Abstract:
A method for maintaining wellbore pressure includes reducing flow rate of a drilling fluid pump fluidly connected to a drill pipe in the wellbore. Flow out of the well is enabled into a first auxiliary line associated with a drilling riser. A seal around the drill pipe is closed. Fluid is pumped down a second auxiliary line at a rate selected to maintain a specific pressure in the wellbore. Drilling fluid flow through the drill pipe is stopped.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority is claimed from U.S. Provisional Application No. 61/318,427 filed on Mar. 29, 2010. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to the field of drilling wellbores through subsurface rock formations. More specifically, the invention relates to methods for controlling wellbore pressure during assembly or disassembly of lengths of drill pipe. 
     2. Background Art 
     Drilling wellbores through subsurface rock formations includes rotating a drill bit disposed at the end of a drill pipe disposed in the wellbore. Various devices are used to rotate the pipe and/or the bit while pumping drilling fluid through the pipe. The drilling fluid performs several functions, namely to cool and lubricate the bit, to lift drill cuttings out of the wellbore, and to provide hydraulic pressure to maintain wellbore mechanical stability and to restrain fluid under pressure in various permeable subsurface formations from entering the wellbore. 
     It is known in the art to use drilling fluid having lower specific gravity than that which would exert sufficient hydraulic pressure to retain fluids in such formations. One such technique is described in U.S. Pat. No. 6,904,981 issued to van Riet and commonly owned with the present invention. Generally, the system described in the &#39;981 patent uses a rotating diverter or rotating control head to close the annular space between the drill string and the wellbore wall. Flow out of the wellbore is automatically controlled so that the fluid pressure at the bottom of the wellbore is maintained at a selected amount. 
     The drill pipe is assembled from a number of individual segments (“joints”) of pipe threadedly coupled end to end. In order to lengthen the wellbore, it is necessary from time to time to add joints to the drill pipe. To remove the drill pipe from the wellbore, for example to replace the drill bit, it is necessary to threadedly disconnect sections (“stands”) of the drill pipe from the part of the drill pipe remaining in the wellbore. When using the system described in the van Riet &#39;981 patent, for example, it is desirable to include a one way (“check”) valve in the drill pipe so that when the upper part of the drill pipe is opened, i.e., disconnected from a kelly or top drive, drilling fluid is prevented from flowing back up the drill string. Annulus pressure can be maintained using a back pressure pump, or by diverting some of the flow from the drilling unit fluid pumps into the annular space. 
     U.S. Pat. No. 6,823,950 issued to von Eberstein, Jr. et al describes a technique for maintaining wellbore pressure during connections for marine drilling systems in which a wellhead is located at the sea floor and a riser fluidly connects the wellbore to a drilling unit on the water surface. The method disclosed in the &#39;950 patent requires filling an auxiliary fluid line associated with the riser system with higher density fluid and/or applying pressure to such line to maintain a selected fluid pressure in the wellbore. 
     A particular disadvantage of using the method described in the &#39;950 patent is that switching from drilling to maintaining wellbore pressure during connections is that it requires the drilling unit operator exercise a high degree of care during the transition from drilling using the drilling unit pumps to the conditions necessary required to make a connection. There may be risk, for example of u-tubing because of the higher density fluid being inserted into the auxiliary line. This may create risk of exceeding formation fracture pressure at some point in the wellbore. 
     What is needed is a technique for maintaining wellbore pressure during the transition from drilling to making connections and during connections that does not require the use of higher density fluid in the auxiliary lines. 
     SUMMARY OF THE INVENTION 
     A method for maintaining wellbore pressure includes reducing flow rate of a drilling fluid pump fluidly connected to a drill pipe in the wellbore. Flow out of the well is enabled into a first auxiliary line associated with a drilling riser. A seal around the drill pipe is closed. Fluid is pumped down a second auxiliary line at a rate selected to maintain a specific pressure in the wellbore. Drilling fluid flow through the drill pipe is stopped. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a floating drilling platform with a dynamic annular pressure control system and fluid circulation system according to the invention. 
         FIG. 2  shows a graph of equivalent drilling fluid densities at the bottom of a well while circulating with respect to the depth of the well and the actual density of the drilling fluid. 
         FIG. 3  is a table showing amount of flow through choke and kill lines needed to maintain an equivalent fluid density in the well as if drilling and circulating through the drill pipe at a selected flow rate. 
         FIG. 4  is a graph showing pressure variation during pipe connections. 
         FIG. 5  is a flow chart of initiating the connection procedure according to the invention. 
         FIG. 6  is a flow chart of initiating drilling according to the invention. 
         FIG. 7  is an example “tripping” procedure. 
         FIG. 8  shows example modifications to the DAPC system in order to use the method of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a floating drilling platform  10  that may be used with a method according to the invention. The floating drilling platform  10  typically includes a marine riser  12  that extends from the floating drilling platform  10  to a wellhead  14  disposed on the water bottom (mud line). The wellhead  14  includes various devices (not shown separately) to close the wellbore. Such wellhead devices may include pipe rams  27 A to seal against the drill pipe (shown at  27  disposed inside the marine riser  12  and in the wellbore  25 ), an annular seal and blind rams to close the wellbore  25  when the drill pipe  27  is removed from the wellbore  25 . In the present example a casing  28  is cemented in place in the wellbore  25  to a selected depth below the water bottom and is coupled at its upper end to the wellhead  14 . 
     What is shown in  FIG. 1  is a dynamic annular pressure control (“DAPC”) system and its components, for example, the system described in U.S. Pat. No. 6,904,981 issued to van Riet and commonly owned with the present invention. The DAPC system may, but not necessarily include a controllable orifice or choke  22  in the drilling fluid return line, a backpressure pump  20  and a DAPC controller  21 . The present invention may be used either with or without the DAPC system. A separate pump  24  or the drilling unit&#39;s drilling fluid pump  29  on the drilling platform  10  may be used to provide fluid flow into the drill pipe  27  and thus into the wellbore  25  at a selected rate. A pressure sensor  26  may be located proximate the wellhead  14  and used to indicate pressure in the wellbore  25 . During assembly or disassembly of a pipe segment from the drill pipe (not shown), fluid may be pumped down one or more of the auxiliary lines  16  associated with the riser and wellhead system (e.g., choke lines, kill lines, booster lines). Fluid may be returned to the surface up one or more of the auxiliary lines  18 . Such procedure will be further explained below with reference to  FIGS. 5 ,  6  and  7 . 
       FIG. 2  shows a graph of equivalent circulating fluid densities at various wellbore depths for various static fluid densities, shown by curves  44  through  60 . The densities are expressed in terms of “mud weight”, which as known in the art is typically expressed in units of pounds weight per gallon volume of drilling fluid. As may be observed by the curves  44  through  60   FIG. 2 , the equivalent circulating density increases (“ECD”) with respect to depth for any particular flow rate of fluid into the wellbore. When fluid flow into the wellbore is stopped, such as for making a connection (i.e., adding or removing a segment to the drill string), the fluid density will drop to its static value. Limits of fluid pressure within the wellbore at any depth are indicated by curves  40  and  42 , which represent, respectively, the formation fracture pressure expressed in mud weight equivalent (gradient) terms and the pressure of fluid in the formations being drilling (formation pore pressure) also expressed in mud weight equivalent terms for consistency with the drilling fluid pressures shown by curves  44  through  60 . 
     Using the system shown schematically in  FIG. 1 , and referring to the tables  FIG. 3 , it can be observed what rate of fluid flow is needed through auxiliary lines (e.g.,  16  and  18  in  FIG. 1 ) to provide the equivalent bottom hole pressure (“BHP”) of drilling fluid circulating through the drill pipe at selected drilling fluid flow rates. 
       FIG. 4  graphically illustrates fluid pressure (expressed in units of pressure) with respect to wellbore depth. Curve  74  shows the fluid pressure with respect to depth when no circulation takes place. Curve  70  represents the formation fluid (pore) pressure with respect to depth, and curve  72  represents the formation fracture pressure with respect to depth during. It may be observed in  FIG. 3  that the drilling fluid has a static gradient that is below the formation fluid pressure gradient. Therefore, using the drilling fluid having static gradient shown in  FIG. 3  would require addition of fluid pressure to the wellbore when drilling operations are interrupted in order to prevent fluid influx from the formation into the wellbore. Curve  68  shows the wellbore fluid pressure with respect to depth while drilling, wherein the drilling platform (or other) pump is operated at a rate of 350 gallons per minute. Curve  62  shows the fluid pressure with respect to depth when pumping fluid into the base of the riser ( 12  in  FIG. 1 ) at 150 gallons per minute. Curves  64  and  66  show, respectively, the fluid pressure with respect to depth while pumping fluid using the system shown in  FIG. 1 , at rates of 50 gallons per minute and 150 gallons per minute. 
       FIG. 5  shows a flow chart of initiating a circulation procedure according to the invention. First, the drilling rig pump rate is reduced, as shown at  80 . The kill line (e.g.,  16  in  FIG. 1 ) may be opened at  82  for pressure monitoring. The pump ( 24  in  FIG. 1 ) may be operated at a low rate at  84  to move fluid down the kill line ( 16  in  FIG. 1 ) if seawater is used to ensure a singular fluid. Then the choke line(s) ( 18  in  FIG. 1 ) may be opened, as shown at  86 , for example, by operating a valve ( 16 A in  FIG. 1 ) proximate the blowout preventer. Different density fluid may be needed to offset choke line friction when the pump ( 24  in  FIG. 1 ) is operated. It is preferable to use multiple riser auxiliary lines for fluid return to the platform if the riser system used makes this possible in order to reduce friction losses in the circulation system. Next, at  88 , the sea floor blowout preventer ( 14  in  FIG. 1 ) is closed to divert return flow through at least one of the auxiliary line(s), e.g., choke line ( 18  in  FIG. 1 ). Such closure may include closing an annular seal (not shown separately) and/or pipe rams (not shown separately) on the blowout preventer. The choke line may be hydraulically connected to the wellbore, for example, by operating a valve ( 18 A in  FIG. 1 ) proximate the blowout preventer. At  90 , the drilling platform&#39;s main drilling pump is stopped to cease pumping fluid through the drill string. The control point pressure in the wellbore ( 25  in  FIG. 1 ) is then maintained by pumping fluid at a selected flow rate down the kill line ( 16  in  FIG. 1 ). 
     During this time, the upper end of the drill pipe may be disconnected from the drilling unit main pumps and a connection may be made or broken (i.e., a segment of drill string may be added or removed from the drill string). The fluid pressure during this time is maintained in the wellbore so that the ECD remains above the formation pore pressure, thereby reducing the possibility of formation fluid entering the wellbore. 
       FIG. 6  shows a flow chart of an example procedure used to resume drilling after maintain pressure as explained with reference to  FIG. 5 . At  92 , the control point pressure is maintained using the pumping technique explained with reference to  FIG. 5 . At  94 , the drilling unit&#39;s main fluid pumps may be restarted to resume drilling flow through the drill pipe. At  96 , dynamic wellbore fluid pressure is maintained at the casing shoe (top of  28  in  FIG. 1 ) or the heel of the wellbore ( 25  in  FIG. 1 ) by control of the fluid flow rate both into the drillstring and into the kill line ( 16  in  FIG. 1 ). The blowout preventer may then be opened, at  98 , to divert return fluid flow from the choke line ( 18  in  FIG. 1 ) and drill pipe back into the riser ( 12  in  FIG. 1 ). At  100 , the choke line(s) are hydraulically isolated from the wellbore, e.g, by closing the valve ( 18 A in  FIG. 1 ). Also at  100 , the pump ( 24  in  FIG. 1 ) may be stopped if it is in use, or stop flow from the drilling rig pump if it is being used to move fluid through the kill line ( 16  in  FIG. 1 ). Then, at  102 , the kill line ( 16  in  FIG. 1 ) is isolated from the wellbore, e.g., by operating the valve ( 16 A in  FIG. 1 ). Finally, at  104 , the choke and kill lines may be flushed with drilling mud if a different density fluid is used during the connection procedure. 
       FIG. 7  explains procedures that may be used with certain operations including axial motion of the drill pipe (e.g., “trips”). At  106 , “wiper” trips will require pumping while moving the drill pipe in and out of the wellbore in order to maintain pressure above formation pore pressure if the blowout preventer is open. At  108 , “stripping” with an annular sealing element in the blowout preventer is one possible option. Rotation of the drill string is not recommended if an annular seal is used. At  110 , stripping from one pipe ram to another pipe ram in the blowout prevented, when the blowout preventer includes multiple pipe rams, is another possible option. Rotation of the drill string is not recommended if multiple pipe rams are used. At  112 , a full trip out of the wellbore or into the wellbore can be performed using the procedure explained with reference to  FIG. 5 . 
     In addition, and referring to  FIG. 8 , one can extrapolate the surface pressure and height of the fluid column, at  114  to obtain pressure below the blowout preventer (“BOP”) if a pressure sensor at the BOP is unavailable. At  116 , the pump ( 24  in  FIG. 1 ) start/stop sequence may be performed based on the pipe ram position. At  118 , the pump may be stopped when the pipe rams are closed. At  120 , the pump may be started when the pipe rams are open. 
     A method according to the invention provides a technique to maintain a selected pressure in the wellbore while making pipe connections. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.