Abstract:
Methods for improving the bonding of a cement sheath to a tubular body in a subterranean well involve anchoring elements mounted on the outside surface of the tubular body—in the annular space between the tubular body and the borehole wall. The cement contains an expansive agent that causes the cement to expand after it sets. The anchoring elements are mounted such that an angle exists between the elements and the tubular-body surface, thereby providing resistance to cement sheath movement away from the tubular-body surface.

Description:
BACKGROUND 
       [0001]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0002]    This disclosure relates to methods for cementing subterranean wells. 
         [0003]    During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that the well penetrates. The latter function is important because it prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well&#39;s production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone. 
         [0004]    The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole is necessary to prevent leaks. Persons skilled in the art know that there are various conditions and events in a wellbore that may interfere with the goal of achieving hydraulic isolation. Such conditions and events include temperature and pressure fluctuations, hydrostatic pressure changes inside casing, mechanical disturbances such as perforating, hydraulic fracturing and seismic movements, and the presence of formation fluids that may be corrosive to the cement sheath, the casing or both. 
         [0005]    Persons skilled in the art also know that, when there is a pressure drop inside cemented casing, the casing diameter may decrease and move away from the cement sheath. As a result, a microannulus may form, providing a fluid-flow path between two subterranean zones or between a subterranean zone and the surface. One method for preventing or sealing a microannulus is to use cement slurries that contain an expanding agent. However, the use of expanding agents is usually limited to wellbore environments in which the Young&#39;s modulus of the cement sheath is lower than that of the formation. If the Young&#39;s modulus of the cement sheath is higher than that of the formation, the cement sheath will preferentially expand into the formation and away from the casing, thereby increasing the size of the microannulus. In many cases this problem may be overcome by using cement systems that are designed to have low Young&#39;s moduli. However, in extreme cases, it is not practical to design cement systems with sufficiently low Young&#39;s moduli because strength would be compromised. 
       SUMMARY 
       [0006]    Embodiments allow improvements by providing well casings that minimize or prevent the expansion of the cement sheath away from the casings. 
         [0007]    In an aspect, embodiments relate to apparatuses for improving bonding of a cement sheath in an annulus of a subterranean well having a borehole wall. The apparatus comprises a tubular body and at least one anchoring element. The anchoring element comprises a stem and a hook. The stem of the anchoring element protrudes from the outer surface of the tubular body by at least 2 mm, but does not protrude farther than about 0.5 times the width of the annulus. The hook is attached to a portion of the stem that is distal to the tubular body, such that the hook and the stem are not collinear. 
         [0008]    In a further aspect, embodiments relate to methods for improving bonding of a cement sheath in an annulus of a subterranean well having a borehole wall. At least one apparatus comprising a tubular body and at least one anchoring element is installed in the wellbore. The element comprises a stem and a hook. The stem protrudes from the outer surface of the tubular body by at least 2 mm, but does not protrude farther than about 0.5 times the width of the annulus. The hook is attached to a portion of the stem that is distal to the tubular body, such that the hook and the stem are not collinear. A cement slurry is prepared that comprises at least one expansive agent. The slurry is placed in the annular space between the outer surface of the tubular body and the borehole wall such that the slurry surrounds the anchoring elements. The slurry is then allowed to set and expand. 
         [0009]    In yet a further aspect, embodiments relate to methods for cementing a subterranean wellbore having a borehole wall. At least one apparatus comprising a tubular body and at least one anchoring element is installed in the wellbore. The element comprises a stem and a hook. The stem protrudes from the outer surface of the tubular body by at least 2 mm, but does not protrude farther than about 0.5 times the width of the annulus. The hook is attached to the portion of the stem that is distal to the tubular body, such that the hook and the stem are not collinear. A cement slurry is prepared that comprises at least one expansive agent. The slurry is placed in the annular space between the outer surface of the tubular body and the borehole wall such that the slurry surrounds the anchoring elements. The slurry is then allowed to set and expand. The tubular body may be casing, liner or coiled tubing or a combination thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates the formation of a microannulus between a casing string and the cement sheath, resulting from a hydrostatic pressure drop inside the casing string. 
           [0011]      FIG. 2  is a graph that illustrates how the Young&#39;s modulus of the formation affects the size of a microannulus when an expanding cement system is present. 
           [0012]      FIG. 3  illustrates the formation of a microannulus between the cement sheath and the formation, arising from a situation during which a non-expanding cement sheath does not separate from the casing string. 
           [0013]      FIG. 4  illustrates the same scenario as  FIG. 3  when an expanding cement sheath is present. Microannulus formation does not occur. 
           [0014]      FIG. 5  illustrates an embodiment of an anchoring element mounted on the casing string, preventing separation of the cement sheath from the casing string. 
           [0015]      FIG. 6  illustrates an embodiment of how anchoring elements may be arranged around the circumference of the casing string. 
           [0016]      FIGS. 7A-7E  illustrate several embodiments of anchoring element shapes. The common theme is that the elements protrude in a manner such that the stem is approximately perpendicular to the casing surface. 
           [0017]      FIG. 8A  illustrates the orientation of the flow direction at an angle alpha to the plane of the anchoring element.  FIG. 8B  illustrates the orientation of an anchoring element in a well with respect to cement-slurry flow. 
           [0018]      FIG. 9  illustrates the orientation of an anchoring element at an angle theta to the flow direction. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The disclosure may be described in terms of treatment of vertical wells, but is equally applicable to wells of any orientation. The disclosure may be described for hydrocarbon production wells, but it is to be understood that the disclosure may be used for wells for production of other fluids, such as water or carbon dioxide, or, for example, for injection or storage wells. It should also be understood that throughout this specification, when a concentration or amount range is described as being useful, or suitable, or the like, it is intended that any and every concentration or amount within the range, including the end points, is to be considered as having been stated. Furthermore, each numerical value should be read once as modified by the term “about” (unless already expressly so modified) and then read again as not to be so modified unless otherwise stated in context. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. In other words, when a certain range is expressed, even if only a few specific data points are explicitly identified or referred to within the range, or even when no data points are referred to within the range, it is to be understood that the inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that the inventors have possession of the entire range and all points within the range. 
         [0020]    As discussed earlier, a microannulus may form if the hydrostatic pressure inside the casing falls. An example of this situation is the replacement of a drilling fluid inside casing by a low-density completion fluid. The casing may shrink away from the cement, giving rise to a microannulus. This scenario is illustrated in  FIG. 1 . The casing  1  is filled with a fluid  2 . When the fluid density falls, the casing shrinks away from the cement sheath  3 , forming a microannulus  4 . The cement sheath may remain bonded to the formation  5 . 
         [0021]    A potential solution to the microannulus problem is to include an expanding agent in the cement system. However, the ability of such a cement system to close a microannulus may depend on the mechanical properties of the cement sheath and the formation. One may use the CemSTRESS™ wellbore modeling application, available from Schlumberger, to perform an analysis of these relationships. The physics of the model is described in the following publication: Thiercelin M J et al.: “ Cement Design Based on Cement Mechanical Response,”  paper SPE 52890 (1998). The software may be used to determine the optimal mechanical properties of the set cement and the necessary amount of cement-sheath expansion. Ultimately, a cement system may be designed that meets the requirements specified by the software. 
         [0022]    A CemSTRESS™ simulation, illustrated in  FIG. 2 , assumes that the Young&#39;s modulus of the formation is 7000 MPa and that there is a 38-μm microannulus between the cement sheath and the casing. The well geometry consists of a 17.8-cm (7-in) casing in an 21.6-cm (8.5-in.) open hole. When the Young&#39;s modulus of the cement sheath exceeds that of the formation, the presence of an expanding cement exacerbates the microannulus problem (i.e., the microannulus becomes wider). To overcome this problem, another method may be required to ensure that the cement sheath remains attached to the casing. 
         [0023]      FIG. 3  illustrates what may happen when a conventional non-expansive cement sheath is made to remain attached to the casing. The microannulus  4  may form at the interface between the cement sheath  3  and the formation  5 . This scenario is also undesirable because the microannulus may allow the passage of formation fluids between subterranean zones or to the surface. Use of an expanding cement may minimize or prevent the formation of a microannulus at the cement-formation interface ( FIG. 4 ). 
         [0024]    There are numerous ways by which an expanding cement may be provided. A thorough overview of expanding cements is presented in the following publications. Nelson E B, Drochon B, Michaux M and Griffin T J: “ Special Cement Systems ,” in Nelson E B and Guillot D (eds.):  Well Cementing -2 nd  Edition. Houston: Schlumberger (2006): 233-268; and patent application EP 2457974 A1. Any of the expanding cement systems described in the literature may be compatible with the methods disclosed in the present application. 
         [0025]    Applicant has determined methods by which the cement sheath may be anchored to well tubulars, which may comprise casing, liner, coiled tubing or a combination thereof. The tubulars may be fabricated from steel, titanium, aluminum, a composite material or plastic. The anchoring mechanism may have the following characteristics.
       Sufficient length to allow a cement slurry to flow and displace drilling fluid between the anchoring elements and the casing, but not too long to negatively affect the running of casing into the borehole.   Sufficient rigidity to survive placement in the borehole and not be deformed by cement expansion. Applicant has determined that elements at least 1 mm thick (if fabricated from steel) may have sufficient rigidity.   Distributed evenly around the casing circumference and along the length where anchoring is required (hence a helicoidal arrangement).   Designed to allow the flow of spacer fluid and cement to displace drilling fluid—oriented perpendicular to, or at low angle to the flow direction or sufficiently thin to prevent entrapment of the drilling fluid.   May be directly welded to the casing or attached by rigid clamps, or the tubular body may be machined such that grooves are created that effectively create protrusions.       
 
         [0031]    It may not be necessary for the anchoring elements to be placed on every joint of casing. The anchoring elements may be confined to key zonal-isolation areas along the casing string, for example along regions where the formation has a low Young&#39;s modulus. The anchoring elements may also be part of a special joint that is run as part of the casing string. The casing or special joint may also include centralizers to help protect the anchoring elements during installation in the well. 
         [0032]    One anchoring mechanism is illustrated in  FIG. 5 . An anchoring element  10  is attached to casing  11  and protrudes into the cement sheath  12 . The length of the anchor stem  13  may be at least 2 mm to allow fluids to flow freely around the element. However, the length of the element may be less than or equal to 0.5 times the width of the annulus between the casing and the formation. For example, in the annulus between a 7-in. (178-mm) casing string and an 8.5-in. (216-mm) open hole (with zero standoff between the casing and the formation), the length of the anchoring element may be less than 9.5 mm. At the end of the anchor stem is a hook  14 . 
         [0033]    An example of a possible distribution of anchoring elements around a casing string is shown in  FIG. 6 . The anchoring elements  20  may be arranged on the casing  21  in a helical pattern. The anchoring elements continue in the same pattern along the back side of the casing (not shown). Ideally, the anchoring elements may be spaced at a minimum of 60° phasing around the casing (assuming a 7-in. casing) In  FIG. 6 , the arrow  22  indicates the circumferential distance between the centers of the anchoring elements. For a 7-in. casing and 60° phasing the distance would be 93 mm. Larger casing diameters may require a minimum phasing of 30° with the aim of maintaining the circumferential spacing below about 100 mm. The vertical spacing of the anchoring elements, indicated by arrow  23 , is ideally between 100 mm and 200 mm independent of casing diameter. 
         [0034]    The shape of the anchoring elements may be such that they feature an angle between the stem  13  and the hook (hereinafter called the hook angle), thus providing mechanical resistance to set cement attempting to separate from the casing. The shapes may comprise those illustrated in  FIGS. 7A-7E . Each figure shows an anchoring element  13  mounted on casing  11 . The hook angle  15  may be ≧90° to allow good mud removal in all orientations and &lt;170° to hold the cement in place. 
         [0035]    In some instances the hook angle  15  may be less than about 90°, but in this case the anchoring element should be oriented with respect to the flow direction to ensure that cement surrounds the anchoring element. 
         [0036]    The stem may have a circular cross section with a diameter between 1 mm and 20 mm, between 1 mm and 10 mm or between 1 mm and 5 mm. The stem may have a rectilinear cross section with the largest dimension between 1 mm and 20 mm, between 1 mm and 10 mm or between 1 mm and 5 mm. In these cases the orientation of the hook element in relation to the cement slurry flow direction is not important, provided that the hook angle is not less than about 90°. 
         [0037]    If the hook angle  15  is less than about 90° ( FIG. 8A ) the anchoring element may be oriented so that cement-slurry flow can easily remove the drilling fluid from the area surrounding the anchoring element. In this case the anchoring element should be oriented such that cement slurry flows past the element at an angle alpha (α) less than about ±45° or greater than ±135°. As shown in  FIG. 8A , if one envisions fluid flowing vertically out of the page toward the reader, the angle α is either 0° or 180°. If one envisions fluid flowing horizontally along the page in a direction perpendicular to the larger dimension of stem  13 , the angle α is either −90° or +90°. The orientation of an anchoring element, and it&#39;s angle α with respect to the direction of cement slurry flow in a well (indicated by the arrows), is shown in  FIG. 8B . 
         [0038]    The stem may be made of sheet like material where the rectangular cross section has a largest dimension between 1 mm and 20 mm, between 1 mm and 10 mm or between 1 mm and 5 mm. As shown in  FIG. 9 , in this case the orientation of the larger dimension of the stem with respect to the flow  90  has to be controlled to allow optimal mud removal. The angle theta (θ) as shown in  FIG. 9  should ideally be less than about ±45° or greater than ±135°. For reference, if the stem of the anchoring device is parallel to the flow  90 , the angle theta (θ) is 0° or 180°. 
         [0039]    In an aspect, embodiments relate to apparatuses for improving bonding of a cement sheath in an annulus of a subterrenean well having a borehole wall. The apparatus comprises a tubular body and at least one anchoring element. The anchoring element comprises a stem and a hook. The stem of the anchoring element protrudes from the outer surface of the tubular body by at least 2 mm, but does not protrude farther than about 0.5 times the width of the annulus. The hook is attached to a portion of the stem that is distal to the tubular body, such that the hook and the stem are not collinear. 
         [0040]    In a further aspect, embodiments relate to methods for improving bonding of a cement sheath in an annulus of a subterranean well having a borehole wall. At least one apparatus comprising a tubular body and at least one anchoring element is installed in the wellbore. The element comprises a stem and a hook. The stem protrudes from the outer surface of the tubular body by at least 2 mm, but does not protrude farther than about 0.5 times the width of the annulus. The hook is attached to a portion of the stem that is distal to the tubular body, such that the hook and the stem are not collinear. A cement slurry is prepared that comprises at least one expansive agent. The slurry is placed in the annular space between the outer surface of the tubular body and the borehole wall such that the slurry surrounds the anchoring elements. The slurry is then allowed to set and expand. 
         [0041]    In yet a further aspect, embodiments relate to methods for cementing a subterranean wellbore having a borehole wall. At least one apparatus comprising a tubular body and at least one anchoring element is installed in the wellbore. The element comprises a stem and a hook. The stem protrudes from the outer surface of the tubular body by at least 2 mm, but does not protrude farther than about 0.5 times the width of the annulus. The hook is attached to a portion of the stem that is distal to the tubular body, such that the hook and the stem are not collinear. A cement slurry is prepared that comprises at least one expansive agent. The slurry is placed in the annular space between the outer surface of the tubular body and the borehole wall such that the slurry surrounds the anchoring elements. The slurry is then allowed to set and expand. The tubular body may be casing, liner or coiled tubing or a combination thereof. 
         [0042]    For all aspects the anchoring elements may be arranged in a helical pattern around the outer surface of the tubular body. 
         [0043]    For all aspects, the cement slurry may comprise an inorganic cement comprising Portland cement, calcium aluminate cement, fly ash, blast furnace slag, lime/silica blends, cement kiln dust, magnesium oxychloride, chemically bonded phosphate ceramics, zeolites, geopolymers and combinations thereof. Organic cement systems comprising epoxy resins, phenolic resins, furan resins and the like are also envisioned. 
         [0044]    For all aspects, the expansive agent in the cement slurry may comprise sodium chloride, calcium sulfate, aluminum sulfate, iron (II) sulfate, magnesium oxide, calcium oxide, aluminum metal, zinc metal, magnesium metal, swellable particles in emulsified oils or combinations thereof. 
         [0045]    Although various embodiments have been described with respect to enabling disclosures, it is to be understood that the preceding information is not limited to the disclosed embodiments. Variations and modifications that would occur to one of skill in the art upon reading the specification are also within the scope of the disclosure, which is defined in the appended claims.