Abstract:
In accordance with aspects of the present disclosure, techniques for monitoring subterranean sequestered CO 2  are disclosed. Tools for gathering relevant data are disclosed, and techniques for interpreting the resultant data also disclosed. For example, electrodes and micro-gravity sensors may be deployed, and their readings interpreted to detect underground CO 2  migration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND 
       [0003]    Carbon dioxide (CO 2 ) is a byproduct of many industrial processes. In some cases, there is a desire to sequester quantities of CO 2  in a manner that prevents it from entering the atmosphere. Sequestration of CO 2  underground is one possibility. When sequestering CO 2  underground, it is sometimes desirable to determine if the CO 2  has migrated from its initial location. For example, it is sometimes desirable to determine whether CO 2  has migrated to underground sources of drinking water. 
       BRIEF SUMMARY 
       [0004]    In accordance with some aspects of the present disclosure, a method of monitoring storage of CO 2  in an underground formation is disclosed. The method can include establishing underground electrodes configured to monitor an electrical property of at least a portion of the formation and establishing underground micro-gravity sensors configured to monitor a density of at least a portion of the formation. The method can also include determining a baseline electrical property of at least a portion of the formation and determining a baseline density of at least a portion of the formation. The method can further include injecting CO 2  into the formation. The method can further include determining an updated electrical property of at least a portion of the formation and determining an updated density of at least a portion of the formation. The method can further include monitoring the underground electrodes and monitoring the underground microgravity sensors. The method can further include detecting a change in the electrical property of at least a portion of the formation and a change in density of at least a portion of the formation, wherein the change in the electrical property and the change in density are indicative of CO 2  migration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a schematic diagram representing electrode placement according to an embodiment. 
           [0006]      FIG. 2  is an example chart depicting admittance versus gas saturation according to an embodiment. 
           [0007]      FIG. 3  is a schematic diagram representing micro-gravity sensor placement according to an embodiment. 
           [0008]      FIG. 4  is an example chart depicting depth versus gravitational change according to an embodiment. 
           [0009]      FIG. 5  is a flowchart depicting an example method according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    CO 2  can be sequestered underground by injecting it into pre-existing boreholes, boreholes drilled specifically for the purpose of CO 2  storage, or both. CO 2  can be sequestered in compressed form at the surface prior to injection. CO 2  is typically injected into a relatively permeable layer of a geological sub-surface formation that lies beneath one or more relatively impermeable layers. CO 2  may be sequestered, for example, 4000-10,000 feet (about 1200-3048 meters) underground. This process is sometimes known as “carbon sequestration” within a technological methodology known as “carbon capture and storage” 
         [0011]    Embodiments allow for monitoring subterranean CO 2  storage. In particular, embodiments allow for monitoring whether sequestered CO 2  has migrated vertically or horizontally underground subsequent to its injection into a subterranean storage location. 
         [0012]    As disclosed herein, a combination of electrical and micro-gravity sensors may be used to monitor whether CO 2  sequestered underground has migrated from its initial storage location. The combined sensor types provide a synergy that allow for monitoring both vertical and horizontal CO 2  displacement. Sensor placement and interpretation of data gathered by the installed sensors are discussed in detail herein as follows. 
         [0013]      FIG. 1  is a schematic diagram representing exemplary electrode placement according to an embodiment. According to certain embodiments, displacement of sequestered CO 2  is detected by monitoring changes in electrical current flow between sub-surface electrodes.  FIG. 1  illustrates a schematic representation of two electrodes formed by borehole metallic casings  102 ,  104 . In general, boreholes  112 ,  114 , such as those used to extract petroleum and those used to inject CO 2 , may be reinforced using metallic casings. Because such casings are typically highly conductive and subterranean, they can provide efficient preexisting electrodes for detecting underground CO 2  migration. 
         [0014]    According to certain embodiments, borehole casing electrodes  102 ,  104  may be selected such that boreholes  112 ,  114 , and thus casing electrodes  102 ,  104 , are separated by a horizontal distance L (depicted in  FIG. 1  as the sum of L/2 and L/2), which may be less than that of the depth of the underground CO 2  plume in the sequestration target zone  108 . Conductive wires  110  may be connected to power source  106  and inserted into boreholes  112 ,  114  such that they make electrical contact with metallic casings  102 ,  104  at a depth interval at least as great as the borehole separation L. That is, the distance from the surface to the electrical contact may be at least as great as the distance between boreholes  112 ,  114 . 
         [0015]    Parameters identified in  FIG. 1  and referred to below in reference to  FIG. 2  include the following. Sequestration target zone  108  has height h w  and a fluidic conductivity denoted by σ w . The z-axis runs vertically in  FIG. 1 . The formation layer  116  above sequestration target zone  108  has a conductivity denoted by σ b   − , and the formation layer  118  below sequestration target zone  108  has a conductivity denoted by σ b   + . 
         [0016]      FIG. 2  is an example chart depicting electrical admittance (i.e., the reciprocal of the electrical impedance) versus gas saturation according to an embodiment. The chart depicted in  FIG. 2  may represent inter-electrode admittance of the system depicted schematically in  FIG. 1 . 
         [0017]    In general, conductivity of sedimentary rocks may be represented according to, by way of non-limiting example: 
         [0000]      σ=ασ W S W   n Φ m   Equation 1
 
         [0018]    In Equation 1, the term σ represents conductivity, Φ represents rock porosity, S W =1−S G , where S G  represents gas saturation, parameter α and cementation factor m vary from 0.6 to 1.5 and from 1.3 to 3, respectively, and saturation exponent n is close to 2. 
         [0019]    Notably, electrical conductivity of rock is highly sensitive to gas saturation. For example, if gas saturation were to vary from 0.0 to 0.95, rock conductivity may vary by as much as a factor of 400. 
         [0020]    To estimate inter-casing admittance versus gas saturation as illustrated by  FIG. 2 , the following non-limiting assumptions are made regarding the system depicted schematically in  FIG. 1 . Porosity of sequestration target zone  108  is assumed to be φ=0.199. The half-spaces above  116  and below  118  sequestration target zone  108  are assumed to be σ b   − =0.418 Siemens/meter (S/m) and σ b   + =0.314 S/m, respectively. The parameter α is assumed to be 1, and the parameters n and m are both assumed to be 2. Sequestration target zone  108  is assumed to lie between 1800 and 2000 meters, such that h w =200 meters. Fluid conductivity at the center of sequestration target zone  108  is assumed to be σ w =12.69 S/m. Again, these assumptions are made for illustrative purposes only; one of ordinary skill in the art is fully capable of adapting the example quantities and computations discussed herein to embrace particular situations encountered in the field. 
         [0021]    Inter-casing admittance as illustrated in  FIG. 2  and calculated for the system of  FIG. 1  according to the exemplary parameters discussed herein may be represented according to, by way of non-limiting example: 
         [0000]        Y=Y   σ     b     −   +Y   w   +Y   σ     b     +   Equation 2
 
         [0022]    In Equation 2, the first and last terms represent contributions of the half-spaces above and below sequestration target zone  108 , respectively. The second term represents admittance of the sequestration target zone itself, which may be estimated according to, by way of non-limiting example: 
         [0000]        Y   w   =πS   w   [F   α ( L, 0)− F   α ( r   0 ,0)] −1   Equation 3
 
         [0023]    In Equation 3, F α (r,z)=2πH 0 [exp−k|z|/(k+α), α=(σ b   + +σ b   − )/S w , S w =σ w h w , r is radial distance, and H 0 [·] denotes a 0-order Hankel transform. The admittance of a conducting half-space penetrated by a semi-infinite casing (an appropriate assumption made here) may be estimated according to, by way of non-limiting example: 
         [0000]        Y   σ     b   =0.25[Ψ σ     b   ( L,z= 0)−Ψ σ     b   ( r   0   ,z= 0)] −1   Equation 4
 
         [0024]    In Equation 4, the term Ψ σ     b    (r,z) may be defined according to, by way of non-limiting example: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0025]    In Equation 5, K n (·) is the modified Bessel function of the second kind and of the n-th order. 
         [0026]    The above equations and assumptions were used to generate the diagram of  FIG. 2 , which plots gas saturation  202  against inter-casing admittance  204  for the system described schematically in  FIG. 1 . The curves represented in  FIG. 2  represent inter-casing separations L of 10 meters, 50 meters, 100 meters, 200 meters, 300 meters, 400 meters and 500 meters. Using this disclosure, one of ordinary skill in the art can place electrodes in borehole casings and be able to estimate the corresponding gas saturation in sequestration target zone  108  based on observed inter-casing admittance. 
         [0027]    In addition to estimating gas saturation based on electrical admittance measurements, a component of certain embodiments includes estimating gas saturation based on density measurements made by micro-gravity sensors. This second component is discussed presently in reference to  FIGS. 3 and 4 . 
         [0028]      FIG. 3  is a schematic diagram representing micro-gravity sensor placement according to an embodiment. In particular,  FIG. 3  depicts borehole  302  below surface  304 . Micro-gravity sensors  306  can be spaced along borehole  302  at 10 meter intervals. In some embodiments, micro-gravity sensors  306  can be spaced at 5 meter intervals within the sequestration target zone, and at 25 meter intervals above and below the sequestration target zone. In some embodiments, a single micro-gravity sensor is positioned in each borehole in the sequestration target zone (e.g.,  108  of  FIG. 1 ); in other embodiments, multiple (e.g., up to several dozen) can be placed both in and out of the sequestration target zone. 
         [0029]    Micro-gravity sensors  306  can be placed in a network of multiple boreholes. In some embodiments, the boreholes are positioned in a square grid arrangement. Boreholes may be spaced at, by way of non-limiting example, 20 meter intervals, 100 meter intervals, or at other intervals. 
         [0030]    The micro-gravity sensor placement parameters discussed herein are representative but non-limiting; other micro-gravity sensor placements are contemplated. 
         [0031]    Micro-gravity sensors  306  are communicatively coupled to computing device  308 . Computing device  308  may detect and store readings from micro-gravity sensors  306  continuously, at periodic intervals, or upon command. Example periodic intervals include daily, weekly, monthly, and quarterly. 
         [0032]    An exemplary micro-gravity sensor is the Deep Density Borehole Gravity Meter (BHGM), available from Micro-g LaCoste, Inc. of Lafayette, Colo. In general, micro-gravity sensors  306  may be capable of resolutions on the order of 1 μGal. 
         [0033]      FIG. 4  is an example chart depicting depth  404  versus gravitational change  402  according to an embodiment. The chart depicted in  FIG. 4  illustrates changes in gravity subsequent to CO 2  injection into the sequestration zone as compared to gravity prior to injection. In general, the presence of CO 2  in a fluid alters the density of the fluid. As CO 2  replaces water in a formation, the density may decrease. On the other hand, as supercritical CO 2  replaces hydrocarbons in a formation, the density may increase. 
         [0034]    In general, changes of at least one micro Gal indicate a movement of CO 2  into or out of the region that experienced the change. Changes that are within the noise threshold of the sensor (e.g., less than one micro Gal) may be disregarded. (As sensor technology advances and sensors become capable of detecting lower and lower differences in gravity, the one micro Gal threshold may be reduced.) 
         [0035]    Two different boreholes are represented in  FIG. 4 : an existing borehole and a nearby borehole used as the CO 2  injection well. The CO 2  plume corresponding to the existing borehole lies at approximately 1990-2000 meters below the surface as shown at portion  406  of the graph, and the CO 2  plume corresponding to the injection well lies at approximately 2050-2085 meters below the surface as shown at portion  408  of the graph. The curves represent the difference in vertical attraction due to gravity by subtracting the gravitational response as observed before and after injection of the CO 2 . For both boreholes, a decrease in vertical gravity is observed at the top of the reservoir. This is because the net density in the reservoir is negative since CO 2  is less dense than the water it has replaced, so the vertical gravitational force is negative. An increase in net gravity acceleration is observed below the reservoir. This is because the net density is negative above the sensor giving rise to a polarity change in acceleration measured. 
         [0036]      FIG. 5  is a flowchart depicting an example method according to an embodiment. At block  500  an initial model is obtained. The model under discussion represents the subterranean structure of the sequestration target zone and surroundings. It typically depicts the various sedimentation and other layers of geological or sequestration significance and their associated electrical and density properties. The initial model may be obtained using, by way of non-limiting example, seismic surveys employing reflective seismology and the electrical and density properties can be obtained by well log or core measurements from nearby wells. 
         [0037]    At block  502 , electrodes are established. This step is discussed in detail above in reference to  FIGS. 1 and 2 . At block  504 , gravity sensors are established. This step is discussed above in reference to  FIGS. 3 and 4 . 
         [0038]    At block  506 , a baseline electrical property is established. This step occurs prior to CO 2  injection. The electrodes discussed in reference to block  502  may be used to that end. The electrical property may be, by way of non-limiting example, a measure of resistivity or admittance. 
         [0039]    At block  508 , a baseline density is established. Again, this step occurs prior to CO 2  injection. The micro-gravity sensors discussed above in reference to block  508  may be used for that purpose. The baseline density may reflect or be derived from micro-gravity readings. 
         [0040]    At block  510 , the initial model is revised. The revision may take into account the baseline electrical property and density readings obtained at blocks  506  and  508 . In some embodiments, the initial model is revised by performing an inversion of the model, known to those of skill in the art. In such an inversion, empirical data may be used to back-calculate parameters of the model. The inversion may utilize the baseline density data, the baseline electrical property data, or both (e.g., a braid or “joint” inversion). It will be appreciated herein and throughout the disclosure, that other types of sensors, for example seismic sensors, can be utilized in the readings obtained. For example, readings including one or more of seismic electrical and seismic density. Then, electrical density and seismic electrical and density joint inversions can be performed. 
         [0041]    At block  512 , CO 2  is injected. This process may proceed over a time period that may span days or months. An exemplary, non-limiting injection rate is two kilograms per second. Other injection rates are also contemplated. 
         [0042]    At block  514 , an updated electrical property is obtained. The updated electrical property may be obtained as discussed above in reference to block  506 . At block  516  an updated density is determined The updated density may be obtained as discussed above in reference to block  508 . 
         [0043]    At block  518 , the model is revised. The model may be revised based on the updated electrical property obtained at block  514  and the updated density obtained at block  516 . The updated model may be generated by way of inversion based on one or both of the updated electrical and density determinations. The revised model is intended to reflect the presence of sequestered CO 2 . Furthermore, the revised model may be compared to the model obtained at block  510  in order to determine the geological differences caused by the new presence of sequestered CO 2 . For example, the graph of  FIG. 4  reflects such differences with respect to gravity. 
         [0044]    At block  520 , electrode readings are monitored, and at block  522  readings from the micro-gravity sensors are monitored. The monitoring may occur continuously, periodically, or on command. If periodically, the monitoring may occur daily, weekly, monthly, quarterly, or yearly. The data detected by the respective sensors may be stored electronically in persistent memory of a computer. 
         [0045]    At block  524 , CO 2  migration is detected. This may be performed by comparing the revised model obtained at block  518  to an inversion model based on the data obtained at blocks  520  and  522 . Alternately, or in addition, the migration may be detected by detecting changes in the parameters themselves. Using both electrical properties and micro-gravity readings, the lateral and vertical extent of such migration may be ascertained. 
         [0046]    Note that many of the steps recited herein may be automated using installed executable software. The software may be implemented on a computer, such as a personal computer executing an operating system. 
         [0047]    While the present disclosure has been described according to its preferred embodiments, it is of course contemplated that modifications of, and alternatives to, these embodiments, such modifications and alternatives obtaining the advantages and benefits of this disclosure, will be apparent to those of ordinary skill in the art having reference to this specification and its drawings. It is contemplated that such modifications and alternatives are within the scope of this disclosure as subsequently claimed herein.