A method of hydraulically fracturing subterranean coal seams to improve the rate and total recovery of gas therefrom. The coal seam is fractured with acid and a proppant-laden fracing fluid in alternating injection stages. The initial injection stage of the fracturing fluid contains from about 0 to about 4 pounds of a spherical proppant having a particle size distribution substantially between 60 and 140 mesh. The subsequent fracing fluid injection stages are alternated with injection stages of a smaller volume of acid. The proppant loading in the fracing fluid is increased with each injection stage until the loading is from about 8 to about 12 pounds of proppant per gallon of fluid. The subsequent and terminal fracing fluid injection stages are with the higher proppant loading. Preferably, the volume of the fracing fluid injection stages is increased in successive stages from about 2000 to about 4000 gallons per stage initially to from about 6000 to about 8000 gallons per stage in the subsequent and terminal stages. The terminal fracing fluid injection stage is followed by a flushing of the well with a proppant-free fluid.

FIELD OF THE INVENTION 
     This invention pertains to hydraulically fracturing earth formations, and more particularly to the hydraulic fracturing of subterranean gas-containing coal formations, i.e. coal seams, for the purpose of increasing the producing rate and total amount of recovery of gas from a well completed in such a formation. 
     BRIEF DESCRIPTION OF THE PRIOR ART 
     Hydraulic fracturing techniques for hydrocarbon formations are well known and have been extensively used for increasing the recovery of oil and gas from hydrocarbon bearing formations. These techniques involve injecting a fracing fluid down the wellbore and into contact with the formation to be fractured. Sufficiently high pressure is applied to the fracing fluid to initiate and propagate a fracture into the formation. Propping materials are generally entrained in the fracing fluid and deposited in the fracture to maintain the fracture open during production. 
     A hydraulic fracturing technique particularly well suited for fracturing low permeability (10 millidarcies or less) gas bearing sandstone formations is described in U.S. Pat. No. 4,186,802. This method includes multiple fracing stages carrying a fine proppant sand of between 60 to 140 mesh size in a sand to fluid ratio mix of 4 pounds/gallon or higher. Each carrier stage is immediately followed by a corresponding spacer stage comprising the fracing fluid without a proppant added. Immediately following the final carrier stage and corresponding spacer stage, a terminating stage carrying a medium proppant sand of a 20 to 40 mesh size is injected, followed by a fracing fluid flush of the tubing string. The fracing fluid was made up of up to 70 percent alcohol by volume in order to reduce the water volume of the fracing fluid which adversely reacted with water sensitive clays within the formation. Up to 20 percent liquified CO 2  by volume was combined with the frac water/alcohol mixture to further reduce the water volume. 
     Coal seams differ from typical subsurface formations from which hydrocarbons are normally recovered, such as carbonate or a sandstone formations. Coal seams are typically much more friable than carbonates or sandstones. Thus, when conventional fracing methods are used, the proppants normally used have a tendency to generate small coal particles from the faces of the fracture which become mixed with the proppant. When the well is put into production, additional coal particles tend to slough off of the faces of the fractures into the proppant. The presence of the coal particles in the proppant tends to plug off the intersticial spaces between the proppant particles and concomitantly reduces the conductivity of the propped fracture. The coal particles also adversely affect the functioning of surface separating and processing equipment. 
     In addition, coal seams are subject to plastic deformation. When conventional 20-40 mesh proppants are used, they are abrasive to the fracture faces. Proppants in the fracture faces and the creep of the coal into the fracture results in reducing the width and conductivity of the fracture. 
     Further, conventional fracturing techniques result in wide fractures at the lowermost portion of the coal seam which narrow as they near the uppermost portion of the coal seam, limiting communication between the upper portions of the coal seam and the fracture. Further complicating the fracture of coal seams, the coal seams are typically saturated with water having a high carbonate concentration. Conventional fracing results in precipitation of the carbonates, further reducing the permeability of the formation at the fracture faces. 
     The present invention provides a method for generating fracture within a subsurface coal seam which have an improved conductivity, an increased production rate and an increased total recovery of gas therefrom in comparison with methods previously attempted fracturing coal seams. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a method for generating fractures within a subsurface coal seam which have an improved conductivity and more uniform width. Broadly, the method comprises injecting stagewise into the formation adjacent the well a proppant-containing fracing fluid alternated with an acidizing solution. 
     The fracing fluid has suspended therein fine proppants with a particle size distribution substantially between 60 and 140 mesh (all mesh sizes herein refer to the U.S. standard sieve series), preferably averaging 100 mesh. The proppants are present in the initial fracing fluid injection stages in an amount ranging from about 0 to about 4 pounds per gallon of fracing fluid. The proppant loading in the fracing fluid is increased in subsequent injection stages until the fracing fluid contains from about 8 to about 12 pounds of proppant per gallon of fluid. Thereafter, the fracing liquid injections are continued at the higher proppant loading. Each fracing liquid stage is immediately followed by injection of an acidizing solution into the formation adjacent the well. 
     The alternating injections of fracing fluid and acid are performed at a rate of from about 15 to about 35 barrels per minute, preferably 20 to 30 barrels per minute, and continuing until at least 3,000 pounds of the fine proppants have been deposited in the formation fracture per vertical foot of the coal seam. Preferably, the terminal injection stage of proppant-containing fracing fluid is followed by a proppant-free fracing fluid or acidizing solution flush of the tubing string. 
     The fracing fluid is preferably water from the coal seam or adjacent formation to which a gelling agent is added at the rate of about 30 pounds per 1,000 gallons. The acid may be any acid typically used for treating subsurface formations, such as acetic, formic, hydrofluoric, or sulfamic, but is preferably hydrochloric acid. Additionally, the fracing fluid or acidizing solution may contain surfactants, suspending agents, sequestering agents, anti-sludge agents, or corrosion inhibitors. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method of the present invention can be carried out by any conventional apparatus used for previously known methods of hydraulic fracturing. Conventional proppant-water mixing equipment and pumping equipment may be utilized in performing the method. The fracturing fluid and acid can be injected through the well tubing, casing or other available or suitable pipe or conduit. The fluid can be injected through perforations in the casing extending through the cement and directly into the formation, the injection being confined to the selected coal seam through conventional isolation techniques. Preferably, however, the well is completed by conventional open-hole techniques to avoid the problem of sand-out which can occur when the fracing fluid must flow through casing performations, especially at the higher proppant loading in the method of the invention. Normally, the shales of the strata overlying and underlying the coal seam are of sufficient hardness to confine the fracture to the coal seam. 
     Although it is possible to use water or other fluid from any suitable source, the fracturing fluid preferably used in carrying out the method of the present invention is water produced from the coal seam or adjacent formation to which is added conventional gels, such as, for example, guar gum, modifed guar gums, polysaccharide derivatives, cellulose derivatives, or synthetic polymers, to obtain a sufficient viscosity to suspend the proppants. Preferably, a substitive guar gum such as HPG (hydroxy propyl guar gum) sold under the designation of WG11 by Halliburton or WG-A2 by Smith Energy is added at the rate of about 30 pounds per 1,000 gallons of formation water. 
     Proppant is added to the fracing fluid in the initial stage at a rate ranging from about 0 (proppant-free) to about 4 pounds per gallon of fracing fluid. 
     The succeeding stages have a proppant loading of from about 2 to about 4 pounds per gallon of fluid initially which is incrementally increased in succeeding stages to a proppant loading of from about 8 to about 12 pounds per gallon of fluid. Thereafter, the proppant loading is at the 8-12 pounds per gallon rate, preferably 10 pounds per gallon. Each incremental increase is preferably from about 0 to about 3 pounds per gallon. 
     The proppant has a particle size distribution substantially between 60 and 140 mesh, preferably averaging 100 mesh. Preferably, the proppant is spherically shaped rather than angularly shaped. Oklahoma 100 mesh sand has been found suitable for most applications. 
     The proppant-containing fracing fluid is injected into the formation in multiple stages. The rate of injection may range from about 15 to about 35 barrels per minute, but best results are obtained at an injection rate of 20-30 barrels per minute. The volume of each fracing fluid injection stage is determined in advance thereof and depends on the size of the fracture desired and the pressure and flow resistance. Normally, 2,000-8,000 gallons per stage produce suitable results. Preferably, the volume of the initial fracing fluid injection stage is from about 2,000 to about 4,000 gallons, and the volume is increased in each following injection stage, as the sand loading is increased, to from about 6,000 to about 8,000 gallons, preferably 7,000 gallons, for subsequent and terminal fracing fluid injection stages. The stages are continued until at least about 3,000 pounds of proppant have been deposited in the formation fracture per vertical foot of the coal seam. With the fracturing method of the invention it is possible to place very large quantities of proppant into the formation. With the method of the invention 500,000 pounds of proppant have been readily deposited to the fractures within the formation and greater quantities may be deposited, when desired. Thus for a coal seam of average width (typically about 30 feet) the fracturing method of the invention may be continued until at least about 15,000 pounds of proppant per vertical foot of the coal seam have been deposited in the formation fractures. 
     The fine, spherical proppant is believed to serve several functions in the invention. As it is injected into the fracture, the spherical shape of the proppant substantially reduces abrasion to the face of the fracture, thereby largely eliminating the problems associated with particles of coal becoming mixed with the proppant. Additionally, spherical proppants having a small particle size exhibit less tendency to become embedded in the face of the fracture and inhibit creep of the coal into the propped fracture. When the pressure on the fracing fluid is reduced and the formation face is allowed to compress the proppants, the proppant particles in the fractures provide a formation consolidating effect, similar to that of gravel packing in a well completed in a poorly consolidated formation by filtering out the coal particles which would otherwise slough off of the fracture faces and plug the interstitial spaces between the proppant particles. The permeability of fine proppants is much greater than that of the coal seam. Thus, if the fracture is wide enough, the conductivity of the propped fracture is sufficient to improve production and overall recovery of gas from the well. 
     Immediately following each proppant-containing fracing fluid injection stage, acidizing solution is injected into the formation. The acidizing solution may contain any conventional acid normally used for treating subsurface formations at typical concentrations. These acids include acetic acid, formic acid, hydrofluoric acid or sulfamic acid. Suitable results are obtained with an aqueous acidizing solution containing 15 percent by weight hydrochloric acid. The acid solution may also contain conventional additives such as surfactants, suspending agents, sequestering agents, anti-sludge agents, or corrosion inhibitors. If desired, the acidizing solution may contain about 1 pound of proppant per gallon of solution. 
     The acid is injected into the formation at about the same rate as the fracing fluid injection stages. The volume of acidizing solution injected depends on the size of the fracture and pressure and flow resistance, but injection of from about 250 to about 1500 gallons, usually about 750 gallons, of an acidizing solution of 15 percent by weight hydrochloric acid between each fracing fluid stage is suitable for most fractures. If desired, the formation may be treated with 500-3000 gallons of the acidizing solution prior to the injection of the initial fracing fluid stage. 
     The acid is believed to serve several functions in the invention. Because the acidizing solution is less dense than the fracing fluid, it tends to flow above the fracing fluid and sand deposited in the lower portion of a vertical fracture, widening and vertically extending the upper portion of the fracture. The acidizing solution also has a tendency to divert from existing fractures and to initiate new fractures which are filled with proppant during the subsequent fracing fluid injection stages. Finally, the acid cleans the well bore and fracture faces by solubilizing any precipitates or contaminants due to drilling or completion fluids or cement which may be present at or adjacent the well bore or fracture faces. 
     The invention is illustrated by way of the following examples of treatment of coal seams in La Plata County, Colo.: 
    
    
     EXAMPLE 1 
     
         ______________________________________Formation Thickness:           82 feetDepth:          2535-2617 feetFracing Fluid:  Formation water, plus 30 pounds           per 1,000 gallons of a hydroxyl           propyl guar gum gelling agent.Propping Agent: 100 mesh Oklahoma sand,           502,603 poundsAcid:           15% HClCasing:         Open HoleAverage Pressure           2010 psiAverage Injection Rate           27 BPMNumber of Fracing Fluid           13StagesVolume of Fracing Fluid           62,834 Gallons(Less Sand Volume)Volume of Acid  8,000 GallonsTotal Fluid Volume           70,834 Gallons______________________________________                         StageEvent         Sand    Fluid Vol.                         Vol.  Rate  PressureNo.   Fluid   (ppg)   (Gal.)  (Gal.)                               (BPM) (psi)______________________________________ 1    Acid    0       2000    2000  26.5  3000 2    Pad     0       3000    3000  26.5  1600 3    Frac    2       2749    3000  27.0  1625 4    Frac    3       2639    3000  26.5  1650 5    Acid    1        500     523  26.5  1800 6    Frac    5       4071    5000  27.0  1800 7    Acid    1        500     523  26.9  1800 8    Frac    7       3790    5000  26.9  1800 9    Acid    1        500     523  27.0  183010    Frac    8       5129    7000  27.0  190011    Acid    1        500     523  27.0  192012    Frac    10      4807    7000  27.0  195013    Acid    1        500     523  27.0  198014    Frac    10      4807    7000  27.0  198015    Acid    1        500     523  27.0  205016    Frac    10      4807    7000  26.5  209017    Acid    1        500     523  26.7  210018    Frac    10      4807    7000  26.3  210019    Acid    1        500     523  26.8  215020    Frac    10      4807    7000  26.8  215021    Acid    1        500     523  27.0  212522    Frac    10      4807    7000  26.9  220023    Acid    0.93     750     782  27.0  215024    Frac    10      4807    7000  26.4  217525    Acid    0.93     750     782  26.8  219026    Frac    10      4807    7000  26.9  214027    Flush   0       3000    3000  26.7  2040______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________Formation Thickness:          72 feetDepth:         3107-3179 feetFracing Fluid: Formation Water, plus 30 pounds per          1000 gallons of a hydroxyl propyl          gelling agent.Propping Agent:          100 mesh Oklahoma sand, 236,380 poundsAcid:          15% HCLCasing:        Open HoleAverage Pressure:          3700 psiAverage Injection Rule:          24.5 BPMNumber of Fracing Fluid          12Stages:Volume of Fracing Fluid:          68,004 Gallons(Less sand volume)Volume of Acid:          10,500 GallonsTotal Fluid Volume:          78,504 Gallons______________________________________                         StageEvent         Sand    Fluid Vol.                         Vol.  Rake  PressureNo.   Fluid   (ppg)   (gal.)  (gal.)                               (BPM) (psi)______________________________________ 1    Acid    0       2000    2000  26    2650 2    Pad     0       3000    3000  25    2800 3    Frac    2       2747    3000  25    3000 4    Frac    3       2638    3000  25    3200 5    Acid    0        750     750  25    3500 6    Frac    5       1710    2100  25    3650 7    Frac    0       4100    4100  25    3700 8    Frac    2       5000    5460  25    3800 9    Frac    0       5850    5850  25    385010    Frac    2       5723    5723  24    385011    Acid    0        750     750  24    390012    Pad     0       2250    2250  NR    NR13    Acid    0        750     750  24    390014    Pad     0       1850    1850  24    370015    Acid    0        750     750  24    370016    Pad     0       2000    2000  24    340017    Acid    0       1000    1000  24    345018    Pad     0       3600    3600  24    340019    Frac    2       2747    3000  24    340020    Acid    0       1000    1000  24    350021    Frac    3       2638    3000  24    360022    Acid    0       1000    1000  24    365023    Frac    5       4071    5000  24    350024    Acid    0        500     500  24    350025    Frac    7       3790    5000  24    340026    Acid    0        500     500  23    340027    Frac    8       5129    7000  23    340028    Acid    0        500     500  23    340029    Frac    10      4807    7000  24.5  330030    Acid    0        500     500  24.5  340031    Frac    10      4354    6340  24.5  340032    Flush   0       2000    2000  24    3400______________________________________ NR = Data not recorded. 
    
     EXAMPLE 3 
     
         ______________________________________Formation Thickness:           15 FeetDepth:          2282-1197 FeetFracing Fluid:  Formation water, plus 30 pounds           per 1000 gallons of a hydroxyl           propyl guar gum gelling agent.Propping Agent: 100 mesh Oklahoma sand, 467,158           poundsAcid:           15% HCLCasing:         Open HoleAverage Pressure:           3300 psiAverage Injection Rule:           23 BPMNumber of Fracing Fluid           13Stages:Volume of Fracing Fluid:           76,450 Gallons(Less sand volume)Volume of Acid: 10,250 GallonsTotal Fluid Volume:           86,700 Gallons______________________________________                         StageEvent         Sand    Fluid Vol.                         Vol.  Rate  PressureNo.   Fluid   (ppg)   (Gal.)  (Gal.)                               (BPM) (psi)______________________________________ 1    Acid    0       2000    2000  25.8  3600 2    Pad     0       3000    3000  25.8  1900 3    Frac    2       2749    3000  25.8  2600 4    Frac    3       2639    3000  25.8  2100 5    Acid    0        750     750  25.8  2400 6    Frac    5       4071    5000  25.8  3600 7    Acid    0        750     750  25    3600 8    Frac    7       3790    5000  25    3600 9    Acid    0       1000    1000  24.8  380010    Frac    8       5129    7000  25    420011    Acid    0         750    750  24    450012    Pad     0       1500    1500  20    460013    Acid    0       1000    1000  21    380014    Pad     0       1500    1500  21    360015    Frac    2       2749    3000  21    340016    Acid    0        750     750  23    360017    Frac    3       2639    3000  23    360018    Acid    0        750     750  23    355019    Frac    4       3383    4000  22.5  365020    Acid    0        750     750  23    370021    Frac    5       4071    5000  23    310022    Acid    0        750     750  23    300023    Frac    7       5306    7000  23    290024    Acid    0        350     350  23    290025    Frac    8       5129    7000  23    297526    Acid    0        350     350  23    300027    Frac    10      4807    7000  23    310028    Acid    0        300     300  23    320029    Frac    10      19228   28000 23.5  300030    Flush   0       3000    3000  23.5  2950______________________________________ 
    
     In Example 1, fracturing of a coal seam proceeds in a manner which is typical according to the invention. A large volume of acid is used in step 1 to initially treat the coal seam and is followed by a pad of proppant-free fracing fluid. In the initial proppant injection stage, fracing fluid to which sand has been added at the rate of 2 pounds per gallon of fracing fluid was injected into the formation, followed by a 3 pound per gallon stage which in turn was followed by an acid stage. Thereafter, the sand loading and/or volume of the fracing fluid injection stage was increased in each stage until a stand loading of 10 pounds per gallon and a volume of 7000 gallons was reached in step 12. The subsequent fracing fluid injection stages were continued at this sand loading and volume until a sufficient amount of sand was deposited on the formation. Following the terminal fracing fluid injection stage, the well was flushed with a volume of sand-free fracing fluid. 
     The well had negligible production before fracing and produced gas at 320 MSCFD thereafter. With continued production, the gas rate increased as water was removed. The majority of coal seams fractured thus far have been adequately fractured according to the invention with minor variations from Example 1 with similar results. 
     In Examples 2 and 3, some difficulty was experienced in that the fractures began to sand out as indicated by the pressure increases at steps 11 and 10, respectively. The sand out was eliminated by alternating acid and pad injections until a pressure reduction was observed, indicating that the fractures were propagating. When propagation of the fractures was observed, the alternating stagewise injection of acid and fracturing fluid was reinitiated at the lower proppant loading and stage volumes. In subsequent stages the volume and proppant loading was incrementally increased according to the invention. Prior to treatment the well of Example 2 had negligible production. After fracturing this well began production at 360 MSCFD. The well of Example 3 had negligible production prior to fracturing. Since treatment the well of Example 3 has not yet been placed on line, so after fracturing production figures are not yet available. 
     It is important in sand out situations that the pressure not be allowed to increase excessively (above about 4500 psi for the particular formations treated in the examples) because of the danger of fracturing the underlying or overlying non-producing formations. It is also important to immediately take preventative measures whenever sand out is threatened because of the danger of sanding out the well and having to abort the fracturing operation. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated process may be made without departing from the spirit of the invention.