Abstract:
Method for increasing production rate of horizontal wells having hydraulic fractures formed along a horizontal segment of the well by drilling drain holes to intersect the hydraulic fractures is provided.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Disclosed herein are improved methods for recovering fluids from horizontal wells that have been hydraulically fractured. More particularly, methods are disclosed for drilling drain holes that intersect hydraulic fractures that have been formed from wells in a hydrocarbon reservoir. 
         [0003]    2. Description of Related Art 
         [0004]    Hydraulic fracturing of vertical wells is a very mature art in the oil and gas industry. The method is well-known: inject a fluid at high rate and a pressure sufficient to overcome earth stresses at the bottom of the well, then inject a proppant material in fluid after a fracture has been opened in the rock surrounding the well. In recent years hydraulic fracturing has received a great amount of attention because it, combined with directional drilling, has made possible large increases in production of oil and gas from “unconventional” reservoirs in the United States. The reservoirs that have been exploited in recent years have been primarily in shale, which was previously thought to be too low in permeability to produce at commercial rates. The shale has natural fractures, and can be fractured with hydraulic fracturing techniques such that in a horizontal well, where tens of spaced-apart hydraulic fractures can be formed along a horizontal segment of a wellbore, very economic production rates can be obtained. (“Horizontal” will be used herein to designate segments of wells or wells that have segments in a hydrocarbon reservoir that are more than sixty degrees from vertical.) This revolution in technology has in recent years made the United States again a world-class producer of hydrocarbons. 
         [0005]    Hydraulic fracturing in shale introduces new aspects to the hydraulic fracturing process that were not known in the classical hydraulic fracturing treatments in vertical wells, first performed in 1947 and first commercially used in 1949 in the United States. In the classical hydraulic fracturing process, a single vertical fracture extends in a productive zone from a vertical wellbore. The fracture is thought to be formed in two “wings” extending from the wellbore into the reservoir, normally of about equal length, extending in the direction perpendicular to the direction of the least horizontal stress in the earth. Amounts of fracturing fluid and proppant are injected to make possible fractures that are propped to a thickness estimated to be about 1/32 inch up to about 1/16 inch. The width of fractures formed while pumping fluid into the earth are often calculated to be up to about ½ inch in some cases. In the matter of hydraulic fracturing of shale reservoirs, the presence of natural fractures in the rock creates further uncertainty in the rate of fluid leak off from the hydraulic fracture while pumping, and the lack of well-defined strata of rock and other factors make prediction of fracture geometry and properties in shale a more difficult proposition. Many technical papers have been published in recent years on the process and results of fracturing of shale reservoirs, but there is still much to be learned. 
         [0006]    One general characteristic that is consistently observed in wells that are hydraulically fractured in shale reservoirs, however, is that the rate of decline of production in the wells is generally greater than the rate of decline of production from wells fractured in conventional reservoirs. The reasons for this and for other observations in fracturing shale wells are complex to explain. A review paper entitled “Examining Our Assumptions—Have Over-Simplifications Jeopardized Our Ability to Design Optimal Fractured Treatments?” (SPE—119143-MS, Society of Petroleum Engineers, January, 2009) discusses many of the factors that can explain results of hydraulic fracturing in shale wells. A recent review paper that discusses results of re-fracturing to address the decline in productivity of fractured wells in shale reservoirs is “Beating the decline through refracturing,” ( World Oil , June 2015, pages 39-43). 
         [0007]    The decline in production rate of a well can be caused by a decline in fluid conductivity of the propped hydraulic fractures in the reservoir around the well. The conductivity of hydraulic fractures can be decreased by several phenomena that have been recognized in the past. One effect is the presence of residue from polymers used in fracturing fluid; another is the crushing of proppant particles in response to the stresses in the earth; and a third, not widely studied, phenomena is the migration of fine solid particles in the hydraulic fracture. Plugging of fractures because of solid particle migration would be exacerbated near a horizontal wellbore, because the fluid velocity flowing into the wellbore becomes extremely high as radial flow from the fracture nears the wellbore. The high velocity increases the tendency for solid particles to migrate in the fracture and plug the conductivity or flow capacity in a fracture. High fluid velocity may also cause erosion or mechanical failure of rock abound the fracture or of the proppant. 
         [0008]    For example, if a horizontal well is producing 100 barrels of liquid per day or 0.385 cu ft per minute from a set of perforations in the horizontal segment of the well, the wellbore has a diameter of 8 inches and a single fracture 1/16 inch wide intersects the wellbore, the total cross-sectional area of the fracture at the wellbore is only 8π× 1/16=1.57 sq inch or 0.011 sq ft. 100 barrels=560 cu ft, so the linear velocity (“Darcy velocity”) of the liquid at the wellbore is 0.385 cu ft/min/0.011 sq ft=35 ft/min or 7 inch/sec. Eight inches away from the wellbore, fluid velocity would be half as large—still a high value. Over time, this high velocity fluid (in turbulent flow) will cause erosion and failure of rock or proppant. Particles produced from the failure could cause a decrease in conductivity of the fracture at or near the wellbore. Well production rate will decrease as fracture conductivity decreases. It is believed that these conditions cause the rapid decline of production rate of fractured horizontal wells. 
         [0009]    U.S. Pat. No. 8,646,526 discloses methods of improving production in a wellbore that includes determining the textural complexity of a formation and the induced fractured complexity in creating a plurality of fractures, then drilling a lateral well originating from a wellbore to maintain conductivity of the formation. More than one lateral well may be drilled. The disclosure also includes drilling a second wellbore in the formation. Method for injecting a material that sets to a solid in hydraulic fractures is also disclosed. 
         [0010]    U.S. Pat. No. 8,333,245 discloses increasing production rate from a reservoir by drilling a plurality of branching wellbores through the reservoir. International Patent WO/2014/107475 discloses developing high pressure shale in tight rock formations using a profusion of sinusoidal open-hole laterals. The object is to drill enough laterals to achieve contact surface areas comparable with that of hydraulically fractured wells. 
         [0011]    Drilling of drain holes in the earth around wells is well-known. For example, U.S. Pat. No. 6,668,948 describes jet drilling of small holes from a cased wellbore. Drain holes are not cased. This technique may be used to extend the drainage radius of a well, somewhat similarly to the hydraulic fracturing technique. “The main body” of a drain hole is defined as that part of the drain hole that is outside the influence of the window in the casing, or generally more than one to two feet from the casing wall. 
         [0012]    What is needed is a method to utilize hydraulic fracturing in shale reservoirs but to decrease, minimize or eliminate the decrease in production rate after the well has been fractured and placed on production. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    Drain holes are drilled from selected locations in a main wellbore to extend selected distances alongside the main wellbore. The drain holes are drilled, preferably using commercial directional drilling technology, after a window is made in the casing of the main wellbore at a selected location. The drain holes may be drilled after the reservoir rock around the main wellbore has been hydraulically fractured and the well has been produced. Alternatively, the drain holes may be drilled before the well has been fractured or after the well has been fractured and before the well is produced. The drain holes may extend across only one hydraulic fracture formed from the main wellbore or they may extend across multiple hydraulic fractures. Drain holes will normally not contain casing and cement. Mechanical or diverter means may be used to plug the drain holes, preferably at the casing in the main wellbore. Offset horizontal wells may also be drilled with open hole segments at selected distances from the horizontal segment of an original horizontal well. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0014]      FIG. 1  is a side view of hydraulic fractures at intervals along a horizontal well and a drain hole drilled parallel to the horizontal segment of the horizontal well from the vertical segment of the horizontal well and a naturally fractured zone. 
           [0015]      FIG. 2  is a side view of hydraulic fractures at intervals along a horizontal well and a drain hole drilled from the horizontal segment of the horizontal well to intersect multiple fractures. 
           [0016]      FIG. 3  is a side view of hydraulic fractures at intervals along a horizontal well and multiple straight drain holes drilled from the horizontal segment of the horizontal well to intersect the multiple fractures. 
           [0017]      FIG. 4  is a side view of hydraulic fractures at intervals along a horizontal well and multiple curved drain holes drilled from the horizontal segment of the horizontal well to intersect the multiple fractures. 
           [0018]      FIG. 5  is a plan view of hydraulic fractures at intervals along a first horizontal well and a drain hole drilled from the horizontal segment of the horizontal well to intersect the multiple hydraulic fractures. 
           [0019]      FIG. 6  is a side view of hydraulic fractures at intervals along a first horizontal well and a second offset horizontal well drilled near the heel of the first well so as to intersect the hydraulic fractures in the first well. 
           [0020]      FIG. 7  is a side view of hydraulic fractures at intervals along a first horizontal well and a second offset horizontal well drilled near the toe of the first well so as to intersect the hydraulic fractures in the first well. 
           [0021]      FIG. 8  is a side view of hydraulic fractures at intervals along a horizontal well and a drain hole drilled parallel to the horizontal segment of the horizontal well from the vertical segment of the horizontal well and straddle packers placed across a hydraulic fracture from the first well and between hydraulic fractures coming from the first well. 
           [0022]      FIG. 9  is a side view of equipment that may be used to drill horizontal wells and drain holes. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Referring to  FIG. 1 , well  10  is a horizontal well drilled into productive reservoir  19 . Reservoir  19  is in rock that has very low permeability—usually shale—or it may be very low permeability sandstone or carbonate. Multiple hydraulic fractures  14  have been formed at spaced-apart locations along horizontal segment  12  of the well. Usually, only by forming multiple hydraulic fractures  14  can an economic production rate be obtained from the well. The number of hydraulic fractures, each one resulting from a “stage” of the hydraulic fracturing treatment of the well, commonly is in the range from 3 to 40 or more. The location of the hydraulic fractures may be determined by perforations in the cemented casing of the well or by packers in open hole outside un-cemented casing that contains openings that can be selectively opened between swellable or inflatable packers. These “well completion” technologies are very well known in the oil and gas industry. The horizontal segment of a horizontal well may also be open hole, with no casing. In this case, the location of a hydraulic fracture may be determined by placement of straddle packers on a work string. 
         [0024]    Damage zones  16  around horizontal segment  12  of well  10 , where fluid enters the casing, is a lower permeability zone that may be formed in or around each fractured interval. This lowered permeability or “damage” zone is attributed to near wellbore effects such as migration of fine particles to this zone or deposition of solids from the flowing fluids entering the well. It is believed that damage zone  16  is normally caused by plugging of the flow channels of fracture  14 . The high velocity of the fluid entering the horizontal well accounts for the increased mobility in solids or deposition of solids in damaged zone  16 . It is believed that damaged zone  16  accounts for the rapid decrease in production rate that is often observed in fractured horizontal wells in low permeability reservoirs. 
         [0025]    Horizontal drain hole  18  may be drilled at any time, but usually it will be drilled after production rate from horizontal well  10  has significantly decreased. Horizontal drain hole  18  is drilled such that it generally follows a path at distance “d” offset from original horizontal segment  12 . The drain hole is drilled through a window formed in the casing in the vertical segment of well  10 . This distance d is preferably greater than the diameter of damage zone  16 , which is often estimated to be in the range of 10 to 20 feet. Drain holes are not cased and cemented. Drain hole  18  may be drilled such that the distance d is relatively constant, or it may be drilled with varying d along the path of the well. The angular location in a vertical plane of horizontal drain hole  18  with respect to the center of horizontal segment  12  may vary along the trajectory of the drain hole. The effect of horizontal drain hole  18  is that a fraction of the fluid that would otherwise enter horizontal segment  12  is diverted to drain hole  18 . This has two positive effects: the fluid velocity into horizontal segment  12  is decreased, because fluid is diverted into drain hole  18 ; and, flow can enter drain hole  18  and well  10  without passing through damage zone  16 . Preferably, drain hole  18  is beyond the radius of damaged zone  16 . Since drain holes are not cased and cemented, fluid can enter where the drain hole intersects a fracture. (In other words, the drain hole is an “open hole.”) A preferred diameter of drain holes is in the range of 3 to 5 inches, with 4¾ inches being a common value because of this diameter being a common bit size, but drain holes may be larger or smaller. The total effect is an increase in production rate from well  10 . 
         [0026]    It is known that natural fracture zones exist in shale and other very low permeability rocks containing hydrocarbons. When such zones are indicated in a reservoir, from additional geophysics, microseismic measurements, or production data, for example, drain holes may be drilled to intersect indicated natural fracture zones, such as zone  19   f  in  FIG. 1 . 
         [0027]    Referring to  FIG. 2 , horizontal well  20  has been drilled into low permeability reservoir  29 . Hydraulic fractures  24  have been formed at selected intervals along horizontal segment  22  of well  20 . Damage zones  26  have formed around horizontal segment  22  near the intersection of fractures  24  with segment  22 . Drain hole  28  has been drilled from a window formed in the casing of horizontal segment  22  at a selected location. Drain hole  28  preferably takes a path alongside original well  20 , but far enough away to be outside the radius of damage zone  26 , preferably more than about 10 to 20 feet. Drain hole  28  may be straight or may undulate or go in selected directions dependent on the geometry of the rock strata in reservoir  29 . 
         [0028]    In another embodiment, illustrated in  FIG. 3 , horizontal well  30  is drilled through low permeability reservoir  39 . Hydraulic fractures  34  have been formed from horizontal segment  32  of well  30 . Damaged zone  36  may exist around horizontal segment  32  at the intersection of hydraulic fractures with the well. In this embodiment, drain holes  38 A,  38 B and  38 C are formed at different locations along horizontal segment  32  by cutting holes in the casing to form windows and drilling drain holes from the windows so as to intersect the hydraulic fractures formed outside damaged zone  36 . Drain holes  38 A, B and C may be as short as about 10′ or as long as hundreds of feet, depending upon the geometry of the reservoir and well  30 . 
         [0029]    In another embodiment, illustrated in  FIG. 4 , horizontal well  40  is drilled through low permeability reservoir  40 . Hydraulic fractures  44  have been formed from horizontal segment  42  of well  40 . Damaged zone  46  may exist around horizontal segment  42  at the intersection of hydraulic fractures with the well. In this embodiment, drain holes  48 A,  48 B and  48 C are formed at different locations along horizontal segment  42  by cutting holes in the casing to form windows and drilling drain holes from the windows so as to intersect the hydraulic fractures formed outside damaged zone  46 . Drain holes  48 A, B and C may be as short as about 10′ or as long as hundreds of feet, depending upon the geometry of the reservoir and well  40  and the drain holes are drilled with a bent sub, which results in curved drain holes. 
         [0030]    In another embodiment, illustrated in  FIG. 5 , a top or plan view, original horizontal well  50  has been drilled into reservoir  59 . Hydraulic fractures  44  have been formed at intervals along original well  50 . Damaged zones  56  may be present near the intersection of hydraulic fractures  54  and original well  50 . Drain hole  51  has been drilled also into reservoir  59 . Drain hole  51  has a horizontal segment alongside and displaced from original well  50  and also intersecting hydraulic fractures  54  formed from original well  50 . Drain hole  51  allows production of fluid from fractures  54  without flow through damaged zones  56 . 
         [0031]    Referring to  FIG. 6 , horizontal well  60  has been drilled into reservoir  69 . The vertical segment of offset well  61  may be in the vicinity of the vertical section of original well  60  and near the heel of the horizontal section of the well. Fractures  64  and damaged zones  66  exist around each fracture. Alternatively, rather than near the heel of the horizontal well, such as illustrated in  FIG. 6 , in an alternate embodiment, illustrated in  FIG. 7 , offset well  71  is drilled vertically into reservoir  79  near the toe of the horizontal section of well  70 . Fractures  74  and damaged zones  76 , have formed along horizontal segment  70 . 
         [0032]    Referring to  FIG. 8 , well  80  is a horizontal well drilled into productive reservoir  89 . Reservoir  89  is in rock that has very low permeability. Multiple hydraulic fractures  84  have been formed at spaced-apart locations along horizontal segment  82  of the well, using the same techniques as described above. Damaged zones  86  exist near the intersection of the hydraulic fractures and the horizontal well. Drain hole  88  has been drilled at a distance d from the horizontal well. A work string (not shown) with straddle packers may be placed in drain hole  88  and the work string lowered to place the straddle packers at locations  87 A and  87 B. In this location, hydraulic fracture  84  between the straddle packers may be re-fractured. Additional proppant may be injected into the fracture. The straddle packers may also be placed so as to place straddle packers at locations  85 A and  85 B in the drain hole. A new hydraulic fracture  83  may be formed from the drain hole between straddle packers  85 A and  85 B. The straddle packers may be released after each fracturing treatment and moved to a new location in the drain hole. Using this procedure, multiple hydraulic fractures may be formed from a drain hole. After fracturing in a drain hole is complete, resin coated sand may be circulated into the drain hole or a slotted liner, pierced plastic pipe, sand screen or uncemented casing may be placed in the drain hole. 
         [0033]    Apparatus generally available in industry for drilling horizontal wells and drain holes, such as described above, is illustrated in  FIG. 9 . Bent motor drilling assembly  90  is illustrated in use in reservoir  99 . Assembly  90  includes instruments for measurements-while-drilling  92 , which are placed in non-magnetic collar  93 . Drilling motor segment  94  is attached to bent sub  95 . Bent subs and drilling motors are well known in the drilling industry. The bent sub may be at selected angles or may be adjustable at angles α, as shown in  FIG. 9 . Bit  96 , usually a drag bit but possibly a rotary bit, as shown, is attached to the bent sub and is powered by drilling motor  94 . The majority of horizontal wells in shale or low permeability formations are drilled with this assembly. The bent assemblies drill along straight trajectories when the drill pipe is rotating and along curved trajectories when the drill pipe is not rotating (i.e., is in the “slide” mode). Thus the trajectory of wells can be controlled by utilizing the rotating and slide modes as needed. The curved sections in horizontal wells are drilled in slide mode. 
         [0034]    The measurement-while-drilling tool measures the azimuth and orientation of the borehole. These measurements are needed to steer the drill bit. Non-magnetic collar  63  allows the MWD tools to make magnetic measurements needed to steer the bit. The bits are normally PDC rotary bits using man-made diamonds in the cutters. These bits allow drilling at high rates and have good side-cutting capabilities, which are important when drilling curved sections in directional wells. 
         [0035]    To drill drain holes out of the cased segment of a well, it is necessary to mill a “window” in the casing and drill through this opening. This is a routine operation performed in the drilling industry. Preferably, drain holes are drilled “underbalanced.” The pressure in the drain hole is preferably maintained below the pressure in the reservoir being drilled. This will minimize or eliminate damage to the fractures and reservoir rock during the drilling operation. 
         [0036]    Wall friction between the drill pipe and the wall of the bore hole increases as the length of the horizontal section increases. When the drill pipe is rotating, wall friction is not a problem. When the drill pipe is not rotating, or is in the slide mode, the wall friction can be high and can limit the length of a horizontal section to one or two miles. 
         [0037]    Service companies have also developed rotary steerable tools (RST) that allow the drill pipe to be rotated in both straight and curved sections in a horizontal well. This overcomes the wall friction problems and allows the drilling of high angle and horizontal sections in excess of eight miles in some wells. Most shale wells are shorter, with horizontal sections not more than 6,000 feet, so they are mostly drilled with bent assemblies because of the high reliability of the bent assemblies and the high cost of the rotary steerable tools. 
         [0038]    The drain holes can be drilled horizontal, slanted, curved or undulating using these tools or any combination of the tools described above. Jet drilling can also be used, such as disclosed in U.S. Pat. No. 6,668,948. 
         [0039]    Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.