Abstract:
There is provided a method of testing a subterranean formation for fracture condition, including the steps of creating a side bore into the wall of a well traversing the formation, sealing the wall around the side bore to provide a pressure seal between the side bore and the well, pressurizing the side bore beyond a pressure inducing formation fracture while maintaining the seal and monitoring the pressure to identify the fracture condition.

Description:
FIELD OF THE INVENTION 
     The present invention is generally related to methods of measuring fracturing and re-opening pressures and stresses in a borehole traversing a subterranean reservoir. 
     BACKGROUND 
     In the co-owned U.S. Pat. Nos. 5,353,637 and 5,517,854 to R. A. Plumb and Y. S. Dave much of background relevant to the present invention is set out in great detail and incorporated herein for reference. In the patents, the need for an accurate measurement of formation breakdown and re-opening pressures is highlighted together with methods to derive further parameters from such measurements. Parameters derived from the pressure measurements include for example the magnitude and direction of maximum and minimum horizontal stresses. 
     As stated in the &#39;637 patent, it is the precise knowledge of differences in stress magnitude that allows engineers to predict the type of fracture treatment that will assure containment in the reservoir beds. However, precise stress magnitude data are rarely obtained, particularly not in shales, which can be very tight. Instead it is commonly assumed that the least principal horizontal total stress in shales is greater than in adjacent reservoir rocks. The &#39;637 patent describes the use of instrumented packers or pairs of packers with hydraulics to pressurize the packers or the volume between packers to measure the important parameters. 
     Another technical area in oil field technology usually considered as unrelated to the production stimulation as described above is the so-called formation sampling. Various techniques for performing formation evaluation (i.e., interrogating and analyzing the surrounding formation regions for the presence of oil and gas) in open, uncased boreholes have been described, for example, in U.S. Pat. Nos. 4,860,581 and 4,936,139, assigned to the assignee of the present invention. An example of this class of tools is Schlumberger&#39;s MDT™, a modular dynamic fluid testing tool. Such a tool may include at least one fluid sample bottle, a pump to extract the fluid from the formation or inject fluid into the formation, and a contact pad with a conduit to engage the wall of the borehole. When the device is positioned at a region of interest, the pad is pressed against the borehole wall, making a tight seal for the pumping operation to begin. 
     To enable the same sampling in cased boreholes, which are lined with a steel tube, sampling tools have been combined with perforating tools. Such cased hole formation sampling tools are described, for example, in the U.S. Pat. No. 7,380,599 to T. Fields et al. and further citing the U.S. Pat. Nos. 5,195,588; 5,692,565; 5,746,279; 5,779,085; 5,687,806; and 6,119,782, all of which are assigned to the assignee of the present invention. The &#39;588 patent by Dave describes a downhole formation testing tool which can reseal a hole or perforation in a cased borehole wall. The &#39;565 patent by MacDougall et al. describes a downhole tool with a single bit on a flexible shaft for drilling, sampling through, and subsequently sealing multiple holes of a cased borehole. The &#39;279 patent by Havlinek et al. describes an apparatus and method for overcoming bit-life limitations by carrying multiple bits, each of which are employed to drill only one hole. The &#39;806 patent by Salwasser et al. describes a technique for increasing the weight-on-bit delivered by the bit on the flexible shaft by using a hydraulic piston. 
     Another perforating technique is described in U.S. Pat. No. 6,167,968 assigned to Penetrators Canada. The &#39;968 patent discloses a rather complex perforating system involving the use of a milling bit for drilling steel casing and a rock bit on a flexible shaft for drilling formation and cement. 
     U.S. Pat. No. 4,339,948 to Hallmark discloses an apparatus and methods for testing, then treating, then testing the same sealed off region of earth formation within a well bore. It employs a sealing pad arrangement carried by the well tool to seal the test region to permit flow of formation fluid from the region. A fluid sampling arrangement in the tool is adapted to receive a fluid sample through the sealing pad from the test region and a pressure detector is connected to sense and indicate the build up of pressure from the fluid sample. A treating mechanism in the tool injects a treating fluid such as a mud-cleaning acid into said sealed test region of earth formation. A second fluid sample is taken through the sealing pad while the buildup of pressure from the second fluid sample is indicated. 
     In U.S. Patent Application Publication 2009/0255669 tools and methods are described for injecting fluid into the formation surrounding wellbore for various purposes such measuring fluid saturations and other formation parameters. 
     Methods and tools for performing downhole fluid compatibility tests include obtaining an downhole fluid sample, mixing it with a test fluid, and detecting a reaction between the fluids are described in the co-owned U.S. Pat. No. 7,614,294 to P. Hegeman et al. The tool includes a plurality of fluid chambers, a reversible pump and one or more sensors capable of detecting a reaction between the fluids. The patent refers also to a downhole drilling tool for cased hole applications. 
     In the light of above known art it is seen as an object of the present invention to improve and extend methods of determining fracture pressures and stresses while reducing requirements for hydraulic and mechanic equipment such as packers. 
     SUMMARY OF INVENTION 
     Hence according to a first aspect of the invention there is provided a method of testing a subterranean formation for fracture condition, including the steps of creating a side bore into the wall of a well traversing the formation, sealing the wall around the side bore to provide a pressure seal between the side bore and the well, pressurizing the side bore beyond a pressure inducing formation fracture while maintaining the seal and monitoring the pressure to identify the fracture condition. 
     The side bore is preferably drilled in direction of the maximum horizontal stress, if this direction is prior knowledge. 
     The method is furthermore best applied to formations of low permeability, which are believed to confine the spread of a fracture to the desired directions. A formation is considered to be of low permeability if the permeability at the test location is less than 100 mD (millidarcy) or even less than 20 mD or 10 mD. 
     This invention allows evaluation of stimulation fluids at reservoir (downhole) conditions. The invention is particularly useful for testing and evaluating formations for a subsequent hydraulic fracturing operation. 
     The method enables fracturing opening and re-opening tests with minimal use of hydraulic fluids. 
     These and other aspects of the invention are described in greater detail below making reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a typically deployment of a formation drilling and sampling tool while performing steps in accordance with an example of the present invention; 
         FIG. 2  illustrates the step of drilling a side bore to an existing well in accordance with an example of the present invention; 
         FIGS. 3A and 3B  illustrate the step of fracturing the formation in the vicinity of a side bore in accordance with an example of the present invention; and 
         FIG. 3C  reproduces pressure vs time profiles similar to those expected to be measured in accordance with an example of the present invention; and 
         FIG. 3D  are simulated contour plots around an injection point representing the approximate size of the high pressure zone at the well bore. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , a well  11  is shown drilled through a formation  10 . The well  11  includes an upper cased section  11 - 1  and a lower openhole section  11 - 2 . The lower openhole section is shown with a layer  12  of formation damaged and invaded through a prior drilling process which left residuals of the drilling fluids in the layer surrounding the well. 
     In this example of the invention, a wireline tool  13  is lowered into the well  11  mounted onto a string of drillpipe  14 . The drill string  14  is suspended from the surface by means of a drilling rig  15 . In the example as illustrated, the wireline tool includes a formation testing device  13 - 1  combined with a formation drilling device  13 - 2 . Such tools are known per se and commonly used to collect reservoir fluid samples from cased sections of boreholes. The CHDT™ open hole drilling and testing tool as offered commercially by Schlumberger can be regarded as an example of such a tool. The connection to the surface is made using a wireline  13 - 3  partly guided along the drill string  14  (within the cased section  11 - 1  of the well  11 ) and partly within the drill string (in the open section  11 - 2 ). 
     The operation of this combined toolstring in a downhole operation in accordance with an example of the invention is illustrated schematically in the following  FIGS. 2-3 . 
     In the example, it is assumed that the stresses around the well  11  have been logged using standard methods such acoustic or sonic logging. At a target depth, the tool  13  is oriented such that it is aligned in directions of the maximum horizontal stress. It is in this direction that fractures typically open first when the whole well is pressurized in a normal fracturing operation. The mounted tool  13  can be rotated by rotating the drill string  14  and thus assume any desired orientation in the well  11 . 
     Making use of the conventional operation mode of the CHDT tool  13 , the body  20  of the tool as shown in more detail in  FIG. 2  includes a small formation drill bit  210  mounted on an internal flexible drill string  211 . While the tool is kept stationary using the sealing pad  22  and counterbalancing arms (not shown), the flexible drill  210  can be used to drill a small side bore  212  into the formation  10  surrounding the well  11 . 
     In the example, a 9 mm diameter hole  212  is drilled to an initial depth of 7.62 cm (3-in) before reaching the final depth of 15.24 cm (6-in). The drilling operation is monitored with real-time measurements of penetration, torque and weight on bit. The bit is automatically frequently tripped in and out of the hole to remove cuttings. The bit  210  trips can be manually repeated without drilling if a torque increase indicates a buildup of cuttings. 
     After the drilling of the side bore  212 , reservoir fluids are produced to clean it of any cuttings that could adversely affect the subsequent injection. After the clean-out, the pressure in the side bore  212  is increased by pumping a (fracturing) fluid either from a reservoir with the tool or from within the well through the tool. 
     As shown in  FIG. 3A , the pump module  230 , which is a positive displacement pump when using the CHDT tool, is activated in reverse after completing the clean-out of the side bore  212  and a fluid is injected from an internal reservoir  231  through an inner flow line  232  of the tool into the side bore  212 . It is important for the present invention that the pad  22  maintains during the injection stages a seal between the well pressure Pw and the formation pressure Pf. The sealing pad in the present example seals an area of 7.3 cm by 4.5 cm. A pressure sensor  233  is used to monitor the pressure profile versus time during the operation. Any loss of seal can be noticed by comparing the pressure in the side bore with the well pressure Pw. 
     The injection pressure can be increased steps of for example 500 kPa increments, with pressure declines between each increment. Eventually the formation breakdown pressure is reached and a fracture  31  as shown in  FIG. 3B  develops at the location of the side bore  212 . Typically the initial fracture pressure is the highest pressure shown in the curves of  FIG. 3C , which illustrates an initial pressure test and subsequent reopening tests as detailed below. 
     In the carbonate formation of 1-10 mD of the example the fracture initiation pressure was established as 19080 kPa. From the first fall off after this fracture initiation the instantaneous fracture shut in pressure is 18700 kPa corresponding to the moment the pump is stopped, followed by the fracture closure pressure of 17920 kPa. At the point the fracture closes the pressure decay changes its characteristic. The pressure at fracture closure is known to be a measure of the minimum horizontal stress. 
     As shown in  FIG. 3C , subsequent increases in the injection rate by increasing the hydraulic motor speed from 300 to 1800 rpm do not alter the injection pressure, which fluctuated around the fracture propagation pressure. This insensitivity to injection rate suggests fracture propagation is dominating with little matrix injection. Of the six injection cycles following the fracture initiation and as illustrated in the curves of  FIG. 3C , the fracture propagation pressure from the last three of 17500 to 17700 kPa were the most consistent, indicating the micro fracture  31  reaches deep enough into the formation to see far field stress conditions, i.e. the formation parameters unperturbed by the drilling of the main well  11 . 
     As the unperturbed stress are typically smaller than those dominant in the damaged zone  12  of the well  11 , the measurement is more representative while easier to perform. 
     There are two natural properties of low permeability formation that have been drilled with high pressure drilling fluid that may favor the application of the above methods. The first is the high pressure gradients that will exist in low permeability rock when subjected to a pressure disturbance. This means the elevated pressure zone surrounding the side bore will not extend far into the formation until a considerable time has elapsed after applying the pressurization to this side bore. The small volume of rock that is pressurized will be covered by the sealing pad that also seals the side bore. 
     The second natural property is the existence of a stress cage or “supercharged” zone  12  around the original drilled wellbore as shown in  FIG. 3B . This zone  12  is created by the hydraulic force of the drilling fluid that supports the original well. The stress cage  12  is an annular volume of elevated stress several wellbore radii thick that surrounds any hole drilled with fluid at a pressure greater than the fluid pressure within the formation itself. The side bore  212  will partially penetrate this stress cage  12 . When the side bore is pressurized up to the breaking strength of the formation, the induced fracture  31  will most likely orientate itself away from this stress cage, propagating away from the main wellbore  11 . 
     This effect is believed to contribute to the fracture not intersecting the main well bore. And in turn it means that the seal of the pad covering the side bore is sufficient for the type of pressurizing and fracturing operation described above without requiring the use of further packers and the like to isolate the main well from the fracturing pressure. 
     A simulation of the isobars around the injection point is shown in  FIG. 3D . The contours shown are flattened, radial cross-sections of pressure at the wellbore wall for an injection rate of 1 cc/s. The vertical (depth) and horizontal distances are both measured in meters. The contours are drawn at successive multiples of 5 bars above initial reservoir pressure, which is 137 bars in the example. They show for example that the approximate diameter of the pressure-zone of 15 bar or more above reservoir pressure is 6 cm while for the zone of 20 bar or more above reservoir pressure it is 3 cm in agreement with the dimensions of the sealing pad used. 
     Using the various fracturing and fracture propagation and closing pressures as established by the present method, more parameters can be deduced as described in detail in the co-owned U.S. Pat. Nos. 5,353,637 and 5,517,854. However, it is worth noting that following the present method ensures that a fracture is only generated at one location of the well  11 , whereas in known methods the fracture appears typically in the two equivalent directions of maximal horizontal stress. This change can be assigned to the inhomogeneous application of pressure in the well. Known methods as represented by the &#39;637 and &#39;854 patents generate a homogenous pressure along the circumference of the well. Following the present method, the pressure is confined to the location of the side bore. 
     By confining the pressure to single location and smaller volume a much smaller volume of fluid is required for the fracturing testing. Conventional fracturing tests on open hole formations with pairs of straddle packers generate fractures by pressurizing the much larger volume of the well between the two packers and create hence much larger fractures. With new method smaller volume of less than for example 100 liters or even less than 50 liters, appear sufficient to perform the tests. For most applications the volume of stored fracturing fluid can be chosen from the range of 5 to 20 liters. These small volumes enable the use of smaller high differential pumps which typically have a slow pump rate without extending the downhole test time. Furthermore dedicated and expensive fluids such as heavy liquids can be applied for testing in accordance with the present invention. 
     Moreover, while the preferred embodiments are described in connection with various illustrative processes, one skilled in the art will recognize that the system may be embodied using a variety of specific procedures and equipment. Accordingly, the invention should not be viewed as limited except by the scope of the appended claims.