Downhole rebound hardness measurement while drilling or wireline logging

A method and apparatus for estimating a rock strength profile of a formation is disclosed. A tool having a testing surface is conveyed into a wellbore in the formation. The testing surface is propelled to impact the formation at a plurality of depths in the wellbore. A measurement of hardness of the formation is obtained from a rebound of the testing surface from the formation at the plurality of depths. The rock strength profile of the formation is estimated using the obtained measurements of hardness at the plurality of depths. A parameter for drilling the wellbore can be affected using the estimated rock strength profile.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure is related to methods and apparatus for estimating a rock strength profile of a wellbore formation in-situ.

2. Description of the Related Art

In petroleum exploration, drilling a wellbore or borehole in an earth formation employs a drill string with a drill bit at an end of the drill string. The speed and effectiveness of drilling is determined in part on the type of rock that is being drilled and its hardness or strength. Various types of rock that can be drilled can range from hard rocks such as granites and dolomites to soft rocks such as sandstones and shales. Various devices for estimating rock hardness are known in the art. However, these require obtaining a core sample and retrieving the sample to a surface location for testing, which can be time-consuming and expensive. Therefore, the present disclosure provides a method and apparatus for estimating in-situ a rock strength profile of a formation.

SUMMARY OF THE DISCLOSURE

In one aspect, the present disclosure provides a method of estimating a rock strength profile of a formation, the method including: conveying a tool having a testing surface into a wellbore in the formation; propelling the testing surface to impact the formation at a plurality of depths in the wellbore; obtaining a measurement of hardness of the formation from a rebound of the testing surface from the formation at the plurality of depths; and estimating the rock strength profile of the formation using the obtained measurements of hardness at the plurality of depths.

In another aspect, the present disclosure provides a method of drilling a wellbore, the method including: conveying a tool having a testing surface into a wellbore in the formation; propelling the testing surface to impact the formation at a plurality of depths in the wellbore; obtaining a measurement of hardness of the formation from a rebound of the testing surface from the formation at the plurality of depths; estimating the rock strength profile of the formation using the obtained measurements of hardness at the plurality of depths; and affecting a parameter for drilling the wellbore using the estimated rock strength profile.

In another aspect, the present disclosure provides an apparatus for estimating a rock strength profile of a formation, comprising: a tool configured to be conveyed into the wellbore; a testing surface disposed on the tool configured to propel against the formation at a plurality of depths in the wellbore and rebound from the formation as a result of the impact; and a processor configured to: obtain a measurement of hardness of the formation from a rebound of the testing surface from the formation at the plurality of depths, and estimate the rock strength profile of the formation from the obtained measurements of hardness at the plurality of depths.

Examples of certain features of the apparatus and method disclosed herein are summarized rather broadly in order that the detailed description thereof that follows can be better understood. There are, of course, additional features of the apparatus and method disclosed hereinafter that will form the subject of the claims.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1is a schematic diagram of an exemplary drilling system100that includes a drill string having a drilling assembly attached to its bottom end that can be operated according to the exemplary methods apparatus disclosed herein.FIG. 1shows a drill string120that includes a drilling assembly or bottomhole assembly (“BHA”)190conveyed in a wellbore126. The drilling system100includes a conventional derrick111erected on a platform or floor112which supports a rotary table114that is rotated by a prime mover, such as an electric motor (not shown), at a desired rotational speed. A tubing (such as jointed drill pipe)122having the drilling assembly190attached at its bottom end extends from the surface to the bottom151of the wellbore126. A drill bit150, attached to drilling assembly190, disintegrates the geological formations when it is rotated to drill the wellbore126. The drill string120is coupled to a drawworks130via a Kelly joint121, swivel128and line129through a pulley. Drawworks130is operated to control the weight on bit (“WOB”). The drill string120can be rotated by a top drive (not shown) instead of by the prime mover and the rotary table114. The operation of the drawworks130is known in the art and is thus not described in detail herein.

In an aspect, a suitable drilling fluid131(also referred to as “mud”) from a source132thereof, such as a mud pit, is circulated under pressure through the drill string120by a mud pump134. The drilling fluid131passes from the mud pump134into the drill string120via a de-surger136and the fluid line138. The drilling fluid131afrom the drilling tubular discharges at the wellbore bottom151through openings in the drill bit150. The returning drilling fluid131bcirculates uphole through the annular space127between the drill string120and the wellbore126and returns to the mud pit132via a return line135and drill cutting screen185that removes the drill cuttings186from the returning drilling fluid131b. A sensor S1in line138provides information about the fluid flow rate. A surface torque sensor S2and a sensor S3associated with the drill string120provide information about the torque and the rotational speed of the drill string120. Rate of penetration of the drill string120can be determined from the sensor S5, while the sensor S6can provide the hook load of the drill string120.

In some applications, the drill bit150is rotated by rotating the drill pipe122. However, in other applications, a downhole motor155(mud motor) disposed in the drilling assembly190also rotates the drill bit150. The rate of penetration (“ROP”) for a given drill bit and BHA largely depends on the WOB or the thrust force on the drill bit150and its rotational speed.

A surface control unit or controller140receives signals from downhole sensors and devices via a sensor143placed in the fluid line138and signals from sensors S1-S6and other sensors used in the system100and processes such signals according to programmed instructions provided from a program to the surface control unit140. The surface control unit140displays desired drilling parameters and other information on a display/monitor141that is utilized by an operator to control the drilling operations. The surface control unit140can be a computer-based unit that can include a processor142(such as a microprocessor), a storage device144, such as a solid-state memory, tape or hard disc, and one or more computer programs146in the storage device144that are accessible to the processor142for executing instructions contained in such programs to perform the methods disclosed herein. The surface control unit140can further communicate with a remote control unit148. The surface control unit140can process data relating to the drilling operations, data from the sensors and devices on the surface, and data received from downhole and can control one or more operations of the downhole and surface devices. Alternately, the methods disclosed herein can be performed at a downhole processor172.

The drilling assembly190also contains formation evaluation sensors or devices (also referred to as measurement-while-drilling, “MWD,” or logging-while-drilling, “LWD,” sensors) determining resistivity, density, porosity, permeability, acoustic properties, nuclear-magnetic resonance properties, corrosive properties of the fluids or formation downhole, salt or saline content, and other selected properties of the formation195surrounding the drilling assembly190. Such sensors are generally known in the art and for convenience are generally denoted herein by numeral165. The drilling assembly190also includes one or more RHMDs167for estimating rock strength of a formation according to the exemplary methods disclosed herein. The drilling assembly190can further include a variety of other sensors and communication devices159for controlling and/or determining one or more functions and properties of the drilling assembly (such as velocity, vibration, bending moment, acceleration, oscillations, whirl, stick-slip, etc.) and drilling operating parameters, such as weight-on-bit, fluid flow rate, pressure, temperature, rate of penetration, azimuth, tool face, drill bit rotation, etc. In addition, the drilling assembly190can also include one or more accelerometers169or equivalent devices for estimating an orientation of the drill string and of the one or more rock hardness measurement devices (RHMD)167in the wellbore. A suitable telemetry sub180using, for example, two-way telemetry, is also provided as illustrated in the drilling assembly190and provides information from the various sensors and to the surface control unit140.

Still referring toFIG. 1, the drill string120further includes energy conversion device160. In an aspect, the energy conversion device160is located in the BHA190to provide an electrical power or energy, such as current, to sensors165, RHMD167and/or communication devices159. Energy conversion device160can include a battery or an energy conversion device that can for example convert or harvest energy from pressure waves of drilling mud which are received by and flow through the drill string120and BHA190. Alternately, a power source at the surface can be used to power the various equipment downhole.

FIG. 2shows an exemplary rock hardness measurement device (RHMD)167suitable for use in the exemplary system of the present disclosure. In an exemplary embodiment, the RHMD is a Schmidt Hammer tool known in the art for non-destructive testing and for measuring strength of various structures such as bridges, dams, foundation, etc. The exemplary RHMD167includes a piston202with an affixed hammer mass204. The piston is configured to slide in and out of casing206through opening210at one end of the RHMD. The piston can reside in a retracted position within the casing206by compressing the compression spring208against end212of the RHMD opposite the opening210. The piston and compression spring are held in the retracted position by a trip screw218in a first position. The trip screw218moves from the first position to a second position to release the compression spring, which upon being released propels the piston202outward from opening210. The RHMD167in one aspect propels the piston202at a surface of a test material. Piston202has a tip214having a surface configured to impact and rebound from the test material. The amount of rebound of the piston/tip from the test material is measured to obtain the hardness of the test material. In one aspect, rebound measuring unit216obtains a measure of the rebound and creates an electronic or digital signal indicating the measured rebound and/or the hardness of the material. The created signal can be sent to a processor coupled to the RHMD167. The processor can use the measured hardness of the material to estimate rock strength of the tested material. In various embodiments, the RHMD167includes means for resetting the piston in its retracted position and thus can be used at a downhole remote location from an operator.

In one embodiment, the tip is configured to obtain a hardness measurement in a wellbore.FIG. 3Ashows a typical piston tip214aused in rock hardness testing. Tip214ais typically a surface of a hemisphere having a diameter of approximately 1 cm. However, wellbore surfaces can be rough due to natural rock surfaces, washout and other forces. Therefore, the tip214aofFIG. 3Acan slip along the wellbore surface or otherwise poorly impact the wellbore surface. The present disclosure therefore provides a piston having a tip214b(FIG. 3B) configured to impact the wellbore surface without slipping. In one embodiment, tip214bis a surface of a sphere having a radius of curvature of approximately 10 cm. Tip214bis therefore able to impact a rough surface and not slip against it. The geometry of the tip enables acquisition of the rebound hardness number in mud cake zones, highly fractured zones and highly weather areas. In addition, tip214bis applicable on rough or curved wellbore surfaces having dented or concaved irregularities.

FIG. 4Ashows a section of a downhole tool190having an exemplary downhole RHMD402according to one embodiment of the present disclosure, such as the RHMD ofFIG. 2. The RHMD402is conveyed downhole with bottomhole assembly and pressed against a surface of the wellbore. The RHMD is activated to propel the piston and tip against the wellbore surface to thereby obtain a rock hardness measurement. RHMD402is coupled to processor406, which can receive the obtained rock hardness measurement from the RHMD and estimate rock strength of the formation at the location. Additionally, a rock strength profile can be estimated at processor406. An orientation device408provides orientation of the drill string and/or RHMD to processor406. Orientation of the RHMD affects the measurements obtained of rock hardness. For example, a horizontal placement of the RHMD wherein the piston is propelled horizontally is substantially unaffected by gravitational force. However, if the piston is propelled in a direction that has a vertical component (i.e., vertical direction), the gravitational force affects the acceleration of the piston and therefore the impact of the piston has against a material to be tested. If the RHMD is oriented so that the piston is propelled downward to the test surface, the piston impacts the test surface with more energy. If the RHMD is oriented so that the piston is propelled upward to the test surface, the piston impacts the test surface with less energy. Processor406therefore computes rock strength using the obtained hardness measurements from the RHMD402and an orientation of the RHMD402, as discussed below with respect toFIG. 5. In one embodiment, computed rock strength can be sent to a surface location using telemetry device204for calculations at surface control unit140.

FIG. 4Bshows another exemplary embodiment of the present disclosure which includes a plurality of RHMDs402a-402fcircumferentially spaced about the tool190. Although six RHMDs are shown for illustrative purposes, the number of RHMDs is not meant as a limitation of the present disclosure. Referring toFIG. 2B, six exemplary RHMDs are spaced at 60° along the circumference of tool190. The circumferentially-spaced RHMDs402a-402fenable an operator to obtain measurements of rock hardness at various azimuthal locations around the tool. In an alternate method of obtaining rock hardness measurements at a plurality of azimuthal locations, the single RHMD402ofFIG. 4Acan be rotated to the circumferential positions of RHMDs402a-402fand activated to obtain measurements at each location. Both tools ofFIG. 2AandFIG. 2Bcan be moved axially to obtain hardness measurements at a plurality of depths of the wellbore. The hardness measurements at the plurality of depths can be used to obtain a rock strength profile of the formation. Typically hardness measurements are obtained at various depths of the wellbore. The rock strength profile can be measured at a single azimuthal location or can be measured at a plurality of azimuthal locations to obtain two- and three-dimensional rock strength profiles.

In various embodiments, the exemplary RHMDs obtain measurements at a particular location in the wellbore by obtaining multiple measurements at the location and nearby locations and averaging values. A maximum and minimum measurement at the particular location may be disregarded and an average taken of the remaining values. Impacting the wellbore formation at the particular location generally affects subsequent rock hardness measurements. Therefore, subsequent measurements related to the particular location can be obtained by moving the RHMD device to a nearby location which may be at a distance of between 10 cm and 30 cm.

In one embodiment, the RHMDs ofFIGS. 4A and 4Bare conveyed within a compartment within tool190to the downhole location and are extended from the compartment to obtain rock hardness measurements. In another aspect, the RHMDs are conveyed on pads coupled to tool190. The pads can be extended from the tool to abut the RHMDs against the wellbore to obtain rock hardness measurements. Although not shown inFIG. 4B, RHMDs402a-402fcan be coupled to one or more orientation devices for estimating orientations of the RHMDs and to a processor for estimating rock strength from obtained rock hardness measurements and orientation measurements. Also, a telemetry device can provide estimated values to a surface location.

FIG. 5shows an exemplary chart500for estimating rock strength of a formation using hardness measurements obtained at a downhole location. Multiple rock hardness scales501,503,505,507and509are shown along an x-axis. Each rock hardness scale corresponds to an orientation of an RHMD. Hence, scale501corresponds to a RHMD in a vertical orientation with the piston directed to be propelled downward. Scale503corresponds to the RHMD orientated at 45° to the downward vertical direction. Scale505corresponds to the RHMD oriented horizontally. Scale507corresponds to the RHMD oriented at 45° to an upward vertical direction. Scale509corresponds to the RHMD oriented vertically with the piston directed to be propelled upward. Uniaxial compressive strength (rock strength) is shown along the y-axis in MegaPascals. A plurality of rock density lines are shown along chart500. Each rock density lines is related to density of various rock types, such as dolomite, granite, sandstone, shale rock, for example. Rock strength is estimated using an appropriate rock hardness scale and rock density. For example, a rock hardness of 48 is obtained at a RHMD which is oriented in a horizontal direction and for which the density of rock is 26 kN/m3. Rock density can be estimated using various methods such as acoustic measurements and/or gamma ray measurements. Therefore, the number 48 is located on scale505, which applied to a horizontally directed piston. Using the rock density line labeled26inFIG. 5, the rock strength is estimated to be about 140 MPa.

In various aspects, the obtained rock strength profile can be used to characterize in-situ wellbore stress conditions in real-time. The methods and apparatus can be used as part of a measurement-while-drilling device or in wireline logging and drilling parameters can be altered based on the rock strength profile. In general, sedimentary reservoir formations show high anisotropic effects because of several fracture networks such as bedding planes, joints, laminations, etc. Therefore, multiple orientations of rebound hardness measurements provide an improved measurement of wellbore strength compared to a single alignment of a rebound hardness measurement.

Therefore, in one aspect, the present disclosure provides a method of estimating a rock strength profile of a formation, the method including: conveying a tool having a testing surface into a wellbore in the formation; propelling the testing surface to impact the formation at a plurality of depths in the wellbore; obtaining a measurement of hardness of the formation from a rebound of the testing surface from the formation at the plurality of depths; and estimating the rock strength profile of the formation using the obtained measurements of hardness at the plurality of depths. A parameter for drilling the wellbore can be affected or altered using the estimated rock strength profile. In one embodiment, the testing surface is configured to rebound from at least one of: (i) washout zone of the formation; (ii) a rough surface of the formation; (iii) a mud cake on the formation. Measurements of hardness of the formation can be obtained at a plurality of azimuthal locations. In one embodiment, the measurements can be obtained by one of: (i) rotating the tool having a single testing surface to obtain hardness measurements at the plurality of circumference locations; and (ii) obtaining the hardness measurements at a plurality of testing surfaces located at selected azimuthal locations of the tool. Rock hardness measurements can be obtained using at least one of: (i) spaced testing surfaces at separate axial locations of the tool; and (ii) radially spaced testing surfaces. An orientation of the testing surface in the wellbore can be obtained with respect to one of a: (i) vertical direction, and (ii) horizontal direction, and the rock strength estimated using the obtained measurement of rock hardness and the estimated orientation. The downhole tool can be used on a drill string or on a wireline. Hardness measurements can be obtained at axially separated depths of the wellbore and an average hardness measurement can be obtained using the axially separated hardness measurements.

In another aspect, the present disclosure provides a method of drilling a wellbore, the method including: conveying a tool having a testing surface into a wellbore in the formation; propelling the testing surface to impact the formation at a plurality of depths in the wellbore; obtaining a measurement of hardness of the formation from a rebound of the testing surface from the formation at the plurality of depths; estimating the rock strength profile of the formation using the obtained measurements of hardness at the plurality of depths; and affecting a parameter for drilling the wellbore using the estimated rock strength profile.

In another aspect, the present disclosure provides an apparatus for estimating a rock strength profile of a formation, comprising: a tool configured to be conveyed into the wellbore; a testing surface disposed on the tool configured to propel against the formation at a plurality of depths in the wellbore and rebound from the formation as a result of the impact; and a processor configured to: obtain a measurement of hardness of the formation from a rebound of the testing surface from the formation at the plurality of depths, and estimate the rock strength profile of the formation from the obtained measurements of hardness at the plurality of depths. The processor can be further configured to affect a parameter for drilling the wellbore using the estimated rock strength profile. The testing surface can be configured to impact at least one of: (i) a washout zone of the formation; (ii) a rough surface of the formation; (iii) a mud cake on the formation. The apparatus can in one embodiment include a plurality of testing surfaces azimuthally spaced around the tool. The apparatus can in another embodiment include the tool having a single testing surface configured to rotate to the plurality of azimuthal locations to obtain the measurement of hardness of the formation at the plurality of azimuthal locations. The testing surface can be at least one of (i) testing surfaces at separate axial locations of the tool; and (ii) radially spaced testing surfaces. In one embodiment, an orientation sensor is configured to estimate an orientation of the testing surface in the wellbore and the processor is configured to estimate the rock strength using the obtained hardness measurement and the estimated orientation. The apparatus can be conveyed in the wellbore on either a drill string or a wireline. The processor can be further configured to obtain an average of hardness measurements obtained at axially separated depths.