Patent Description:
Earth formations may be used for various purposes such as hydrocarbon production, geothermal production, and carbon dioxide sequestration. In order to efficiently employ resources for using an earth formation, it is necessary to know one or more properties or parameters of the earth formation. One example of a property is unconfined compressive strength (UCS). By knowing the UCS of formation rock, a production engineer for example can determine how fast to pump hydrocarbons from a well without producing sand grains. Many types of other actions may also be performed by knowing the properties of formation rock. Therefore, it would be well received in drilling and production industries if techniques were developed to accurately and efficiently estimate properties of earth formations.

<CIT> discloses a method and system for analysis of a digital core image obtained from a sample.

Disclosed is a method for estimating a property of an earth formation and performing an action related to the earth formation with action related equipment using the estimated property according to claim <NUM>.

Also disclosed is a system for estimating a property of an earth formation and performing an action using the estimated property according to claim <NUM>.

A detailed description of one or more embodiments of the disclosed apparatus and method presented herein by way of exemplification and not limitation with reference to the figures.

Disclosed are methods and apparatus for estimating a property of an earth formation. The methods and apparatus involve simulating formation rock features using a mathematical model of the rock features and calibrating the model using tests of actual samples of the rock. Accurate property values are thus estimated from an accurate rock model. In addition, drilling and/or production actions may be performed on the formation based upon knowledge of the estimated property.

It is noted that testing of rock samples alone may not be sufficient to provide the desired accuracy of estimated property values. The collection of rock parameters from rock tests can be biased due to different reasons. One reason is the rock sampling itself. Common practice involves collecting rock cores e.g. from a wellbore. With this procedure only certain sections of the wellbore may be sampled producing a subset of sample data. From the rock core further subsets are generally created called "plugs". In some cases the plugs are directly collected from the wellbore wall.

On the plugs the rock tests are carried out. The selection of the plugs as samples is based on a variety of decisions which are not discussed in detail here. Important to note however, is that the heterogeneity of the rock column may not be fully represented as the material properties, the coherency or degree of damage (e.g., micro-cracks) may differ from one location to another thus making the plug not fully representative of the earth formation.

The second source of bias may be introduced by the rock tests themselves. Multi-stage rock tests for example are carried out to measure rock stress/strain curves under increasing stress conditions. In these tests the sample is loaded and unloaded repeatedly. In each cycle, deformation is halted before significant inelastic behavior occurs to avoid rock failure that may fully destroy the sample. A destroyed sample would make further measurements impossible. All of these loading cycles however introduce damage to the rock material by crack closing as well as the creation of micro cracks which are linked to the onset of inelastic behavior. Because of this cumulative damage the rock mechanical properties of the sample may change throughout the test and introduce bias.

Apart from these multistage tests, other tests (e.g. unconfined compressive strength measurements) that provide useful information may require the full destruction of the sample during the test procedure, thus, not allowing any other subsequent tests on the same plug.

The main shortcomings testing alone are in summary (a) the lack of fully representing the rock's variability to due to the sampling and (b) the introduction of damage into the sample during the testing up to when its destruction inhibits further rock tests.

To overcome these shortcomings and provide other advantages, the methods and apparatus disclosed herein link the physical rock tests to a simulation of the very same tests or type of tests. The simulation is based on a mathematical material model of the formation rock that has the capability to track and quantify the damage history and damage directionality of the material. This model may be referred to herein as a "micro-crack evolution model. " In one or more embodiments, the model is based on a modified Mohr-Coulomb model having a term representing dilatation in an out-of-plane (i.e., out of fracture plane) orientation. <CIT> entitled "Mechanisms-Based Fracture Model For Geomaterials" discloses an example of the mathematical material model.

Based on the geometry and property distribution of the physical sample, a digital model (i.e., a mathematical model that is implemented by a processor such as in a computer processing system) is created. The digital model is calibrated against a non-destructive test or a test in which the physical sample is damaged incrementally (e.g. multi-stage test). Model validation is achieved if the parameters of the digital rock model adequately represent the rock deformation parameters of the physical sample. For example, model validation may be achieved if the results of a simulated multi-stage test are the same as the results of an actual multi-stage test for each incremental test.

The verified digital model can then be used to forward model multiple destructive tests. Furthermore, the model is able to separate the micro-properties of the rock matrix as well as the bulk macro-properties of the rock as a result of its internal damage. The model's ability to add a variable degree and distribution of damage to the model allows creation of a rock property catalogue with respect to verifying degrees of damage.

In general, the term "macro-property" relates to a property that is visible to the naked eye, while the term "micro-property" relates to a property that is not readily visible to the naked eye. With respect to each other, macro-properties are physically larger than micro-properties. It can be appreciated that properties may be sample dependent and/or scale dependent (i.e., dependent on physical size of the sample). Hence, micro-properties may be differentiated from macro-properties.

<FIG> is a flow chart for a method <NUM> for estimating a property of an earth formation. Block <NUM> calls for obtaining a sample of rock from the earth formation. The sample may be a sidewall core sample or a core sample of rock being drilled to drill a borehole. In one or more embodiments, a plurality of plugs may be extracted from one core sample. The physical sample may be referred to hereafter as a "plug" but may also represent another form of rock or material sample. The sample may be extracted from a formation using a coring tool such as illustrated in <FIG>.

Block <NUM> calls for scanning the sample with a volumetric imaging device to obtain a three-dimensional volume representation of the sample. The volumetric characteristics (such as voids, macro-cracks or micro-cracks) of the sample or plug are captured with the volumetric imaging device (e.g. computer tomography). The volumetric imaging device in one or more embodiments captures the radiological density of the plug and stores this information as a volumetric data set. <FIG> depicts aspects of one example of a volumetric imaging device <NUM>. The volumetric imaging device <NUM> includes an energy emitter <NUM> configured to emit energy into a sample <NUM> of the rock of the earth formation. The volumetric imaging device or scanner <NUM> also includes an imager <NUM> configured to receive energy from the sample <NUM> due to interaction of the emitted energy with the sample <NUM> and to provide a three-dimensional image of the sample <NUM> using the received energy. A processor <NUM> may process data received from the imager in order to provide the three-dimensional image. Non-limiting embodiments of the energy emitted by the scanner <NUM> include X-rays. Other types of energy may also be used. In one or more embodiments, the volumetric imaging device is a computed tomography (CT) scanner or mico-CT scanner.

Block <NUM> calls for determining internal rock damage of the sample with a processor using the three-dimensional volume representation of the sample. The volumetric data is used to derive a representative, statistical and/or discrete data set on matrix material of the sample that quantifies void, micro-crack, and macro-crack distribution in the plug.

Block <NUM> calls for performing one or more tests on the sample using a rock test device. The one or more tests may include non-destructive rock tests and/or destructive rock tests that cause rock damage such as a multi-stage rock test. In this block, the plug is subjected to one or more rock tests which may include subjecting the sample to a force in order to capture rock parameters. These may be tests which, ideally, are non-destructive and do not introduce significant damage to the material. The non-destructive and/or destructive tests may include measuring a deformation parameter of the material when a force is applied to the plug. Single-stage rock tests and/or multistage stage rock tests to capture the stress/strain behavior may be carried out next. This complements the rock property catalogue but is expected to introduce damage to the plug. <FIG> depicts aspects of a rock test device <NUM>. The rock test device <NUM> may include a force generator <NUM> such as a piston for applying a force to the sample <NUM> and a sensor <NUM> for sensing a corresponding change of the sample <NUM> due to the applied force. Non-limiting embodiments of the sensor <NUM> include a strain sensor <NUM>, a size measuring sensor <NUM> and an acoustic microphone or transducer <NUM>. A processor <NUM> may process data received from the sensors to provide a rock deformation measurement.

Block <NUM> calls for measuring a rock deformation parameter of the sample using a sensor. The terms "deformation" or "damage" relate to physical changes to a rock that results from one or more forces applied to the rock. Non-limiting embodiments of the deformation parameter include a stress curve, a strain curve, and/or locations in the sample of stress and/or strain. Physical deformation such as strain may be measured with the strain sensor <NUM> coupled to the sample <NUM>. External damage may be sensed by the size measuring sensor <NUM> configured to sense the size of the sample it undergoes an applied force. The internal damage of the plug may be monitored by measuring the acoustic emissions related to micro-cracking. That is, as a test force is imposed on the sample, the acoustic transducer <NUM> (or multiple transducers) receives acoustic energy (e.g., sound waves) related to the deformation or cracking of the sample. Using triangulation or other techniques, a location of a source of the acoustic energy (i.e., location of the deformation or cracking) can be determined. Also a repeated volumetric imaging and characterization run can provide supporting information on the degree and the distribution of internal damage. The data sets are to be used for verification purposes discussed in block <NUM> further below. The rock parameters obtained for the plug represent the bulk properties and are referred to hereafter as the physical "macro-properties".

Block <NUM> calls for constructing a mathematical model of the sample that replicates the determined internal rock damage and damage distribution of the sample. In one or more embodiments, the mathematical model is a digital three-dimensional model that is configured to be implemented by a processor such as in a computer processing system. In other embodiments, the model may have less than or greater than three dimensions. The model is also configured to model the evolution of micro-cracks and macro-cracks as the cracks develop due to applied forces. From the volumetric characterization of the plug, a digital representation is built for the purpose of a digital mechanical simulation. The representative, statistical and/or discrete distribution of matrix material, void space and damage (e.g. micro cracks) of the plug are represented in the digital model to replicate the plug's internal structure and damage. As noted above, one example of the digital model is a mechanism-based material model such as the modified Mohr-Coulomb model having a term representing dilatation in an out-of-plane orientation.

Block <NUM> calls for simulating the one or more tests using the mathematical model. The digital model is populated with plug's macro-properties derived from the physical tests carried out on the plug. The performed rock tests are simulated with the digital model of the plug using the mechanism-based material model capable of tracking the damage history and damage directionality. The parameters of the digital rock tests are collected. These may include the overall bulk characteristics of the test such as stress and/or strain curves and acoustic properties as well as discrete characteristics such as strain localization patterns, internal damage and/or acoustic emissions.

Block <NUM> calls for obtaining a rock deformation parameter using the one or more simulated tests corresponding to the measured rock deformation parameter.

Block <NUM> calls for comparing the rock deformation parameter obtained from the one or more simulated tests to the corresponding measured rock deformation parameter. The results of the physical rock tests with bulk parameters and - upon availability - discrete observations (e.g. acoustic emissions, introduced physical damage as revealed by volumetric imaging etc.) are compared with the outcome of the digital simulation to determine if the outcome of the digital simulation matches the results of the one or more physical rock tests. The term "match" relates to determining if the outcome of the digital simulation is within a selected range, such as within <NUM>% deviation for example, of the outcome of the physical rock tests. If several tests of the same type are performed, the range may be selected to provide a <NUM>% confidence interval. The selected range in general will be determined by an amount of desired accuracy of the digital model.

Block <NUM> calls for adjusting one or more parameters of the mathematical model based upon the rock parameter obtained from simulation not being with a selected range of the measured rock parameter. If the bulk properties returned from the mathematical model simulation are different from those of the plug or if the discrete observations are different, then the mathematical model properties are adjusted. This will apply to the mathematical model's matrix properties, which are updated and the simulation is re-run until a match is achieved. The mathematical model can be calibrated with one or more parameters to predict material behavior that is consistent with experimentally observed material behavior. The parameters used to adjust the mathematical model are generally dependent on the specific mathematical model used. In the case of the modified Mohr-Coulomb model, such parameters include the initial damage parameter, initial values of fracture tensor components, and/or calibration coefficients for the relation between equivalent stress and strain rate, for grade dependence, for strain-rate dependence, and the like.

Block <NUM> calls for providing a latest mathematical model as a verified mathematical model based upon the rock deformation parameter obtained from simulation being with a selected range of the measured rock deformation parameter. That is, the latest iteration of the model for the latest run simulation that provides a match is designated as the verified mathematical model. In other words, the digital model can be called verified if the bulk properties and the bulk behavior are matching. It should be noted that blocks <NUM>-<NUM> are generally implemented using a processor.

The method <NUM> may also include performing an action on the earth formation using a parameter obtained from the verified mathematical model. For example, the verified mathematical model may be used to estimate rock strength parameters of a greater earth formation. Inserted as parameters in a subsurface geomechanical simulation, the rock strength parameters determine under which operational constraints the earth formation stays intact. From the estimated rock strength parameters inserted into the model those skilled in the art can optimize the development of hydrocarbon production by e.g. adjusting hydrocarbon pump rates, hereby avoid damage to the rock which may otherwise produce grains of sand. From the estimated intact rock strength combined with the in-situ stress the user can calculate the minimum bottom hole pressure for solid free production in a so-called sand production prediction analysis. The sand production prediction analysis uses rock mechanic calculations in combination with an experimental or empirical derived critical deformation parameter (such as a critical amount of strain) to calculate at which bottom hole pressure or reservoir pressure the wellbore wall will start to fail and grains will be transported in the produced fluids or gas. By controlling the production rates by changing valve settings at the well, the user can possibly avoid the solid production. In another example, the estimated rock strength parameter is used to determine a mud or drilling fluid weight that would avoid a collapse of a borehole being drilled. Based on the rock strength parameters of the earth formation, those skilled in the art will change the weight of the drilling fluid by adjusting its density to fall within safe operational limits. The weight limits being large enough to avoid the borehole collapsing upon itself due to the weight and stresses of the earth formation surpassing the limits of the rock strength. And the weight limits being small enough to avoid the formation of fractures as an effect of the drilling fluid pressure surpassing the rock strength parameters of the earth formation.

It can be appreciated that the method <NUM> providing the verified digital rock model has several advantages. One advantage is that the verified digital rock model has an accurate representation of the rock's micro- and macro properties which capture the effects of micro-cracks and damage. On the digital sample further forward modeling can be applied to run multiple simulated "destructive" tests with different configurations to obtain a more complete catalogue of rock properties. In contrast, a physical sample would only support a single destructive test from which limited information can be obtained (e.g. tensile strength, UCS). The possibility of repeated destructive tests allows one skilled in the art to obtain additional information, e.g. the rock strength as a function of confining pressure and/or of changes in temperature. This additional information helps to create a more complete model of material behavior, which is more useful for the application to subsurface conditions than limited information (e.g., no confining pressure at room temperature).

Another advantage is that the digital rock model can separate the matrix properties from the macro-properties. For example, the unconfined compressive strength (UCS) from samples without damage is expected to have higher values as opposed to the samples which have fractures. Under assumption of the matrix properties, a mechanism based material model can add any degree and distribution of damage to the sample to forward model the bulk rock properties of differently damaged rocks.

Next, examples of production equipment are discussed. <FIG> depicts aspects of production equipment for producing hydrocarbons from an earth formation. A production rig <NUM> is configured to perform actions related to the production of hydrocarbons from the borehole <NUM> (may also be referred to as a well or wellbore) penetrating the earth <NUM> having the earth formation <NUM>. The formation <NUM> may contain a reservoir of hydrocarbons that are produced by the production rig <NUM>. The production rig <NUM> may include a pump <NUM> configured to pump hydrocarbons entering the borehole <NUM> to the surface. The pump <NUM> may include a valve (not shown) for controlling the flow rate of hydrocarbons being pumped. The borehole <NUM> may be lined by a casing <NUM> to prevent the borehole <NUM> from collapsing. The production rig <NUM> may include a reservoir stimulation system <NUM> configured to stimulate the earth formation <NUM> to increase the flow of hydrocarbons. In one or more embodiments, the reservoir stimulation system <NUM> is configured to hydraulically fracture rock in the formation <NUM>. The production rig <NUM> may also include a well rejuvenation system <NUM> configured to rejuvenate the borehole <NUM> (e.g., increase hydrocarbon flow into the borehole <NUM>). In one or more embodiments, the well rejuvenation system <NUM> includes an acid treatment system configured to inject acid into the borehole <NUM>.

The production rig <NUM> may also be configured to extract of core sample of the formation <NUM> a downhole coring tool <NUM>. The downhole tool <NUM> may be conveyed through the borehole <NUM> by an armored wireline that also provides communications to the surface. The core sample may be extracted using an extendable core drill <NUM>. Once the core sample is extracted, it is stored and conveyed to the surface for analysis. In general, a plurality of core samples is extracted in order to adequately represent the properties of rock present in the formation. For example, a higher number of samples would be required if the properties change significantly with depth as opposed to not changing significantly with depth.

<FIG> also illustrates a computer processing system <NUM>. The computer processing system <NUM> is configured to implement the methods disclosed herein. Further, the computer processing system <NUM> may be configured to act as a controller for controlling operations of the production rig <NUM> to include core sample extraction and analysis. Non-limiting examples of control actions include turning equipment on or off, setting setpoints, controlling pumping and/or flow rates, and executing processes for formation stimulation and well rejuvenation. In general one or more of the control actions may be determined using a formation parameter obtained from the verified model. In one or more embodiments, the computer processing system <NUM> may update or receive an update of the verified model in real time and, thus, estimate the formation parameter and provide control actions in real time.

Next, examples of drilling equipment are discussed. <FIG> depicts aspects of drilling equipment. A drill rig <NUM> is configured to drill the borehole <NUM> into the earth <NUM> according to a desired trajectory or geometry. The drill rig <NUM> includes a drill string <NUM> and a drill bit <NUM> disposed at the distal end the drill string <NUM>. The drill rig <NUM> is configured to rotate the drill string <NUM> and thus the drill bit <NUM> in order to drill the borehole <NUM>. In addition, the drill rig <NUM> is configured to pump drilling mud (i.e., drill fluid) of a selected weight through the drill string <NUM> in order to lubricate the drill bit <NUM> and flush cuttings from the borehole <NUM>. A geo-steering system <NUM> is coupled to the drill string <NUM> and is configured to steer the drill bit <NUM> in order to drill the borehole <NUM> according to the desired trajectory. A controller <NUM> is configured to control operations of the drill rig <NUM> to include controlling the geo-steering system <NUM>. In one or more embodiments, the geo-steering system can control the direction of drilling by exerting a force on the borehole wall using extendable pads. The computer processing system <NUM> may provide inputs into the controller <NUM> based upon formation parameters estimated using the verified model. In one or more embodiments, the computer processing system may receive updates of the verified model in real time and, thus, estimate the formation parameter and provide inputs to the controller <NUM> in real time.

In support of the teachings herein, various analysis components may be used including a digital and/or an analog system. For example, the computer processing system <NUM>, the controller <NUM>, and/or the geo-steering system <NUM> may include digital and/or analog systems. The system may have components such as a processor, storage media, memory, input, output, communications link (wired, wireless, optical or other), user interfaces, software programs, signal processors (digital or analog) and other such components (such as resistors, capacitors, inductors and others) to provide for operation and analyses of the apparatus and methods disclosed herein in any of several manners well-appreciated in the art. It is considered that these teachings may be, but need not be, implemented in conjunction with a set of computer executable instructions stored on a non-transitory computer readable medium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the present invention. These instructions may provide for equipment operation, control, data collection and analysis and other functions deemed relevant by a system designer, owner, user or other such personnel, in addition to the functions described in this disclosure. Processed data such as a result of an implemented method may be transmitted as a signal via a processor output interface to a signal receiving device. The signal receiving device may be a display monitor or printer for presenting the result to a user. Alternatively or in addition, the signal receiving device may be memory or a storage medium. It can be appreciated that storing the result in memory or the storage medium will transform the memory or storage medium into a new state (containing the result) from a prior state (not containing the result). Further, an alert signal may be transmitted from the processor to a user interface if the result exceeds a threshold value.

Further, various other components may be included and called upon for providing for aspects of the teachings herein. For example, a sensor, transmitter, receiver, transceiver, antenna, controller, optical unit, electrical unit or electromechanical unit may be included in support of the various aspects discussed herein or in support of other functions beyond this disclosure.

Elements of the embodiments have been introduced with either the articles "a" or "an. " The articles are intended to mean that there are one or more of the elements. The terms "including" and "having" are intended to be inclusive such that there may be additional elements other than the elements listed. The conjunction "or" when used with a list of at least two terms is intended to mean any term or combination of terms. The term "configured" relates one or more structural limitations of a device that are required for the device to perform the function or operation for which the device is configured.

The flow diagram depicted herein is just an example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.

While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.

It will be recognized that the various components or technologies may provide certain necessary or beneficial functionality or features. Accordingly, these functions and features as may be needed in support of the appended claims and variations thereof, are recognized as being inherently included as a part of the teachings herein and a part of the invention disclosed.

Claim 1:
A method (<NUM>) for estimating a property of an earth formation and performing an action related to the earth formation with action related equipment using the estimated property, the method (<NUM>) comprising:
obtaining a sample (<NUM>) of rock from the earth formation;
scanning the sample (<NUM>) with a volumetric imaging device (<NUM>) to obtain a three-dimensional volume representation of the sample (<NUM>);
determining internal rock damage of the sample (<NUM>) using the three-dimensional volume representation of the sample (<NUM>);
performing one or more tests on the sample (<NUM>) using a rock test device (<NUM>); the method characterized by:
measuring a deformation parameter of the sample (<NUM>) using a deformation sensor (<NUM>);
constructing a mathematical model of the sample (<NUM>) that replicates the determined internal rock damage of the sample (<NUM>);
simulating the one or more tests using the mathematical model;
obtaining a rock deformation parameter using the one or more simulated tests corresponding to the measured deformation parameter;
comparing the rock deformation parameter obtained from the one or more simulated tests to the corresponding measured deformation parameter;
adjusting parameters of the mathematical model based upon the rock deformation parameter obtained from simulation not being within a selected range of the measured deformation parameter; and
providing the mathematical model as a verified mathematical model based upon the rock deformation parameter obtained from simulation being within a selected range of the measured parameter;
the method further comprising estimating a unconfined compressive strength (UCS) of the earth formation (<NUM>) using the verified mathematical model; and
performing at least one of:
(a) pumping hydrocarbons from the earth formation (<NUM>) using a pump (<NUM>) at a flow rate determined by the unconfined compressive strength (UCS) in order to avoid sand grains from being pumped with the hydrocarbons; and
(b) pumping drilling fluid for drilling a borehole (<NUM>) using drilling equipment, the drilling fluid having a weight that is selected using the unconfined compressive strength (UCS) that avoids collapse of the borehole (<NUM>);
wherein the determining, constructing, obtaining a rock deformation parameter, comparing adjusting, and providing are performed using a processor (<NUM>, <NUM>).