Abstract:
A method of cement lining a wellbore to enable a fixed sensor internal of the lining to sense characteristics of the external formations surrounding the wellbore, wherein, at the region of the sensor position, means are utilised before completion of setting of the cement to create, in use, a permanent fluid pressure connection through the cement lining between the external formation and the sensor.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a method of cement lining a wellbore and also to a system for performing such method.  
         BACKGROUND TO THE INVENTION  
         [0002]    In operation of a wellbore, it is a known practice to provide one or more fixed sensors inside the cement lining for acquiring data related to conditions in and characteristics of ground formations surrounding the wellbore. In order to enable fluid connection between the surrounding formation and a fixed sensor internal of the cement lining, it is known to perforate the cement lining after the cement has set, for example using small explosive charges.  
           [0003]    An aim of the present invention is to provide an alternative and improved method of providing fluid pressure communication between the external formation and a fixed sensor inside the lining.  
         SUMMARY OF THE INVENTION  
         [0004]    According to one embodiment of the invention, a method of cement lining a wellbore if provided to enable a fixed sensor internal of the lining to sense characteristics of the external formations surrounding the wellbore. At the region of the sensor position, means are utilised before completion of setting of the cement to create, in use, a permanent fluid pressure connection through the cement lining between the external formation and the sensor.  
           [0005]    The invention may be practised in various ways.  
           [0006]    According to a first embodiment, a cement curing retarder is introduced into the cement slurry in the region of the sensor totally to prevent curing of the cement in that region. In use, the region of uncured cement then provides fluid communication between the formation and the sensor.  
           [0007]    Examples of suitable retarders include substances the molecules of which contain a substantial number of —OH groups and high temperature retarders from the family of organophosphate chelating agents. It is important to select a retarder which will not hydrolise during the period over which the sensor is to be used (i.e. over which readings are to be taken from the sensor), which may be several years.  
           [0008]    According to a second embodiment, a system is used to increase the permeability of the cement in the region of the sensor, typically by the introduction of gas bubbles into the cement before it has set. The increased porosity of the cement due to the presence of the gas bubbles then provides fluid communication between the formation and the sensor in use.  
           [0009]    A suitable system for inducing gas bubbles includes the installation of a small gas container, preferably pressurised, adjacent the sensor and release of the gas by opening a valve, by triggering a small explosive charge, or by chemical reaction if the gas is stored in the container in liquid or solid state. A preferred gas is carbon dioxide, which will slowly react with the cement, leaving interstices in the cement which will become occupied by water, oil or other liquid.  
           [0010]    According to a third embodiment, a method is provided to induce cracks in the cement during curing.  
           [0011]    Three preferred ways of inducing cracks are firstly the use of a sonic transducer, such as a solenoid or piezo-electric device, to create shear waves in the cement during curing, in the region of the sensor; secondly the addition to the cement during curing, locally to the sensor, of a substance which changes the coefficient of expansion of the cement, additives which are able to increase the rate of expansion of the cement to create cracks being magnesium and aluminium salts, and thirdly the use of bristles, especially metal bristles, attached to the sensor to link the sensor with the external formation.  
           [0012]    The curing of the cement is an exothermal reaction, and the bristles induce local cracking by presenting a different rate of expansion as the cement cools down. It is possible to use bristles made of dissimilar metals, which give rise to electrolysis in the cement slurry as it sets, resulting in gas production which promotes the opening of channels in the cement in order to facilitate fluid communication between the formation and the sensor.  
           [0013]    According to another embodiment of the invention, there is provided a section of wellbore casing having fixed thereto a sensor and means operative when the wellbore casing is being cement lined for inducing cracks in the cement slurry, as it is setting, in use to enable fluid pressure connection between the sensor and the external ground formation.  
           [0014]    The means for inducing cracks in the cement lining may be means for performing any one of the aforedescribed methods.  
           [0015]    Although the cement can become solid before it has fully set or cured, as used herein, the completion of the curing or setting process of the cement means the time when the cement reaches its final shear strength. In other words, at a time when the cement has reached its full hardness. In some cases it can take many days or even weeks for the cement to completely cure or set. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    In the accompanying drawings, according to embodiments of the invention:  
         [0017]    [0017]FIG. 1 shows a section of wellbore casing having fixed thereto a sensor and means for injecting fluid into the cement slurry when the casing has been lowered into the wellbore, and the wellbore is being cement lined;  
         [0018]    [0018]FIG. 2 shows a section of wellbore casing within a wellbore, having fixed thereto a sensor and means for transmitting shock waves into the cement slurry to create local cracking;  
         [0019]    [0019]FIG. 3 is an elevational view;  
         [0020]    [0020]FIG. 4 is a plan view of a section of wellbore casing having fixed thereto a sensor and bristle means for inducing local cracks in the cement lining; and  
         [0021]    [0021]FIG. 5 is a diagram to aid understanding of one possible feature of the bristle means of FIGS. 3 and 4. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Referring to FIG. 1, a section of wellbore casing  10  has fixed thereto by clamps  12  a sensor  14  having a sensor port  16  and adjacent thereto a container  18  filled with a substance, generally a fluid possibly under pressure, to be expelled adjacent the sensor. A sensor cable  20 , fixed by clamps  22  to the wellbore casing and, in use, for conveying data from the sensor via a connector  23  to the surface, may also be used to cause the substance to be expelled at  24  from the container  18  at the appropriate time, for example by transmitting a signal to open a valve or to trigger a small explosive charge or to activate a motor-driven piston or a spring-loaded plunger.  
         [0023]    After the casing has been installed in the wellbore, and is being cement lined, the substance is expelled from the container  18  into the cement slurry adjacent the sensor  14 , as the cement is curing, to treat the cement so that fluid pressure communication is enabled between the sensor and the external ground formation in use.  
         [0024]    The container  18  may house a substance more especially but not exclusively a fluid, of any one of several different types.  
         [0025]    One option is a substance which will prevent the cement from curing at all locally to the sensor. Examples of such substances, among the many available, are sucrose, mannitol and catechol, but in all cases the particular curing retarder selected should be one which will prevent the cement from curing, locally to the sensor, for the period over which the sensor is to be used (i.e. over which readings from the sensor are to be taken).  
         [0026]    Another option for the substance which is to be expelled from the container is a pressurised gas which will locally increase the permeability of the cement by creating bubbles therein. An inert gas such as nitrogen may be used, or a gas such as carbon dioxide which will slowly react with the cement to leave interstices in the cement which will fill with water, oil or other liquid.  
         [0027]    Instead of storing the gas in the container under pressure, it could be stored in a liquid or solid state, and it could be released by chemical reaction, as for example by use of a small explosive charge locally to increase the porosity of the cement.  
         [0028]    Still another option is to release from the container  18  a substance in the form of a cement additive which locally increases its rate of expansion, whereby to create cracks in the cement as it cures. Examples of such additives are magnesium, magnesium hydroxide, aluminium dioxide and calcium aluminite.  
         [0029]    [0029]FIG. 2, which uses similar reference numerals to FIG. 1 for similar parts, shows another method of inducing cracks in the cement as it is curing. In this method, the sensor  14  has associated with it a sonic transducer  30 , such as a piezo-electric or solenoid device, which can be triggered, again preferably by a signal transmitted down the sensor data cable  20 , to induce pressure or shear waves in the cement slurry, locally to the sensor, as the cement is curing. The pressure or shear waves cause the creation of cracks  32  in the cement  34 , which, in use, provide fluid pressure communication through the cement lining between the sensor  14  and the external ground formation  36 .  
         [0030]    [0030]FIGS. 3 and 4, again using similar reference numerals for similar parts, show still another method of inducing cracks in the cement, locally to the sensor  14 , as the cement cures. In this method the sensor is associated with a small mass  40  of porous material from which project bristles  42 , preferably metallic bristles. FIG. 3 shows the section of casing carrying the sensor and the bristles, before it is installed in the wellbore. FIG. 4 shows the casing section in the installed condition. It can be seen that the bristles  42  provide a link between the sensor  14  and the external ground formation  36  around the wellbore  44 . As the curing of the cement is an exothermal reaction, the temperature of the cement slurry rises during setting whereby the bristles  42  create an expansion differential as the cement subsequently cools. This induces the formation of local micro-cracks in the cement and, in use, provides fluid pressure communication between the sensor  14  and the external ground formation  36 .  
         [0031]    Referring to FIG. 5, if the bristles are made of two dissimilar metals, some  42 A made of zinc and some  42 B made of aluminium for example, the cement slurry can act as an electrolite, promoting corrosion of the annode (zinc) and the production of hydrogen at the cathode (aluminium). Both the corrosion and the gas generation assist the opening of channels in the cement which, in use, provide fluid pressure communication between the sensor and the external ground formation. A switch  44  can be used to start and stop the reaction and a resistor  46  to control the speed of the reaction. The switch  44  is preferably not closed until the cement lining is being installed. The pH value of the cement slurry affects the speed of reaction, and the resistor  46  can be used to optimise the speed of reaction for a given value of the pH of the cement.  
         [0032]    Various electrochemical couples are able to achieve forced metal corrosion, including Pb—Al, Ag—Ni, Zn—Mg, Cu—Mg, H2-Ag, H2-Al, H2-Mg, O2-Pb, O2-Ni and O2-Zi.  
         [0033]    In FIGS.  1  to  3 , the cable which transmits data from the sensor to the surface during use is shown clamped to the outside of the wellbore casing.  
         [0034]    The above-described embodiments are illustrative of the invention only and are not intended to limit the scope of the present invention.