WO2015181579A1

WO2015181579A1 - Subsurface molten salt heater assembly having a catenary trajectory

Info

Publication number: WO2015181579A1
Application number: PCT/IB2014/061697
Authority: WO
Inventors: Scott NGUYEN; Harold Vinegar
Original assignee: Genie Ip B.V.
Priority date: 2014-05-25
Filing date: 2014-05-25
Publication date: 2015-12-03

Abstract

An L-shaped or U-shaped subsurface heater assembly (e.g. a molten salt heater) having a catenary trajectory is disclosed herein. In some embodiments, the heater is configured to operate at an operating temperature T_OPERATE of at least 450 degrees Celsius, or at least 500 degrees Celsius, or at least 550 degrees Celsius, or at least 600 degrees Celsius. In some embodiments, at least a heel section follows a catenary trajectory characterized by a catenary coefficient a having a value of at least 200 or at least 300 or at least 400. In some embodiments, at least the heel section of the heater is constructed of a nickel-free and/or chromium steel alloy, such as P91 steel. Although the creep rupture strength of this steel alloy is significantly less than those of more expensive austenitic stainless steels, the subsurface heater has high-temperature stability against creep rupture due to the catenary trajectory. Methods of operating the presently disclosed subsurface heater assembly to produce and/or mobilize hydrocarbon fluids are disclosed herein.

Description

SUBSURFACE MOLTEN SALT HEATER ASSEMBLY HAVING A CATENARY

TRAJECTORY

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of PCT/US2014/35053 filed on April 22, 2014 which claims priority to U.S. Application 61/814,346 filed on April 22, 2013, both of which are hereby incorporated in their entirety by reference.

FIELD OF THE INVENTION

The present invention relates to methods and systems of heating a subsurface formation, for example, in order to produce hydrocarbon fluids therefrom.

BACKGROUND

L-Shaped Conduit-in-Conduit Heater Assemblies

FIGS. 1-3 (prior art) are illustrations of a conventional subsurface molten salt L- shaped conduit-in-conduit heater assembly 100 disposed within a subsurface hydrocarbon-containing formation 220 such as a bitumen formation, a kerogenous chalk, an oil-shale formation, a heavy oil formation, a tight oil formation, or a coal formation.

In one embodiment, pyrolysis fluids and/or mobilization fluids are recovered from the subsurface formation 220 via a production well (NOT SHOWN).

FIGS. 1 and 3 are side-views of the heater assembly, while FIG. 2 illustrates a cross-section of a conduit-in-conduit. FIG. 1 illustrates various details of the heater assembly, while FIG. 3 is more schematic and illustrates certain implementation details of one example of a conventional conduit-in-conduit heater assembly.

In all of FIGS. 1-3, a hot heat-transfer fluid (i) first enters the L-shaped heater assembly 100 via inlet 204, (ii) subsequently flows within a core/inner section 320 of the heater assembly towards a toe 350 of the L-shaped heater assembly; (iii) at the toe, changes horizontal direction and flows back towards the surface through an annular- shaped outer section 324 of the heater assembly; and (iv) exits the heater assembly via outlet 208. As illustrated in FIG. 2, the heater comprises (i) pipe PI 334 (also referred to defining a core region or inner-flow region 320, (ii) pipe P2 332; (iii) insulating layer 308 in between pipes PI 334 and P2 332; (iii) pipe P3 302; (iv) annular-shaped outer-flow region 324 and (iv) wellbore 330 outside of an outer surface of pipe P3 302 and formation 220. The fluid within the inner region 320 is hotter than that in outer flow region 324.

The heat transfer fluid may be heated in any manner known in the art. In one example, the heat-transfer fluid is heated in a surface-level furnace - e.g. powered by fossil fuel (e.g. by non-condensable hydrocarbon gases recovered from the formation 220) or powered by alternative energy sources such as solar or nuclear. Alternatively or additionally, it is possible to heat the heat-transfer fluid within the subsurface by forcing an electrical current through a subsurface electrical-resistor in thermal communication with the heat-transfer fluid.

Although a number of heat-transfer fluids may be used (e.g. hot synthetic oil, superheated steam, and others), for applications where heater efficiency is important, the preferable choice of heat-transfer fluid is molten salt, due to its very high heat-capacity and high temperature stability.

In FIG. 3, the L- shaped heater assembly includes three sections - a vertical section

410, a heel section 420, and a horizontal-section 430.

In many applications related to in-situ pyrolysis or to in-situ mobilization, the maximum depth of the heater is around 500 meters - for example, in a range between 200 and 500 meters.

The example of FIG. 3 illustrates a medium radius horizontal well with a true vertical depth that is about 300 meters below the surface 216. The total length of the heater, or the 'measured depth' (MD) is about 4,600 feet or about 1,400 meters.

The 'true vertical depth' corresponds to the conventional definition of a 'depth.' When a point P on a heater has a 'measured depth' (MD) of x meters, this means that a length of a path between point P and the surface along the trajectory of the heater is x meters. Thus, the term 'measured depth' actually corresponds to a 'path length.'

A first feature of the conventional medium radius horizontal subsurface heater of

FIG. 3 is that the vertical section 410 remains exactly vertical for slightly more than 50% of its true-depth— the kick-off point where the heater turns in a horizontal direction is at a depth of 152.4 meters,. A second feature of the conventional subsurface heater of FIG. 3 is that in the heel section, the heater follows a curve shaped exactly like a circular section— in the example of FIG. 3, the radius of curvature is 160 meters, causing heater to turn by 35.7 degrees per 100 meters of drilling depth.

Reference once again to FIG. 1, additional numbered elements include: overburden 218, outer insulation 466 (for minimizing heat-losses to overburden 218), sliding seals 450 and 470, casing 458, packer 462, and wellhead 454.

Recovery of Unconventional Oil

The world's supply of conventional sweet, light crude oil is declining, and discoveries and access to new resources for this premium oil are becoming more challenging. To supplement this decline and to meet the rising global demand, unconventional oils are being produced and brought to market. Sources of unconventional oils include tar sands, tight oil formations, oil shale formations, kerogenous chalks, heavy oil formations and coal formations.

Despite the need for oil derived from unconventional sources, many policymakers are concerned about the collateral C0₂ footprint associated with both the production of hydrocarbon fluids from unconventional sources and the exploitation of such hydrocarbon fluids.

In reference to tar sands, Wikipedia states:

According to the Canadian asoc-anon of Petio eurn Producers and Environ ent Canada the industrial activity undertaken to produce oil sands make up about 5% of Canada's greenhouse gas emissions, or 0.1 % of global greenhouse gas emissions. It predicts the oil sands will grow to make up 8% of Canada's greenhouse gas emissions by 2015... A 2012 study by Swart and Weaver estimated that if only the economically viable reserve of 170 Gbbl (27x10⁹ m³) oil sands was burnt, the global mean temperature would increase by 0.02 to

0.05 °C.

There is an ongoing need for technology for producing unconventional oil in an environmentally-friendly manner, with minimal greenhouse gas emissions. Unfortunately, there is often a trade-off between the goal of minimizing environmental impact and that of producing unconventional oil in a manner that is economically feasible. In one example, reliance on heaters powered by renewable energy sources such as wind or solar may increase the overall per barrel cost due to the intermittent nature of renewable energy.

Another example relates to molten salt heaters. Molten salt heaters are extremely efficient because they operate at very high temperatures (e.g. at least 550 or at least 600 degrees Celsius) - therefore, thermal energy is transferred to the subsurface formation very efficiently. For situations where molten salt is heated by a fossil-fuel furnace (e.g. powered by natural gas recovered from the subsurface formation), there is no need to first convert energy from fossil fuel into electricity, which significantly negatively impacts energy efficiency.

Unfortunately, molten salt heaters tend to be much more capital-intensive than their electrical counterparts. For example, because they operate at higher temperature, and because they require a molten salt flow system, the cost of constructing and installing subsurface molten salt heaters may significantly exceed that of electrical heaters.

Although the environmental benefits of molten salt heaters are quite clear, unless the technology exists to make them economically viable, operators will prefer the less efficient electrical heaters, even if their carbon footprint is much greater than that of molten salt heaters.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to an L-shaped heater assembly that operates at shallow depths (e.g. at most 1,000 meters, or at most 500 meters) so as to produce unconventional oil from these shallow depths.

The present inventors are now disclosing that, generally speaking, conventional L-shaped molten salt heaters operating at high-temperatures (e.g. at least 550 or at least 600 degrees Celsius) unfortunately require expensive high nickel content stainless steel in order to be stable, over a lifetime of 100,000 hours, against creep rupture stress, and therefore are not economically feasible. However, by modifying a shape of the heater-assembly so that at least a heel- section of the heater follows a catenary trajectory, it is surprisingly possible to construct a generally L-shaped molten salt heater from less capital intensive high-chrome alloy steel (e.g. P91 or 410 Cb) such that the molten salt heater is stable against creep rupture over an operating lifetime of at least 100,000 hours at 550 degrees Celsius or even at 600 degrees Celsius.

It is thus possible, for the first time, to achieve the environmental and efficiency related benefits of molten salt heaters operating at high temperature for an extended period of time in a manner that is economically feasible.

Embodiments of the present invention relate to an L-shaped or U-shaped molten salt subsurface heater constructed of relatively inexpensive subsurface tubulars constructed of high chrome alloy steels such as P91 or 410 Cb (as opposed to more expensive high nickel content stainless steel) that is capable of operating for extended periods of time (e.g. at least 100,000 hours) at relatively high temperatures (e.g. at least 450 degrees Celsius or at least 500 degrees Celsius or at least 550 degrees Celsius or at least 600 degrees Celsius). In particular, the presently-disclosed heaters are stable, at high temperatures, against creep-stresses due to heater operation for 100,000 hours.

After analyzing these stresses, including stress due to dynamic salt pressure, vertical pipe weight, and frictional forces, the present inventors have concluded that the maximum static and dynamic salt pressure occurs at the heel of the L-shaped or U-shaped heater. By modifying the shape of the heater so that the heater substantially follows a catenary trajectory at its heel and for at least a portion of a vertical section of the heater, it is possible to construct a subsurface heater that is capable of enduring these stresses, even at high temperatures using inexpensive high chrome alloy steel.

The present inventors have performed numerical simulations of a number of designs, and have concluded that the catenary subsurface heater surprisingly achieves these objectives without relying on expensive steel alloys such as stainless steel with high nickel content. By disclosing a heater shape capable of meeting these design requirements without employing expensive stainless steel alloys, it is now possible to enjoy the heat-transfer efficiency benefits of an L-shaped (or U-shaped) molten salt heater while controlling capital costs. Without the presently-disclosed heater designs, it is possible that the economics of in-situ production of hydrocarbon fluids would have favored electrical heaters, which are less capital intense but suffer from a much lower operating energy efficiency.

In contrast, the presently-disclosed molten salt heaters are both economically feasible and capable of producing unconventional hydrocarbon fluids with reduced emission of greenhouse gases.

A molten salt heater assembly for heating a subsurface formation comprises: a. a quantity of molten salt; b. a generally L-shaped conduit-in-conduit assembly within the subsurface formation, the L-shaped conduit-in-conduit assembly comprising a heel section connecting generally vertical and generally horizontal sections of the conduit-in- conduit assembly; and c. a flow system configured to force the molten salt to flow through the L-shaped conduit-in-conduit assembly to heat the subsurface formation, wherein at least the heel section of the L-shaped conduit-in-conduit assembly follows a catenary trajectory characterized by a catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

In some embodiments, at least a portion of the generally vertical section of the conduit-in-conduit assembly (e.g. at least 20% or at least 30%) follows the catenary trajectory characterized by the catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

A molten salt heater assembly for heating a subsurface formation comprises: a. a quantity of molten salt; b. a generally U-shaped conduit-assembly within the subsurface formation comprising first and second generally vertical sections, and a generally horizontal section connecting the generally vertical sections so as to define first and second heel sections, c. a flow system configured to force the molten salt to flow through the generally U-shaped-shaped conduit assembly to heat the subsurface formation, wherein at one or both of the heel sections, the U-shaped conduit-assembly follows a catenary trajectory characterized by a catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

In some embodiments, at least a portion of one or both generally vertical sections of the heater assembly (e.g. at least 20% or at least 30%) follows the catenary trajectory characterized by the catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

In some embodiments, the molten salt heater further comprises a furnace configured to heat the molten salt.

In some embodiments, a subsurface heater is configured to heat the molten salt.

In some embodiments, the subsurface heater is an electrically resistive heater. In some embodiments, the subsurface heater is configured to operate at an operating temperature of at least 450 degrees Celsius, or at least 500 degrees Celsius, or at least 550 degrees Celsius or at least 600 degrees Celsius.

In some embodiments, configured to sustain, under normal operating conditions, an operating temperature of at least 450 degrees Celsius, or at least 500 degrees Celsius, or at least 550 degrees Celsius, or at least 600 degrees Celsius.

Apparatus for heating a subsurface formation comprising: a generally L-shaped heater assembly arranged within the subsurface formation, the L-shaped heater assembly comprising: i. a generally vertical section; ii. a generally horizontal section; and iii. a heel section connecting the generally vertical and horizontal sections, wherein: A. the apparatus is configured to operate at an operating temperature T_0PERATE of at least 450 degrees Celsius, or at least 500 degrees Celsius, or at least 550 degrees Celsius, or at least 600 degrees Celsius; and B. the heel section of the L-shaped heater assembly follows a catenary trajectory characterized by a catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

In some embodiments, at least a portion of the generally vertical section of the heater assembly (e.g. at least 20% or at least 30%) follows the catenary trajectory characterized by the catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

Apparatus for heating a subsurface formation comprising: a generally U-shaped heater assembly arranged within the subsurface formation, the U-shaped heater assembly comprising: i. first and second generally vertical section; ii. a generally horizontal section connecting the first and second vertical sections so as to define first and second heel sections; and wherein: A. the apparatus is configured to operate at an T _OPERATE operating temperature of at least 450 degrees Celsius, or at least 500 degrees Celsius, or at least 550 degrees Celsius, or at least 600 degrees Celsius; and B. one or both of the heel sections of the U-shaped heater assembly follows a catenary-trajectory characterized by a catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

The apparatus heats the subsurface so that thermal energy from the heated L- shaped or U-shaped heater assembly (i.e. at least a portion thereof) is transferred to the subsurface.

In some embodiments, the apparatus is configured to sustain normal operations

(i.e. without failing from creep rupture) at a temperature equal to or exceeding the operating temperature for at least 20,000 hours or at least 40,000 hours or at least 60,000 hours or at least 80,000 hours or at least 100,000 hours.

In some embodiments, the apparatus is configured as an advection-based heater. In some embodiments, a heat-transfer fluid of the advection-based heater is selected from the group consisting of synthetic oil, molten salt, molten metal.

In some embodiments, a heat-transfer fluid of the advection-based heater is a hot gas or a supercritical fluid.

In some embodiments, the hot gas is selected from the group consisting of C0₂, propane, flue gas and steam.

In some embodiments, the heater-assembly is an electrical heater (for example, a Curie heater).

In some embodiments, wherein the subsurface formation is a hydrocarbon- containing formation such as a bitumen formation, a kerogenous chalk, an oil shale formation, a heavy oil formation, a tight oil formation, or a coal formation.

In some embodiments, at least one of the heel sections (i.e. a single heel section for an L-shaped heater ; or one of two heel-sections for a U-shaped heater; or both heel sections for a U-shaped heater) is constructed from a steel alloy.

In some embodiments, at least a majority of the subsurface heater is constructed from a steel alloy. In some embodiments, the steel alloy is a molybdenum and/or vanadium strengthened chrome steel alloy.

In some embodiments, the steel alloy is substantially nickel free and/or has a chromium: nickel ratio of at least 3: 1 or at least 5: 1 or at least 10: 1 or at least 50: 1 or at least 100: 1.

In some embodiments, the steel alloy is a martensitic steel (e.g. a high- temperature martenistic steel).

In some embodiments, the steel alloy is selected from the group consisting of P91, P92 and 410 Cb.

In some embodiments, a molybdenum content of the steel alloy is about 1% and/or a vanadium content is about 0.25%— e.g. to strengthen the alloy.

In some embodiments, at least one of the heel sections or at least a majority of the subsurface heater is constructed from a material having a room-temperature (i.e. 20° C) yield stress of at most 5,500 atmospheres, or at most 5,000 atmospheres or at most 4,500 atmospheres or at most 4,300 atmospheres or at most 4.250 atmospheres or at most 4,200 atmospheres.

In some embodiments, at least one of the heel sections or at least a majority of the subsurface heater is constructed from a material characterized by a ratio between (i) a 550° C yield stress; and (ii) a room-temperature yield stress, of at least 0.3 or at least 0.4 or at least 0.5 or at least 0.6 or at least 0.65 or at least 0.7.

In some embodiments, the catenary trajectory is characterized by a catenary coefficient selected to minimize a total heater assembly length while satisfying a creep- rupture condition, the creep-rupture condition requiring that a maximum Van Mises stress of the heater assembly does not exceed a creep-rupture strength of the material from which at least the heel section is constructed.

In some embodiments, a depth of the horizontal section is d and a ratio between:

(i) a total horizontal displacement of the heater assembly at depths of less than d/2; and

(ii) a total horizontal displacement of the heater assembly at depths above that d of the horizontal section but below d/2 is at least 0.1 or at least 0.2 or at least 0.3 or at least 0.4 or at least 0.5 or at least 0.6 or at least 0.65 or at least 0.7. In some embodiments, a depth of the horizontal section is d and a ratio between:

(i) a total horizontal displacement of the heater assembly at depths of less than d/4; and

(ii) a total horizontal displacement of the heater assembly at depths above that d of the horizontal section but below d/4 is at least 0.05 or at least 0.1 or at least 0.2.

In some embodiments, a total length of the subsurface heater assembly is at least

500 meters or at least 1,000 meters and/or wherein a length of the horizontal section is at least 500 meters or at least 1,000 meters.

In some embodiments, a maximum depth of the heater assembly is at most 1000 meters or at most 750 meters or at most 500 meters or at most 250 meters and/or wherein a minimum depth of the generally horizontal section is at least 50 meters or at least 100 meters.

In some embodiments, a value of the heel-section catenary coefficient a is at most 1000 or at at most 800 or at most 700 or at most 600.

Some embodiment relate to use of any presently-disclosed apparatus or heater assembly to generate pyrolysis fluids from hydrocarbons of the subsurface formation and/or to mobilize hydrocarbon fluids of the subsurface formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 describe a conventional L- shaped subsurface heater.

FIG. 4 describes an analysis of stresses in an L-shaped subsurface heater.

FIG. 5 describes material properties of P91 steel.

FIG. 6 describes a novel subsurface heater having a catenary trajectory in a heel section thereof.

FIG. 7 illustrates two heater trajectories for two values of the a coefficient.

FIG. 8 illustrates the maximum Von Mises stresses as a function of the a coefficient.

FIG. 9 describes, for subsurface heaters constructed from the P91 steel alloy, the Von Mises Stress as a function of measured depth for a catenary heater and a non-catenary heater.

FIG. 10 illustrates a U-Shaped heater comprising first and second catenary-shaped heel sections.

DETAILED DESCRIPTION OF EMBODIMENTS The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the exemplary system only and are presented in the cause of providing what is believed to be a useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how several forms of the invention may be embodied in practice and how to make and use the embodiments.

For brevity, some explicit combinations of various features are not explicitly illustrated in the figures and/or described. It is now disclosed that any combination of the method or device features disclosed herein can be combined in any manner - including any combination of features - and any combination of features can be included in any embodiment and/or omitted from any embodiments.

FIG. 4 illustrates an analysis of stresses upon pipes of an L-shaped subsurface heater assembly. The total stresses, known as the Von Mises stresses, include (i) hoop stress (for example, caused by dynamic salt pressure 640A, 640B); (ii) axial tensile stress (for example, caused by weight 610, in the vertical section 410 of the heater, of the pipe or salt); and (iii) axial compressive stress (for example, in the horizontal section 420 of the heater, a thermal expansion force 630A is counteracted by friction 630B between an outer surface of pipe P3 302 and an inner surface of wellbore 330). The magnitude of stress 630B is linearly proportional to the coefficient of friction between an outer surface of pipe P3 302 and an inner surface of wellbore 330 — it was assumed that this coefficient of friction or 'friction factor' was 0.3.

In some embodiments, insulating layer 308 is constructed of a carbon granular material under vacuum. The insulating material may be microporous silica, for example, available in both sheet and granular form. Operation under vacuum may improve the thermal insulation performance by a factor of 2-5. Examples are Pyrogel χχ™ from Aspen Aerogels or Microtherm® Super. In these embodiments, another example of hoop stress, albeit of a smaller magnitude than the dynamic salt pressure 640, relates to a vacuum pressure to the vacuum of layer 308.

The present inventors have investigated the Von Mises stresses for both conventional L- shaped molten salt heaters (e.g. see FIG. 1) and for novel molten salt heaters (e.g. see FIG. 4) where (i) the heater follows a catenary trajectory throughout at least a majority (e.g. an entirety of) a heel section 420 and (ii) within at least a significant portion (at least 20%, or at least 30% or at least 40% or at least 50%) of a vertical portion 410 of the heater. In particular, the present inventors have employed computer software to numerically simulate the Von Mises stresses on operating L-shaped conduit-in-conduit heaters for a variety of heater shapes, and where the pipes P1-P3 334, 332, 302 are constructed from different steel alloys.

The simulations assumed that the heater was constructed from a P91 alloy. For this alloy, the creep-rupture stress as a function of temperature is illustrated in FIG. 5 for a design life of 20,000 hours, 40,000 hours, 60,000 hours and 100,000 hours. Since it is intended to operate the molten salt heater at a temperature of about 550 degrees Celsius, the creep-rupture stress of the material is about 1361 atmospheres as illustrated in FIG. 5.

As will be discussed below with reference to FIG. 9, the results of the numerical simulation indicate that when the subsurface heater is constructed from the P91 steel alloy, the heater is incapable of operating for 100,000 hours at a sustained temperature of 550 degrees Celsius without suffering from creep rupture when constructed according to the conventional design of FIG. 9 - this creep rupture is defined by a Von Mises stress exceeding the yield stress of 1361 atmospheres (illustrated in FIG. 5). In contrast, as will be discussed below, the catenary heater design of FIG. 6 does not suffer from this deficiency, and is capable of operating for at least 100,000 hours at a temperature of 550 degrees Celsius without suffering from creep rupture.

By way of introduction, it is noted that catenary trajectories are given by: y = a cosh where "y" is the vertical coordinate, "x"is the horizontal coordinate, and "a" is catenary coefficient. Catenary trajectories are thus characterized by an a coefficient— the numerical simulations were performed for a number of values of the a coefficient.

The catenary heater of FIG. 6 is considered close to 'ideal' for this depth and horizontal length and is characterized by a catenary coefficient of about 500.

As shown in FIG. 7, the heater trajectory may depend on a value of a.

The numerical simulations were run for a variety of L-shaped heaters, each including a catenary-shaped heel section having its own respective a coefficient. The results are illustrated in FIG. 8, which show that the maximum Von Mises stress is a monotonically decreasing function dependent on the a coefficient of the heater. While smaller a coefficients are desirable in order to minimize the overall length of the heater (also referred to as the 'measured depth' or MD), the numerical simulations have indicated that when the subsurface heater is constructed from a P91 steel alloy and is required to operate for at least 100,000 hours at 550 degrees Celsius, a minimum -coefficient value required for the subsurface heater to meet these specifications is about 500.

FIG. 9 illustrates the Von Mises stress as a function of location along the subsurface heater, expressed as the 'measured depth' for three heater pipes— inner pipe PI 334, middle pipe P2 332, and outer pipe P3 302. As illustrated in FIG. 9, (i) the Von Mises stresses in pipe P3 exceeds the stresses on pipes PI and P2; and (ii) the maximum Von Mises stress occurs within a heel 420 of the subsurface heater.

FIG. 9 illustrates a comparison of Von Mises stresses (i.e. assuming a friction factor of 0.3) for two cases: (i) a first case of a conventional L-shaped heater such as that illustrated in FIG. 3 (this is illustrated by the 'dot-dash' line of FIG. 9) and (ii) a second case related to an 'ideal' catenary heater of FIG. 6.

For the former case relating to the heater of FIG. 3, a value of the Von Mises stress exceeds 1361 atmospheres (beyond a 'range of allowable creep stress' for P91 alloy steel at an operating temperature of 550 degrees Celsius) for a majority of the measured depth range between 304 meter and 610 meters. In contrast, as illustrated in FIG. 9, the Von Mises stress of pipe P3 of the heater of FIG. 6 never exceeds the allowable creep stress for P91 alloy steel at an operating temperature of 550 degrees Celsius. The stresses for all numerical simulations were computed assuming that the heater is constructed of a P91 steel alloy. Creep-rupture properties of P91 are illustrated in FIG. 5. Another physical property of P91 is the yield stress. In particular, itg is noted that the yield stress of P91 steel at room temperature (about 20 degrees Celsius) is relatively low— about 4,000 atmospheres. At 550 degrees Celsius, the yield stress is about 2,870 atmospheres.

Although the yield stress of P91 at high temperatures of 550 degrees Celsius is less than that observed at room temperature, the drop is relatively moderate. Thus, although P91 is probably not appropriate for applications related to an extended reach drilling of a production well where it is critical for the alloy to have a high yield stress, it is appropriate for the presently-disclosed subsurface heaters, where room-temperature yield stress is less important, but stability against creep rupture at high temperatures for long times (e.g. 100,000 hours) is an important property.

Other alloys with similar properties are P91, P92, 410 Cb, and others having a chromium content of between about 9% and 13%. The high nickel austenitic stainless steels, such as 316H, 347H, etc. have a much higher creep rupture strength but are too expensive because of the high nickel content.

For the present disclosure, when steel alloy is "substantially nickel free," a wt% nickel content of the steel alloy is at most 1.5% or at most 1.25% or at most 1% or at most 0.5% or exactly 0%.

A "chromium:nickel ratio" of a steel alloy refers to a ratio between: (i) a wt% content of chromium within the steel alloy; and (ii) a wt% content of nickel within the steel alloy.

Although embodiments of the present invention have been discussed in terms of generally L-shaped heaters, the skilled artisan will appreciate that presently-disclosed teachings may be applied to so-called U-shaped heaters. An example of a U-shaped heater having where heels thereof follow a catenary trajectory is illustrated in FIG. 10. In alternate embodiments (not illustrated in the figures), only one of heels follows a catenary trajectory. Reference is once again made to FIGS. 3 and 6. For the heater of FIG. 3 where the vertical section is exactly vertical until the 'kick-off depth of about 50% of a depth of the horizontal section. For this heater, the 'horizontal displacement' of the heater for depths of less than d/2 (in this case less than 198 meters) is zero.

In contrast, for the heater of FIG. 6, it is possible to define a ratio of the horizontal displacement for depths less than d/2 to the horizontal displacement for depths between d/2 and d. The ratio of the displacement disp(d/2) to disp(d) exceeds 0.1.

Some embodiments relate to patterns of heaters and/or production wells and/or injection wells. Some embodiments relate to methods of hydrocarbon fluid production and/or methods of heating a subsurface formation. Unless specified otherwise, any feature or combination of feature(s) relating to heater and/or production well locations or patterns may be provided in combination with any method disclosed herein even if not explicitly specified herein. Furthermore, a number of methods are disclosed within the present disclosure, each providing its own set of respective features. Unless specified otherwise, in some embodiments, any feature(s) of any one method may be combined with feature(s) of any other method, even if not explicitly specified herein.

Furthermore, any 'control apparatus' may be programmed to carry out any method or combination thereof disclosed herein.

In the description and claims of the present application, each of the verbs, "comprise" "include" and "have", and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb.

All references cited herein are incorporated by reference in their entirety. Citation of a reference does not constitute an admission that the reference is prior art.

The articles "a" and "an" are used herein to refer to one or to more than one. (i.e., to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

The term "including" is used herein to mean, and is used interchangeably with, the phrase "including but not limited" to.

The term "or" is used herein to mean, and is used interchangeably with, the term

"and/or," unless context clearly indicates otherwise. The term "such as" is used herein to mean, and is used interchangeably, with the phrase "such as but not limited to".

The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons skilled in the art.

Claims

WHAT IS CLAIMED

1. A molten salt heater assembly for heating a subsurface formation, the heater assembly comprising:

a. a quantity of molten salt;

b. a generally L-shaped conduit-in-conduit assembly within the subsurface formation, the L-shaped conduit-in-conduit assembly comprising

a heel section connecting generally vertical and generally horizontal sections of the conduit-in-conduit assembly; and

c. a flow system configured to force the molten salt to flow through the L-shaped conduit-in-conduit assembly to heat the subsurface formation,

wherein at least the heel section of the L-shaped conduit-in-conduit assembly follows a catenary trajectory characterized by a catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

2. A molten salt heater assembly for heating a subsurface formation, the heater assembly comprising:

a. a quantity of molten salt;

b. a generally U-shaped conduit-assembly within the subsurface formation comprising first and second generally vertical sections, and a generally horizontal section connecting the generally vertical sections so as to define first and second heel sections,

c. a flow system configured to force the molten salt to flow through the generally U-shaped-shaped conduit assembly to heat the subsurface formation,

wherein at one or both of the heel sections, the U-shaped conduit-assembly follows a catenary trajectory characterized by a catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

3. The heater assembly of any preceding claim further comprising a furnace configured to heat the molten salt.

4. The heater assembly of claim 3 wherein the furnace is a solar furnace.

5. The heater assembly of any preceding claim further comprising a subsurface heater configured to heat the molten salt.

6. The heater assembly of claim 5 wherein the subsurface heater is an electrically resistive heater.

7. The heater assembly of any preceding claim, further comprising an electrically resistive heater in thermal communication with the subsurface formation and a wind turbine configured to supply wind-generated electricity to the electrically resistive heater.

8. The heater assembly of any preceding claim configured to operate at an operating temperature of at least 450 degrees Celsius, or at least 500 degrees Celsius, or at least 550 degrees Celsius or at least 600 degrees Celsius.

9. The heater assembly of any preceding claim configured to sustain, under normal operating conditions, an operating temperature of at least 450 degrees Celsius, or at least 500 degrees Celsius, or at least 550 degrees Celsius, or at least 600 degrees Celsius.

10. Apparatus for heating a subsurface formation comprising:

a generally L-shaped heater assembly arranged within the subsurface formation, the L-shaped heater assembly comprising:

i. a generally vertical section;

ii. a generally horizontal section; and

iii. a heel section connecting the generally vertical and horizontal sections, wherein:

A. the apparatus is configured to operate at an operating temperature Ί OPERATE of at least 450 degrees Celsius, or at least 500 degrees Celsius, or at least 550 degrees Celsius; and B. the heel sections of the L- shaped heater assembly follows a catenary- trajectory characterized by a catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

11. Apparatus for heating a subsurface formation comprising:

a generally U-shaped heater assembly arranged within the subsurface formation, the U-shaped heater assembly comprising:

i. first and second generally vertical section;

ii. a generally horizontal section connecting the first and second vertical sections so as to define first and second heel sections; and

wherein:

A. the apparatus is configured to operate at an Ί OPERATE operating temperature of at least 450 degrees Celsius, or at least 500 degrees Celsius, or at least 550 degrees Celsius; and

B. one or both of the heel sections of the U-shaped heater assembly follows a catenary-trajectory characterized by a catenary coefficient a having a value of at least 200 or at least 300 or at least 400.

12. The apparatus of any of claims 10-1 1 wherein the apparatus is configured to sustain normal operations at a temperature equal to or exceeding the operating temperature for at least 20,000 hours or at least 40,000 hours or at least 60,000 hours or at least 80,000 hours or at least 100,000 hours.

13. The apparatus of any of claim 10-12, configured as an advection-based heater.

14. The apparatus of claim 13 wherein a heat-transfer fluid of the advection-based heater is selected from the group consisting of synthetic oil, molten salt, molten metal.

15. The apparatus of claim 13 wherein a heat-transfer fluid of the advection-based heater is a hot gas or a supercritical fluid.

16. The apparatus of claim 14 wherein the hot gas is selected from the group consisting of flue gas and steam.

17. The apparatus of claim 10-12 configured as an electrical heater.

18. The apparatus of claim 17 configured as a Curie heater.

19. The apparatus of any of claim 17-18 further comprising a wind turbine configured to supply wind-generated electricity to the electrically resistive heater.

20. The apparatus or heater assembly of any preceding claim wherein the subsurface formation is a hydrocarbon-containing formation.

21. The apparatus or heater assembly of any preceding claim wherein the hydrocarbon- containing formation is selected from the group consisting of a bitumen formation, a kerogenous chalk, an oil shale formation, a heavy oil formation, a tight oil formation, or a coal formation.

22. The apparatus or heater assembly of any preceding claim wherein at least one of the heel sections is constructed from a steel alloy.

23. The apparatus or heater assembly of any preceding claim wherein at least a majority of the subsurface heater is constructed from a steel alloy.

24. The apparatus or heater assembly of any of claims 22-23 wherein the steel alloy is a chrome steel alloy.

25. The apparatus or heater assembly of any of claims 22-24 wherein the steel alloy (i) is substantially nickel-free and/or (ii) has a chromium: nickel ratio of at least 3: 1 or at least 5: 1 or at least 10: 1 or at least 50: 1 or at least 100: 1.

26. The apparatus or heater assembly of any of claims 22-25 wherein the steel alloy is a high- temperature martenistic steel strengthened by molybdenum and/or vanadium additions.

27. The apparatus or heater assembly of any of claims 22-26 wherein the steel alloy is selected from the group consisting of P91, P92 and 410 Cb.

28. The apparatus or heater assembly of any preceding claim wherein at least one of the heel sections or at least a majority of the subsurface heater is constructed from a material having a room-temperature yield stress of at most 5,500 atmospheres, or at most 5,000 atmospheres or at most 4,500 atmospheres or at most 4,300 atmospheres or at most 42,250 atmospheres or at most 4,200 atmospheres.

29. The apparatus or heater assembly of any preceding claim wherein at least one of the heel sections or at least a majority of the subsurface heater is constructed from a material characterized by a ratio between (i) a 550° C yield stress; and (ii)a room-temperature yield stress, of at least 0.3 or at least 0.4 or at least 0.5.

30. The apparatus or heater assembly of any preceding claim wherein the catenary- trajectory is characterized by a catenary coefficient selected to minimize a total heater assembly length while satisfying a creep rupture condition, the creep-rupture condition requiring that a maximum Von Mises stress of the heater assembly does not exceed a creep rupture strength of the material from which at least the heel section is constructed.

31. The apparatus or heater assembly of any preceding claim wherein a depth of the horizontal section is d and a ratio between: (i) a total horizontal displacement of the heater assembly at depths of less than d/2; and (ii) a total horizontal displacement of the heater assembly at depths above that d of the horizontal section but below d/2 is at least 0.1 or at least 0.2 or at least 0.3.

32. The apparatus or heater assembly of any preceding claim wherein a depth of the horizontal section is d and a ratio between: (i) a total horizontal displacement of the heater assembly at depths of less than d/4; and (ii) a total horizontal displacement of the heater assembly at depths above that d of the horizontal section but below d/4 is at least 0.05 or at least 0.1 or at least 0.2.

33. The apparatus or heater assembly of any preceding claim wherein a total length of the subsurface heater assembly is at least 500 meters or at least 1,000 meters and/or wherein a length of the horizontal section is at least 500 meters or at least 1,000 meters.

34. The apparatus or heater assembly of any preceding claim wherein a maximum depth of the heater assembly is at most 1000 meters or at most 750 meters or at most 500 meters or at most 250 meters and/or wherein a minimum depth of the generally horizontal section is at least 50 meters or at least 100 meters.

35. The apparatus or heater assembly of any preceding claim wherein a value of the heel- section catenary coefficient a is at most 1000 or at most 900 or at most 800 or at most 700 or at most 600.

36. The apparatus or heater assembly of any preceding claim, stable against creep-rupture for a period of time of at least 100,000 hours at an operating temperature of at least 550 degrees Celsius or at least 600 degrees Celsius.

37. Use of the apparatus or heater assembly of any preceding claim to generate pyrolysis fluids from hydrocarbons of the subsurface formation and/or to mobilize hydrocarbon fluids of the subsurface formation.

38. The method of claim 37 wherein the subsurface formation is selected from the group consisting of a tar sands formations, a kerogenous chalk, an oil shale formation, a tight oil formation, and a coal formation.