Abstract:
Oils present in oil reservoirs can be upgraded by high temperature cracking through the injection of oxygen into the reservoir and combusting the oils to generate heat. By employing injection wells and production wells, the oxygen may be placed into the reservoir increasing the flux of oxygen present as well as the temperature and cracking severity needed to produce upgraded oil.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the injection of oxygen or oxygen-enriched air into an oil reservoir causing in-situ combustion to occur and the oil to crack into lighter fractions.  
         BACKGROUND OF THE INVENTION  
         [0002]    In conventional in-situ combustion (ISC) processes, also called fire flooding or heavy oil air injection (HOAI), vertical wells are used for injection of air and typically water for the production of oil. The distance between the wells is often substantial and oil and water vaporized by the combustion and upgrading process condense in the cooler parts of the reservoir, travel through the heavy oil and are produced via well techniques. Due to the highly viscous oils through which these lighter fluids must travel, it may be difficult to maintain production and pressure may build on the injection side. This may be one reason for failure of field applications of such technology in the past. A short distance process may be utilized in which a vertical or horizontal injection well and a horizontal producer well are used so that displacement of oil can be achieved along the horizontal producer well. A combustion front propagates through the reservoir above the horizontal well allowing good communication of the upgraded oil and the production well. One example of such a process is the Toe to Heel Air Injection (THAI) process. A catalyst may be placed in the producer well to obtain further upgrading of the oil, as in the CAPRI process.  
           [0003]    The Combustion Override Split-production Horizontal well (COSH) process also uses air injected into the reservoir to generate steam and heat in-situ. A well arrangement is used to segregate and control fluid flows and thereby reduce early oxygen breakthrough as well as sanding and gas locking of downhole pumps. The well arrangement makes use of an air injection well, gas producer well which removes excess nitrogen and other gases from near the top of the pay zone, and horizontal well to recover oil from a lower portion of the pay zone. The COSH process has an advantage over the THAI and CAPRI processes in that problems which arise from the handling of nitrogen and other gases are reduced.  
           [0004]    Other examples of short distance displacement processes include Steam Assisted Gravity Drainage (SAGD) and Vapor Extraction (VAPEX). In the SAGD process, steam enters through a horizontal injection well and travels a relatively short distance to a horizontal production well. The heating of heavy viscous oils between these wells allows the oils to flow to the production well. The VAPEX process is similar to SAGD but hydrocarbon vapor is used instead of steam. Asphaltene precipitation is caused by the mixing of solvent and oil and provides for an in-situ upgrading of the oil.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides for a method for cracking oil in an underground oil reservoir comprising injecting oxygen into the oil reservoir and igniting the oil therein. The combustion associated with the high influx of oxygen will generate high temperatures which will cause the oil to crack into lighter fractions to form coke or carbonaceous solids from the heaviest compounds in the oil such as asphaltenes.  
           [0006]    The present invention also provides for injecting oxygen into the oil reservoir such that in-situ combustion can take place for recovering the oil through one or more production wells. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0007]    The present invention provides for an in-situ combustion process for recovering oil from an underground oil containing reservoir. The process comprises injecting oxygen into the oil reservoir. This oxygen will react to combust the oil which causes heat generation. The resulting high temperatures will cause the oil to crack into lighter fractions. The invention is most applicable for heavy oil or tar sands. By injecting oxygen and igniting the oil in the presence of the oxygen, high temperatures greater than 400° C. and preferably greater than 500° C. cause the oil to crack to form lighter more valuable products, as well as coke or carbonaceous solids. In addition, higher temperatures are desirable to facilitate the formation of CO 2 . Less desirable oxygen containing hydrocarbons may be formed at lower temperatures.  
         [0008]    For purposes of the present invention, oxygen can mean pure 100% oxygen gas, but it can also include oxygen-enriched air which contains oxygen in an amount greater than 25%. Purification of the oxygen allows for a significantly higher flux of oxygen to be placed into the well due to the reduction of associated nitrogen which would be present in air. This aids in increasing the temperature which increases the cracking severity but also provides for reducing the need to handle nitrogen in the gases at the production well.  
         [0009]    In one embodiment of the present invention, a horizontal producer well and vertical injection well is employed. The horizontal producer well will result in a short distance displacement process whereby oil and water vaporized by the intense heat of the oil combustion front can travel freely to the horizontal producer well. This will allow the large flux of oxygen into the reservoir to continue unhampered by upstream blockages. Injectivity of oxygen-containing gas is increased by the combustion of carbonaceous materials near the injection well. In a second embodiment, two sets of horizontal wells may be employed whereby oxygen is injected in one set of horizontal wells and oil is produced from a lower perpendicular set of horizontal wells. The oxygen, when injected into the reservoir, may be ignited by an electronic device or other form of heat, such as steam, which will increase local temperature in the reservoir  
         [0010]    Water may also be injected with the oxygen once the combustion zone has been established. The steam generated in this manner is an efficient means to transfer heat to the oil. The cracked oils will be of higher quality in that they are relatively light, virtually free of metals and have a lower sulfur content than the untreated oils. The temperatures greater than 400° C. by which the oils are cracked will also improve carbon dioxide production. The carbon dioxide is known to reduce viscosity and interfacial tension in the oil as well as to cause swelling of the oil in order to enhance production. The use of oxygen necessarily means that less nitrogen or no nitrogen is present in the reservoir such that a much higher flux of oxygen is provided and consequently higher temperatures and higher concentrations of carbon dioxide are present.  
         [0011]    One means of obtaining the oxygen employed in the present invention is from an air separation plant which can be on-site or very close to the actual production wells. In preferred embodiments of the present invention, the well arrangement having a first vertical gas injection well located near the top of the oil bearing portion of the reservoir and a horizontal oil production well located near the bottom of the reservoir. The oxygen would be injected into the first well, combusted and drive the cracked oils into the horizontal oil production well where the oil can be recovered by conventional means. In another preferred embodiment, several horizontal gas injection wells running substantially parallel to each other are located near the top of the oil bearing portion of the reservoir and several oil producing wells also horizontal and substantially parallel to each other are situated perpendicular to the gas injection wells. This provides additional advantages in that the gas injection wells could be used one at a time to upgrade the reservoir oils in the vicinity of that particular gas injection well. The injection and production wells may be formed of any material that is commonly employed in the oil production industry. One example would be a perforated stainless steel tubing of dimensions sufficient to deliver oil from the reservoir.  
         [0012]    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 of 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.