Teoría de los sectores económicos – productivos

James H.Cobbs P.E., B.Sc. Pet.Eng., M.S.P.E., M.N.S.P.E., M.O.S.P.E., M.N.A.F.E., F.A.A.A.S.

David C.Cobbs P.E., B.A.(Phy.), M.S.P.E., M.S.E.G., M.N.S.P.E., M.O.S.P.E.

Cobbs Engineering, Inc., Tulsa, Oklahoma, U.S.A.

SYNOPSIS

Because of their widespread distribution, the chalks of England and Europe are likely to see increasing use of drilled shafts either into or through them.

Seven drilled shafts have been constructed in chalk with three of them sealed successfully by initial grouting Four of the shafts were problems where remedial grouting caused serious economic consequences.

These shafts have been studied, along with drilled shaft sealing in general, to present the best possible grouting practice for insuring successful sealing with the initial grout placement.

INTRODUCTION

Because of its widespread occurrence in both England and Europe, shafts drilled into or through chalk probably will become common. The presence of aquifers within the total chalk interval and the unique characteristics of the chalk present both opportunities and problems of shaft sealing which need consideration by designers, contractors and owners if the maximum economic potential of drilled shafts is to be realized.

Seven shafts have been drilled in chalk at three sites. One site with one shaft is in the state of Alabama in the United States, one site with two shafts near Rouen, France, and four shafts near Killingholm, Humberside, England. Of the seven drilled shafts, the first three were successfully sealed by the initial grouting while the other four all suffered sealing problems with serious economic consequences. The quite variable results achieved in these seven shafts demonstrate the need to study and better understand what constitutes good grouting technique as applied specifically to chalk.

CHARACTERISTICS OF CHALK

Chalk is not a uniform material but varies, depending on the amount of foreign material included in it. White chalk characteristically is composed of two principal materials, molluscan debris and foraminifera which range in size from 10–100 microns within a matrix composed of coccoliths in the range of 0.5−4 microns in size. Commonly the smaller size will constitute from 75– 90% of the total chalk volume while the larger grains will constitute 10–25% of the chalk. The chalk will characteristically be very deficient in grains of greater than 100 microns and grains in the 4–10 micron range. The grains within the chalk usually are weakly cemented with calcareous material, primarily calcite. The degree of cementation can be variable itself, contributing to the variations which have been observed in the chalk. Hancock¹, Black², and Boswell³ have described the charactristics of the chalk extensively.

Another characteristic of the chalk, described by Jenner and Burfitt⁴, is the positive electrical charge carried on the individual chalk grains. Each grain then is surrounded by a halo of pearlized water.

It is the very fine grain size and the electropositive surface charge which the grains carry that makes chalk one of the most unforgiving materials for successful sealing of shaft liners.

The arbitrary and indefinite division usually applied between silt and colloidal size particles is 1 micron. Colloidal activity depends on the specific surface of the particle which varies with particle shape and on the surface potential. In some cases smaller size particles will not be colloidal and in others particles of greater than 1 micron and smaller size most assuredly will be colloidal in behavior and it is quite likely that the boundary between silt size and colloidal size particles will approach 4 microns, the upper limit of the coccolith fragments.

The weak cementing and fine particle size of the chalk presents problems in separating drilled solids from the circulating drilling mud during the drilling operation. Commution of the chalk is a requirement for drilling and additional commution can be expected from collisions of the drilled particles within the circulating system as they are circulated to the surface. Any drilled solids not removed from the circulating media on the surface will suffer further commution as they are recirculated in

the drilling operation. It is not uncommon to have the drilled solids accumulate within the drilling mud until they constitute 30% or more by volume of the total solids content of the mud. Ideal drilling muds will maintain less than 10% total solids content as determined by recovery on a 200 mesh screen and in some instances efforts are made to maintain total solids as low as 5%.

It is not the purpose of this paper to discuss the techniques for removal of drilled solids from mud systems but rather to point out that high concentrations of chalk solids can develop in the drill ing mud which will adversely affect the sealing of the shaft liner with grout.

CHARACTERISTICS OF DRILLING MUD

During the grouting phase of the shaft construction there is a complex interaction between the drilling mud, the Portland cement grout and the included drilled solids in the system. The characteristics of the mud will make an important contribution to the overall effectiveness of the grouting operation so it is appropriate to familiarize the reader with the basic objectives to be achieved with a mud.

Eight principal functions of a drilling mud are: 1) to remove cuttings from the bottom of the hole and carry them to the surface; 2) to cool the bit; 3) to plaster the wall of the hole with an impermeable cake; 4) to control subsurface pressures; 5) to hold cuttings in suspension when circulation is interrupted; 6) to release cuttings at the surface; 7) to support part of the weight of the drill pipe and casing in the hole; 8) to reduce to a minimum any adverse effects on the formation adjacent to the borehole.

The relative importance of any of these functions or combinations of functions is determined by the size of the hole being drilled, the formations being penetrated and the fluid content of those formations. Each of the functions of the mud and mud properties most influential on these functions are discussed individually and the ideal mud then is described.

Removal of cuttings

The removal of cuttings from the face of the bore is probably the most important function of the mud. Adequate cleaning of the face insures longer bit life and greater drilling efficiency. The mud rising from the bottom of the hole carries the cuttings to the surface. Under the influence of gravity, the cuttings are always falling relative to the motion of the mud but with adequate velocity, density and viscosity the net velocity of the cuttings will be ascending.

Cooling the bit

Heat is generated by friction on the bit where the teeth react with the formation being drilled. There is little chance for this heat to be conducted away through the formation so it must be removed by the circulating fluid. The heat having been transmitted from points of friction to the mud is lost to the atmosphere at the surface.

Hole wall

A good mud should deposit a tough filter cake on the borehole wall opposite formations with permeability to aid in consolidation of the formation and to retard the passage of fluid into or out of the formation. This property of the mud is improved by increasing the colloidal fraction of the mud with addition of bentonite and chemically treating the mud to improve the solids distribution.

Pressure control

Proper restraint of formation fluid pressures and support of the formation depends on the density of the mud. Under normal circumstances for shaft drilling, water as a circulating fluid or mud with some additional contained solids will be more than sufficient to balance formation pressure and prevent the entry of unwanted fluids into the borehole. Occasions of abnormal pressure which require specialized solids of higher specific gravity are rarely a factor in shaft drilling.

Cuttings suspension

Good drilling muds have properties that cause the solids being carried to the surface to be held in suspension due to a gellation or thixotropy which develops when circulation is interrupted. On resumption of circulation, the mud will revert to its fluid condition and carry the suspended particles to the surface.

Cuttings release at the surface

A mud with high suspending ability for removal of cuttings from the borehole will be one which only reluctantly releases the cuttings at the surface. Either long surface retention time or mechanical solids removal equipment is required to remove cuttings effectively from the suspending mud.

Support

With increasing hole size and depth, the weight supported by surface equipment becomes increasingly important. Since both drill pipe and the casing or lining material to be installed are buoyed by a force equal to the weight of mud displaced, an increase in mud density will result in a reduction of the total weight which the surface equipment must support.

Support of adjacent formation

When the borehole penetrates a formation of high in situ stress, the formation may spall because of the asymmetry of the in situ horizontal stress field. Some formations are reactive with water and can hydrate and slough when exposed to the drilling mud. Also when drilling highly fractured formations, the formation may tend to slough because of sliding along the naturally occurring or induced fracture planes which is promoted by the lubrication of any penetrating liquid from the drilling mud.

Increasing the density of the drilling mud will work to stabilize the borehole wall and minimize, if not prevent, spalling or sloughing as a result of the in situ stress or natural fracturing within the formation. Formation reaction with the drilling mud can be controlled by the appropriate chemical treatment of the mud prior to the penetration of a reactive formation.

The ideal mud is one which has sufficient density to offer support to the borehole yet reverts to very low density when flowing beneath the drill bit to effectively scour cuttings from the bottom of the hole. This mud will also have a relatively high viscosity when carrying cuttings to the surface along with adequate gel strength to hold these cuttings in suspension when circulation is interrupted. When this hypothetical mud reaches the surface it will revert to low density, low viscosity and low gel strength so that the cuttings will drop rapidly from the mud. It is obviously impossible to achieve this hypothetical ideal mud, so a series of compromises must be made to get the desirable properties for a most nearly optimum mud.

Drilling muds almost universally contain bentonite as the gelling material to impart the desirable rheological properties to the mud and this is usually the most satisfactory and economical material. In some instances, water alone can be used for the circulating fluid but frequently this is not the case.

Muds containing bentonite are characteristically intolerant of contamination by the calcium ion. Because the reaction of Portland cement to water liberates a large number of free calcium ions, the drilling mud is normally quite intolerant of cement contamination unless special protective measures are taken^5–7.

REACTIONS OF PORTLAND CEMENT

The metamorphosis of Portland cement and water from a slurry of anhydrous particles to a set grout is a complex series of chemical reactions. During reaction time a number of intermediate compounds are formed which create a liquor rich in these intermediates. As the reaction proceeds, the intermediate compounds further react as does the water itself so that the finished mater- ial is a hydrous crystalline mass with almost no free water and considerable strength. For our consideration the intermediate compound of greatest importance is calcium hydroxide which supersaturates the cement liquor. Part of the calcium hydroxide will crystallize as free calcium hydroxide subject to leaching but the majority will react with other intermediates to become a part of the strength-creating series of compounds. During the time while the cement liquor is rich in calcium hydroxide, it can react with surrounding materials as well as with the cementacious materials. This is the critical period during which unwanted reactions can occur.

Nearly all bentonite drilling mud formulations are intolerant of high concentrations of calcium ions. This is especially true when the pH of the mud is increased by the addition of hydroxil ions. The effect on the mud of increasing the calcium ion concentration is to flocculate it with a very rapid increase in its apparent viscosity, yield point and gel strength. This action tends to form a layer of flocculated mud at the mud/grout interface, promoting viscous fingering or channeling of grout through the mud and leaving pockets or channels of mud beneath the advancing front of grout.

The flocculation of the bentonite in the mud is undesirable but with the usual concentration of 5% or less by volume, it may not by itself present an unmanageable problem. The greatest problem is probably the loss of the power to suspend drilled solids included in the mud.

The calcium ion, having a positive charge, tends to cause a collapse of the diffused double layer around the bentonite particles where gravitational attraction then brings them together to form flocs.

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Just as the calcium ion causes flocculation of the bentonite in a drilling mud, the hydroxil ion in the cement liquor will neutralize the diffuse double layer around the positively charged chalk particle and aggregates of chalk grains will begin to form in the flocs of increasing mass. Further increases in the anionic concentration will cause the individual flocs to band together in an extensive gel structure.

Robinson⁹, along with Witten and Cates¹⁰, describe the process of viscous fingering of one liquid through another.

Effective shaft sealing then becomes a matter of preventing or minimizing the adverse effects of the contact between the grout slurry and the drilling mud. Good grouting practices have been established through the years by painful trial and error.

The basic good practice does not vary regardless of the formations to be sealed, but because of the unforgiving nature of chalk the observance of good grouting practice is even more imperative than when other formations are to be sealed.

THE EFFECT OF THE CASING

The objective of grouting a casing or a liner into a borehole is to support the casing and seal the annulus so that there will be essentially no movement of unwanted fluids either up or down the annulus. To achieve these goals requires that the in situ annular fluid be replaced by the grout material.

To achieve nearly perfect replacement of in situ fluid by grout material begins with the design of the casing itself. Provision for grouting must be made and provision for maintaining an adequate standoff of the casing from the borehole wall must also be provided. The displacement of the in situ fluid by the grout material is analogous to the tremi placement of concrete where the difference in density causes the in situ fluid to essentially float on top of the displacing grout. Since both the mud and chalk are thixotropic, if there is continuity of either left in the annular space, it will provide a channel for the migration of fluids within the annulus. If a water-bearing zone is crossed by a stringer or channel of mud or chalk solids it can be displaced by the formation water to provide a leakage channel with potentially undesirable consequences.

If the casing or liner is in contact with the wall of the borehole there will be a dead area on either side of the contact where the in situ material cannot be displaced by the ascending grout. Fig. 1 is a plan section through a borehole which diagramatically illustrates trapping in the vicinity of a casing borehole contact point. Even eccentricity of the casing within the borehole will cause a non-uniform rise of grout with the potential for trapping channels of undesir able material within the body of the grout. Vaughn¹¹, studied the effect of the flow of non-Newtonian fluids in eccentric annuli, primarily directed toward the grouting of oil and gas well casing where the clearance between the borehole and casing is much smaller than it is in a shaft. Even though the eccentricity is less in shaft grouting than oil and gas well grouting, the effect is still present and it is illustrated in Fig. 2.

To prevent actual borehole contact and/or unacceptable degrees of eccentricity of casing within the borehole, standoff or centralizing devices are installed on the casing prior to its installation. These devices normally are a T section with several of them arranged circumferentially around the casing with the long axis parallel to the longitudinal axis of the casing. Fig. 3 illustrates a ring of centralizers on a section of casing. The centralizers are not normally extended to establish full contact with the borehole wall but rather extend from the casing wall enough to assure minimum standoff and minimize the eccentricity of the casing in the borehole.

Because some eccentricity of the casing in the borehole is inevitable, to minimize the effects of the eccentricity, as illustrated in Fig. 3, multiple grout lines are commonly used to assure maximum circumferential continuity of the grout around the casing. The number of grout lines used varies with the diameter of the casing and should never be less than four. In the larger diameter casings it is common to use six or eight grout lines equally spaced circumferentially around the casing.

Fig.1 Effect of casing borehole contact

To assure the placement of grout lines during the grouting operation, grout line guides are installed on the casing. These guides usually are pipe with a sufficient diameter to accept the grout lines. On occasion, square tubing has been used for grout line guides but most commonly it is standard pipe. These guides function only to provide guidance for the grout lines so they are penetrated by a series of holes through which the grout can flow during the grouting operation. These openings usually are flame cut with the area of each hole being equal to or greater than the cross sectional flow area of the grout line. The penetrations of the grout line guides are commonly in a spiral pattern around the guide with spacing between holes usually about 2 ft. (600 mm). Fig. 4 shows the placement of grout line guides around a section or joint of casing.

In two of the four shafts which required remedial work, continuous grout line guides were used. Only short sections of pipe attached to the casing of approximately 3 m vertical intervals were used on the other two. Unfortunately, the grout lines either failed to thread through all of the guides or the guides caused the lines to snag. As a result, several intervals of grout line were pulled apart and dropped in the annulus. This probably contributed to an uneven grout rise and the trapping of mud and chalk

In two of the four shafts which required remedial work, continuous grout line guides were used. Only short sections of pipe attached to the casing of approximately 3 m vertical intervals were used on the other two. Unfortunately, the grout lines either failed to thread through all of the guides or the guides caused the lines to snag. As a result, several intervals of grout line were pulled apart and dropped in the annulus. This probably contributed to an uneven grout rise and the trapping of mud and chalk