Abstract:
A method of forming a cementitious plug in a well is disclosed. According to the method, a well is selected for treatment which lacks sufficient formation pressure to cause well fluid to naturally flow to the surface of the well. In addition, a formation penetrated by the well has unobstructed access between it and the surface. Having thus selected a well for treatment, a liquid slurry comprising a cementitious material, whose density is greater than the density of the well fluid, is introduced into the well. The slurry is permitted to drive the well fluid into the formation. Sufficient slurry is added to the well to fill the well to the surface. The slurry is then permitted to set into a hardened mass.

Description:
This application claims the benefit of Provisional Application 60/311,965 filed Aug. 13, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention broadly relates to well cementing. The invention further relates to well plugging. The invention more particularly relates to a method of forming a cement plug in a well bore. 
     2. Description of the Prior Art and Problems Solved 
     The term, “primary cementing,” is employed by persons skilled in the art of well cementing to refer to the formation of a sheath of cement in the annular space between the wall of a bore hole drilled in the earth and the exterior wall of a casing positioned in the bore hole. The sheath is ordinarily formed as a part of the initial construction of a well, such as a well which produces hydrocarbons, for example, liquid petroleum and natural gas, from a subterranean earth formation. The purpose of the sheath is to stabilize the bore hole, support the casing in the bore hole and segregate subterranean formations which contain hydrocarbons from subterranean formations which contain water, particularly potable water. The sheath of cement can extend from the bottom of the casing to the surface of the earth. A method of forming the sheath is well known. 
     The term “casing” is employed in the previous paragraph to broadly refer to at least one and usually two or more tubular conduits of decreasing diameter which, together in a telescoping mode, extend from the surface of the earth to the bottom of the bore hole. In one typical example, a so-called surface casing having a large diameter continuously extends from the surface of the earth to a point below the deepest formation which contains potable water. A second casing, sometimes called a production casing whose outside diameter is less than the inside diameter of the surface casing, continuously extends from the surface of the,earth to a target formation, such as one which contains hydrocarbons. A sheath of cement is placed in the entire annular space between the surface casing and the bore hole and a second sheath of cement is placed in the annular space from the bottom of the production casing to a point above the target formation. 
     When the producing life of a well is complete, such as when recovery of hydrocarbons from the well is no longer economically sound, the well is abandoned. Abandoned wells pose a variety of hazards, one of which is the potential of undesirable fluids, such as hydrocarbons and/or salt water, which originate from subterranean formations penetrated by the bore hole, to migrate to and contaminate potable water in other subterranean formations which are also penetrated by the bore hole. To prevent such contamination, regulatory agencies in the several states require that abandoned wells be plugged, such as by placing a mass of hardened cementitious material in the well bore at least at points adjacent hydrocarbon producing formations and also at points adjacent potable water formations. Such plugs completely occupy the well bore volume adjacent the formations and function as a barrier to migrating fluids. 
     The current method of plugging a well broadly comprises forming a slurry of cement in water at the well head, introducing a continuous connected string of delivery pipe (sometimes called tubing), into the well bore until the bottom of the tubing attains a desired point of delivery of the slurry, pumping the slurry down the tubing to the bottom thereof and back up the exterior thereof, continuing pumping until a desired quantity of slurry has been deposited in the well bore to form a plug therein, and then withdrawing the tubing from the well bore. In the current method when the tubing is withdrawn from the well bore, the top of the slurry on the exterior of the tubing is preferably at the same level as the slurry in the interior of the tubing. This is referred to as a balanced plug. Before a second balanced plug can be placed, the cement in the preceding plug must first be permitted to set to a minimum hardened condition. Accordingly, if multiple plugs are required, then they cannot all be placed in a continuous operation due to the need to wait on cement to set. 
     Note use of the terms “bore hole” and “well bore.” For purposes of disclosure, the term “bore hole” is employed to describe the linear hole actually drilled in the earth. The wall of the bore hole is the earthen rock exposed by the drill. The term “well bore” is employed to describe the containment vessel for the conduit or the intended conduit through which fluids pass between the surface of the earth and subsurface formations penetrated by the bore hole. It is common to install a continuous string of casing in the interior of the bore hole. The volume of the interior of the casing is the well bore. The volume between the wall of the bore hole and the exterior surface of the casing is referred to as the annular space. Thus, primary cementing involves placing cement in the annular space and well plugging involves placing cement in the well bore. In the absence of a casing it is clear that there is no annular space and there is no distinction between bore hole and well bore. 
     Persons skilled in the art know that considerable surface equipment is required to perform the current method of well plugging. Such equipment comprises a derrick to suspend tubing in the hole, transports for delivering to and storing dry cement and water at the well head, equipment at the well head for blending and mixing the cement and water to form the slurry and a high volume/high pressure pump at the well head to pressure the slurry down the suspended tubing and back up the exterior thereof to a predetermined destination. 
     The current method is employed to produce cement plugs in wells regardless of depth, and is particularly useful to form plugs in wells whose internal pressures are sufficiently high to cause fluids to naturally flow to the surface of the earth. Such wells require the use of methods and equipment which function to control such pressures and to prevent the flow of fluids from the well while the cementing operation is proceeding. 
     A need thus exists for a method of forming cement plugs in wells whose internal pressures are not sufficiently high to cause formation fluids to flow to the surface of the earth. 
     THE INVENTION 
     Summary of the Invention 
     By this invention there is provided a method of well cementing which comprises forming a plug of cement in the well bore. According to the method of the invention, a suitable bore hole is first selected. Upon selection of a suitable bore hole, a liquid comprising a cementitious slurry is introduced into the well bore at the surface of the earth. The liquid is permitted to descend in the well bore by gravity, form a column of slurry to a desired point in the well bore and then permitted to harden therein to form a plug. The bulk density of the introduced liquid is selected so that it, when multiplied by the distance from the surface of the earth to a designated location in the well bore, produces a pressure which is in excess of the natural pressure at the face of any subsurface formation actually contacted by the liquid. The method of this invention thus depends upon hydrostatic pressure generated by introduced liquid and not on mechanical pressure generated by a surface pump. 
     A suitable bore hole is one which penetrates at least one subsurface formation which produces a well fluid other than fresh water, wherein the natural pressure of the formation is not great enough to cause the well fluid to flow from the formation through the bore hole to the surface of the earth. The formation must possess sufficient permeability and porosity to permit the well fluid to be injected into it within an acceptable period of time by pressure induced at the formation face by hydrostatic pressure in the well bore and the formation must also possess sufficient structural strength to avoid being fractured by such induced pressure. 
     The method of this invention features positioning all plugs required within the entire well bore in one continuous operation without stopping to wait for a preceding plug to set. Thus, the reference in the preceding paragraph to an “acceptable period of time” of injection of well fluid into a subsurface formation is the amount of time that a slurry must remain in a flowable liquid state before it begins to set. It is believed that such an “acceptable period of time” is in the range of from about 8 to about 10 hours. A set time in the range of 8 to 10 hours can be selected by the addition to the slurry of known set time additives. 
     As mentioned, the formation into which well fluid is injected must also possess sufficient structural strength to avoid being fractured by the total hydrostatic pressure produced at the formation. In this regard, if the total hydrostatic pressure at a formation divided by the distance to the formation from the surface, i.e. the pressure gradient, is a value in the range of from about 0.4 to about 0.5 lb/sq.in. per foot of depth, then it is believed that a fracture will not be produced in the formation. For example, a liquid having a bulk density equal to the density of water (62.43 lb/cu.ft.) in a well having a depth of about 6500 feet produces a pressure gradient of about 0.4335 lb/sq.in. per foot of depth. 
     In addition to the low probability that a fracture will be induced by the hydrostatic pressure created by liquid in a well that is less than or equal to about 6500 feet deep, the well bore temperature in such wells is considered by persons skilled in the art to be low. Minimal performance requirements are demanded of cement at low temperature applications, so a wide range of cement compositions will operate. 
     Accordingly, by the method of this invention, a suitable well is first selected. Such a well is one which is no longer productive of useful well fluids, such as oil and gas, and has a subsurface formation containing such well fluids which is penetrated by a bore hole. The term well fluid can also include water produced from the formation which is sometimes referred to as produced formation water. The formation is not blocked by any device in the bore hole and, thus, has unobstructed access to the surface of the earth via the well bore. The natural pressure in the formation is not great enough to cause well fluids to flow to the surface of the earth. Such a condition can be evidenced by a static column of well fluid in the well bore which does not reach the surface of the earth. 
     Having thus located such a candidate well, the next step in the selection method is to determine whether the formation has sufficient porosity and permeability to accept low viscosity fluids in an acceptable period of time without undergoing a fracture. Accordingly, a simple injectivity test is performed by filling the well bore to the surface of the earth with a measured quantity of a fluid having a known density and a known viscosity; permitting fluid to flow into the mentioned formation; measuring the time required for fluid in the well bore to attain a static condition; and measuring the level of the attained static column of fluid. The measured quantity of fluid introduced into the well bore is that quantity which is equal to the volume of the well bore between the formation and the surface of the earth less the quantity of the static column of well fluid initially present therein. The quantity of fluid which actually enters the formation is determined by appropriate mathematical combination of fluid in the well bore before the test, the quantity of fluid in the well bore after the test and the measured quantity of fluid introduced into the well bore during the test. The density of the fluid added during the test is at least equal to, and is preferably greater than, the density of the fluid initially at rest in the well bore. The natural pressure within the formation can be calculated by those skilled the art by use of the density of the well fluid and the height of the static column of fluid above the formation. The porosity and permeability of the formation is then determined by application of, for example, the D&#39;arcy Equation which is known by those skilled in the art of reservoir evaluation. 
     Having thus selected a candidate well, the method of this invention is further comprised of forcing at least a portion of the well fluid initially standing in the well bore from the well bore into the subsurface formation or formations of its source while, simultaneously, entirely replacing such portion, in one aspect, with a single quantity of a first liquid comprising a cementitious material, or, in a second aspect, with a combination of the first liquid, a second liquid and dense spacing discs or plugs followed by a single quantity of the first liquid. 
     The combination of first liquid, second liquid and dense spacing discs or plugs is defined herein as “a cementing unit” which consists of two spacings discs, a single quantity of first liquid and a single quantity of second liquid, wherein the spacing discs are placed between successive quantities of second liquid and, first liquid or between well fluid and first liquid as the case may be. 
     The single quantity of first liquid is defined herein as “a final unit” which consists of a single quantity of cementitious material and one spacing disc. 
     In the mentioned second aspect, the method of this invention is comprised of a series of steps which operate to force the well fluid into the subsurface formation or formations of its source in stages by employing at least one cementing unit and a final unit, wherein at least one cementing unit is employed per subsurface formation containing an undesirable well fluid. The final unit is used to block formations containing potable water. The entire well bore from the bottom thereof to the surface is filled with the cementing units and the final unit. 
     Guided by the known relationship that pressure is the product of height and density, to create hydrostatic pressure sufficient to force the well fluid into the formation the bulk density of the cementing units and final unit can be equal to or greater than the bulk density of the well fluid. Furthermore, the hydrostatic pressure created by the weight of the combination of cementing units and final unit at the formation must be greater than the natural reservoir (pore) pressure of the formation. Methods of preparing the first liquid, which is comprised of cementitious material, and the second liquid, which is a spacer fluid, each having a desired density, are well known in the art of well cementing. 
     Descriptions of the cementitious materials, spacer fluids and spacing discs as well as a more detailed account of the steps employed in the method of this invention are provided below in connection with the drawings and appended example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a drawing of an abandoned well which penetrates various subsurface formations, including those having fluids originating therefrom. 
     FIG. 2 is a drawing of the abandoned well shown in FIG. 1 which has been plugged in accordance with the method of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, bore hole  2  is shown penetrating the surface  4  of the earth and extending therein and completely passing through subsurface formations  6 ,  8 ,  10 ,  12 ,  14 ,  16 ,  18 , and  20  and entering, but not completely passing through, subsurface formation  22 . Bore hole  2  thus terminates in formation  22 . 
     A tubular metal casing  24  is installed in bore hole  2  to form annulus space  26  between wall  28  of bore hole  2  and exterior surface  30  of casing  24 . A sheath of cement  32  occupies annulus  26  and extends from the bottom  34 , not shown, of casing  24  in subsurface formation  22  to the top of bore hole  2  at surface  4  of the earth. Sheath  32  can be produced by means known in the art. As is well known in the art, sheath  32  supports casing  24  in bore hole  2 , stabilizes bore hole  2  and isolates and protects subsurface formations  6 ,  8 ,  10 ,  12 ,  14 ,  16 ,  18 ,  20  and  22  which are penetrated by bore hole  2 . The structure thus described can be referred to as a well wherein the interior of casing  24  can be adapted to admit and contain fluids originating from formations penetrated by bore hole  2  to enable such fluids to be conducted to surface  4 . For purposes of definition, the portion of bore hole  2  containing fluid originating from formations penetrated by bore hole  2  is referred to as the well bore. In FIGS. 1 and 2 the well bore is the interior of casing  24  and is referred to as well bore  38 . 
     The top  36  of casing  24  is permanently sealed by cap  40 . As shown in FIG. 1, no equipment is connected to well bore  38  to enable the transport of fluids from formations penetrated by bore hole  2  to surface tanks or treating facilities. The well is thus considered to be abandoned. 
     The abandoned well shown in FIG. 1 penetrates several subsurface formations including formations  8  and  12 , which do contain fresh, or potable, water, and formations  16  and  20 , which do not contain fresh water. Well bore  38  is in direct communication with formation  22 , and is also in direct communication with formations  16  and  20 . In this regard holes  42  and  44 , called perforations, made by well known means, exist in the wall of casing  24  and in cement sheath  32  and extend from well bore  38  into formation  16 . Similarly, perforations  46  and  48  extend from well bore  38  through casing  24  and sheath  32  into formation  20 . An unobstructed path of communication thus exists between formations  16  and  20  and surface  4  via perforations  42 ,  44 ,  46  and  48  and well bore  38 . 
     A static column of well fluid  50  originating from either one or both of formations  16  and  20  occupies a portion of well bore  38 . Well fluid  50  is comprised of water, which is not fresh, and can also include hydrocarbons and other components. Formation  22  is in direct contact with well fluid  50 . In one aspect, the hydrostatic pressure produced by well fluid  50  at formation  22  is balanced by the pore pressure of formation  22 , accordingly well fluid  50  does not enter formation  22 . In another aspect, formation  22  is not sufficiently permeable and porous to permit well fluid  50  to drain therein by gravity or by applied pressure less than that required to fracture the same. In this later case, formation  22  could be cement placed in bore hole  2  during primary cementing. Well fluid  50  thus poses a contamination threat to fresh water contained in formations  8  and  12 . 
     The surface  52  of well fluid  50  in well bore  38  is below the top of bore hole  2  at surface  4  of the earth. The natural internal pressure of formations  16  and  20 , that is the pore pressure, is insufficient to cause the surface  52  of well fluid  50  to extend to surface  4 , but is of sufficient intensity to maintain well fluid  50  in a static condition as shown in FIG.  1 . 
     Referring now to FIG. 2, the abandoned well shown in FIG. 1 has been plugged by masses of hardened cementitious material  54 ,  56  and  58  positioned in separate portions of well bore  38  adjacent to formations  8  and  12 , which do contain fresh water, and formations  16  and  20 , which do not contain fresh water. Cementitious material  54 ,  56  and  58  are thus positioned in well bore  38  to protect formations  8  and  12  from fluids, such as salt water and hydrocarbons liquid, which migrate from formations  16  and  20 , and to prevent such fluid migration from formations  16  and  20 . Notice that cementitious material  54  continuously extends from a point below the bottom of formation  12  to cap  40  at top  36  of casing  24  to thereby shield formations  8  and  12  from migrating fluid. Also notice that cementitious material  56  continuously extends from a point below perforations  42  and  44  which penetrate formation  16  to a point above formation  16  to thereby prevent well fluid  50  from entering well bore  38  from formation  16 . Notice further that cementitious material  58  continuously extends from a point at or slightly above perforations  46  and  48  which penetrate formation  20  to a point above formation  20 . The cooperation of cementitious material  58  and formation  22  prevents well fluid  50  from entering well bore  38  from formation  20 . 
     A plug  60  is positioned in well bore  38  at a point at or slightly above perforations  46  and  48 . The bottom of plug  60  is believed to be adjacent to the lowest portion of the lower of perforations  46  and  48 . The side surface of plug  60  is slidably pressed against the interior surface of casing  24  and the bottom surface of plug  60  is supported by well fluid  50  at surface  52   a . Cementitious material  58  is supported by the top surface of plug  60 . The top surface of cementitious material  58  is positioned at a point above the top of formation  20  and at a point below the bottom of formation  16 . 
     A plug  62  is positioned in well bore  38  at a point below perforations  42  and  44 . The side surface of plug  62  is slidably pressed against the interior surface of casing  24  and the bottom surface of plug  62  is supported by the top surface of cementitious material  58 . Spacer fluid  64  is positioned in well bore  38  and is supported by the top surface of plug  62 . The top surface of spacer fluid  64  terminates at a point below perforations  42  and  44 . The combination of plug  60 , cementitious material  58 , plug  62  and spacer fluid  64  is defined herein as a “cementing unit.” 
     A plug  66  is positioned in well bore  38  at a point below perforations  42  and  44 . The side surface of plug  66  is slidably pressed against the interior surface of casing  24  and the bottom surface of plug  66  is supported by the top surface of spacer fluid  64 . Cementitious material  56  is supported by the top surface of plug  66 . The top surface of cementitious material  56  is positioned at a point above the top of formation  16  and at a point below the bottom of formation  12 . 
     A plug  68  is positioned in well bore  38  at a point below the bottom of formation  12 . The side surface of plug  68  is slidably pressed against the interior surface of casing  24  and the bottom surface of plug  68  is supported by the top of cementitious material  56 . Spacer fluid  70  is positioned in well bore  38  and is supported by the top surface of plug  68 . The top surface of spacer fluid  70  terminates at a point below the bottom of formation  12 . The combination of plug  66 , cementitious material  56 , plug  68  and spacer fluid  70  is defined herein as a “cementing unit.” 
     A plug  72  is positioned in well bore  38  at a point below the bottom of formation  12 . The side surface of plug  72  is slidably pressed against the interior surface of casing  24  and the bottom surface of plug  72  is supported by the top of spacer fluid  70 . Cementitious material  54  is supported by the top surface of plug  72 . The bottom surface of cementitious material  54  is positioned at a point below the bottom of formation  12  and the top surface of cementitious  54  extends to cap  40  at top  36  of casing  24 . It is clear that cementitious material  54  extends in a continuous mass from a point below formation  12  to a point above formation  8  and terminates  69  at cap  40 . The combination of plug  72  and cementitious material  56  is defined herein as the “final unit.” 
     It is clear that the abandoned well shown in FIG. 1 is plugged with two cementing units and one final unit as shown in FIG.  2 . 
     Operation of the Invention 
     The porosity, permeability and formation pressure of formations  16  and  20  are first determined by an injectivity test as previously described to verify that the abandoned well is eligible for plugging by the method of this invention. 
     A quantity of cementitious material is then introduced into measuring tank  74  through conduit  76  which includes valve  78 . The quantity of cementitious material thus introduced is equal in volume to the volume of cementitious material  58  required to occupy well bore  38  from a point at perforations  46  and  48  to a point above formation  20 . 
     Plug  60  is then introduced into well bore  38  via line  79  which includes valve  80 . The combination of line  79  and valve  80  is referred to in the well cementing art as a “plug launcher.” The outer surface of plug  60  is adapted to contact and slide along the inner surface of casing  24 . In addition, plug  60 , including the outer surface thereof, is still further adapted to prevent the passage of fluid through or around the plug. Plug  60  thus operates to segregate well fluid  52  in contact with the bottom surface thereof from contacting and otherwise mixing with cementitious  58  in contact with the top surface thereof. 
     Thereafter, valves  78  and  80  are closed, valve  82  between tank  74  and top  36  of casing  24  is opened and pump  84  is activated to thereby transfer the cementitious material previously measured into tank  74  into well bore  38  via conduits  86 ,  88  and  90 . The cementitious material is placed on and supported by the upper surface of plug  60 . The combination of the hydrostatic pressure developed by cementitious material  58  and the pressure generated by pump  84  causes plug  60  to slide within casing  24  and to force at least a portion of well fluid  52  into either one or both of formations  16  and  20  via perforations  42  and  44  and perforations  46  and  48 , respectively. The density of cementitious material  58  is preferably equal to or greater than the density of well fluid  52  in order to minimize the pressure required by pump  84  to force well fluid  52  to enter formations  16  and  20 . 
     A quantity of spacer fluid is then introduced into measuring tank  74  through conduit  92  which includes valve  94 . The quantity of spacer fluid thus introduced is equal in volume to the volume of spacer fluid  64  required to occupy well bore  38  from a point adjacent the top surface of cementitious material  58  to a point below perforations  42  and  44 . 
     Plug  62  is then introduced into well bore  38  via line  79 . Plug  62  and plug  60  are identical in all respects. Plug  62  operates to segregate cementitious fluid  58  in contact with the bottom top surface thereof from contacting and otherwise mixing with spacer fluid  64  in contact with the top surface thereof. 
     Thereafter, valves  94  and  80  are closed, valve  82  between tank  74  and top  36  of casing  24  is opened and pump  84  is activated to thereby transfer the spacer fluid previously measured into tank  74  into well bore  38  via conduits  86 ,  88  and  90 . The spacer fluid is placed on and supported by the top surface of plug  62 . The combination of the hydrostatic pressure developed by spacer fluid  64 , cementitious material  58  and the pressure generated by pump  84  causes plugs  62  and  60  to slide within casing  24  and to force a still further portion of well fluid  52  into either one or both of formations  16  and  20  via perforations  42  and  44  and perforations  46  and  48 , respectively. The bulk density of cementitious material  58  and spacer fluid  64  is preferably equal to or greater than the density of well fluid  52  in order to minimize the pressure required by pump  84  to force well fluid  52  to enter formations  16  and  20 . 
     A single cementing unit consists of the combination of plugs  60  and  62 , cementitious material  58  and spacer fluid  64 . Upon the introduction of this cementing unit a portion of well fluid  52  has been forced into formations  16  and  20 . At this time it believed that the bottom of plug  60  is approaching perforations  46  and  48 . 
     A second cementing unit, consisting of the combination of plugs  66  and  68 , cementitious material  56  and spacer fluid  70  is then introduced into well bore  38  in the manner described for introduction of the first cementing unit. Upon the completion of the introduction of the second cementing unit, it is believed that a still further portion of well fluid  52  is forced into formations  16  and  20 , that the bottom of plug  60  is positioned slightly above, if not adjacent to the lowest portions of perforations  46  and  48 , and the top surface of spacer fluid  70  is positioned above the bottom of formation  12 . 
     A quantity of cementitious material is then introduced into measuring tank  74  through conduit  76 . The quantity of cementitious material thus introduced is equal in volume to the volume of cementitious material  54  required to occupy well bore  38  from a point at or slightly below formation  12  to cap  40 . 
     Plug  72  is then introduced into well bore  38  via line  79 . Plug  72  and plug  60  are identical in all respects. Plug  72  operates to segregate cementitious fluid  54  in contact with the top surface thereof from contacting and otherwise mixing with spacer fluid  70  in contact with the bottom surface thereof. 
     Thereafter, valves  78  and  80  are closed, valve  82  between tank  74  and top  36  of casing  24  is opened and pump  84  is activated to thereby transfer the cementitious material previously measured into tank  74  into well bore  38  via conduits  86 ,  88  and  90 . The cementitious material is placed on and supported by the top surface of plug  72 . The combination of the hydrostatic pressure developed by cementitious materials  54 ,  56  and  58 , spacer fluids  64  and  70  and the pressure generated by pump  84  cause plugs  60 ,  62 ,  66 ,  68  and  72  to slide within casing  24  and to force well fluid  52  into either one or both of formations  16  and  20  via perforations  42  and  44  and perforations  46  and  48 , respectively. The bulk density of cementitious materials  54 ,  56  and  58  and spacer fluids  64  and  70  is preferably greater than the density of well fluid  52  in order to minimize the pressure required by pump  84  to force well fluid  52  to enter formations  16  and  20 . 
     The final unit consists of the combination of plug  72  and cementitious material  54 . 
     Upon the completion of the introduction of the final unit, it is believed that all of well fluid  52  which can be forced into formations  16  and  20  has been forced into formations  16  and  20 . It is also believed that the bottom of plug  60  is positioned at or slightly below the lowest portions of perforations  46  and  48 . It is further believed that the top surface of spacer fluid  70  is positioned below the bottom of formation  12 . It is still further believed that top surface of cementitious material  54  is in contact with the bottom surface of cap  40 . 
     To complete the method, cementitious materials  58 ,  56  and  54  are permitted to set to thereby form the hardened cementitious material as shown in FIG.  2 . 
     The above description features the use of a single measuring tank  74 . Accordingly, the method as described is conducted as a batch process because the tank is employed to contain cementitious material and spacer fluid in alternation. However, the process can be performed in at least a partial continuous flow process by the use of an additional measuring tank and appropriate connecting plumbing. In the continuous process one tank is dedicated to cementitious material and the second is dedicated to spacer fluid. 
     The first liquid can be, and is preferably, delivered to the sight of the well to be plugged in a standard concrete ready-mix truck. This mode of delivery permits the slurry to be prepared at a remote location to thereby avoid the necessity of equipment at the site of the well to store the ingredients and mix the slurry. 
     The cementitious material useful herein can be any material having hydraulic activity which is defined as a material which hardens in the presence of water. Examples of such materials include Portland cement, fly ash, lime, gypsum, granulated blast furnace slag and mixtures thereof. A preferred cementitious material is ASTM Type 1(API Class A) which is readily available in construction concrete yards. 
     In addition, the cementitious material can have, and preferably does have, mixed therewith a quantity of filler, such as graded sand, pozzolan, mortar sand, of the type normally employed in general concrete construction operations, and mixtures thereof. The ratio of cementitious material to filler useful herein is an amount in the range of from about 0.25 to 5, preferably 0.5 to 4 and still more preferably from about 1 to about 2 pounds of filler per pound of cementitious material. The particle size of the filler is in the range of from about 20 to 2000, preferably 50 to 500 and still more preferably from about 100 to about 200 microns. Stated differently, the particle size of the filler is usually in the range of 10 to 325 mesh U.S. Sieve Series or 44 to 2000 microns. 
     It is evident from above that the filler can be present in the first liquid in quantities of up to about 500% of the cementitious material and is thus an important feature of the cement hydration reaction. The filler not only functions as a diluent, but also bonds with the cement to create a solid matrix. The filler in the concentrations involved acts to reduce shrinkage and enhance the strength of the set mass. In addition, the particle size of the filler can enable the filler to act as a bridging agent to prevent or reduce slurry loss if fracture does occur. Still further, the filler aids the effectiveness of low shear mixing ordinarily employed in ready mix applications which permits the preparation and pumping of low viscosity cement which is associated with high set strength cement and reduced water shrinkage. 
     The cementitious material or the combination of cementitious material and filler is mixed with water to produce the first liquid, a slurry, which can be transferred by pump  84  as shown in FIG.  2 . The ratio of cementitious material to water useful herein is an amount in the range of from about 0.36 to 0.56, preferably 0.40 to 0.53 and still more preferably from about 0.44 to about 0.50 pounds of water per pound of cementitious material. 
     Cement set time retarders can also be employed in the first liquid to control the setting of cement employed in the cementing units and final unit to avoid premature hardening while the method is being performed. Set time retarders and the methods of their use are well known in the art of well cementing. 
     The first liquid prepared according to the above recipe has a density in the range of from about 100 to about 150 pounds of slurry per cubic foot of slurry. 
     The pump, such as pump  84  shown in FIG. 2, used to transfer the first liquid (and the second liquid) from measuring tank  74  to well bore  38  is any positive displacement, transfer pump capable of pumping a viscous fluid suspending large-diameter solids. Such pumps useful herein are known as concrete pumps and are capable of being towed on a trailer by a pickup truck. 
     The second liquid functions to space adjacent quantities of slurry and is thus also referred to as a spacer fluid. Spacer fluids remain in the liquid phase and do not harden. The density of spacer fluids employed herein can be less than, equal to or greater than the density of the first liquid. It is merely preferred that the bulk density of the total quantity of first liquid and the total quantity of second liquid employed to form a plug in a particular well bore be greater than the density of the well fluid in that particular well. 
     Spacer fluids known in the art are useful herein. Such fluids, which are preferably inert to the environment in which they are placed, include drilling fluid, water, produced formation water and gelled water containing additives. Examples of such additives are corrosion inhibitors, weighting agents and dispersants. The density of known spacer fluids useful herein can be in the range of from about 63 to about 150 pounds per cubic foot of fluid. 
     The dense spacing discs or plugs are placed between successive quantities of second liquid and first liquid or between well fluid and first liquid as the case may be. The discs, which are ordinarily insoluble solid plugs, are well known in the art of well cementing as wiper plugs and are readily available from a variety of well service company suppliers. The wiper plugs operate to support liquid placed on their top surfaces, to prevent intermixing of the liquids between which the plugs are placed and are designed to fit tightly against the interior wall of a casing and yet readily slide against such wall upon the application of hydrostatic pressure. Examples of such plugs include Haliburton five wiper plugs and Industrial Rubber 
     EXAMPLE 
     A cementitious plug was placed in the well bore of an abandoned well in accordance with the method of this invention. The well contained a 4.5 inch casing and was 2179 feet deep. The annular space was cemented from the bottom to the surface and the casing was perforated at 1681 feet and 1689 feet below the surface. An injectivity test was performed in which 70 barrels of salt water were pumped into the casing. The casing could not be filled with water. The well was on a vacuum. It was reported that the water entered the perforations at about 3 barrels per minute at 0 psi. 
     A cement slurry was prepared and transported to the well location in a ready mix truck. The slurry contained Class A cement, 200% sand by weight of cement, 0.6% lignosulfonate set time retarder by weight of cement and sufficient water to produce a slurry having a density of 18 pounds of slurry per gallon of slurry (134.63 pounds per cubic foot). 
     A rubber plug was placed in the casing. Then, 6.5 barrels of the cement slurry were pumped into the casing on top of the plug. A second rubber plug was placed in the casing on top of the slurry and then 15.6 barrels of salt water spacer fluid were placed on top of the second plug. 
     Thereafter, a third rubber plug was placed in the casing on top of the spacer fluid which was followed by a quantity of slurry required to fill the remainder of the casing, about 350 feet. The well bore was filled to the surface. Operations were terminated. 
     The well was checked the next day. It was observed that the cement had set to a hard mass and that the surface of the mass was about 3 feet below the surface of the earth.