Abstract:
A method of cementing a well using a cementing spacer. The method includes pumping a drilling fluid into a well. A cementing spacer is then pumped into the well to serve as a buffer between the drilling fluid and a cement. The cementing spacer includes substantially unviscosified water and a weighting agent. Cement is then pumped into the well to displace the cementing spacer and the drilling fluid to complete the cementing of the well.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/031,083 filed on Feb. 26, 1998. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates generally to cementing spacers for use in cementing wellbores. More specifically, the invention relates to the use of Stokes Law cementing spacers when cementing wells.  
           [0004]    2. Background Art  
           [0005]    When drilling an oil or gas well, drilling fluid having a prescribed density is used during the drilling operation for several purposes including, for example, balancing a formation fluid pressure (which generally increases as the depth of a well increases) present in geologic formations that are penetrated by the wellbore. The drilling fluid, or “drilling mud,” is typically pumped down a drillstring, through a drill bit, and is returned to the surface though an annulus formed between the drillstring and a wall of the wellbore. This process is known as “circulation” of the drilling fluid.  
           [0006]    If the density of the drilling fluid is excessive, the hydrostatic pressure exerted by the drilling fluid on the formations can result in fractured formations and a resultant loss of drilling fluid into the “broken down” formations. Loss of drilling fluid into the formation typically results in “lost circulation” (e.g., the loss of a return fluid communication path to the surface through, for example, the wellbore annulus) and eventually a pressure underbalance with respect to formation fluid pressure. Lost circulation can result in uncontrolled discharge or “blowouts” of pressurized formation fluids to the surface because pressure control of the well has been lost. For example, when drilling fluid is lost into the formation, the wellbore pressure drops and permits higher pressure formation fluids to flow into the wellbore in the form of a “kick.” The kick may propagate to the surface and result in a blowout that can damage rig equipment and injure or kill rig personnel.  
           [0007]    These conditions may generally be avoided by appropriate selection of the density of the drilling fluid used to drill the well. The density of the drilling fluid is usually controlled by the addition of “weighting agents” in the form of, for example, particulate solids of heavy earth materials, such as barite. The weighting agents are added to the drilling fluid in a known ratio with respect to the fluid volume in the wellbore to produce a carefully regulated drilling fluid with a known density.  
           [0008]    During the drilling process, it is often necessary to periodically lower steel casing or well liners into the wellbore to line the walls thereof in order to maintain stability of the wellbore. Moreover, the casing may be required to protect shallower formations from the high wellbore pressures required to maintain fluid pressure balance or overbalance with respect to formations near the bottom of the wellbore. The casing, which is typically steel, must fit inside the wellbore diameter.  
           [0009]    After the casing is placed in the wellbore, an external casing annulus is formed between an outer surface of the casing and the wall of the wellbore. In order to prevent fluid communication along the external casing annulus, oil well cement is typically pumped into the external casing annulus. Cementation of the casing in the wellbore is important because undesirable fluid communication between the bottom of the wellbore and the surface through the external casing annulus can result in formation fluid leakage to the surface or to other subsurface formations, and can result in other types of well damage resulting in a loss of production potential. The oil well cement is placed in the external casing annulus by pumping a substantially fluid cement slurry down the casing, out of the bottom of the casing, and up into the external casing annulus.  
           [0010]    During the cementing process, the cement slurry must completely displace the drilling fluid from the external casing annulus because drilling fluid that is not displaced may provide a path for the flow of formation fluids up the external casing annulus after the cement has set. Moreover, slurries of oil well cement are often not chemically compatible with common drilling fluids. For example, if the cement slurry comes into direct contact with the drilling fluid during the displacement process, the cement slurry and the drilling fluid may mix together and form a viscous material. When the cement slurry is pumped into the external casing annulus, the cement slurry may bypass the viscous material, thereby leaving channels of viscous material that do not set up to form a solid, impermeable cement barrier to formation fluids. Accordingly, a cement “spacer” fluid is often pumped into the wellbore between the drilling fluid and the cement slurry to improve the displacement of the drilling fluid and to prevent direct contact and mixing of the drilling fluid and the cement slurry.  
           [0011]    Again, however, the density of such a cement spacer cannot exceed certain limits or the lost circulation condition will be encountered, and it cannot fall below other certain limits or an underbalanced condition will occur. Thus it is necessary to be able to control the density of the spacer fluid used in cementing operations in a manner similar to that used to control the density of drilling fluid during drilling operations.  
           [0012]    Prior art spacers are generally made by mixing a suitable liquid base fluid with a viscosifier which may be, for example, a soluble polymer or bentonite clay and a weighting agent including, for example, solid particles of barite or calcium carbonate. The weighting agent may also include low-density particles such as hollow glass or ceramic spheres or foamed nitrogen. The most common spacer base fluid is plain water. “Plain water” includes, for example, any source of chemically suitable water that is readily available for such applications, including fresh water, seawater, saltwater, and brine. Alternatively, a suitable organic solvent may be used as the spacer base fluid. Organic solvents are often advantageous for use in displacing oil based drilling fluids. When used in this manner, organic solvents may also include viscosifiers and weighting agents.  
           [0013]    In prior art cement spacer fluids, the viscosifier is used to support the particles of weighting agent so as to prevent settling of the weighting agent during the pumping operation. A performance objective of the viscosifier is often to develop “gel strength” under static conditions to aid in the support of the weighting agent particles. Particle settling in cement spacer fluids is usually evaluated in laboratories with settling tests, similar to the API free water test used for cement slurries. In the test, the volume of free water, which accumulates on the top of the spacer under specified conditions, is determined. A common practice is to require that the free water be below some maximum volume.  
           [0014]    Therefore, it is desirable to have a cementing spacer that is designed to displace drilling fluid in a wellbore and serve as a buffer between the drilling fluid and a cement slurry used to cement, for example, casing in a wellbore. Moreover, it is desirable to have a cementing spacer that can be readily formed at a well site.  
         SUMMARY OF INVENTION  
         [0015]    A method of cementing a well using a cementing spacer. The method comprises pumping a drilling fluid into a well and pumping a cementing spacer into the well to displace the drilling fluid, wherein the cementing spacer comprises substantially unviscosified water and a weighting agent. Cement is then pumped into the well to displace the cementing spacer and the drilling fluid to complete the cementing of the well.  
           [0016]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
       
    
    
     DETAILED DESCRIPTION  
       [0017]    Embodiments of the invention have been developed from a study of particle settling calculations based on the Stokes-Einstein equation. The particle settling calculations show that the sedimentation rate, or particle settling velocity, of particles of a weighting agent in a base fluid is relatively slow when compared to the depth of a typical well. For example, a total sedimentation distance of about 40 feet in a 4 hour period was calculated for particles of a calcium carbonate weighting agent in a water base fluid.  
         [0018]    Accordingly, if a controlled density cementing spacer comprising calcium carbonate and water (and substantially no viscosifier) is pumped into an external casing annulus in a substantially vertical wellbore, particles of calcium carbonate in the cementing spacer typically will settle no more than about 40 feet by the time the cement has set. This degree of settling will not cause any operational problems with respect to cementing the well.  
         [0019]    In an embodiment of the invention, the cementing spacer comprises a weighting agent (such as, for example, calcium carbonate, barite, ferrite, hematite, etc.) and water. Note that as previously disclosed, “water” may include fresh water, salt water, seawater, brine, or any other chemically suitable source of water that will not adversely react with drilling mud or the cement in the wellbore. The cementing spacers described above are typically referred to as “Stokes Law” mixtures. The resulting cementing spacers have numerous advantages discussed below when compared to prior art spacers that use viscosifiers to support the weighting agent.  
         [0020]    One form of the Stokes-Einstein equation is shown below as Equation 1:  
             V   =       2          gr   2          (       d   p     -     d   f       )           9      v               (   1   )                               
 
         [0021]    wherein V is a particle settling velocity (cm/sec), g is gravitational acceleration (980 cm/sec 2 ), r is a particle radius (cm), d p  is a particle density (g/cm 3 ), d f  is a fluid density (g/cm 3 ), and v is a fluid viscosity (poise).  
         [0022]    Numerical solutions of Equation 1 have been determined for different types of particulate weighting agents (such as, for example, calcium carbonate, barite, ferrite, hematite, etc.) and different particle diameters. Moreover, the solutions have been determined using water or organic solvents. The results show that, compared to a depth of a well (e.g., the overall height of a cement annulus from a casing bottom to a well head) and to a length of a typical casing string, the sedimentation velocity (or sedimentation rate) of particles of the weighting agent is substantially slow.  
         [0023]    Therefore, based on calculations performed using Equation 1, it has been determined that cementing spacers comprising water and a weighting agent have a substantially slow particle settling velocity so that they may be pumped into a well using typical rig operating techniques and do not require the addition of a viscosifier (such as bentonite or viscosifying polymers) to impede particle settlement or otherwise affect the rheology of the cementing spacer. A small amount of viscosifier may be present in the cementing spacer as long as the amount does not substantially affect the rheology of the cementing spacer (e.g., as long as the amount of viscosifier does not substantially affect the settling properties of the weighting agent). These cementing spacers do not adversely affect cement slurries used to cement wells, and avoidance of the use of viscosifiers may have several advantages, including:  
         [0024]    Cementing spacers are less expensive because they comprise fewer components.  
         [0025]    Cementing spacers have predictable properties resulting in less pilot testing and quality control requirements.  
         [0026]    The reduction or absence of gel strength development, combined with the settling motion of the cementing spacer particles, maintains hydrostatic pressure on the cement slurry as it sets and thereby provides a better seal through producing zones.  
         [0027]    Cement bond well logs are improved.  
         [0028]    Cementing spacers have a substantially Newtonian rheology and experience turbulent flow at lower pumping rates and thereby improve the displacement of drilling fluid (in the external casing annulus) by the cement slurry.  
         [0029]    Less mixing occurs at the interface between the turbulent flow cementing spacer and the drilling fluid, which also improves the displacement of the drilling fluid.  
         [0030]    While the cementing spacers comprise substantially unviscosified water and a weighting agent, other non-viscosifying additives may be used as well. For example, friction reducing additives may be used with the invention. Friction reducing additives may also serve to either minimize or enhance solid packing of particles of the weighting agent.  
         [0031]    Moreover, during extended settling conditions (e.g., settling conditions that continue for some time after the cement has set), particles of the weighting agent (which may comprise, for example, barite) in the cementing spacer settle and may form a “plug” (e.g., a “barite plug”) proximate the top of a cement column. The plug forms an additional seal and further prevents fluid transmission from the bottom of the wellbore to the surface. The additional sealing properties of the plug may be useful, for example, in meeting regulatory requirements associated with, for example, external casing pressure and/or microannular gas leakage (a condition that results from the formation of a small microannulus or gap between the set cement and the casing and/or the formation which may allow slow leakage of gas to the surface).  
         [0032]    Stokes Law calculations also apply to the “particle rise” of particles of low density weighting agents that may be added to the cementing spacer. For example, the use of hollow glass or ceramic spheres, foamed nitrogen, etc., to lower or reduce the density of the cementing spacer may also be used in embodiments of the invention. Further, the cementing spacers may be used to recover expensive oil based drilling fluids from wells for future reuse.  
         [0033]    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.