Abstract:
A method for stimulating or fraccing an oil or a natural gas well by adding a liquefied hydrocarbon such as liquid petroleum gas and liquefied natural gas, a proppant and a diluent such as carbon dioxide, nitrogen or mixtures thereof to the well. The order of addition is typically liquefied hydrocarbon then diluent but this order can be reversed and in other circumstances the liquefied hydrocarbon and diluent can be mixed together and fed to the well as a mixture.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from provisional patent application Ser. No. 61/776,956 filed Mar. 12, 2013. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In the production of natural gas from shale or other “tight-gas” formations, hydraulic fracturing (or “frac” or “fraccing”) is used to break up the rock around the well bore and reduce the resistance to gas flow. The frac technique generally requires injecting into the well large amounts of fluids that are compressible like nitrogen or carbon dioxide or incompressible such as water or liquefied petroleum gas. The fluids are pumped to high pressure to create large compressive forces around the well bore. These forces break the rock and create tiny fissures for gas flow. 
         [0003]    The stimulation of natural gas containing formations, such as shale gas has been the subject of intensive study. Several methods of fracturing these deposits are known and incompressible fluids such as water with other chemicals and solids; e.g., mineral acids and proppants are employed on the order of 1 to 2% by volume to the injected fluid. The use of water results in the generation of a large amount of waste that is laden with chemicals and creates an expensive disposal issue. Thousands of tons of fluid may be injected during each frac job and much of this fluid is returned to the surface when the flow is reversed (hereafter called “produced fluids”) and natural gas is produced from the well. Alternatively, high pressure frac fluid sources including carbon dioxide and nitrogen help overcome the liquid waste disposal cost associated with hydraulic fracturing. However, the frac gas generated upon drilling has a high initial content of carbon dioxide and nitrogen. This mixture can be difficult and expensive to separate and inevitably leads to natural gas losses and even venting of the initial diluted natural gas to the atmosphere because it cannot be readily flared. 
         [0004]    Recently GASFRAC has tested the use of liquid petroleum gas (LPG) which has high propane content and lowers the amount of frac fluid necessary as well as being easier to separate from the gas that results from drilling. The LPG is readily recoverable and can be reused at later stages in the overall process. However, injection of LPG at high pressures carries some risk involving explosions. The use of other liquid hydrocarbons such as gelled hydrocarbons is also possible and would facilitate the complete use of the resulting well gas; however flammability concerns could limit its use. 
         [0005]    Hydraulic fracturing is used to produce gas and oil from low permeability formations, such as for example, unconventional natural gas reservoirs. Most fracturing treatments use water and polymers with a gelling agent as a fracturing fluid. 
         [0006]    A hydraulic fracturing process can be energized by the addition of a compressible, sometimes soluble, gas phase into the treatment fluid. When the well is produced, the energized fluid expands and gas comes out of solution. Energizing the fluid creates high gas saturation in the invaded zone, thereby facilitating gas flowback. 
         [0007]    During a flowback period, a mixture of gas is used as the energized solution (e.g., N 2  and/or CO 2 ) and natural gas comes out. Before selling the natural gas to a pipeline operator, the concentration of energizing gas needs to be dropped to acceptable levels dictated by the pipeline operator (usually 2 to 3%). Although the concentration of energizing gas decreases quite rapidly in the beginning of the process, achieving the required levels may take considerable time (up to 30 to 45 days). 
         [0008]    In order to address the issue of environmental pollution from flaring during a flowback period, various techniques have to be employed to facilitate natural gas clean-up by separating it from the energizing gas. Such techniques may include N 2  and/or CO 2  separation by using membrane and/or adsorbent (PSA/VSA) technologies, which involves the deployment and operation of relevant equipment, which in turn adds cost to the natural gas production process. 
         [0009]    Alternative technologies for shale gas development utilize, for example, liquefied petroleum gas (LPG) in place of conventional fracturing fluids, and specifically, in place of high volume, high pressure slick water-based fracturing fluids. The unique properties of the LPG fracturing process result in significant savings on material expenses, increased well productivity and fracture as well as flow-back mixture clean up. 
         [0010]    The gelled LPG used in the fracturing process has the ability to generate the necessary fracture system, carry the proppant through the wellbore and place into the oil and gas reservoir being stimulated. The LPG used in the process is highly soluble in well formation hydrocarbons. As a result, the LPG process results in less damage to formations than conventional hydraulic fracturing processes. Unlike conventional treatments where as much as 50% of the carrier remains in the reservoir and hinders well performance, virtually 100% of the LPG can be recovered, The obvious advantage of LPG fracturing is that gelled propane would replace the use of water, thereby reducing the amount of fresh water used and the associated environmental concerns. In addition, propane that is injected into the formation can be recovered and reused, therefore eliminating the need to treat or dispose of large volumes of wastewater that may have high concentrations of naturally occurring salts, metals, radionuclides and other constituents commonly found in shale reservoirs. 
         [0011]    Additionally the injection of a hydrocarbon into the shale creates less “damage” due to “swelling” as compared to water, which may impede hydrocarbon flow; therefore LPG fracturing has the potential to increase well production. While there are a variety of potential benefits to using LPG fracturing for shale energy development, there may be some potential disadvantages, such as increased costs for conducting the fracturing treatment, increased explosion hazards, and limited capacity to utilize this technology on a wide commercial basis. One advantage water-based fracturing technologies have though is that water is virtually incompressible, therefore the pressure is transferred more directly to fracture the shale more effectively, whereas LPG may require more surface pressure to exert the necessary downhole pressure. 
         [0012]    LPG fracturing is a promising technology that may become more common as advancements in its use occur and has the potential to reduce water use and increase well yields. 
         [0013]    The proposed invention offers safer and a potentially more economical solution to conventional and/or energized hydraulic fracturing by utilizing a combination of high-efficiency fracturing fluids (e.g., water, LPG, etc.) with environmentally safe and inert gases, like nitrogen. 
         [0014]    These methods improve natural gas and natural gas liquids recovery from fraccing operations; reduced flammability of the overall combination makes for a safer operation; reduced use of diluents such as carbon dioxide or nitrogen allows for better natural gas recovery; cost reduction improvements over the use of liquid petroleum gas by itself; and augmentation of the hydrocarbon pressure by using nitrogen or carbon dioxide so that high pressure equipment does not need to be employed. 
       SUMMARY OF THE INVENTION 
       [0015]    A method for stimulating an oil or a natural gas well comprising adding to the well a combination of a liquefied hydrocarbon selected from the group consisting of liquid petroleum gas and liquefied natural gas, a proppant and a diluent selected from the group consisting of carbon dioxide, nitrogen and a mixture of carbon dioxide and nitrogen. 
         [0016]    The stimulating of the oil or natural gas well will lead to a fracturing of the fissures in the well thereby leading to an increased yield of oil or natural gas. 
         [0017]    The invention may also be employed in fraccing operations of shale gas formations. These methods allow for the recovery of natural gas and natural gas liquids from frac gas. The well can also be a horizontal well. 
         [0018]    The methods of the invention will provide for the recovery of natural gas and natural gas liquids from the oil or natural gas well. 
         [0019]    In the methods of the invention, the combination is added to the well sequentially. The liquefied hydrocarbons are typically added first followed by addition of the diluent to the well. Sufficient time is typically allowed for between feeding the liquefied hydrocarbon to the well before feeding the diluent to the well. The liquefied hydrocarbons are fed to the well at a first pressure and their pressure is increased by the addition of the diluent at a higher pressure. In preferred embodiments, the liquefied hydrocarbon is in gel form. 
         [0020]    In an alternative embodiment, the diluent is fed to the well before the liquefied hydrocarbon. 
         [0021]    Water may be fed to the well along with the liquefied hydrocarbons in certain circumstances. In one embodiment, the water and liquefied petroleum gas is first added to the well, followed by natural gas and then followed by a feed stream of nitrogen. 
         [0022]    In a further alternative embodiment, the liquefied hydrocarbon and the diluent are not fed to the well sequentially but are mixed together before being fed to the well simultaneously. 
         [0023]    The liquefied hydrocarbons, whether liquefied petroleum gas or liquefied natural gas are typically fed to the well at higher pressures in the range of 5000 to 10,000 prig (345 to 690 bar). 
         [0024]    The proppant when present is selected from the group consisting of silica sand, resin-coated sand and man-made ceramics. 
         [0025]    The use of the diluents alters the flammability of the hydrocarbon combination and will allow for easier separation from the well gas that is derived from drilling operations as described in  FIGS. 1 ,  2  and  3 . Carbon dioxide is preferably used because of its higher density, higher heat capacity and easier separation from methane and other hydrocarbons. 
         [0026]    Liquefied hydrocarbons can be effectively used in the stimulation of gas wells. Liquid petroleum gas which has higher propane content can be employed advantageously because of its higher density and higher temperature. Rich liquefied natural gas which has a higher ethane and C3+ content and which is more readily available and economical being derived from peak shavers will remain a liquid at high pressures even at elevated temperatures as noted in  FIG. 4 . 
         [0027]    The proppants are typically selected from the group consisting of silica sand, resin-coated sand and man-made ceramics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a graph showing the effect of carbon dioxide concentration on methane flammability. 
           [0029]      FIG. 2  is a graph showing the effect of carbon dioxide feed concentration versus ignition response. 
           [0030]      FIG. 3  is a graph showing carbon dioxide feed concentration versus flame propagation. 
           [0031]      FIG. 4  is a chart of vapor liquid equilibriums at various temperatures and pressures for rich liquefied natural gas. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    The invention is applicable to horizontal wells utilized in unconventional gas production and consists in the gradual introduction of the fracturing fluid of choice into the well under moderate pressure, followed by introduction of the inert gas under high pressure, sufficient to increase the overall pressure of the fracturing fluid to required levels. 
         [0033]    The fluid introduced first could be water or a liquid hydrocarbon, which are easy to separate. This step is followed by the addition of a high pressure energized fluid like N 2  and/or CO 2 , or natural gas, or even LPG. It is also possible to have multiple fracturing fluid combinations, for example: water/LPG, followed by natural gas, followed by N 2 . The top fluid serves to boost the pressure and ideally to provide a non-flammable blanket. By changing the N 2  and/or CO 2  concentration profile in the flowback mixture from exponential decay to a more rapid, near-square shaped decay, this will allow for easier separation of the fracturing fluid from the recovered hydrocarbons. It should be noted that some diffusion/mixing of the fluids is inevitable, which would prevent a user from obtaining a perfect square-shaped drop in N 2  and/or CO 2  concentration profile. The methods of the present invention will yield beneficial results. The invention will improve flowback fluid cleanup. A better recovery of fracturing fluids can be achieved. The inventive methods are also advantageous in water sensitive formations that suffer from water saturation and clay swelling issues. The inventive methods will work to enhance production of oil in low-permeability and low porosity well formations as well as shallow formation and those nearly depleted gas reservoirs. Additionally, in those geographical regions where water shortages are prevalent or those with stricter water regulations, the present invention will provide for enhanced gas or oil recovery without using more water as well as limiting the amount of chemical employed. 
         [0034]    The reduced water consumption is beneficial when water is the main fracturing fluid due to its replacement with inert gas (e.g., nitrogen) in the vertical portion of the well. 
         [0035]    The fracturing operation can be safer even when using more risky but more formation friendly fracturing fluids (e.g., LPG) due to the compression taking place below the ground under a blanket of a more environmentally safe inert gas such as nitrogen. This may also result in partial or almost complete elimination of gelling agents, which otherwise must be added to LPG to reduce the chance of explosion. 
         [0036]    The invention further reduces the flowback period and subsequent elimination of the need for natural gas clean-up due to reduced content of CO 2  and/or N 2  in the flowback mixture. 
         [0037]    This latter benefit can be appreciated because the nitrogen for example is only present in the portion of the well and is not used in actually fracturing the rock formation. Although some mixing and diffusion is unavoidable, the nitrogen is not mixed with the natural gas when used as the energized solution. This means that once the fracturing is complete and the flowback is started, nitrogen gas will flow out first, relatively quickly and at a relatively constant concentration as opposed to traditional flowback scenario. 
         [0038]    The invention is further explained by way of non-limiting examples, provided below. 
         [0039]    Using homogeneous energized fluid in the entire wellbore (baseline—longest flowback period). 
         [0040]    In this case, a fracturing fluid containing energizing component(s), CO 2  and/or N 2 , is prepared at the surface and pumped down hole at a selected pressure that exceeds the reservoir pressure. In the typical fracturing application, the first and the last step of the process are done without proppant (first is to pump-in the fracturing fluid without the proppant (pad stage) to create fractures, and the last is to clean-up remaining proppant from the wellbore (flush stage). During intermediate stages, the proppant is introduced into the fracturing fluid and its load is gradually increased. This results in quite a lengthy flowback period, required to bring the concentration of the energizing components down to the required level which is usually 2 to 3%. 
         [0041]    Gradual introduction of the fracturing fluid components (shortest flowback period). 
         [0042]    In this case, the components of the fracturing fluid are introduced gradually in stages. The first component (water or liquid hydrocarbons) is followed by a high pressure energized fluid like N 2  and/or CO 2  serving to boost the pressure and ideally provide a non-flammable blanket. In this case, during flowback, N 2  and/or CO 2  will flow out first and their concentration in the flowback mixture will decrease much more rapidly as schematically depicted below. It should be noted that some diffusion/mixing of the fluids is inevitable, which would prevent an operator from obtaining a perfect square-shaped drop in N 2  and/or CO 2  concentration profile. 
         [0043]    Using a mixture of hydrocarbon(s) and energizing components to pump down hole (intermediate flowback period). 
         [0044]    In this case, by using the energizing or diluent component (N 2  and/or CO 2 ), it becomes possible to safely pump the potentially flammable hydrocarbons (for example, natural gas) down hole at required pressures due to diluting them below their flammability limits. Although this will not result in the shortest flowback period, it does still reduce the concentration of the energizing component (N 2  and/or CO 2 ) and therefore, minimizes the cleanup effort. 
         [0045]    In general during the practice of the invention, the liquefied hydrocarbon mixture is pumped up to pressures of about 5000 to 10,000 psig before their subsequent use for fraccing. In the methods of the invention, gelled liquid petroleum gas or gelled liquefied natural gas are the preferred hydrocarbons and can be employed individually or as a mixture of the two. Proppants such as silica sand, resin-coated sand and man-made ceramics are added to the hydrocarbon mixture and will act in the oil or gas well formation to keep fissures open and enhance gas recovery. A high pressure intermediate gas which is typically carbon dioxide or nitrogen can be added either simultaneously with the pressurized hydrocarbon or separately and will serve as a blanket. The resulting frac mixture becomes less flammable as a result. 
         [0046]    Moreover, in a different embodiment, the liquefied hydrocarbon can be pumped at a lower pressure for fraccing and its pressure can be subsequently boosted with the addition of higher pressure carbon dioxide or nitrogen. This pressure boosting by the carbon dioxide and/or nitrogen diluent will provide benefits of both using the liquefied natural gas or liquid petroleum gas in gel form with the properties of the diluent in fraccing operations while minimizing the risks associated with the high pressure pumping of hydrocarbons. 
         [0047]    Carbon dioxide or nitrogen when used alone in well stimulation and fraccing operations can be effective. However, separation of these components after a few days time after the well has been drilled becomes more difficult. For example, carbon dioxide fraccing may yield an initial gas content that is lean in methane which can only be enriched via cleanup once the carbon dioxide content is lowered to 50%. Nitrogen fraccing faces the same limitation. The use of hydrocarbons with either of carbon dioxide or nitrogen facilitates natural gas recovery by limiting the initial amount of carbon dioxide or nitrogen introduced into the well to a more manageable level. 
         [0048]    While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.