Abstract:
A proppant for enhancing the recovery of petroleum from rock formations comprises pellets of sulphur and a binding and strengthening additive, preferably comprising cement. A process for manufacturing the proppant comprises first melting sulphur to form a liquid and forming a mixture of the liquid sulphur with cement powder. The mixture is then cooled and pelletized to form pellets of a sulphur and cement composite. The pellets are then dried and sorted by size.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to proppants used in fracturing processes for petroleum extraction. More specifically, the invention relates to proppants made principally from sulphur and to methods for producing such proppants.  
           [0003]    2. Description of the Prior Art  
           [0004]    In the field of petroleum extraction from subterranean reservoirs by means of drilled wells, it is often found that reservoirs having low fluid permeability result in wells that have low production rates. As a solution to this problem, hydraulic fracturing has become widely used as a method of enhancing production from petroleum bearing rock formations (reservoirs). Generally, fracturing involves pumping a viscous fluid, under high pressure, into the reservoir so as to form fractures in the rock. Such fractures serve to increase the permeability of the reservoir thereby leading to increased petroleum production rates.  
           [0005]    In the hydraulic fracturing process, it is known to utilize proppants that are mixed into the viscous fluid to form a slurry or suspension. Using high pressure hydraulic pumps, the slurry is pumped into the well, at a pressure sufficient to create fractures in the rock formation containing the petroleum. Once the fractures are created, the pressure is released and the proppant remains in the fractures and serves to maintain the fractures in an open state, or, in other words, to “prop” open the fractures. Proppants in use today are generally sand grains or high strength, spherical pellets that create a highly permeable pathway through which the petroleum can flow more readily.  
           [0006]    Known proppants include natural materials such as sand or synthetic materials such as glass, metal or ceramic pellets, usually close to spherical in shape. Such known proppants have associated problems. For example, although sand is quite inexpensive and available in large quantities, its function is impeded in situations where the reservoir is deep. In such cases, the tremendous pressures (for example 8000 psi) cause the sand to be crushed and agglomerate, thus sealing the fractures. In effect, the sand no longer serves its intended function. This problem is also encountered when using some synthetic proppants such as glass beads. Synthetic proppants made from ceramics are considerably stronger than sand and glass and are able to withstand very high pressures without crushing. However, ceramic proppants are considerably more expensive than sand and the like, thereby resulting in high production costs.  
           [0007]    U.S. Pat. No. 4,547,468 and RE 34,371 teach known synthetic proppants that have the deficiencies as indicated above. Further U.S. Pat. No. 6,364,018 teaches the use of crushed nut shells and wood particles as a proppant material.  
           [0008]    It is also commonly known to provide the proppant material with a surface coating comprising a resinous material. Such coatings serve to provide the proppant with a durable surface that is capable of withstanding the mechanical and chemical stresses that are associated with the fracturing process. Examples of such coatings are discussed in published Canadian patent application 2,198,812.  
           [0009]    In petroleum production, it is also known that sulphur is a common contaminant of the produced petroleum that must be extracted before the petroleum can be transported or used. Generally, petroleum production companies are forced to store the sulphur extracted from the produced petroleum due to the low demand for such product or otherwise dispose of such product. Storage of the extracted sulphur is complicated by the flammability of such product. Therefore, in either case, the presence of sulphur compounds in the produced petroleum proves to be an extra cost for petroleum production companies.  
           [0010]    It is an object of the present invention to provide a proppant that overcomes deficiencies known in the art.  
         SUMMARY OF THE INVENTION  
         [0011]    In one embodiment, the present invention provides a proppant for enhancing recovery of petroleum from rock formations comprising pellets of a mixture of sulphur and a first binding and strengthening additive.  
           [0012]    In another embodiment, the invention provides a process for forming a proppant for enhancing recovery of petroleum from rock formations, the proppant comprising pellets of a mixture of sulphur and a binding and strengthening additive, the process comprising:  
           [0013]    heating said sulphur to form a liquid;  
           [0014]    forming a mixture of liquid sulphur and the binding and strengthening additive;  
           [0015]    heating the mixture to maintain said mixture in a liquid state;  
           [0016]    cooling and pelletizing the mixture;  
           [0017]    retrieving and drying said pellets. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein:  
         [0019]    [0019]FIG. 1 is a schematic diagram of a manufacturing process for proppants according to an embodiment of the invention.  
         [0020]    [0020]FIGS. 2 a  and  2   b  are front and side view photographs of a proppant particle comprising sand.  
         [0021]    [0021]FIGS. 3 a  and  3   b  are front and side view photographs of a proppant particle comprising a ceramic material.  
         [0022]    [0022]FIGS. 4 a  and  4   b  are front and side view photographs of a proppant particle comprising a sulphur material of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    The present invention provides a proppant for a hydraulic fracturing process that comprises generally spherical pellets made from sulphur and cement as well as a method of forming such pellets. The sulphur used in forming the proppant can be derived from the sulphur extracted from the petroleum produced from nearby wells. In the usual case, such sulphur is generally disposed of with a high associated cost. Therefore, by utilizing such by-product to form the proppant, these disposal costs are avoided. In the preferred embodiment, the sulphur used in the present invention is obtained from petroleum during the petroleum refining process. In this manner, the raw material for the proppants of the invention is readily obtained and, in fact, is a by-product of the petroleum extraction process. In the preferred embodiment, the sulphur used in the process of the present invention is in a liquid or elemental form.  
         [0024]    It will be understood that the term “petroleum” as used herein includes oil, gas and any other hydrocarbon material that is extracted from subterranean accumulations.  
         [0025]    A preferred process for manufacturing the proppant pellets of the present invention involves first mixing the liquid sulphur with a cement powder. Preferably, the cement powder is sulphate resistant and is provided to achieve a proportion of 3 parts sulphur to 1 part cement powder (by weight) in the mixture. It will be understood that various other proportions of the sulphur and cement components are possible. The mixture is heated to about 118° C. to melt the sulphur and is agitated mechanically. This temperature corresponds to the melting point of sulphur at atmospheric pressure. Other temperatures to achieve this result will be apparent to persons skilled in the art depending upon the relevant environmental conditions. The mixture is then dripped into a cooling chamber containing cold water. In a preferred embodiment, the dripping is achieved by passing the mixture through a sieve plate or the like. Upon contact with the cold water, the sulphur/cement mixture is solidified into pellets. The pellets can then be separated by draining off the water. Following this, the pellets are passed through a tumble dryer where they are allowed to dry and cure under heat (for example 65° C.). The formed pellets are then subjected to a screening process to size the pellets.  
         [0026]    The cement used in the above process serves as a binder and strengthening agent. It will be understood that other binders and strengthening agents, such as silicone and the like, can also be used in the present invention.  
         [0027]    In the preferred embodiment, the pellets, thus formed, are then coated with an epoxy material such as Chem-Rez™.  
         [0028]    [0028]FIG. 1 illustrates, schematically, a system  10  for forming the pellets. As shown, the sulphur separated from the petroleum stream is placed in a tank  12 . Tank  12  includes an agitator  14  and is heated. A cement powder, which is preferably sulphate resistant, is then added to the tank. The cement powder (preferably type  50 , sulphate resistant cement) is contained in a holding tank or hopper  16  and is provided into the tank  12  through piping  18 . The mixture of the sulphur and cement powder is heated to about 118° C. while being agitated.  
         [0029]    In one embodiment other additives such as silicone  19  can also be added to the mixture.  
         [0030]    The heated mixture is then pumped via pump  20  to a feed tank  22  through piping  24 . The feed tank  22  is provided with a plurality of apertures  26  on its base  28 . The sulphur/cement mixture is then allowed to drip through the holes into a cooling chamber  30 . It will be understood that the diameter of the apertures  26  should not be large enough to form a stream of the mixture but only drops. Further, it will be understood that the diameter of the apertures  26  will also affect the size of the drops formed and, therefore, the size (i.e. diameter) of the pellets. Further, the pellet size will also depend upon the composition of the mixture.  
         [0031]    Cooling chamber  30  is supplied with a spray of cold water through a plurality of ports  32  located on the side walls of the chamber  30 . The water is allowed to circulate or swirl within the chamber  30 . As the drops  34  of the sulphur/cement mixture, still in a heated liquid state, enter into the cooling chamber  30  and contact the cooling water, they are solidified into pellets  36 . As illustrated, the cooling chamber  30  is preferably vertically elongated so as to maximize the exposure of the sulphur/cement mixture to the cooling water, thereby ensuring adequate solidification and proper formation of the pellet-like shape.  
         [0032]    Upon solidification, the sulphur pellets drop to the bottom of cooling chamber  30  due to gravity. A siphon  38  is used to extract the formed pellets  36  from the bottom of chamber  30 . The pellets along with some water are moved to a dewatering station  40 . The station  40  includes a dewatering screen  42  onto which the pellets and water are deposited. The screen  42  serves to drain water while retaining the pellets  36 . The pellets are collected on a tray  44 . The drained water is collected in a tank  46 , which includes a divider  48  that separates the collecting tank  46  into two sections. The first section  50  is where the water from the dewatering station  40  is first deposited. The water, which still includes some fine pellets that may have passed through the dewatering screen  42 , is then pumped via pump  52  to a cyclone separator  54 , which separates the second sample of solid pellets  36   a  from the water and collects them on a tray  56 . The water is then transported to the second section  58  of the collecting tank  46 . The water is then preferably pumped via pump  60 , and through piping  61 , back to the cooling chamber  30  after cooling if necessary.  
         [0033]    The pellets  36  and  36   a  are then collected from trays  44  and  56  and dried in a dryer (not shown). The dryer also allows the pellets to cure and is preferably set at a temperature of about 65° C. Following the drying step, the pellets can then be separated into the desired size groups using known screening methods. The sized pellets are then stored for later use.  
         [0034]    In a preferred embodiment, the pellets are subsequently coated with any known coating including, for example, an epoxy material. Such coating serves to increase the strength of the pellet and to provide a barrier to degradation or corrosion due to the presence of harsh chemicals in the well. Further, the coating provides a smooth outer surface for the pellets, which enhances their flow properties and to reduce their abrasive effects on the pumps and piping used to convey the pellets. As known in the art, the physical properties of the coating can be adjusted by varying the resin/hardener ratio.  
         [0035]    Proppants according to the present invention can be used as commonly known in the art. That is, the proppants of the present invention are first mixed with a fracturing fluid to form a slurry or suspension that is then pumped under pressure through the well-head and into the rock formation containing the petroleum to be extracted. The high pressure fluid creates fractures in the rock and creates highly permeable pathways through which the petroleum can flow. The proppant is carried into the fractures by the fluid and is then left in the fractures thereby propping them open.  
         [0036]    The use of sulphur in forming the proppant according to the present invention offers various advantages. Firstly, as discussed above, the invention provides a viable alternative to expensive disposal of the sulphur extracted from the produced petroleum. Furthermore, the specific gravity of sulphur is such that it is well suited to work with fracturing fluids.  
         [0037]    The process of the present invention also offers various advantages. For example, unlike other proppants such as sand, the invention provides a process whereby the size of the proppant particles is highly controlled. Further, unlike sand or ceramic materials, proppants of the present invention are manufactured to highly spherical geometries, which enhances their flow properties as well as porosity when positioned inside a fracture. FIGS.  2  to  4  illustrate the particle geometries for sand and ceramic proppants as well as that for proppants of the present invention. As can be seen, the proppant particles according to the present invention are considerably more spherical in shape. In addition, the surface coating of the particles offers a means of controlling the hardness of the pellets, thereby allowing the formation of pellets that are particularly suited for specific depth ranges.  
         [0038]    Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.