heaps at different stages of the method with heat transfer between the heaps using suitable carrier fluids .

4. The method defined in any one of the preceding claims wherein step (b) includes controlling the thermal profile within the heap so that the bulk temperature of the material within the heap is at least 200°C.

5. The method defined in any one of the preceding claims wherein step (b) includes controlling the thermal profile within the heap so that the bulk temperature of the material within the heap is in the range of 200-400⁰C.

6. The method defined in any one of the preceding claims wherein step (b) includes controlling the thermal profile by virtue of the properties of the oil shale supplied to the heap and/or the geometry and/or other properties of the heap (including particle size distribution and packing density within the heap) .

7. The method defined in any one of claims 1 to 6 wherein step (b) includes controlling the thermal profile by external heat sources , such as the use of electrical resistance heating positioned within the heap and/or by supplying heated gas to the heap.

8. The method defined in claim 7 wherein step (b) comprises controlling the thermal profile within the heap by a combination of (a) indirect heating via heating elements extending into the heap and (b) direct heating using a heated fluid, such as a heated liquid, such as water.

9. The method defined in claim 8 wherein the heated fluid is a heated gas .

10. The method defined in claim 8 wherein the heated fluid is a heated liquid.

11. The method defined in claim 10 wherein the step (b) includes supplying the heated liquid into an upper section of the heap, whereby the heated liquid flows downwardly through the heap.

12. The method defined in claim 10 or claim 11 wherein step (b) includes supplying the heated liquid so that there is substantially uniform flow of the heated liquid through the heap so that there is heat transfer via the heated liquid to substantially all, i.e. at least 70%, of the material in the heap.

13. The method defined in any one of claims 10 to 12 wherein the liquid is selected on the basis that the boiling point of the liquid is above the temperature of the material in the heap, thereby to retain the liquid in a liquid phase and avoid pressure build-up in the heap that would occur if the liquid boils and forms a gas phase .

14. The method defined in any one of claims 10 to 13 wherein the liquid is selected on the basis that the liquid has a sufficiently low viscosity to flow through the heap at the operating temperature of the heap.

15. The method defined in any one of claims 8 to 14 wherein the heating elements to indirectly heat the material in the heap in step (b) any one or more than one of electrical resistance heating, radio frequency heating, burning fuel, super-heated steam, and other heated fluids.

16 The method defined in any one of the preceding claims wherein step (c) includes discharging liquid that includes the liquid hydrocarbon product from the heap from a lower part of the heap.

17. The method defined in claim 16 wherein step (c) includes discharging and optionally capturing, volatiles from the heap and thereafter recovering heat from the volatiles and using the heat for example to heat the liquid supplied to the heap in step (b) .

18. The method defined in any one of the preceding claims includes crushing or otherwise processing the mined oil shale and forming a required particle size distribution for forming the heap in step (a) .

19. A system for of recovering hydrocarbons in liquid form from oil shale that includes :

(a) a heap, preferably a sealed heap, of oil shale;

(b) a system for controlling a thermal profile of the material within the heap to allow at least partial decomposition of solid hydrocarbons in the oil shale to form a liquid hydrocarbon product; and

20. The system defined in claim 19 wherein the thermal profile control system (b) is adapted to control the thermal profile of the material within the heap by heating the oil shale in the heap by a combination of (i) indirect heating via heating elements extending into the heap and (ii) direct heating using a heated fluid, such as a heated liquid, such as water, to allow the at least partial decomposition of solid hydrocarbons in kerogen in the oil shale to form the liquid hydrocarbon product.