Cementing wells with crack and shatter resistant cement

Improved cement compositions and methods of cementing subterranean zones penetrated by well bores are provided. In accordance with the methods, a crack and shatter resistant cement composition is introduced into a subterranean zone by way of the well bore penetrating it comprised of a hydraulic cement, sufficient hydrophilic fibers to make the cement composition crack and shatter resistant upon setting and sufficient water to form a pumpable slurry. Thereafter, the cement composition is allowed to set in the subterranean zone.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to cementing subterranean zones
 penetrated by well bores, and more particularly, to such methods whereby
 cement compositions which are crack and shatter resistant are utilized.
 2. Descrintion of the Prior Art
 Hydraulic cement compositions are commonly utilized in subterranean well
 completion and remedial operations. For example, hydraulic cement
 compositions are used in primary cementing operations whereby pipe strings
 such as casings and liners are cemented in well bores. In performing
 primary cementing, a hydraulic cement composition is pumped into the
 annular space between the walls of a well bore and the exterior surfaces
 of a pipe string disposed therein. The cement composition is permitted to
 set in the annular space thereby forming an annular sheath of hard
 substantially impermeable cement therein. The cement sheath physically
 supports and positions the pipe string in the well bore and bonds the
 exterior surfaces of the pipe string to the walls of the well bore whereby
 the undesirable migration of fluids between zones or formations penetrated
 by the well bore is prevented.
 Multi-lateral wells have recently been developed which include vertical or
 deviated principal well bores having one or more ancillary laterally
 extending well bores connected thereto. Drilling and completion equipment
 is available which allows multiple laterals to be drilled from a principal
 cased and cemented well bore. Each of the lateral well bores can include a
 liner cemented therein which is tied into the principal well bore. The
 lateral well bores can be drilled into predetermined producingformations
 or zones at any time in the productive life cycle of the well.
 In both conventional wells having single well bores and multi-lateral wells
 having several bores, the cement compositions utilized for cementing
 casings or liners in the well bores must have sufficient ductility and
 toughness to resist cracking or shattering as a result of pipe movements,
 impacts and/or shocks subsequently generated by drilling and other well
 operations. Set cement in wells, and particularly, a set cement. Sheath in
 the annulus between a pipe string and the walls of a well bore often fails
 by cracking or shattering during perforating and/or drilling operations.
 When the set cement cracks or shatters, rubble is often produced in the
 well or annulus.
 Various types of fibers have been utilized in construction cement
 compositions heretofore including fibers formed of glass, steel, graphite,
 polyesters, polyamides and polyolefins. Polyolefin fibers are generally
 the most preferred in that they are readily available, are low in cost and
 have high resistance to corrosion and degradation. Fibrillated net-shaped
 polyolefin fibers are particularly suitable for use in cement compositions
 because they resist being pulled out of the set cement. The fibers
 fimction to control shrinkage cracking in the early stages of the cement
 setting process, and after setting, the fibers provide ductility and
 toughness to the cement composition whereby it resists cracking or
 shattering. When cracking or shattering does occur, the fibers hold the
 cracked or shattered set cement together and prevent the formation of
 rubble.
 A problem heretofore experienced in the use of fibers in well cement
 compositions is that the fibers are hydrophobic and are difficult to dry
 blend with cement. The fibers agglomerate in the dry cement when it is
 conveyed causing plugging to occur, and when the cement and fibers are
 combined with mixing water, the fibers form mats which prevent their
 dispersion into and throughout the cement slurry. The lack of dispersion
 of the fibers in the cement slurry make it difficult to pump.
 Thus, there are needs for improved well cement compositions and methods
 wherein the cement compositions contain fibrillated fibers which can be
 easily mixed and conveyed with dry cement and subsequently dispersed in
 the aqueous cement slurries formed.
 SUMMARY OF THE INVENTION
 The present invention provides improved cement compositions and methods of
 cementing a subterranean zone which meet the needs described above and
 overcome the deficiencies of the prior art. The methods of the invention
 are basically comprised of the steps of introducing a crack and shatter
 resistant cement composition into a subterranean zone to be cemented and
 allowing the cement composition to set therein. The crack and shatter
 resistant cement compositions utilized in accordance with this invention
 are basically comprised of a hydraulic cement, sufficient hydrophilic
 fibers to make the cement composition crack and shatter resistant upon
 setting and sufficient water to form a pumpable slurry of the cement and
 fibers. The hydrophilic fibers utilized in accordance with this invention
 are preferably hydrophilic fibrillated polyolefin fibers.
 The methods of this invention are particularly suitable for cementing a
 pipe string such as casing or a liner in a well bore whereby the set
 cement can withstand the formation of perforations therein as well as
 other impacts and shocks subsequently generated by drilling and other well
 operations without cracking or shattering and forming rubble. Such methods
 are basically comprised of the following steps. A crack and shatter
 resistant cement composition of this invention is pumped into the annulus
 between the pipe string and the walls of the well bore. The cement
 composition is then allowed to set into a hard crack and shatter resistant
 impermeable mass having ductility and toughness.
 It is, therefore, a general object of the present invention to provide
 improved cement compositions and methods of cementing subterranean zones
 penetrated by well bores using the cement compositions.
 Other and further objects, features and advantages of the present invention
 will be readily apparent to those skilled in the art upon a reading of the
 description of preferred embodiments which follows.
 DESCRIPTION Of PREFERRED EMBODIMENTS
 The present invention provides improved crack and shatter resistant cement
 compositions and methods of cementing subterranean zones using the
 compositions. The improved crack and shatter resistant cement compositions
 are basically comprised of a hydraulic cement, sufficient hydrophilic
 fibers to make the cement composition crack and shatter resistant upon
 setting and sufficient water to form a pumpable slurry of the cement and
 fibers. The methods are basically comprised of the steps of introducing a
 crack and shatter resistant cement composition of this invention into the
 zone by way of the well bore and then allowing the cement composition to
 set in the zone.
 The term "crack and shatter resistant cement composition" is used herein to
 mean a cement composition that sets into a hard impermeable mass having
 ductility and toughness and that resists cracking and/or shattering as a
 result of perforating operations, pipe movements, impacts, shocks and the
 like. If cracking or shattering does occur, the pieces formed are held
 together by the hydrophilic fibers in the cement composition.
 While the cement compositions and methods of this invention are useful in a
 variety of well completion and remedial operations, they are particularly
 useful in primary cementing, i.e., cementing casings and liners in well
 bores. The crack and shatter resistant cement compositions of this
 invention are readily and easily prepared without the conveying, mixing,
 fiber dispersal and pumping problems encountered heretofore.
 A variety of hydraulic cements can be utilized in the crack and shatter
 resistant cement compositions of this invention including those comprised
 of calcium, aluminum, silicon, oxygen and/or sulfur which set and harden
 by reaction with water. Such hydraulic cements include Portland cements,
 pozzolana cements, gypsum cements, high aluminum content cements, silica
 cements and high alkalinity cements. Portland cements are generally
 preferred for use in accordance with the present invention. Portland
 cements of the types defined and described in the API Specification For
 Materials And Testing For Well Cements, API Specification 10, Fifth
 Edition, dated Jul. 1, 1990 of the American Petroleum Institute are
 particularly suitable. Preferred API Portland cements include classes A,
 B, C, G and H, with API classes G and H being the most preferred.
 While fibers formed of various materials can be utilized in accordance with
 the present invention, the fibers utilized must resist degradation in a
 hydraulic cement composition. For example, fibers formed of polyesters,
 polyamides and glass suffer from the disadvantage that they degrade in the
 presence of hydrated lime. Hydrated lime is released in a cement
 composition as the cement therein is hydrated. Polyolefin fibers are
 suitable for use in cement compositions in that polyolefin fibers do not
 degrade or otherwise loose their strength over time in a set cement
 composition. However, fibers formed from polyolefins are hydrophobic and
 are very difficult to dry blend with hydraulic cements and disperse in
 water. Because they are hydrophobic, the polyolefin fibers cluster
 together when mixed with water and do not disperse therein. When cement
 slurries containing such non-dispersed fibers are pumped in high pressure
 pumps, difficulties are encountered due to the fiber clusters plugging off
 lines, valves and the like.
 In accordance with the present invention, hydrophilic polyolefin fibers are
 included in the crack and shatter resistant cement compositions. The
 hydrophilic polyolefin fibers do not degrade in cement compositions and
 are readily dry mixed with cement and dispersed in the cement mixing
 water. Particularly suitable such hydrophilic polyolefin fibers are
 commercially available from the Forta Corporation of Grove City, Pa.
 The preferred polyolefin fibers are polypropylene or polyethylene fibers
 which are in a fibrillated net configuration which maximizes the long term
 durability and toughness of a cement composition including the fibers. The
 fibrillated net-shaped fibers fimction exceptionally well in preventing
 cracking or shattering of cement compositions containing them, and if
 cracking or shattering does occur, in holding the cracked or shattered
 cement together, i.e., the individual pieces produced are held together by
 the fibers thereby preventing rubble formation.
 The normally hydrophobic polyolefin fibers are converted to hydrophilic
 fibers by treating the hydrophobic fibers with a surface active agent. The
 most preferred hydrophilic fibers for use in accordance with the present
 invention are hydrophilic polypropylene fibrillated net-shaped fibers
 having lengths in the range of from about 0.5 inch to about 1.5 inches.
 Generally, the hydrophilic fibers utilized are included in a cement
 composition of this invention in an amount in the range of from about 0.1%
 to about 1% by weight of hydraulic cement in the composition, more
 preferably in an amount in the range of from about 0.125% to about 0.5%.
 The water utilized in the cement compositions of this invention can be
 fresh water, unsaturated aqueous salt solutions or saturated aqueous salt
 solutions such as brine or seawater. The water is generally present in the
 cement compositions in an amount in the range of from about 30% to about
 100% by weight of hydraulic cement in the compositions, more preferably in
 an amount in the range of from about 35% to about 60%.
 As will be understood by those skilled in the art, the crack and shatter
 resistant cement compositions of this invention can include a variety of
 additives for improving or changing the properties of the cement
 compositions. Examples of such additives include, but are not limited to,
 set retarding agents, fluid loss control agents, dispersing agents, set
 accelerating agents and formation conditioning agents.
 Set retarding agents are included in the cement compositions when it is
 necessary to extend the time in which the cement compositions can be
 pumped so that they will not thicken or set prior to being placed at a
 desired location in a well. Examples of set retarding agents which can be
 used include, but are not limited to, lignosulfonates such as calcium and
 sodium lignosulfonate, organic acids such as tartaric acid and gluconic
 acid, copolymers and others. The proper amount of set retarding agent
 required for particular conditions can be determined by conducting a
 "thickening time test" for the particular retarder and cement composition.
 Such tests are described in the API Specification 10 mentioned above. A
 particularly preferred set retarder for use in accordance with the present
 invention is a copolymer or copolymer salt of 2-acrylamido-2-methylpropane
 sulfonic acid and acrylic acid. The copolymer comprises from about 40 to
 about 60 mole percent 2-acrylamido-2-methylpropane sulfonic acid with the
 balance comprising acrylic acid, and the copolymer or salt thereof
 preferably has an average molecular weight below about 5,000. When used, a
 set retarder is included in the cement composition of this invention in an
 amount in the range of from about 0.1% to about 2% by weight of hydraulic
 cement in the composition.
 Examples of fluid loss control agents which can be used include, but are
 not limited to, cellulose derivatives, modified polysaccharides,
 polyacrylamides, guar gum derivatives, 2-acrylamido-2-methylpropane
 sulfonic acid copolymers, polyethyleneimine and the like.
 An example of a dispersing agent which can be utilized is comprised of the
 condensation polymer product of an aliphatic ketone, an aliphatic aldehyde
 and a compound which introduces acid groups into the polymer, e.g., sodium
 bisulfite. Such a dispersant is described in U.S. Pat. No. 4,557,763
 issued to George et al. on December 10, 1985.
 Examples of set accelerating agents which can be utilized include, but are
 not limited to, calcium chloride, zinc formate and triethanolamine, and
 examples of formation conditioning agents include, but are not limited to,
 potassium chloride and sodium chloride.
 A method of the present invention for cementing a subterranean zone
 penetrated by a well bore comprises the steps of:
 (a) introducing a crack and shatter resistant cement composition into the
 zone by way of the well bore, the cement composition comprising a
 hydraulic cement, sufficient hydrophilic fibers to make the cement
 composition crack and shatter resistant upon setting and sufficient water
 to form a pumpable slurry of the cement and fibers; and
 (b) allowing the cement composition to set in the zone. A more preferred
 method of cementing a subterranean zone penetrated by a well bore
 comprises the steps of:
 (a) pumping a crack and shatter resistant cement composition into the zone
 by way of the well bore, the cement composition comprising Portland
 cement, sufficient hydrophilic polyethylene fibrillated net-shaped fibers
 to make the cement composition crack and shatter resistant upon setting
 and sufficient water to form a pumpable slurry of the cement and fibers;
 and
 (b) allowing the cement composition to set in the zone. A preferred method
 of this invention for cementing a pipe string, such as casing or a liner,
 in a well bore whereby the set cement can withstand the formation of
 perforations therein as well as other impacts and shocks subsequently
 generated by drilling or other well operations without cracking or
 shattering and forming rubble is comprised of the steps of:
 (a) pumping a crack and shatter resistant cement composition into the
 annulus between the pipe string and the walls of the well bore, the cement
 composition comprising Portland API Class G or H cement, hydrophilic
 polyethylene fibrillated net- shaped fibers present in an amount in the
 range of from about 0.125% to about 0.5% by weight of cement in the
 composition and water present in an amount in the range of from about 38%
 to about 46% by weight of cement in the composition; and
 (b) allowing the cement composition to set into a hard crack and shatter
 resistant impermeable mass having ductility and toughness.
 In order to further illustrate the methods of the present invention the
 following example is given.

EXAMPLE
 A base cement composition comprised of Portland API Class H cement and
 fresh water present in an amount of about 38% by weight of the cement
 having a density of 16.4 pounds per gallon was prepared. A portion of the
 base cement composition without fibers as well as portions thereof with
 hydrophobic polypropylene fibers and hydrophilic polypropylene fibers were
 tested for mechanical properties in accordance with API RP 10B. The fibers
 utilized and their quantities along with the results of the tests are set
 forth in the Table below.
 TABLE
 Mechanical Properties.sup.1 Of Fiber Containing Cement
 Compositions
 Quantity of Plastic Failure
 Tests
 Fibers, % by
 1000 psi Brazilian Young's
 Test Type of weight of Compressive Tensile Unconfined
 Confined Tensile Modulus Poisson's
 No. Fibers cement Strength, psi Strength.sup.2, psi Strength,
 psi Strength, psi Strength, psi (Ex 10.sup.6) Ratio
 1 None -- 4120 467 6910
 8286 292 1.91 0.193
 2 Hydrophobic 0.125 3610 504 6639
 8541 837 1.67 0.138
 Polypropylene
 3 Hydrophobic 0.25 3590 512 6006
 7972 635 1.48 0.14
 Polypropylene
 4 Hydrophobic 0.5 3760 492 5523
 7444 645 1.47 0.11
 Polypropylene
 5 Hydrophilic 0.125 3970 556 5523
 8605 727 1.5 0.13
 Polypropylene
 6 Hydrophilic 0.25 3750 493 5373
 7623 705 1.45 0.124
 Polypropylene
 7 Hydrophilic 0.5 3280 456 4933
 7025 669 1.53 0.117
 Polypropylene
 .sup.1 The test samples were cured at 140.degree. F. for 72 hours prior to
 testing
 .sup.2 Briquetts were prepared for tensile strength measurements
 From the Table it can be seen that the mechanical properties of the test
 cement portions containing hydrophilic fibers are essentially the same as
 the test cement portions containing hydrophobic polypropylene fibers. In
 addition, the hydrophilic fibers were easily dry blended with the cement
 and readily dispersed in the mixing water while the hydrophobic fibers
 were not.
 Thus, the present invention is well adapted to attain the objects and
 advantages mentioned as well as those which are inherent therein. While
 numerous changes may be made by those skilled in the art, such changes are
 encompassed within the spirit of this invention as defined by the appended
 claims.