Abstract:
A method of fracturing a rock formation is provided including placing through a wellbore penetrating the rock formation propellants in a cavity located at a radial distance from the wellbore and igniting the propellants to cause a pressure sufficient to fracture the formation.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method of treating subterranean reservoirs particularly hydrocarbon reservoirs. More specifically, the invention pertains to methods of increasing the exposed surface of such reservoirs, particularly for the purpose of enhancing recovery of hydrocarbon. 
       BACKGROUND 
       [0002]    It has long been recognized that in order to increase recovery from a hydrocarbon reservoir, it is beneficial to increase the exposure of the reservoir to the well or wells drilled through it. This recognition led to methods such as perforating, fracturing and acidizing. 
         [0003]    Whilst many of those methods are not considered to be relevant for the present invention, it is worth noting that propellants have been used as a substitute for hydraulic fracturing. In conventional hydraulic fracturing a fluid is pressurized from the surface to generate a pressure sufficiently high to generate fractures in the subterranean formation below. In some instances, particularly where the economics were not favorable for the deployment of heavy pumping equipment, propellants have been used. Lowered into the borehole, the propellants when ignited with the correct pressure built-up create the conditions for fracturing the reservoir rock surrounding the well. Likewise propellants have been used to assist as secondary means other explosives or fluids in the fracturing process. 
         [0004]    Such known use of propellants is described for example in the co-owned U.S. Pat. No. 5,355,802 issued to Petitjean, the U.S. Pat. No. 5,295,545 to Passamaneck and the more recent U.S. Pat. No. 7,073,589 to Tiernan and Passamaneck, as well as the patents referenced in these patents. 
         [0005]    As hydrocarbon fields are growing more mature, it has also been found that these established methods are no longer sufficient to exploit a reservoir to the extent theoretically possible. In response to this challenge a plethora of new methods have been proposed to increase recovery beyond that afforded by established methods. These methods are generally referred to as “Enhanced Oil Recovery” or EOR methods. 
         [0006]    It is therefore an object of the present invention to provide novel EOR methods. Ideally the new methods are suitable for all reservoirs but in particular for carbonate rocks. 
       SUMMARY OF INVENTION 
       [0007]    According to a first aspect of the invention, a method of fracturing a rock formation is provided including placing through a wellbore penetrating said rock formation propellants into a cavity located at a radial distance from said wellbore and igniting the propellants to cause a pressure sufficient to fracture said formation. 
         [0008]    According to a second aspect of the invention, a method of enhancing access to a subterranean rock formation is provided including placing through a wellbore penetrating said rock formation propellants into a cavity located at a radial distance from said wellbore and igniting the propellants to cause a pressure sufficient to fracture said formation, thereby creating more cavities for an iterative placement and ignition of further propellant or other fracturing methods. 
         [0009]    Yet another aspect of the invention relates to the beneficial effects gained by applying the above methods to hydrocarbon bearing reservoirs. With the increased access afforded by these methods many known EOR methods can be applied with higher efficiency leading to improved recovery of hydrocarbons from reservoirs. In a preferred embodiment, such improved EOR methods include the use of heated fluids such as steam pumped through the network of natural fractures as found in many, mostly carbonate, rocks. Access and range of such network increases by making use of the fractures created by the propellants in accordance with the methods of this invention. 
         [0010]    According to these aspects of the invention, the rock formation which is preferably a carbonate rock with a recoverable hydrocarbon fluid content is fractured or even rubblized at locations away from the main well. As result of applying methods in accordance with the invention, the rock surface accessible through macroscopic flow channels such as fractures is increased. The increase in accessible formation can be exploited to increase the amount of fluids drained or produced from or alternatively, expose more rock surface to treatment fluids. 
         [0011]    A well in accordance with the present invention is defined as a drilled hole designed to allow access of standard well tools such as tubing or wireline conveyed instruments or completion and production equipment. The cavities as defined herein are not wide enough to allow for such access. Instead, the creation and/or access to the cavities requires specialized tools of comparatively small diameter, such as a wireline or tubing conveyed lateral drilling tools. Alternatively the cavities may be generated on the force or flow of pressurized fluids or prior ignition of propellants. 
         [0012]    Hence the cavities in accordance with the present invention have a maximum effective diameter of 13 cm [4 inches] or even only 7 cm [2 inches] or less. The effective diameter is defined as a cross-section of a however irregularly shaped opening which is sufficiently wide to allow passage of a cylindrical object of such diameter. 
         [0013]    The cavity or cavities for the propellant can be any opening at a radial distance from the well. The cavity can be either naturally occurring or artificially created. Cavities comprise fissures, fractures, channels or boreholes. To increase the precision of placement and the overall control of the process, it is a preferred variant of the invention to use microboreholes as cavity. 
         [0014]    Such microboreholes are known per se for the purpose of extracting core samples from or positioning sensors into a reservoir. Apparatus for drilling microboreholes and known applications of microboreholes are described for example in the U.S. Pat. No. 4,226,288 to Collins, the co-owned U.S. Pat. No. 5,692,565 to MacDougall et al., U.S. Pat. No. 6,896,074 to Cook et al. and U.S. Pat. No. 7,191,831 to Reid et al. 
         [0015]    A propellant is a source of both energy and working fluid. Typically it can be further distinguished from explosives by the rise time of the pressure build-up after ignition. This rise time is in the order of 0 to 0.4 ms for explosives and in the order of 0.4 ms to 1 ms or even 5 ms for propellants. The pressure rise time for hydraulic fracturing is at least an order of magnitude longer. 
         [0016]    Preferred propellants for the present application are solid propellants mixed with oxidizers such as ammonium perchlorate. The commercially available series of Arcite® propellants widely used as fuel to inflate airbags and in some of the known downhole applications of propellants is seen as a particularly safe and suitable products for use in the present invention. 
         [0017]    These and other aspects of the invention are described in greater detail below making reference to the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0018]      FIG. 1  is a flow diagram illustrating steps in accordance with an example of the present invention; 
           [0019]      FIG. 2  shows the preparation of a microborehole for use in accordance with an example of the present invention; 
           [0020]      FIG. 3A  shows a microborehole loaded with propellants in accordance with an example of the present invention; 
           [0021]      FIG. 3B  illustrates the effect of igniting the propellant on the formation; and 
           [0022]      FIG. 4  illustrates an improved EOR operation in accordance with an example of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The following example of a method in accordance with the present invention is illustrated using the block diagram of  FIG. 1  and the drawings of  FIGS. 2-4 . 
         [0024]    In the example it is assumed that propellants are to be deposited into a newly drilled microborehole (Step  11  of  FIG. 1 ). This step is illustrated in  FIG. 2 . This figure shows a main well  21  in a carbonate rock formation  20 . The main well  21  is used to access the desired depth in the reservoir  20  with a wireline suspended drilling unit  22 . The drilling unit is suspended from a wireline surface unit  23  through a well head  24  located at the top end of the well  21 . 
         [0025]    At the desired depth, the wireline suspended drilling unit  22  is deflected by means of a temporary packer  25  and a deflection vane  26  into the formation to drill a microborehole  27 . 
         [0026]    This microborehole  27  is drilled to the target location within the formation  20 , at which stage the drilling unit  22  is withdrawn and a propellant depositing unit  31  is lowered into the drilled microborehole. This step  12  of  FIG. 1  is illustrated in  FIG. 3A . The depositing step leaves a propellant cartridge  32  unit in the microborehole  27 . A detonator line  33  connects the propellant cartridge  32  with the depositing unit and hence with the surface. As an alternative to the detonator line  33 , the propellant may be ignited using delayed ignition energy release mechanism co-placed with the propellant. 
         [0027]    A suitable propellant is a mixture of ammonium perchlorate as the oxidizer and Actite 386 M as the fuel. Alternatively, a combination of potassium perchlorate and Arcite 497 L can be used. However it should be understood that numerous other oxidizer/fuel combination are also applicable. 
         [0028]    The cartridge with the propellant is then ignited (Step  13  of  FIG. 1 ). The ignition releases a pressure pulse with a rise time of more than 0.4 ms. The pressure pulse fractures the surrounding formation as shown in  FIG. 3B . This figures shows the elements of  FIG. 3A  after the ignition of the propellant cartridge  32 . 
         [0029]    The steps of  FIG. 1  as described above can be repeated re-using for example the drilled microborehole, drilling further microboreholes or using a cascading set of microboreholes. 
         [0030]    In  FIG. 4 , the treatment of the reservoir as described above is shown to have created a network  40  of partly connected or intersecting fractures. This network can be exploited to improve EOR methods as shown. The example of  FIG. 4  illustrates a Thermally Assisted Gas-Oil Gravity Drainage (TA-GOGD) similar to the recovery process as implemented by Shell/PDO in Oman Qarn Alam field. A steam injector well  41  is drilled to the depth of the network  40  of fractures. 
         [0031]    To produce from the reservoir  20 , steam is injected via the injector well  41  through the network  40  of fractures into the reservoir  20 . The heat increases the temperature and hence decreases the viscosity of the oil trapped in the reservoir rock. As the steam is distributed through the network  40  of fractures, a greater volume of the reservoir  20  is exposed compared to conventional applications of TA-GOGD. Thus a greater volume of oil can be drained and pumped to the surface.