Abstract:
This invention relates to a method for completing and producing hydrocarbons from a well through the use of extreme overbalanced pressure during perforation of the casing string, followed by an underbalanced surge to produce the hydrocarbons through the tubing string.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of drilling for hydrocarbons such as oil and gas. More specifically, this invention relates to a method for completing and producing hydrocarbons through the use of extreme overbalance pressure followed by an underbalance surge. 
     BACKGROUND OF THE INVENTION 
     When drilling for hydrocarbons such as oil and gas in a subterranean formation, a wellbore is drilled into the formation for the purpose of gaining access to the hydrocarbons. Drilling mud is often used to assist in the drilling process and to hydrostatically suppress the flow of fluids into the well from a pressurized formation. A casing is installed and cemented to the face of the wellbore. During this process the drilling mud is, for the most part, replaced with water. The casing blocks the flow of fluid from the formation into the wellbore. Charges are then detonated so as to cause perforations in the well casing and cement. These perforations extend into the formation and permit the influx of the hydrocarbons into the well. The perforation is typically carried out using a perforating gun. 
     Perforation can be conducted in “overbalanced” conditions where the well pressure is greater than the pressure at which the formation will fracture. The term “underbalance” is used to refer to a situation where the pressure of liquid in the wellbore is less than the pressure existing in the formation. 
     After the perforations have been created, the drilling mud is gradually displaced by the influx of hydrocarbons and the hydrocarbons eventually flow into the wellbore and then up to the surface. 
     Conventional drilling methods and assemblies suffer from certain disadvantages. For example, it can be problematic to clean the perforations of mud filtrate, cement contaminates and perforation debris. In standard extreme overbalance perforating applications it is not possible to surge the well prior to flowing the well back to surface. 
     These and other deficiencies in the prior art have been addressed by the present invention. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides a method of completing and producing hydrocarbons in a well, the well having a casing string extending into a wellbore and penetrating a hydrocarbon-bearing formation, a tubing string extending downhole into the casing string, the strings forming an annulus between them which annulus is sealed in one portion by a packer, the tubing string having a bore and ports in its surface located downhole of the packer, the tubing string having valve means located downhole of the ports for selectively trapping or releasing pressure in the tubing string downhole of the valve means, means for perforating the casing string being located at the tubing string&#39;s lower end and downhole of the valve means, the casing string initially blocking communication of the hydrocarbons with the wellbore, the method comprising: blocking the ports in the tubing string so as to prevent fluid communication between the casing string and the tubing string; creating a fluid column in the tubing string&#39;s lower end; adding a sufficient volume of a gas to the fluid column to achieve a predetermined downhole pressure which will exceed the fracture gradient of the formation; perforating the casing at the hydrocarbon-bearing formation so as to fracture the formation; maintaining pressure below the packer through the use of the valve means; removing sufficient volumes of the gas from the well so that it is in an underbalanced state; opening the ports so as to release the pressure below the packer and allow it to surge into the tubing string, thus permitting a mixture of hydrocarbons and gas to surge up the tubing string to the surface. 
     The gas used may be nitrogen, carbon dioxide or any other suitable gas. The valve may comprise a flapper valve. If desired, an additional volume of gas may be pumped into the tubing string after perforation to further fracture the formation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the installed tubing conveyed perforating assembly of the present invention, prior to perforation of the assembly and the introduction of gas. 
     FIG. 2 is a perspective view of the assembly in which fluid and nitrogen gas has been introduced into the tubing. 
     FIG. 3 is a perspective view of the assembly after detonation has occurred and the firing head vent has opened. 
     FIG. 4 is a perspective view of the assembly after a volume of nitrogen has been pumped into the tubing. 
     FIG. 5 is a perspective view of the assembly after the check valve has closed. 
     FIG. 6 is a perspective view of the assembly after the nitrogen pressure has been bled out of the tubing, trapping high pressure below the check valve and the packer. 
     FIG. 7 is a perspective view of the assembly illustrating a drop bar falling within the tubing to initiate the venting device. 
     FIG. 8 is a perspective view of the assembly illustrating the surge of a mixture of hydrocarbons and nitrogen through the bar vent and up the tubing to the surface, the drop bar having come to rest after initiating the bar venting device. 
     FIG. 9 is a cross-sectional view of the bar actuated vent in the closed position; 
     FIG. 10 is a cross-sectional view of the check valve tool; and 
     FIG. 11 is a cross-sectional view of the firing head. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a conventional well  10  comprising a casing string  12 , and tubing string  14  extending downwardly inside the bore of the casing forming an annulus  16  between the casing and tubing. The casing extends into a formation  20  which contains hydrocarbons such as oil or gas. A cement sheath holds the casing in place in the hole which has been drilled in the ground. 
     The tubing has a bore  24 . Together, the annulus  16  and the tubing bore  24  form the wellbore. Packer  28  is positioned between casing  12  and tubing  14  to seal annulus  16 . Fluid may be inserted into the annulus  16  between the tubing and the casing above the packer  28  in order to reduce the difference in pressure between the formation  20  and the inside of the casing. Located downhole of the packer  28  is bar actuated vent  42 , shown in more detail in FIG.  9 . Vent  42  is in the shape of a hollow cylinder with a pin  46  extending into the interior of the vent. Ports  50  permit fluid communication between the interior of the tubing and the annulus  16 . In FIGS. 1 and 9, vent  42  is in the closed position. In the closed position, sleeve  54  obstructs the ports  50  in the vent  42  and prevents fluid communication with the annulus  16 . As described below, vent  42  may be opened by breaking the pin  46  after which spring  56  causes sleeve  54  to move upwards into recess  58  until shoulder  60  prevents further upward movement of the sleeve  54 . In this open position, ports  50  are unobstucted by the sleeve  54  so as to permit the displacement of wellbore fluid. 
     It can thus be seen that vent  42  can replace a standard surge tool and results in the tubing being kept dry. The vent  42  can be used in an underbalance application or in an extreme overbalance perforation system and does not depend on tubing hydrostatic pressure to operate. The vent  42  can be used in wells with open perforations to achieve an underbalance when the guns fire to add a surge. 
     Check valve tool  62 , shown in FIGS. 1 and 10, is located downhole of the bar actuated vent. Check valve tool  62  is a valve of the type known as a flapper valve and is of a generally cylindrical shape. In FIG. 1, the check valve tool is in the closed position. FIG. 10 illustrates the check valve tool in various positions. Flappers  64  are hinged at the sides of the check valve tool and, as shown in FIG. 10, open downward. In FIG. 10, each of two flappers  64  are shown in five different positions, moving in a counter-clockwise motion from a closed position (horizontal) to an open position (vertical). 
     Automatic venting firing head  70  and perforating gun  72  are also shown in FIG. 1, downhole of the check valve tool. 
     FIG. 2 illustrates the introduction of fluid and liquid nitrogen into the tubing. The fluid is formation specific. For example, fluids such as a mixture of potassium chloride and water, methanol or frac oil are just some of the fluids which may be used depending upon the physical composition of the formation. The process is initiated by trickling an amount of fluid into the tubing on top of the firing head  70 , thus creating fluid column  74 . Typically, it is necessary to use anywhere from 1 to 5 barrels of fluid so as to create a fluid column of roughly 200 to 300 meters in height. Each barrel contains approximately 42 gallons or 0.1590 cubic meters of fluid. The amount of fluid which is trickled into the tubing varies from well to well, depending upon such factors as the depth of the well, type of formation and the pressures which are encountered in the well. After the creation of the fluid column, a volume of nitrogen  76  is then introduced to the tubing The volume of nitrogen which is needed again varies depending upon the type of well but is calculated to create an overbalance condition with respect to fracturing the formation. In FIG. 2, check valve tool  62  is open. Referring to FIG. 10, flappers  64  would be facing vertically downward when check valve tool  62  is in the open position. 
     FIG. 3 illustrates the detonation of the charges and the opening of the firing head. As is well known in the industry, charges are detonated so as to cause perforations in the well casing and cement. These perforations extend into the formation and permit the influx of the hydrocarbons into the well. Activation of the firing head causes a drop in pressure within the tubing. 
     FIG. 11 illustrates firing head  70  in its closed position. The firing head vent is set to operate at a predetermined pressure thereby firing the gun and opening the vent. Firing head  70  can be used with time delay fuses to provide time to adjust tubing pressures before the guns can fire. As shown in FIG. 11, ports  78  are obstucted by sleeve  80  when the firing head vent is in its closed position. In operation, sleeve  80  slides downward into recess  82  so as to open ports  78 . 
     In FIG. 4, a further volume of nitrogen  76  is pumped into the tubing to build up pressure from the surface. Depending upon the volume of nitrogen which is pumped into the tubing for this purpose, the newly introduced nitrogen displaces part or all of the fluid column  74  in the tubing and the nitrogen and fluid column are pumped through the perforations in the casing and into the formation. The volume of nitrogen which is needed for this purpose varies depending upon the characteristics of the well. For example, one to two tubing volumes of nitrogen might be needed for this purpose. Pumping might not occur at all if fracture extension is not desired. 
     The downhole pressure should be monitored during the pumping of the nitrogen. When pumping has ceased, the check valve contains pressure below the valve. In FIG. 5, the check valve is now in the closed position. Referring to FIG. 10, the two flappers  64  would be in the horizontal position. The closure of the check valve  62  allows time for the formation to cure and the tubing pressure to be bled down before performing an underbalance surge with the use of a mechanically deployed bar or tube venting device. 
     FIG. 6 illustrates the following step. After the check valve tool  62  has closed, the master valve on the surface is opened and pressure is evacuated out of the tubing. Bleeding off the nitrogen pressure out of the tubing creates a zone of high pressure  90  trapped in the region in the tubing below the check valve tool  62  and in the annulus  16  below the packer  28 . 
     As shown in FIG. 7, drop bar  94  is introduced into the tubing such that it falls down inside the tubing and comes into contact with pin  46  in the bar actuated vent. Drop bar  94  is of a cylindrical shape with a flare on top so that it sits below the top of the bar actuated vent but above the check valve. When the drop bar comes into contact with pin  46 , the pin breaks and pressure trapped in the cavity together with the aid of spring  56  pushes sleeve  54  upward into a recess  58  in the tool housing, resulting in the opening of ports  50 . 
     As shown in FIG. 8, the opening of the bar actuated vent  42  allows the trapped pressure  90  below the packer to surge into the lower pressure tubing and causes the hydrocarbon/nitrogen mixture  96  to surge up the tubing in the direction of the surface. 
     The well response is then observed and pressure readings taken at regular intervals. 
     Several advantages over conventional completion processes become apparent upon an examination of the above description. Unlike the present method, conventional methods do not permit a full underbalance surge to be accomplished. Conventional systems bleed off the gas pressure at surface at the same time that the pressure downhole is slowly being bled of By evacuating all of the pressure out of the tubing and then dropping a bar to open up the vent, a full underbalance surge will take place at the formation. 
     The process as described herein benefits the production of oil and gas by providing a means to back surge and clean the perforations of mud filtrate, cement contaminates and perforation debris. This surge also cleans the compacted zone surrounding the perforation tunnel, reduces damage to the reservoir rock and allows the oil or gas zone to produce effectively. In contrast with conventional extreme overbalance perforating assemblies, a means to surge the new perforation will now take place. 
     The foregoing description is given to assist in understanding the invention. It will be readily apparent to those skilled in the art that many variations, applications, modifications and extensions of the basic principles involved in the disclosed invention may be made without departing from its spirit or scope. For example, carbon dioxide is a suitable substitute for nitrogen as is any other product or gas which is able to compress and apply a pressure to the equipment downhole while, at the same time, having physical characteristics that will not damage the formation and will be compatible with formation fluids.