A method of drilling into a geological region including a subsurface clathrate reservoir includes drilling a borehole into the geological region including the subsurface clathrate reservoir and dissociating at least a portion of the clathrate in a region near the borehole. After the dissociating, material within at least a portion of the region near the borehole in which the clathrate has been dissociated is compacted to form a compacted region at least partially surrounding the borehole within the clathrate reservoir. After the compacting, well casing is placed into the borehole within the compacted region and the well casing is cemented into the borehole in the compacted area.

FIELD 
     The present invention relates generally to exploitation of clathrate reservoirs and more particularly to improving recoverability of clathrate reservoirs. 
     BACKGROUND 
     Clathrates are substances in which a lattice structure made up of first molecular components (host molecules) that trap or encage one or more other molecular components (guest molecules) in what resembles a crystal-like structure. In the field of hydrocarbon exploration and development, clathrates of interest are generally clathrates in which hydrocarbon gases are the guest molecules in a water molecule host lattice. They can be found in relatively low temperature and high pressure environments, including, for example, deepwater sediments and permafrost areas. Clathrates are also referred to as hydrates, gas hydrates, methane hydrates, natural gas hydrates, CO2 hydrates and the like. For the purposes of this invention the term Clathrates will be used. 
     Clathrates generally form a significant portion of the structural support for the reservoir in which they occur, particularly with respect to cementing and/or occupying pore space. As clathrates dissociate, the constituents become mobile and cease acting as support, weakening the formation and potentially causing localized compaction of the reservoir. In a production environment, such localized subsurface compaction can lead to effects on equipment in the local area, both subsurface and on the surface. For example, in the subsurface environment, casings and drill strings may be collapsed due to high compressive loading caused by compaction of the reservoir, subsidence of the reservoir overburden strata and uplift of the reservoir underlying strata. On the surface, subsidence caused by subsurface clathrate dissociation and reservoir compaction can lead to sinkholes, subsidence, and other related motions that can cause damage to surface equipment such as well-heads, pipelines, equipment and other facilities in the immediate vicinity. The inventors have recognized a need to reduce or remediate this possibility. 
     SUMMARY 
     An aspect of an embodiment of the present invention includes a method of drilling into a geological region including a subsurface clathrate reservoir, including drilling a borehole into the geological region including the subsurface clathrate reservoir and dissociating at least a portion of the clathrate in a region near the borehole. After the dissociating, material within at least a portion of the reservoir region near the borehole in which the clathrate has been dissociated is compacted to form a compacted region at least partially surrounding the borehole within the clathrate reservoir. After the compacting, well casing is placed into the borehole within the compacted region and the well casing is cemented into the borehole in the compacted reservoir area. 
     An aspect of an embodiment may include a system for drilling into a geological region including a subsurface clathrate reservoir, including a drill, configured and arranged to drill a borehole into the geological region including the subsurface clathrate reservoir, a source of dissociation-promoting material configured and arranged to deliver the dissociation-promoting material to at least a portion of the clathrate in a region near the borehole, a device configured and arranged to place a well casing into the borehole after a dissociation and compacting process have been performed to form a compacted region of the reservoir at least partially surrounding the borehole within the clathrate reservoir, and a source of cement configured and arranged to cement production tubing in the borehole for use in producing hydrocarbons from the clathrate reservoir. 
     An aspect of an embodiment of the present invention includes a system including a drill bit or other mechanical device configured and arranged to direct drilling fluid in a radial direction relative to the borehole such that dissociation of surrounding clathrates is increased as a result of radial force from drilling fluid flow. 
     Aspects of embodiments of the present invention include computer readable media encoded with computer executable instructions for performing any of the foregoing methods and/or for controlling any of the foregoing systems. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Other features described herein will be more readily apparent to those skilled in the art when reading the following detailed description in connection with the accompanying drawings, wherein: 
         FIG. 1   a  illustrates a subsurface region where no clathrates are present having a deposit of unconsolidated sediment. 
         FIG. 1   b  illustrates a subsurface region where no clathrates are present having a deposit of consolidated sediment. 
         FIG. 1   c  illustrates a subsurface region where no clathrates are present having a deposit of fully compacted sediment. 
         FIG. 1   d  illustrates a subsurface region where clathrates are present having a deposit of unconsolidated sediment and clathrates. 
         FIG. 1   e  illustrates a subsurface region where clathrates are present having a deposit of consolidated sediment and clathrates. 
         FIG. 1   f  illustrates a subsurface region where clathrates are present having a deposit of fully compacted sediment and clathrates. 
         FIG. 1   g  illustrates a first consequence of localized clathrate dissociation in a subsurface region where clathrates are present. 
         FIG. 1   h  illustrates a first part of a second consequence of localized clathrate dissociation in a subsurface region where clathrates are present. 
         FIG. 1   i  illustrates a second part of a second consequence of localized clathrate dissociation in a subsurface region where clathrates are present. 
         FIG. 2   a  illustrates a subsurface region where clathrates are present during drilling prior to clathrate dissociation. 
         FIG. 2   b  illustrates a subsurface region where clathrates are present without preconditioning after drilling prior to clathrate dissociation. 
         FIG. 2   c  illustrates a subsurface region where clathrates are present without preconditioning during production and clathrate dissociation. 
         FIG. 2   d  illustrates dissociation-induced compaction in a subsurface region where clathrates are present without preconditioning. 
         FIG. 3   a  illustrates a subsurface region where clathrates are present during drilling prior to pre-conditioning. 
         FIG. 3   b  illustrates a subsurface region where clathrates are present after drilling prior to pre-conditioning. 
         FIG. 3   c  illustrates a subsurface region where clathrates are present after drilling and during promotion of clathrate dissociation. 
         FIG. 3   d  illustrates pre-conditioning a subsurface region where clathrates are present. 
         FIG. 3   e  illustrates a subsurface region where clathrates are present during re-drilling after pre-conditioning. 
         FIG. 3   f  illustrates a completed well in a subsurface region where clathrates are present after pre-conditioning and re-drilling. 
         FIG. 3   g  illustrates production in a subsurface region where clathrates are present after pre-conditioning and well completion. 
         FIG. 4  presents data extracted from the U.S. National Energy Technology Laboratory methane hydrate newsletter “Fire in the Ice” Volume 10, Issue 2 pages 9-11 “Relative Gas Volume Ratios for Free Gas and Gas Hydrate Accumulations” by Boswell et al. illustrating a potential volume change in fluids (gas+water) immediately after dissociation occurs. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1   a - 1   i  illustrate a subsurface region (which may represent a region below a land surface or the sea floor) in which hypothetical clathrate reservoirs might occur.  FIGS. 1   a - 1   c  illustrate a structure where no clathrates are present.  FIGS. 1   d - 1   f  illustrate a similar structure in which clathrates are also present.  FIGS. 1   g - 1   i  illustrate a similar structure as depicted in  FIGS. 1   d - 1   f  that at a point in time undergoes localized clathrate dissociation. 
     As shown in  FIG. 1   a , near the surface lies a deposit  20  of unconsolidated sediment containing solid sediment particles  14  and liquid water  18  in the pore spaces between the solid sediment particles  14 . Deposit  20  lies between overburden  2  and underlying strata  4 , as do all deposits in  FIGS. 1   a - 1   i . As the solid sediment particles  14  and liquid water  18  in deposit  20  become buried over geologic time, deposit  20  moves deeper below the surface and the solid sediment particles  14  and liquid water  18  within deposit  20  become exposed to increasing pressure with depth of burial. Under this increasing pressure the solid sediment particles  14  of deposit  20  remain relatively immobile while the liquid water  18  is able to flow out to lower pressure regions. This causes the solid sediment particles  14  to move closer together and at times come in contact with nearby solid sediment particles  14  (i.e., become more consolidated, or compacted), leaving less pore space between solid sediment particles  14  and likewise, relatively less liquid water  18  in the deposit. Eventually the burial process will convert the characteristics of deposit  20  into those of consolidated deposit  22  where most of the solid sediment particles  14  are in closer proximity or contact with each other, as illustrated in  FIG. 1   b . Proceeding deeper, the sediments become fully compacted with all solid sediment particles  14  in very tight contact with each other to form a deposit  24 , as illustrated in  FIG. 1   c , still containing some water, but in very small pore spaces and comprising (for example) a rock-like sandstone deposit. Throughout this process the contacts between the solid sediment particles  14  bear more and more of the weight of all the sediment and water above them. As a very broad generalization with many exceptions there is a linear relationship between the compaction of a deposit and the depth of burial, with shallow deposits being relatively unconsolidated and deeper deposits becoming increasingly more compacted. 
     As shown in  FIG. 1   d , deposit  30  is nearly identical to deposit  20  in  FIG. 1   a  with the exception that some of the liquid water  18  between the solid sediment particles  14  has come into contact with guest gas molecules at the appropriate temperature and pressure and together are converted into solid clathrate particles  16 . The solid clathrate particles  16  generally mimic the behavior of their neighboring solid sediment particles  14 . Deposit  30  therefore has solid sediment particles  14 , solid clathrate particles  16  and liquid water  18  present in the space between particles. As the solid sediment particles  14 , solid clathrate particles  16  and liquid water  18  in deposit  30  become buried over geologic time, deposit  30  moves deeper below the surface and the solid sediment particles  14 , solid clathrate particles  16  and liquid water  18  within deposit  30  become exposed to increasing pressure with depth of burial. Under this increasing pressure the solid sediment particles  14  and solid clathrate particles  16  of deposit  30  remain relatively immobile while the liquid water  18  is able to flow out to lower pressure regions. Some liquid water  18  may also be reduced by continued conversion into solid clathrate particles  16 . This causes the solid sediment particles  14  and solid clathrate particles  16  to move closer together and at times come in contact with nearby solid sediment particles  14  and /or solid clathrate particles  16  (i.e., become more consolidated, or compacted), leaving less pore space between solid sediment particles  14  and solid clathrate particles  16  and likewise, relatively less liquid water  18  in the deposit. As shown in  FIG. 1   e , deposit  32  may lose less thickness relative to deposit  22  in  FIG. 1   b  due to this conversion of liquid water  18  in pore spaces into more and/or larger solid clathrate particles  16 , preventing normal compaction. Eventually the burial process will convert the characteristics of deposit  30  illustrated in  FIG. 1   d  into those of consolidated deposit  32  illustrated in  FIG. 1   e  where most of the solid sediment particles  14  and solid clathrate particles  16  are in contact with each other. Proceeding deeper, as illustrated in  FIG. 1   f , the sediment and clathrates become fully compacted with all solid sediment particles  14  and solid clathrate particles  16  in very tight contact with each other to form a reservoir  34 , still containing some water, but in very small pore spaces and forming (for example) a rock-like clathrate and sandstone reservoir. Throughout this process the contacts between the solid sediment particles  14  and solid clathrate particles  16  bear more and more of the weight of all the sediment and water above them. As a very broad generalization with many exceptions  FIGS. 1   d - 1   f  illustrate a case where the presence of clathrates causes a form of compaction of a deposit that is different from that of  FIGS. 1   a - 1   c.    
       FIGS. 1   g - 1   i  contain cases illustrating two consequences to the reservoir  34  of  FIG. 1   f  if the solid clathrate particles  16  undergo localized dissociation. Upon dissociation, the solid clathrate particles  16  change from incompressible and relatively immobile solids into very mobile fluids (generally, liquid water  18  and compressed free (guest) gasses  12  liberated from the clathrate lattice). This dissociation causes an instantaneous increase in local pressure as the compressed free gasses  12  attempt to expand to various multiples of their pre-dissociative space as detailed in  FIG. 4 . What was once a reservoir consolidated and under an in situ pressure roughly in proportion with its neighboring non-clathrate deposits suddenly contains either: Case A as shown in  FIG. 1   g -a localized high pressure pocket between the zones of dissociation  50  if there are no paths of relief; or Cases B (i) and (ii), shown in  FIGS. 1   h  and  1   i , respectively—where B(i) shows formation of a localized void  52  in the remaining undissociated reservoir  34  between the dissociation fronts  50  as dissociated liquid water  18  and compressed free gasses  12  move from the local high pressure area to lower pressure areas by whatever means that are available (permeability, faulting, flowing along or inside drill pipes, etc.). This immediately causes case B(ii) where the surrounding overburden  2  and underlying strata  4  displace into and fill the localized void  52  due to the pressure differential and form a compacted zone  54  in order to support the weight of all the deposits above them. The localized results of dissociation may be expected to hold in  FIGS. 1   g - 1   i  or in any region in which clathrates form a part of the structural support of a formation. Note in particular that this newly compacted region does not contain any clathrates. 
     As will be appreciated, localized dissociation of a previously structurally stable sediment and clathrate subsurface reservoir will in many cases result in subsurface collapses. Such collapses can have both local (subsurface) effects and distant (surface) effects.  FIGS. 2   a - 2   d  illustrate one potential subsurface result of dissociation-induced compaction during drilling and production operations. 
       FIG. 2   a  illustrates the situation prior to dissociation. A drill string  40  with drill bit  42  has been introduced into the clathrate reservoir  34  that is intended to be produced. The clathrate reservoir  34  includes solid sediment particles  14  along with solid clathrate particles  16  and minor amounts of liquid water  18 . The clathrate reservoir  34  surrounding the drill string  40  and drill bit  42  is considered to be compacted equivalent to neighboring deposits at similar depths, and therefore relatively stable. 
       FIG. 2   b  illustrates the situation after the drill string  40  and drill bit  42  are removed and production tubing and/or casing  44  is installed in one of the common manners. 
     As will be appreciated, efforts to produce the gasses stored in the clathrate and sediment reservoir  34  will entail intentionally inducing dissociation to free the gas from the clathrate host matrix. Such efforts may include, for example, decreasing pressure, adding heat, adding clathrate inhibiting materials and/or molecular substitution into the deposit  34  or any combination of these. See, for example, U.S. Pat. No. 7,537,058 describing production from a hydrate reservoir. As production begins, a zone of dissociation  50  shown in  FIG. 2   c  is formed immediately in a highly localized radial zone around the production tubing and/or casing  44 . This illustrates a key distinction between production of hydrocarbons from conventional hydrocarbon gas reservoirs and production of hydrocarbon gas clathrate reservoirs. Conventional hydrocarbon gas reservoirs are essentially large pressure vessels and as they are produced the reservoir pressure relatively uniformly drops and there is a relatively uniform compaction throughout the reservoir. Hydrocarbon gas clathrate reservoirs on the other hand produce hydrocarbon gas in essentially the opposite way: production commences by establishment of a dissociation front immediately at the wellbore and the dissociation front gradually moves out radially from the wellbore, as does compaction. 
     As the clathrates  16  are dissociated into liquid water  18  and compressed free gas  12 , the remaining reservoir sediment becomes progressively less consolidated as illustrated by  FIG. 2   c . At some point, the structural support of the dissociated area is exceeded by the hydrostatic and lithostatic pressure and the overburden  2  and underlying strata  4  surrounding production tubing and/or casing  44  may displace into the resultant void and create a compacted zone  54 , crushing the production string as illustrated in  FIG. 2   d . Generally, the production tubing and/or casing and surrounding sealing cement will collapse radially and/or axially. Other failure modes may include flow of gas up the exterior of the collapsed drill string and cement, potentially blowing out to the surface or sea floor. 
     In order to reduce or eliminate this effect, steps may be taken to pre-condition (pre-compact) the reservoir in way of the selected production well location after the initial drilling and prior to installation of the production string such that catastrophic collapse during initial production can be avoided as illustrated in  FIGS. 3   a - 3   g . As illustrated in  FIG. 3   a , the reservoir is drilled. Then the drill pipe  40  and drill bit  42  are repositioned somewhere between the total well depth and a point near or above the top of the clathrate reservoir ( FIG. 3   b ) and one and/or more methods that promote dissociation are applied to the reservoir ( FIG. 3   c ) to create a void  52 . 
     In one example of promoting dissociation, hot water, hot drilling mud or other heated fluid may be injected or circulated, raising the temperature of the clathrates, causing dissociation. Alternately, or in addition, clathrate inhibiting chemicals may be injected. Such inhibiting chemicals include, for example, salts, methanol and glycols including but not limited to monoethylene glycol and diethylene glycol. 
     In another approach, mobile fluids present in the reservoir, water for example, may be pumped out to reduce the reservoir's pressure to a point below the pressure of clathrate stability, causing dissociation. One method of achieving this is to use underbalanced drilling techniques. Another example could be deployment of a submersible pump located at the end of the drill string. 
     In one embodiment, the dissociation process may be begun during the initial drilling operation by adding heat and/or inhibiting chemicals to the drilling fluid circulating through the zone of interest and/or utilizing underbalanced drilling techniques. 
     As will be appreciated, dissociation induced by any of the foregoing methods will tend to proceed outwardly in a radial direction from the outer edges of the original borehole. By way of example, dissociation may be induced in a radius of a few meters around the borehole, for example, between about 1 m and about 10 m. In a particular embodiment, the treated region is lm surrounding the borehole. In an embodiment, dissociation is induced along a complete vertical extent of the reservoir. 
     Withdrawing the drill pipe to the top of the clathrate reservoir prior to inducing dissociation maintains the drill pipe in a state of tension during localized slumping downward of the overburden in the drilling pipe's vicinity, a situation for which it is well-engineered. 
     In application, it may be useful to limit the progress of the dissociation to control the volumes of gas and/or water generated with limitations of the drilling system in mind. Embodiments of these methods may include reducing the applied heat and/or inhibiting chemicals and/or increasing the bottom whole pressure such that the rate of dissociation is reduced or stopped as appropriate. 
     Gas released in the dissociation process will generally escape through the borehole along with the circulating fluids. The gas may be collected, combined with other hydrocarbon production, or alternately it may be flared and/or otherwise vented. 
     Likewise, fluid (e.g., water) released by dissociation may be collected. This collection serves both to remove water from the area to be compacted, preventing it from re-forming clathrates and to further decrease relative pressures in the zone, improving the dissociation rate and increasing compaction. The collected fluid may be treated and may then be disposed of or used for other purposes. For example, it may be re-injected into other subterranean formations, either for disposal or for use in flooding for sustained conventional oil production in a later stage recovery process. 
     Once the clathrate is dissociated in a region surrounding the borehole, the empty borehole will generally collapse. In one approach, prior to collapse or induction of dissociation, additional stabilizing material may be injected into the borehole. For example, gravel, sand or similar filler materials may be injected into the bottom of the borehole or into a region surrounding the borehole prior to dissociation and collapse, either to reduce the displacement of overlaying or underlying strata and/or to create and/or maintain a zone of high permeability in the wellbore area. In either case, the collapsed region has become consolidated to form the compacted region  54  ( FIG. 3   d ), which region no longer contains hydrates. 
     After the consolidation steps are completed, and the clathrate reservoir area below the drill string is appropriately consolidated, the well may be re-drilled through the now-consolidated area ( FIG. 3   e ), completed ( FIG. 3   f ) and produced ( FIG. 3   g ). Likewise, surface facilities, pipelines and other massive equipment may be safely sited directly above the compacted area. 
     In the case of large reservoirs, it may be useful to make use of multiple boreholes for production, injection and/or monitoring. In these cases, it should be appreciated that pre-compaction methods in accordance with embodiments of the present invention may be applied to one or more of the boreholes, and that in a particular embodiment, each borehole. 
     As will be appreciated, the method as described herein may be performed using a computing system having machine executable instructions stored on a tangible medium. The instructions are executable to perform each portion of the method, either autonomously, or with the assistance of input from an operator. In an embodiment, the system includes structures for allowing input and output of data, and a display that is configured and arranged to display the intermediate and/or final products of the process steps. A method in accordance with an embodiment may include an automated selection of a location for exploitation and/or exploratory drilling for hydrocarbon resources. 
     Those skilled in the art will appreciate that the disclosed embodiments described herein are by way of example only, and that numerous variations will exist. The invention is limited only by the claims, which encompass the embodiments described herein as well as variants apparent to those skilled in the art. In addition, it should be appreciated that structural features or method steps shown or described in any one embodiment herein can be used in other embodiments as well.