1. Field of the Invention
This invention relates generally to methods and compositions for cementing, and more specifically to methods and flexible cement compositions for cementing in high stress and high temperature environments.
2. Description of Related Art
Cementing is a common technique employed during many phases of wellbore operations. For example, cement may be employed to cement or secure various casing strings and/or liners in a well. In other cases, cementing may be used in remedial operations to repair casing and/or to achieve formation isolation. In still other cases, cementing may be employed during well abandonment. Cement operations performed in wellbores under high stress conditions may present particular problems, among other things, difficulty in obtaining good wellbore isolation and/or maintaining mechanical integrity of the wellbore. These problems may be exacerbated in those cases where wellbore and/or formation conditions promote fluid intrusion into the wellbore, including intrusion of water, gas, or other fluids.
In a wellbore, cement may be used to serve several purposes. Among these purposes are to selectively isolate particular areas of a wellbore from other areas of the wellbore. For example, in primary cementing, cement is commonly placed in the annulus created between the outside surface of a pipe string and the inside formation surface or wall of a wellbore in order to form a sheath to seal off fluid and/or solid production from formations penetrated by the wellbore. This isolation allows a wellbore to be selectively completed to allow production from, or injection into, one or more productive formations penetrated by the wellbore. In other cases cement may be used for purposes including, but not limited to, sealing off perforations, repairing casing leak/s (including leaks from damaged areas of the casing), plugging back or sealing off the lower section of a wellbore, sealing the interior of a wellbore during abandonment operations, etc.
One important objective of a primary cement job is to provide good isolation between producing zones up to the surface and in a manner that will endure through the entire life of the well. No fluid movement, either gas or liquid, is normally desirable at any time through the cemented annulus. In this regard, possible paths for fluid movement in the annulus include the interface between cement/rock and cement/casing and the cement matrix. Cement adherence to the formation and casing is primary affected by cement shrinkage and by stress changes induced by down-hole variations on pressure and temperature, especially inside the casing but also at the formation.
Conventional well cement compositions are typically brittle when cured. These conventional cement compositions often fail due to stresses, such as compressional, tensile and/or shear stresses, that are exerted on the set cement. Wellbore cements may be subjected to shear and compressional stresses that result from a variety of causes. For example, stress conditions may be induced by relatively high temperatures and/or relatively high fluid pressures encountered inside cemented wellbore pipe strings during operations such as perforating, stimulation, injection, testing, production, etc. Stress conditions may also be induced or aggravated by fluctuations or cycling in temperature or fluid pressures during similar operations. Variations in temperature and internal pressure of the wellbore pipe string may result in radial and longitudinal pipe expansion and/or contraction which tends to place stress on, among other things, the annular cement sheath existing between the outside surface of a pipe string and the inside formation surface or wall of a wellbore. Such stresses may also be induced in cement present in other areas of the wellbore in the pipe.
In other cases, cements placed in wellbores may be subjected to mechanical stress induced by vibrations and impacts resulting from operations, for example, in which wireline and pipe conveyed assembly are moved within the wellbore. Hydraulic, thermal and mechanical stresses may also be induced from forces and changes in forces existing outside the cement sheath surrounding a pipe string. For example, overburden and formation pressures, formation temperatures, formation shifting, etc. may cause stress on cement within a wellbore.
Conventional wellbore cements typically react to excessive stress by failing. As used herein, xe2x80x9ccement failurexe2x80x9d means cracking, shattering, debonding from attached surfaces (such as exterior surfaces of a pipe string and/or the wellbore face), or otherwise losing its original properties of strength and/or cohesion. Stress-induced cement failure typically results in loss of formation isolation and/or wellbore mechanical integrity. This in turn may result in loss of production, loss of the wellbore, pollution, and/or hazardous conditions.
Injection or production of high temperature fluids may cause thermal expansion of trapped fluids located, for example, between a pipe string and a cement sheath, between a cement sheath and the formation, and/or within the cement sheath. Such trapped fluids may create excessive pressure differentials when heated and/or cooled, resulting in cement failure. Thermal cycling (such as created by intermittent injection or production of fluids that are very warm or cool relative to the formation temperature), typically increase the likelihood of cement failure.
In still other cases, mechanical and/or hydraulic forces exerted on the exterior of a cement sheath may cause stress-induced cement failure. Such forces include, but are not limited to, overburden pressures, formation shifting, and/or exposure to overpressured fluids within a formation. Increased pressure differential, such as may be caused when the interior of a cemented pipe string is partially or completely evacuated of liquid, also tends to promote cement failure, especially when combined with relatively high pressures exerted on the exterior of a cement sheath surrounding the cemented pipe string.
In addition, any type of thermal, mechanical or hydraulic stress that acts directly on a set cement composition, or which tends to cause deformation of a wellbore tubular in contact with a set cement composition may promote, or result in, failure of a conventional cement composition.
Natural fiber-containing cementing systems and methods are provided in which cement slurries may be formulated to provide hardened cement compositions possessing relatively high resilience, elasticity, and/or ductility at relatively high temperatures. In one embodiment, such hardened cement compositions may be characterized as having an increased ratio of flexural strength to compressive strength as compared to conventional cement compositions. As used herein, a xe2x80x9chardened cement compositionxe2x80x9d means a cured or set cement slurry composition.
The disclosed cement formulations may be advantageously used to cement wellbores in relatively high temperature environments where high stress resistance is required. These include oil/gas, water and geothermal wells in which high stress conditions exist or in which cement will be subjected to conditions of high stress including, but not limited to, those types of wellbores discussed above. Specific examples of such wells include, but are not limited to, wells having slimhole completions, highly deviated or horizontal wells, wells exposed to thermal and/or pressure cycling, high perforation density completions, wells completed in formations subject to relatively high overburden and/or fluid pressures, and wells having junction points between a primary wellbore and one or more lateral wellbores. Such cement systems are typically characterized by the ability to provide the ductility needed to withstand impacts and shocks of well operations and/or stresses induced by temperature and/or fluid production/injection, while at the same time providing relatively high compressive strength.
As disclosed herein, a natural fiber-containing cementing system may comprise a hydraulic cement, water, and at least one natural mineral fiber material, such as at least one fibrous calcium silicate material. Examples of suitable calcium silicate fibers include, but are not limited to, wollastonite pyrophillite, algamatolite, etc. or a mixture thereof Other cementing additives including, but not limited to, fibers, aluminum silicate (such as a metakaolin), fluid loss additives, set retarders, dispersants, etc. may also be optionally employed.
In one embodiment using the disclosed cement compositions containing natural mineral fiber material, a surprising increase in the ratio of flexural strength/compressive strength (i.e., above about 0.35) may be advantageously achieved at downhole temperatures above about 180xc2x0 F., and particularly at downhole temperatures above about 240xc2x0 F., with a fibrous mineral content (e.g., wollastonite) of from about 10% to about 150% by weight of base cement (xe2x80x9cBWOCxe2x80x9d).
In one respect, disclosed is a method of cementing within a wellbore, including introducing a cement slurry including a hydraulic cement base and a natural mineral fiber into the wellbore; and allowing the cement slurry to cure within the wellbore to form a hardened cement composition within the wellbore; wherein a temperature of at least a first portion of the well bore is greater than about 180xc2x0 F.; wherein the natural mineral fiber is present in the cement slurry in an amount greater than about 10% by weight of cement, and is also present in the cement slurry in an amount selected to be effective to result in at least a portion of the cured cement composition having a ratio of flexural strength to compressive strength that is greater than or equal to about 0.35 at the temperature of the at least a first portion of the well bore that is greater than about 180xc2x0 F. Examples of natural mineral fibers that may be employed may include, but are not limited to, at least one of wollastonite, pyrophillite, algamatolite, or a mixture thereof.
In one embodiment of this method, the natural mineral fiber may be present in the cement slurry in an amount selected to be effective to result in at least a portion of the cured cement composition having a ratio of flexural strength to compressive strength that is greater than or equal to about 50% higher than the ratio of flexural strength to compressive strength of a cured conventional cement composition having substantially the same composition, but without the natural mineral fiber component, at the temperature of the at least a first portion of the wellbore that is greater than about 180xc2x0 F.
In another embodiment of this method, a temperature of the at least a first portion of the well bore is less than about 180xc2x0 F. when the cement slurry is introduced into the wellbore and allowed to cure; and further including allowing the temperature of the at least a first portion of the wellbore to rise above about 180xc2x0 F.; wherein the natural mineral fiber is present in the cement slurry in an amount selected to be effective to result in an increase in the compressive strength of at least a portion of the cured cement composition when the temperature of the at least a first portion of the wellbore is allowed to rise above about 180xc2x0 F.
In another embodiment, disclosed is a fiber-containing cement composition, comprising a hydraulic cement base and a natural mineral fiber; wherein said natural mineral fiber is present in an amount greater than about 10% by weight of cement; wherein said natural mineral fiber is also present in said fiber-containing cement composition in an amount selected to be effective so as to result in cement slurry and a cured cement composition formed from said cement slurry having a ratio of flexural strength to compressive strength that is greater than or equal to about 0.35 when said cement slurry is exposed to a temperature of greater than about 180xc2x0 F.; and wherein said natural mineral fiber comprises at least one calcium silicate natural mineral fiber.