Abstract:
Water is removed from passages in a subterranean hydrocarbon formation by feeding into the passages a composition comprising 5 to 70 wt. % of alkanol(s) having 1 to 3 carbon atoms and 30 to 95 wt. % dense phase carbon dioxide, under a pressure greater than zero up to 25,000 psig and at a temperature of 0° F. to 200° F., allowing the feed composition to remain in the well, whereby the feed composition solubilizes with said water in said passages, and then removing from said well a liquid product composition comprising said solubilized water and alkanol.

Description:
[0001]    This application claims priority from U.S. Provisional Application Ser. No. 60/872,724, filed Dec. 4, 2006, the entire content of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to treating oil and gas production wells to improve their operational capacity. 
       BACKGROUND OF THE INVENTION 
       [0003]    Production rates from wells in formations that contain oil or gas can be increased by hydrofracturing, in which a fluid is applied at high pressure within the formation to induce fractures and fissures in the formation and thereby stimulate release of the desired hydrocarbons from the formation. The fluid typically contains a liquid vehicle and additional solid and/or liquid components that enhance the penetration of the hydrofracturing fluid into the areas of the formation that are to be hydrofractured, and enhance the effectiveness of the fracturing that is carried out. 
         [0004]    However, if excessive amounts of water have accumulated in the formation, especially in the passages of the formation into which the hydrofracturing fluid would be applied, then the desired hydrofracturing cannot be carried out as fully because the water blocks the hydrofracturing fluid from reaching as far into the passages of the formation as would be possible if the blocking water were not present. 
         [0005]    Thus, there remains a need for an effective method of removing induced and conic water from the areas of an oil or gas formation so as to improve the ability to stimulate production of hydrocarbons from the formation by hydrofracturing. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The present invention solves these and other problems, and provides numerous advantages described herein, in a method for dehydrating a subterranean oil-containing or gas-containing formation that contains water in passages in the formation, comprising 
         [0007]    (a) feeding into a well that extends to said passages in said formation a feed composition comprising 5 to 70 wt. % of an alkanol component selected from the group consisting of alkanols containing 1 to 3 carbon atoms and mixtures thereof, and 30 to 95 wt. % dense phase carbon dioxide, under a pressure from zero up to 25,000 psig and at a temperature of 0° F. to 200° F., 
         [0008]    (b) allowing the feed composition to remain in the well, whereby the feed composition solubilizes with said water in said passages, and then 
         [0009]    (c) removing from said well a liquid product composition comprising said solubilized water and alkanol. 
         [0010]    As used herein, “dense phase carbon dioxide” means carbon dioxide in the form it takes at a temperature 10° F. below its critical temperature or higher and at a pressure 100 psi below its critical pressure or higher. 
         [0011]    As used herein, a substance “solubilizes” or becomes “solubilized” when it forms a pumpable liquid with another one or more materials, whether by forming a solution or dispersion of the substance in the one or more other materials, or a solution or dispersion of the one or more other materials in the substance. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    The composition used in the present invention contains, at least, dense phase carbon dioxide and an alkanol component. The term “downhole composition” will be used in the following description to refer to the composition when it is ready to be fed down an oil or gas well, meaning that it contains all components that it will contain as it is fed down the well and that it is at the desired temperature and pressure at which it will be fed down the well. 
         [0013]    The preferred dense phase carbon dioxide is supercritical carbon dioxide, wherein the carbon dioxide is at a temperature above its critical temperature of 87.8° F. and above its critical pressure of 1070 psia. However, carbon dioxide that is at a temperature somewhat below its critical temperature and/or at a pressure somewhat below its critical pressure can be used effectively as well. 
         [0014]    The dense phase carbon dioxide comprises at least 30 wt. % of the downhole composition, and more preferably at least 50 wt. % of the downhole composition. Higher amounts, up to 90 wt. % or even up to 95 wt. % of the downhole composition, can be used with good results. 
         [0015]    The alkanol component comprises at least 5 wt. % of the downhole composition. The alkanol component can comprise 10 wt. % or more of the downhole composition, up to 50 wt. % or even up to 70 wt. %. Generally, higher alkanol concentrations are preferred with higher amounts of water creating pore throat blockages in the capillaries in the formation passages to be dehydrated by the method of the present invention. The preferred alkanol component is methanol. 
         [0016]    In one preferred embodiment of the present invention, the dense phase carbon dioxide and the alkanol component are the only substances present in the downhole composition. However, other compounds that are effective as solubilizers for the water in the formation can be present. It is preferred however that aromatic compounds, that is, compounds containing one or more than one benzene ring, should not be present in the downhole composition. Examples of such aromatic compounds include benzene, toluene, and xylenes. 
         [0017]    One preferred manner for preparing for and carrying out the present invention includes the following steps. The alkanol component and the carbon dioxide are combined in a vessel that can accommodate the pressures that are applied. This can be accomplished by, first, partially filling a suitable vessel with an amount of the alkanol component necessary to obtain the desired alkanol/carbon dioxide ratio. Then, carbon dioxide is fed into the vessel in an amount necessary to provide the desired ratio of the alkanol component to the carbon dioxide. The carbon dioxide can be provided from a conventional cylinder or larger storage/supply container which typically holds carbon dioxide under pressure. Any other components such as surfactant, additional solvent, and the like, are added to the vessel, or injected into the stream of carbon dioxide and alkanol while going downhole, in the desired amounts thereof. 
         [0018]    Next, the mixture of alkanol and carbon dioxide (and any other components present) is heated, for instance by pumping it through a heat exchanger, to achieve the desired temperature. Satisfactory temperatures are generally in the range of 70° F. to 170° F. 
         [0019]    The mixture is then injected downhole. Preferably a nearly constant pump pressure is maintained, varying flow as necessary. Pressure up to 10,000 psig and even up to 25,000 psig is applied. The mixture is held downhole, by not releasing the pressure on the well opening, for a period of time sufficient to allow the mixture to solubilize water in the formation passages. Suitable hold times range from several minutes up to 24 hours. One way to determine an effective length of time is to determine the pressure at the well opening when injection of the feed mixture is completed, and to permit the mixture to remain in the well until the pressure has decreased by a predetermined amount such as by 25%. The decrease in pressure indicates that the feed mixture has penetrated into the formation and solubilized water as desired. 
         [0020]    After the desired hold time, the pressure on the mixture is released, allowing the mixture of solubilized water and alkanol to flow out of the well. Water removed from the formation passages can be replaced by hydrofracturing fluid fed into the well. 
         [0021]    Preparing the downhole composition for injection into the well requires a high pressure pump having a minimum 5000 psig working pressure; and a heater capable of heating the composition to the preferred supercritical temperature. One suitable heater is a direct-fired heater. Another useful heater uses an indirect line heater using glycol as the heating medium and tubes rated to the maximum discharge of the pump at 300° F. Preferably the temperature of the downhole composition is set at a value within 10° F. of the temperature at the bottom of the well, if possible or at least mid hole temperature, to minimize the risk of imposing thermal shock on the piping at the bottom of the well. 
         [0022]    Injection of the carbon dioxide into the wellbore to control its pressure can be carried out in any of several ways, preferably accompanied by controlling its critical pressure for maximum heating results. The preferred way which in some cases, which is also more economical, is through use of tubing which is brought to the oil or gas well location coiled on a portable unit, rigged up next to the well, and uncoiled and fed into the wellbore, preferably to the full formation depth or short thereof. The heated composition is then injected into the coil tubing. The interior dimensions of the coil tubing would be sized for to allow the carbon dioxide to remain above its critical pressure allowing best heating control and maximum heat. A second way, whose adaptation would depend principally on economics, would be to set a packer using the wells production tubing and then injecting the mixture down the tubulars. 
         [0023]    After the composition has been held in the well for the desired period of time, the pressure being applied to the well at its opening is decreased or removed, whereupon a flowable, pumpable liquid composition is removed from the well. This removal can occur without external aid, relying on the internal pressures within the well itself or on remaining energy from the injected carbon dioxide, or can be aided by a suitable pump that withdraws the liquid from the well. If the well is allowed to flow back by itself the flow back should be controlled using a choke valve, increasing the flow back incrementally after the pressure begins to drop. 
         [0024]    The composition that is recovered from the oil or gas well is a liquid that comprises water and alkanol that had been a component of the downhole composition. 
         [0025]    The method of the present invention has been shown to be effective in removing undesired water from within the well, resulting in improved production rates and economic value for the treated well when the well is subsequently subjected to a given hydrofracturing treatment. 
         [0026]    A preferred aspect of the present invention is the ability to use polar and non-polar solvents, alcohol and carbon dioxide, in wells and formations that do not contain accumulations of hydrocarbon solids (by which is meant hydrocarbons, namely compounds composed predominantly of carbon and hydrogen, that are solid at the conditions existing in an oil or gas well or at the conditions existing at the earth&#39;s surface outside the well, and which typically comprise paraffins and asphaltenes). The absence of such solids means that alkanol and carbon dioxide fed into the formation are not consumed in solubilizing solid hydrocarbons, so that more of the mixture of alkanol and carbon dioxide is available for solubilization and removal of water from the formation. 
         [0027]    Another advantage is that the carbon dioxide is believed to establish an acidic pH in the composition that is fed downhole, and in the formation passages into which the composition passes, that reduces the tendency of clays present in the passages to swell. This reduced swelling would also enhance the ability of the hydrofracturing fluid to reach further into the formation passages where hydrofracturing is to occur. 
         [0028]    In addition, oil produced after treatment of an oil well by the method of the present invention has contained acceptable parts per million of residual alcohol levels. 
         [0029]    Another advantage is that the method of the present invention is carried out at the same well from which production of oil or gas will resume or will be enhanced, as distinguished from other technologies wherein compositions are injected into one or more injection wells some distance from the production well itself in the hope of increasing the flow of hydrocarbons out of the production well without putting anything down the production well.