BACKGROUND Field of the Invention The present invention relates generally to methods and systems for production of hydrocarbons, hydrogen, and/or other products from various subsurface formations such as hydrocarbon containing formations. In particular, certain embodiments described herein relate to methods and systems for inhibiting sloughing material from affecting equipment and/or operation in heater or production wellbores. Description of Related Art Hydrocarbons obtained from subterranean formations are often used as energy resources, as feedstocks, and as consumer products. Concerns over depletion of available hydrocarbon resources and changes in the overall quality of produced hydrocarbons have led to development of processes for more efficient recovery, processing and/or use of available hydrocarbon resources. In situ processes may be used to remove hydrocarbon materials from subterranean formations. Chemical and/or physical properties of hydrocarbon material within subterranean formations may need to be changed to allow hydrocarbon material to be more easily removed from the subterranean formations. Chemical and physical changes may include: in situ reactions that produce removable fluids, composition changes, solubility changes, density changes, phase changes, and/or viscosity changes of the hydrocarbon material within the formation. A fluid may be, but is not limited to, a gas, a liquid, an emulsion, a slurry, and/or a stream of solid particles that has flow characteristics similar to liquid flow. Heaters may be placed in wellbores to heat the formation during an in situ process. Examples of in situ processes utilizing downhole heaters are illustrated in U.S. Patent Nos. 2,634,961 to Ljungstrom; 2,732,195 to Ljungstrom; 2,780,450 to Ljungstrom; 2,789,805 to Ljungstrom; 2,923,535 to Ljungstrom; and 4,886,118 to Van Meurs et al. Some formation layers may have material characteristics that lead to sloughing in a wellbore. Sloughing of material in the wellbore may lead to overheating, plugging, equipment deformation, and/or fluid flow problems in the wellbore. Inhibiting sloughing has the technical advantage of allowing efficient and easy operation of wells in the formation. Summary of the Invention The invention provides a method for treating heater wellbores and installing heaters in a subsurface formation, comprising: providing one or more explosives into portions of one or more wellbores selected for the explosion in the formation, the wellbores formed in one or more zones in the formation; controllably exploding the explosives in one or more of the wellbores such that at least some of the formation surrounding the selected wellbores has an increased permeability; and providing one or more heaters in the one or more wellbores. The invention also provides in combination with one or more of the above inventions: (a) allowing heat to transfer from the one or more heaters to the one or more zones of the formation; (b) providing heat from one or more heaters to at least a portion of the formation, wherein one or more of the heaters are in one or more of the wellbores sized, at least in part, such that a space between the wellbore and one of the heaters in the wellbore has a width that inhibits particles of a selected size from freely moving in the space; and (c) controlling heating of the zones of the formation such that a heating rate of one or more zones is maintained below 20°C/day for at least 15 days, below 10°C/day for at least 30 days, or below 5°C/day for at least 60 days, thereby inhibiting sloughing of material proximate the heater during and/or subsequent to the heating. The invention also provides in combination with one or more of the above inventions: (a) assessing a permeability of a part of the formation and selecting the wellbores for explosion, sizing the wellbores, and/or controlling the heating of the zones based on the assessed permeability; and (b) assessing a clay content of a part of the formation and selecting the wellbores for explosion, sizing the wellbores, and/or controlling the heating of the zones based on the assessed clay content. The invention also provides in combination with one or more of the above inventions: wherein at least one of the wellbores has a liner placed between the heater in the wellbore and the formation, and wherein the liner comprises openings that are sized such that fluids can pass through the liner but particles of a selected size cannot pass through the liner. Brief Description of the Drawings Advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings in which: FIG. 1 depicts an illustration of stages of heating the hydrocarbon containing formation. FIG. 2 shows a schematic view of an embodiment of a portion of an in situ conversion system for treating the hydrocarbon containing formation. FIG. 3 depicts an embodiment for providing the controlled explosion in the opening. FIG. 4 depicts an embodiment of the opening after a controlled explosion in the opening. FIG. 5 depicts an embodiment of a liner in an opening. FIG. 6 depicts an embodiment of a liner in a stretched configuration. FIG. 7 depicts an embodiment of a liner in an expanded configuration. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Detailed Description of the Invention The following description generally relates to systems and methods for treating hydrocarbons in the formations. Such formations may be treated to yield hydrocarbon products, hydrogen, and other products. "Hydrocarbons" are generally defined as molecules formed primarily by carbon and hydrogen atoms. Hydrocarbons may also include other elements such as, but not limited to, halogens, metallic elements, nitrogen, oxygen, and/or sulfur. Hydrocarbons may be, but are not limited to, kerogen, bitumen, pyrobitumen, oils, natural mineral waxes, and asphaltites. Hydrocarbons may be located in or adjacent to mineral matrices in the earth. Matrices may include, but are not limited to, sedimentary rock, sands, silicilytes, carbonates, diatomites, and other porous media. "Hydrocarbon fluids" are fluids that include hydrocarbons. Hydrocarbon fluids may include, entrain, or be entrained in non-hydrocarbon fluids (for example, hydrogen, nitrogen, carbon monoxide, carbon dioxide, hydrogen sulfide, water, and ammonia). "Heavy hydrocarbons" are viscous hydrocarbon fluids. Heavy hydrocarbons may include highly viscous hydrocarbon fluids such as heavy oil, tar, and/or asphalt. Heavy hydrocarbons may include carbon and hydrogen, as well as smaller concentrations of sulfur, oxygen, and nitrogen. Additional elements may also be present in heavy hydrocarbons in trace amounts. Heavy hydrocarbons may be classified by API gravity. Heavy hydrocarbons generally have an API gravity below 20°. Heavy oil, for example, generally has an API gravity of 10-20°, whereas tar generally has an API gravity below 10°. The viscosity of heavy hydrocarbons is generally at least 100 centipoise at 15 0C. Heavy hydrocarbons may also include aromatics or other complex ring hydrocarbons. "API gravity" refers to API gravity at 15.5 °C (60 °F). API gravity is as determined by ASTM Method D6822. "ASTM" refers to American Standard Testing and Materials. A "formation" includes one or more hydrocarbon containing layers, one or more non-hydrocarbon layers, an overburden, and/or an underburden. The "overburden" and/or the "underburden" include one or more different types of impermeable materials. For example, overburden and/or underburden may include rock, shale, mudstone, or wet/tight carbonate. In some embodiments of in situ conversion processes, the overburden and/or the underburden may include a hydrocarbon containing layer or hydrocarbon containing layers that are relatively impermeable and are not subjected to temperatures during in situ conversion processing that result in significant characteristic changes of the hydrocarbon containing layers of the overburden and/or the underburden. For example, the underburden may contain shale or mudstone, but the underburden is not allowed to heat to pyrolysis temperatures during the in situ conversion process. In some cases, the overburden and/or the underburden may be somewhat permeable. "Formation fluids" and "produced fluids" refer to fluids removed from the formation and may include pyrolyzation fluid, synthesis gas, mobilized hydrocarbon, and water (steam). Formation fluids may include hydrocarbon fluids as well as non-hydrocarbon fluids. "Carbon number" refers to the number of carbon atoms in a molecule. A hydrocarbon fluid may include various hydrocarbons with different carbon numbers. The hydrocarbon fluid may be described by a carbon number distribution. Carbon numbers and/or carbon number distributions may be determined by true boiling point distribution and/or gas-liquid chromatography. A "heat source" is any system for providing heat to at least a portion of the formation substantially by conductive and/or radiative heat transfer. A "heater" is any system for generating heat in a well or a near wellbore region. Heaters may be, but are not limited to, electric heaters, circulated heat transfer fluid or steam, burners, combustors that react with material in or produced from the formation, and/or combinations thereof. The term "wellbore" refers to a hole in the formation made by drilling or insertion of a conduit into the formation. As used herein, the terms "well" and "opening", when referring to an opening in the formation, may be used interchangeably with the term "wellbore". "Pyrolysis" is the breaking of chemical bonds due to the application of heat. Pyrolysis includes transforming a compound into one or more other substances by heat alone. Heat may be transferred to a section of the formation to cause pyrolysis. "Pyrolyzation fluids" or "pyrolysis products" refers to fluid produced during pyrolysis of hydrocarbons. Fluid produced by pyrolysis reactions may mix with other fluids in the formation. The mixture would be considered pyrolyzation fluid or pyrolyzation product. Pyrolyzation fluids include, but are not limited to, hydrocarbons, hydrogen, carbon dioxide, carbon monoxide, hydrogen sulfide, ammonia, nitrogen, water, and mixtures thereof. "Condensable hydrocarbons" are hydrocarbons that condense at 25 0C at 101 kPa absolute pressure. Condensable hydrocarbons may include a mixture of hydrocarbons having carbon numbers greater than 4. "Non-condensable hydrocarbons" are hydrocarbons that do not condense at 25 °C and 101 kPa absolute pressure. Non-condensable hydrocarbons may include hydrocarbons having carbon numbers less than 5. Hydrocarbons in formations may be treated in various ways to produce many different products. In certain embodiments, such formations are treated in stages. FIG. 1 illustrates several stages of heating a hydrocarbon containing formation. FIG. 1 also depicts an example of yield ("Y") in barrels of oil equivalent per ton (y axis) of formation fluids from the formation versus temperature ("T") of the heated formation in degrees Celsius (x axis). Desorption of methane and vaporization of water occurs during stage 1 heating. Heating of the formation through stage 1 may be performed as quickly as possible. For example, when the hydrocarbon containing formation is initially heated, hydrocarbons in the formation desorb adsorbed methane. The desorbed methane may be produced from the formation. If the hydrocarbon containing formation is heated further, water in the hydrocarbon containing formation is vaporized. Water may occupy, in some hydrocarbon containing formations, between 10% and 50% of the pore volume in the formation. In other formations, water occupies larger or smaller portions of the pore volume. Water typically is vaporized in a formation between 160 0C and 285 0C at pressures of 600 kPa absolute to 7000 kPa absolute. In some embodiments, the vaporized water produces wettability changes in the formation and/or increased formation pressure. The wettability changes and/or increased pressure may affect pyrolysis reactions or other reactions in the formation. In certain embodiments, the vaporized water is produced from the formation. In other embodiments, the vaporized water is used for steam extraction and/or distillation in the formation or outside the formation. Removing the water from and increasing the pore volume in the formation increases the storage space for hydrocarbons in the pore volume. In certain embodiments, after stage 1 heating, the formation is heated further, such that a temperature in the formation reaches (at least) an initial pyrolyzation temperature (such as a temperature at the lower end of the temperature range shown as stage 2). Hydrocarbons in the formation may be pyrolyzed throughout stage 2. A pyrolysis temperature range varies depending on the types of hydrocarbons in the formation. The pyrolysis temperature range may include temperatures between 250 0C and 900 0C. The pyrolysis temperature range for producing desired products may extend through only a portion of the total pyrolysis temperature range. In some embodiments, the pyrolysis temperature range for producing desired products may include temperatures between 250 0C and 400 0C or temperatures between 270 0C and 350 °C. If a temperature of hydrocarbons in a formation is slowly raised through the temperature range from 250 °C to 400 °C, production of pyrolysis products may be substantially complete when the temperature approaches 400 0C. Heating the hydrocarbon containing formation with a plurality of heat sources may establish thermal gradients around the heat sources that slowly raise the temperature of hydrocarbons in the formation through a pyrolysis temperature range. In some in situ conversion embodiments, a portion of a formation is heated to a desired temperature instead of slowly heating the temperature through a temperature range. In some embodiments, the desired temperature is 300 °C, 325 °C, or 350 0C. Other temperatures may be selected as the desired temperature. Superposition of heat from heat sources allows the desired temperature to be relatively quickly and efficiently established in the formation. Energy input into the formation from the heat sources may be adjusted to maintain the temperature in the formation substantially at the desired temperature. The heated portion of the formation is maintained substantially at the desired temperature until pyrolysis declines such that production of desired formation fluids from the formation becomes uneconomical. Parts of a formation that are subjected to pyrolysis may include regions brought into a pyrolysis temperature range by heat transfer from only one heat source. In certain embodiments, formation fluids including pyrolyzation fluids are produced from the formation. As the temperature of the formation increases, the amount of condensable hydrocarbons in the produced formation fluid may decrease. At high temperatures, the formation may produce mostly methane and/or hydrogen. If the hydrocarbon containing formation is heated throughout an entire pyrolysis range, the formation may produce only small amounts of hydrogen towards an upper limit of the pyrolysis range. After all of the available hydrogen is depleted, a minimal amount of fluid production from the formation will typically occur. After pyrolysis of hydrocarbons, a large amount of carbon and some hydrogen may still be present in the formation. A significant portion of carbon remaining in the formation can be produced from the formation in the form of synthesis gas. Synthesis gas generation may take place during stage 3 heating depicted in FIG. 1. Stage 3 may include heating a hydrocarbon containing formation to a temperature sufficient to allow synthesis gas generation. For example, synthesis gas may be produced in a temperature range from 400 0C to 1200 0C, 500 0C to 1100 0C, or 550 0C to 1000 0C. The temperature of the heated portion of the formation when the synthesis gas generating fluid is introduced to the formation determines the composition of synthesis gas produced in the formation. The generated synthesis gas may be removed from the formation through a production well or production wells. FIG. 2 depicts a schematic view of an embodiment of a portion of the in situ conversion system for treating the formation that contains hydrocarbons. Heat sources 20 are placed in at least a portion of the formation. Heat sources 20 may include electric heaters such as insulated conductors, conductor-in-conduit heaters, surface burners, flameless distributed combustors, and/or natural distributed combustors. Heat sources 20 may also include other types of heaters. Heat sources 20 provide heat to at least a portion of the formation to heat hydrocarbons in the formation. Energy may be supplied to heat sources 20 through supply lines 22. Supply lines 22 may be structurally different depending on the type of heat source or heat sources used to heat the formation. Supply lines 22 for heat sources may transmit electricity for electric heaters, may transport fuel for combustors, or may transport heat exchange fluid that is circulated in the formation. Production wells 24 are used to remove formation fluid from the formation. Formation fluid produced from production wells 24 may be transported through collection piping 26 to treatment facilities 28. Formation fluids may also be produced from heat sources 20. For example, fluid may be produced from heat sources 20 to control pressure in the formation adjacent to the heat sources. Fluid produced from heat sources 20 may be transported through tubing or piping to collection piping 26 or the produced fluid may be transported through tubing or piping directly to treatment facilities 28. Treatment facilities 28 may include separation units, reaction units, upgrading units, fuel cells, turbines, storage vessels, and/or other systems and units for processing produced formation fluids. The in situ conversion system for treating hydrocarbons may include barrier wells 30. Barrier wells are used to form a barrier around a treatment area. The barrier inhibits fluid flow into and/or out of the treatment area. Barrier wells include, but are not limited to, dewatering wells, vacuum wells, capture wells, injection wells, grout wells, freeze wells, or combinations thereof. In some embodiments, barrier wells 30 are dewatering wells. Dewaterrng wells may remove liquid water and/or inhibit liquid water from entering a portion of the formation to be heated, or to the formation being heated. In the embodiment depicted in FIG. 2, the dewatering wells are shown extending only along one side of heat sources 20, but dewatering wells typically encircle all heat sources 20 used, or to be used, to heat the formation. As shown in FIG. 2, in addition to heat sources 20, one or more production wells 24 are placed in the formation. Formation fluids may be produced through production well 24. In some embodiments, production well 24 includes a heat source. The heat source in the production well may heat one or more portions of the formation at or near the production well and allow for vapor phase removal of formation fluids. The need for high temperature pumping of liquids from the production well may be reduced or eliminated. Avoiding or limiting high temperature pumping of liquids may significantly decrease production costs. Providing heating at or through the production well may: (1) inhibit condensation and/or refluxing of production fluid when such production fluid is moving in the production well proximate the overburden, (2) increase heat input into the formation, and/or (3) increase formation permeability at or proximate the production well. In some in situ conversion process embodiments, an amount of heat supplied to the formation from a production well per meter of the production well is less than the amount of heat applied to the formation from a heat source that heats the formation per meter of the heat source. Some formation layers may have material characteristics that lead to sloughing in a wellbore. For example, lean clay-rich layers of an oil shale formation may slough when heated. Sloughing refers to the shedding or casting off of formation material (for example, rock or clay) into the wellbore. Layers rich in expanding clays have a high tendency for sloughing. Clays may reduce permeability in lean layers. When heat is rapidly provided to layers with reduced permeability, water and/or other fluids may be unable to escape from the layer. Water and/or other fluids that cannot escape the layer build up pressure in the layer until the pressure causes a mechanical failure of material. This mechanical failure occurs when the internal pressure exceeds the tensile strength of rock in the layer and produces sloughing. Sloughing of material in the wellbore may lead to overheating, plugging, equipment deformation, and/or fluid flow problems in the wellbore. Sloughed material may catch or be trapped in or around the heater in the wellbore. For example, sloughed material may get trapped between the heater and the wall of the formation above an expanded rich layer that contacts or approaches the heater. The sloughed material may be loosely packed and have low thermal conductivity. Low thermal conductivity sloughed material may lead to overheating of the heater and/or slow heat transfer to the formation. Sloughed material in a hydrocarbon containing formation (such as an oil shale formation) may have an average particle diameter between 1 millimeter ("mm") and 2.5 centimeter ("cm") cm, between 1.5 mm and 2 cm, or between 5 mm and 1 cm. Volumes of the subsurface formation with very low permeability (for example, 10 microdarcy ("μdarcy") or less, 20 μdarcy or less, or 50 μdarcy or less) may have a tendency to slough. For oil shale, these volumes are typically lean layers with clay contents of 5% by volume or greater. The clay may be smectite clay or illite clay. Material in volumes with very low permeability may rubbilize during heating of the subsurface formation. The rubbilization may be caused by expansion of clay bound water, other clay bound fluids, and/or gases in the rock matrix. Several techniques may be used to inhibit sloughing or problems associated with sloughing. The techniques include initially heating the wellbore so that there is an initial slow temperature increase in the near wellbore region, pretreating the wellbore with a stabilizing fluid prior to heating, providing a controlled explosion in the wellbore prior to heating, placing a liner or screen in the wellbore, and sizing the wellbore and equipment placed in the wellbore so that sloughed material does not cause problems in the wellbore. The various techniques may be used independently or in combination with each other. In some embodiments, the permeability of a volume (a zone) of the subsurface formation is assessed. In certain embodiments, clay content of the zone of the subsurface formation is assessed. The volume or zones of assessed permeability and/or clay content are at or near a wellbore (for example, within 1 m, 0.5 m, or 0.3 m of the wellbore). The permeability may be assessed by, for example, Stoneley wave attenuation acoustic logging. Clay content may be assessed by, for example, a pulsed neutron logging system (such as RST (Reservoir Saturation Tool) logging from Schlumberger Oilfield Services (Houston, TX, USA)). The clay content is assessed from the difference between density and neutron logs. If the assessment shows that one or more zones near the wellbore have a permeability below a selected value (for example, at most 10 μdarcy, at most 20 μdarcy, or at most 50 /xdarcy) and/or a clay content above a selected value (for example, at least 5% by volume, at least 3% by volume, or at least 2% by volume), initial heating of the formation at or near the wellbore may be controlled to maintain the heating rate below a selected value. The selected heating rate varies depending on type of formation, pattern of wellbores in the formation, type of heater used, spacing of wellbores in the formation, or other factors. Initial heating may be maintained at or below the selected heating rate for a specified length of time. After a certain amount of time, the permeability at or near the wellbores may increase to a value such that sloughing is no longer likely to occur due to slow expansion of gases in the layer. Slower heating rates allow time for water or other fluids to vaporize and escape the layer, inhibiting rapid pressure buildup in the layer. A slow initial heating rate allows expanding water vapor and other fluids to create microfractures in the formation instead of wellbore failure, which may occur when the formation is heated rapidly. As a heat front moves away from the wellbore, the rate of temperature rise lessens. For example, the rate of temperature rise is typically greatly reduced at distances of 0.1 m, 0.3 m, 0.5 m, 1 m, 3 m, or greater from the wellbore. In certain embodiments, the heating rate of a subsurface formation at or near the wellbore (for example, within 3 m of the wellbore, within 1 m of the wellbore, within 0.5 m of the wellbore, or within 0.3 m of the wellbore) is maintained below 20 °C/day for at least 15 days. In some embodiments, the heating rate of a subsurface formation at or near the wellbore is maintained below 10 °C/day for at least 30 days. In some embodiments, the heating rate of a subsurface formation at or near the wellbore is maintained below 5 °C/day for at least 60 days. In some embodiments, the heating rate of a subsurface formation at or near the wellbore is maintained below 2 °C/day for at least 150 days. In certain embodiments, the wellbore in the formation that has zones or areas that lead to sloughing is pretreated to inhibit sloughing during heating. The wellbore may be treated before the heater is placed in the wellbore. In some embodiments, the wellbore with a selected clay content is treated with one or more clay stabilizers. For example, clay stabilizers may be added to a brine solution used during formation of a wellbore. Clay stabilizers include, but are not limited to, lime or other calcium containing materials well known in the oilfield industry. In some embodiments, the use of clay stabilizers that include halogens is limited (or avoided) to reduce (or avoid) corrosion problems with the heater or other equipment used in the wellbore. In certain embodiments, the wellbore is treated by providing a controlled explosion in the wellbore. The controlled explosion may be provided along selected lengths or in selected sections of the wellbore. The controlled explosion is provided by placing the controlled explosive system into the wellbore. The controlled explosion may be implemented by controlling the velocity of vertical propagation of the explosion in the wellbore. One example of a controlled explosive system is Primacord® explosive cord available from The Ensign-Bickford Company (Spanish Fork, Utah, USA). A controlled explosive system may be set to explode along selected lengths or selected sections of a wellbore. The explosive system may be controlled to limit the amount of explosion in the wellbore. FIG. 3 depicts an embodiment for providing a controlled explosion in an opening. Opening 32 is formed in hydrocarbon layer 34. Explosive system 36 is placed in opening 32. In an embodiment, explosive system 36 includes Primacord®. In certain embodiments, explosive system 36 has explosive section 38. In some embodiments, explosive section 38 is located proximate layers with a relatively high clay content and/or layers with very low permeability that are to be heated (such as lean layers 40). In some embodiments, a non- explosive portion of explosive system 36 may be located proximate layers rich in hydrocarbons and low in clay content (such as rich layers 42). In some embodiments, the explosive portion may extend adjacent to lean layers 40 and rich layers 42. Explosive section 38 may be controllably exploded at or near the wellbore. FIG. 4 depicts an embodiment of an opening after the controlled explosion in the opening. The controlled explosion increases the permeability of zones 44. In certain embodiments, zones 44 have a width between 0.1 m and 3 m, between 0.2 m and 2 m, or between 0.3 m and 1 m extending outward from the wall of opening 32 into lean layers 40 and rich layers 42. In one embodiment, the width is 0.3 m. The permeabilities of zones 44 are increased by microfracturing in the zones. After zones 44 have been created, heater 46 is installed in opening 32. In some embodiments, rubble formed by the controlled explosion in opening 32 is removed (for example, drilled out or scooped out) before installing heater 46 in the opening. In some embodiments, opening 32 is drilled deeper (drilled beyond a needed length) before initiating a controlled explosion. The overdrilled opening may allow rubble from the explosion to fall into the extra portion (the bottom) of the opening, and thus inhibit interference of rubble with a heater installed in the opening. Providing the controlled explosion in the wellbore creates microfracturing and increases permeability of the formation in a region near the wellbore. In an embodiment, the controlled explosion creates microfracturing with limited or no rubbilization of material in the formation. The increased permeability allows gas release in the formation during early stages of heating. The gas release inhibits buildup of gas pressure in the formation that may cause sloughing of material in the near wellbore region. In certain embodiments, the increased permeability created by providing the controlled explosion is advantageous in early stages of heating a formation. In some embodiments, the increased permeability includes increased horizontal permeability and increased vertical permeability. The increased vertical permeability may connect layers (such as rich and lean layers) in the formation. As shown by the arrows in FIG. 4, fluids produced in rich layers 42 from heat provided by heater 46 flow from rich layers to lean layers 40 through zones 44. The increased permeability of zones 44 facilitates flow from rich layers 42 to lean layers 40. Fluids in lean layers 40 flow to the production wellbore or a lower temperature wellbore for production. This flow pattern inhibits fluids from being overheated by heater 46. Overheating of fluids by heater 46 may lead to coking in or at opening 32. Zones 44 have widths that extend beyond a coking radius from a wall of opening 32 to allow fluids to flow coaxially or parallel to the opening at a distance outside the coking radius. Reducing heating of the fluids may also improve product quality by inhibiting thermal cracking and the production of olefins and other low quality products. More heat may be provided to hydrocarbon layer 34 at a higher rate by heater 46 during early stages of heating because formation fluids flow from zones 44 and through lean layers 40. In certain embodiments, a perforated liner (or a perforated conduit) is placed in the wellbore outside of the heater to inhibit sloughed material from contacting the heater. FIG. 5 depicts an embodiment of a liner in the opening. In certain embodiments, liner 48 is made of carbon steel or stainless steel. In some embodiments, liner 48 inhibits expanded material from deforming heater 46. Liner 48 has a diameter that is only slightly smaller than an initial diameter of opening 32. Liner 48 has openings 50 that allow fluid to pass through the liner. Openings 50 are, for example, slots or slits. Openings 50 are sized so that fluids pass through liner 48 but sloughed material or other particles do not pass through the liner. In some embodiments, liner 48 is selectively placed at or near layers that may lead to sloughing (such as rich layers 42). For example, layers with relatively low permeability (for example, at most 10 μdarcy, at most 20 /xdarcy, or at most 50 jttdarcy) may lead to sloughing. In certain embodiments, liner 48 is a screen, a wire mesh or other wire construction, and/or a deformable liner. For example, liner 48 may be an expandable tubular with openings 50. Liner 48 may be expanded with a mandrel or "pig" after installation of the liner into the opening. Liner 48 may deform or bend when the formation is heated, but sloughed material from the formation will be too large to pass through openings 50 in the liner. In some embodiments, liner 48 is an expandable screen installed in the opening in a stretched configuration. Liner 48 may be relaxed following installation. FIG. 6 depicts an embodiment of liner 48 in a stretched configuration. Liner 48 has weight 52 attached to a bottom of the liner. Weight 52 hangs freely and provides tension to stretch liner 48. Weight 52 may stop moving when the weight contacts a bottom surface (for example, a bottom of the opening). In some embodiments, the weight is released from the liner. With tension from weight 52 removed, liner 48 relaxes into an expanded configuration, as shown in FIG. 7. In some embodiments, liner 48 is installed in the opening in a compacted configuration and expanded with a mandrel or pig. Typically, expandable liners are perforated or slotted tubulars that are placed in the wellbore and expanded by forcing a mandrel through the liner. These expandable liners may be expanded against the wall of the wellbore to inhibit sloughing of material from the walls. Examples of typical expandable liners are available from Weatherford U.S., L.P. (Alice, TX) and Halliburton Energy Services (Houston, TX). In certain embodiments, the wellbore or opening is sized such that sloughed material in the wellbore does not inhibit heating in the wellbore. The wellbore and the heater may be sized so that an annulus between the heater and the wellbore is small enough to inhibit particles of a selected size (for example, a size of sloughed material) from freely moving (for example, falling due to gravity, movement due to fluid pressures, or movement due to geological phenomena) in the annulus. In some embodiments, selected portions of the annulus are sized to inhibit particles from freely moving. In certain embodiments, the annulus between the heater and the wellbore has a width at most 2.5 cm, at most 2 cm, or at most 1.5 cm. Different methods to reduce the effects of sloughing described herein may be used either alone or in combinations thereof. Further modifications and alternative embodiments of various aspects of the invention may be apparent to those skilled in the art in view of this description. In particular, the different methods to inhibit the effects of sloughing disclosed herein may be combined or utilized individually. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. In addition, it is to be understood that features described herein independently may, in certain embodiments, be combined.