Patent Description:
For a detailed description of exemplary embodiments, reference will now be made to the accompanying drawings in which:.

Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, different companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function.

In the following discussion and in the claims, the terms "including" and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to. " Also, the terms "couple" or "couples" are intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections.

"Rock type" shall mean a rock having certain rock properties. Rock type may alternatively be referred to as "petrofacies.

"Rock properties" shall mean a physical feature or measured value of a rock type. Porosity and permeability are examples of rock properties. Likewise, sonic velocity (speed of sound through the rock) and gamma readings are example of rock properties.

"Depositional facies" shall mean a depositional structure, such as a point bar, channel, splay, and marine bar. Depositional facies shall not imply any particular rock type or rock property, although a certain depositional facies may often be associated with certain rock types and rock properties. Depositional facies may alternatively be referred to as "geological facies.

"Geocellular model" shall mean a model of an underground formation, the model comprising a plurality of cells or tessellations that represent a predetermined volume.

"Distance" in reference to cells of a geocellular model shall mean a conceptual distance represented by the geocellular model, and shall not be read to require an actual physical distance between cells.

"Variogram" shall mean a function that defines a spatial dependence of a rock property. For example, a variogram may indicate a high probability of presence of the rock property in directions along a north-south line, and a low probability of presence of the rock property in directions along an east-west line.

"Regionalized variable" shall mean a variable describing a property which has geographic meaning and thus can be estimated or simulated in a geographic space.

The following discussion is directed to providing a better understanding of the invention, and to show how the same may be carried into effect. It should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims.

The present invention relates to a method as defined in claim <NUM>, to a computer system as defined in claim <NUM> and to a non-transitory computer-readable medium as defined in claim <NUM>. More generally, the invention is directed to creating a geocellular model of an underground formation, involving creating the geocellular model by obtaining rock properties from a plurality of well logs, and assigning the obtained rock properties to a three-dimensional model in order to more accurately estimate rock types. The specification first turns to a discussion of the related art.

Knowing the properties and locations of underground rock formations is useful for making decisions as to where and how to perform hydrocarbon drilling operations. In particular, a geologist making drilling decisions may consider various rock types in an underground formation, where each rock type may be comprised of rock properties describing composition and structure. For example, a section of an underground formation may be comprised of the following different rock types: sandstone; limestone; shale; and granite, where each rock type has rock properties that differ from one another. In addition, where each rock type may have differing rock properties from other rock types, among a singular rock type, different locations within a rock type may have varying rock properties. In order to ascertain information regarding the underground formation, rock properties for each rock type may be measured and subsequently recorded in a well log. Well logging is a technique used to identify properties associated with earth formations surrounding a wellbore. The interrogation of a formation surrounding a wellbore to identify one or more property of a rock type may be by, for example, sound, electrical current, electromagnetic waves, or high energy nuclear particles (e.g., gamma particles and neutrons).

More specifically, at various times in the creation of a well, various tools may be run within the well to create well logs to measure rock properties, where the rock properties may be indicative of the ability of the formation to economically produce hydrocarbons. For example, well logs may include natural gamma logs (i.e., created by a tool that measures natural gamma radioactivity), gamma-gamma logs (i.e., created by a tool that releases interrogating energy in the form of gamma rays or particles); formation porosity; formation resistivity; formation permeability; acoustic impedance; and spectral information of the underground formation at a particular depth.

Rock properties are only measured within a limited radius around the well in which measurements are taken. The rock properties are aggregated into a well log, where a geologist makes a determination as to the rock type surrounding the well based on the rock properties in the well log. However, because the geologist is able to make a determination as to the rock type only within a certain distance from the well (based on information obtained from the well logs), in order to create a model of the underground formation in the related art, rock types can be distributed between measured wells as if the rock types were regionalized variables. For example, if the rock type measured in one well is shale, and the rock type measured in a nearby offset well is also shale, then the rock type between the two wells may be assumed to be shale as well, regardless if shale is or is not present between the two wells and regardless of any additionally measured rock properties.

Furthermore, because knowledge of rock type does not necessarily imply a depositional facies, distributing rocks types using this method does not guarantee that the resulting geometries of the created models will make sense in the way they would if the depositional facies were known. Further, in distributing rock types, valuable information in the form of varying rock properties for a singular rock type will be lost.

Some or all of an underground hydrocarbon bearing formation (herein after "underground formation" or "formation"), including petrofacies, may be modeled by the creation of a three-dimensional geocellular model. <FIG> shows a perspective view of a portion of a geocellular model <NUM>. As the name implies, a geocellular model comprises a plurality of cells, for example cells <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, where all of the cells considered together approximate the physical extent of the formation (or a relevant section of the formation). As shown in <FIG>, the example cells <NUM>-<NUM> are cubical and have approximately equal volume. For example, cell <NUM> may represent <NUM>,<NUM> cubic meters (<NUM> cubic feet) of earth. However, the cells may have varying volumes and varying shapes.

It is to be understood that each cell (e.g., cells <NUM>-<NUM>) is a mathematical construct, not a physical construct. The illustration of <FIG> showing the geocellular model <NUM> is merely to orient the reader to the idea of a geocellular model which may later represent a portion of an underground formation.

In order to, ultimately, more accurately describe the rock types within an underground formation, the geocellular model <NUM> will be provided with and will contain data describing the rock properties represented by the location of each individual cell using analytical modeling (e.g., interpolation, simulation, and other geostatistical principles).

For example, each cell may contain a value indicative of the porosity of the portion of the formation associated with the respective cell in the geocellular model <NUM>. Each cell contains more than one value; for example, each cell may contain a value indicative of the porosity, as well as a value indicative of permeability and/or gamma radiation and/or resistivity. Because actual values are possible only where values have been measured ( with downhole measuring tools), cells without associated measured values will be algorithmically assigned with interpolated and/or simulated rock property values. <FIG> provides a more detailed snapshot of the geocellular model <NUM>.

<FIG> shows a perspective view of a portion of the geocellular model <NUM> located below the earth's crust <NUM>. The geocellular model <NUM> is graphically displayed over a portion of a formation. The layers between the surface <NUM> and the geocellular model <NUM> are not shown so as to avoid unduly complicating the figure, and also to provide a clear view of the cross-section of the depositional facies contained within the geocellular model <NUM>.

The location of a cell of the geocellular model <NUM> correlates conceptually to the corresponding location of the formation <NUM>. In particular, the geocellular model <NUM> of <FIG> is overlaid over a portion of formation <NUM> which illustratively contains two rock deposits, rock deposit <NUM> (shown in hatched lines) and rock deposit <NUM>, which lie between <NUM> feet below the surface and <NUM> feet below the surface (in the present description, <NUM> foot <NUM>,<NUM> meters). The depositional facies of rock deposit <NUM> is defined geometrically by a channel and two splays. For purposes of discussion, rock deposit <NUM> is considered to be sandstone, and rock deposit <NUM> is considered to be shale.

<FIG> also shows several wellbores drilled into the hydrocarbon bearing formation <NUM>. Wellbores <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are associated with wellheads <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, respectively, to illustrate that the wellbores have been previously drilled. The illustrative wellbores are displayed as vertical but they can have any geometry, including wellbores where a portion of the wellbore is horizontal. In addition, it should be noted that, although wellbores <NUM>-<NUM> are shown as previously drilled, data regarding the underground formation <NUM> may have been gathered at any stage of the creation of each wellbore.

Although the rock deposits <NUM> and <NUM> are shown as having specific depositional facies, the depositional facies of either rock deposit is not precisely known from or depicted in the geocellular model. Using methods described later in the discussion, cells of the geocellular model <NUM> will be populated with rock properties in order to provide a more accurate model of the underground formation <NUM>.

By using measuring tools within with each wellbore, data regarding the rock properties within a certain radial distance of the measuring tools is determined and used to create a well log, such as the example well log shown in <FIG>.

Turning briefly to <FIG>, a portion of a well log <NUM> for an example well is shown. In particular, well log <NUM> displays plotted data measured and obtained between the example vertical depths of <NUM> feet to <NUM> feet. In the example well log <NUM>, three types of measurements have been plotted with regard to the rock properties surrounding the well: porosity <NUM>; gamma radiation <NUM>; and permeability <NUM>. In the particular example, from a depth of <NUM> to <NUM>, the measured porosity is ranges from <NUM>% to <NUM>%, the measured gamma radiation ranges from <NUM> API (where "API" is an "American Petroleum Institute" unit) to <NUM> API; and the permeability is <NUM> millidarcy (md) to <NUM> md. At a depth from <NUM> to <NUM> feet, the measured porosity ranges from <NUM>% to <NUM>%; the measured gamma radiation ranges from <NUM> API to <NUM> API; and the permeability ranges from <NUM> md to 200md. Based on the measured rock properties, a rock type may be determined over a length of well bore and represent the area around the well.

Although each rock type (e.g., sandstone, shale, etc.) is characterized by a general set of innate rock properties, as can be seen in the example well log, it can also be seen that a single rock type may have varying rock properties based on location and/or depth. Furthermore, between rock types, such as between limestone and shale, the rock properties may vary further. By way of mechanisms discussed more below, well log information, such as the well log information obtained and plotted in <FIG>, will be used to create the geocellular model <NUM> shown in <FIG>.

Turning now to <FIG> shows an overhead view of a cross section of rock deposit <NUM> and rock deposit <NUM> at a depth of <NUM> feet (i.e., the top of the geocellular model <NUM>). Also visible in the overhead view is the location of the wells <NUM>-<NUM>, shown as dots.

Data obtained from tools associated with wells <NUM>, <NUM>, <NUM>, <NUM> and <NUM> may indicate that the rock type surrounding those wells is sandstone. However, outside of a certain radial distance from the center of each well (e.g., <NUM> feet), it is difficult to know if the rock type is actually sandstone. Likewise, data obtained from tools associated with well <NUM> indicates that the rock type surrounding well <NUM> (within a certain radial distance) is shale in the example of <FIG>.

If no other information is provided to the geologist besides that of the corresponding well logs, it is difficult to determine an accurate representation of the depositional facies of rock deposits <NUM> and <NUM>, nor the rock properties outside a certain radius from the measuring tools. Thus, looking at <FIG>, if well <NUM> was not present, the geologist may believe the rock type between wells <NUM> and <NUM> to also be sandstone. Likewise, the engineer may believe the rock type between wells <NUM> and <NUM> to be solely sandstone.

In order to produce a more accurate model, the geologist creates the geocellular model by obtaining a rock property from the well log and associating that value with a corresponding cell in the geocellular model <NUM>. For each rock property obtained from the well log, the rock property will be associated with the property's associated location in the geocellular model <NUM> (i.e., in the cell which corresponds to the location in the formation from which the rock property value was measured). In embodiments and implementations of the invention, each well log provides more than one rock property, and each cell is associated with more than one rock property, as will be described below.

For example, returning to <FIG>, the rock properties associated with well <NUM> and obtained from the well log can be associated with cells <NUM> and the cell directly below cell <NUM>. Likewise, the rock properties associated with well <NUM> can be associated with cell <NUM> and the cell directly below cell <NUM>. Although the geocellular model <NUM> is shown as having two layers comprising a total of <NUM> cells, in reality, the geocellular model may be on the order of tens to thousands of layers with thousands or even tens of thousands of cells.

In addition, rock property values distributed into a cell are applicable to the entire cell volume. In other words, in cells representing locations in the formation <NUM> where more than one rock type is present, such as cell <NUM>, the distribution of rock property values will not provide the geologist with enough information to determine two distinct rock types. Although the distribution of rock property values into the cells of the geocellular model may help to provide a more accurate depiction of the depositional facies, the distribution will not provide an indication of the exact depositional facies.

In an example embodiment, from the well log associated with well <NUM>, a porosity value is obtained. The porosity value is then stored in the respective locational cell in the geocellular model: cell <NUM>, and in <FIG>, the cell directly below cell <NUM>. Because no well log data is available for the location corresponding to cell <NUM> in this example, cell <NUM> does not yet receive a value to store. Continuing to the right (towards cell <NUM>), a porosity value is obtained from well log data corresponding to well <NUM> between the depths for <NUM> feet and <NUM> feet. The obtained porosity value is then stored in the corresponding locational cell in the geocellular model: cell <NUM>, and in <FIG>, the cell directly below cell <NUM>. Each cell in the geocellular model is correspondingly associated with measured data values where possible. For example, each cell may have one or more of the following example measurements associated: porosity, gamma, permeability, resistivity, and additional seismic data.

For cells in which there is no actual rock property measurement available (e.g., porosity, gamma radiation, permeability has not been measured or it not present in a well log), values may be assigned by interpolating between the cells. Returning again to <FIG>, well <NUM> is drilled through the hydrocarbon formation <NUM> at a location that corresponds to cell <NUM> in the geocellular model <NUM>. Through the use of downhole measuring tools, a well log of data is aggregated (e.g., the well log of <FIG>), and certain rock properties are known for the portion of sandstone present in the hydrocarbon formation <NUM> corresponding to the location of cell <NUM> within the geocellular model. Similarly, well <NUM> is drilled through the hydrocarbon formation <NUM> at a location that corresponds to cell <NUM> in the geocellular model <NUM>, and a well log of data is aggregated with respect to rock properties measured within the portion of sandstone present in the hydrocarbon formation <NUM> corresponding to the location of cell <NUM> within the geocellular model. Within the volume of cell <NUM>, however, no corresponding well has been drilled into the hydrocarbon formation <NUM>, and thus, there are no downhole measurements taken and no corresponding well log.

For cells in which there is no corresponding measurement data, an interpolation technique may be used to distribute rock properties. A previously unknown rock property value in a cell is interpolated by considering the known rock properties of two nearby cells. For example, a known rock property value for well <NUM> and corresponding to cell <NUM> is considered, as is a known rock property for well <NUM> corresponding to cell <NUM>. In addition, the respective distances between cell <NUM> and cell <NUM> to cell <NUM> (i.e., the cell with no known rock property value), is considered. In this example, cells <NUM> and <NUM> are equidistant from cell <NUM>, and thus the rock properties of cell <NUM> and <NUM> may be interpolated into <NUM>.

In embodiments of the invention, at least one variogram, or a series of variograms may be calculated with respect to the measured data. A variogram is a statistical function that is indicative of the difference between data points as a function of distance and direction (azimuth). Once the variograms have been calculated, the data points representing the non-measured rock properties may be calculated using a kriging or co-kriging technique. One of ordinary skill in the art is aware of the kriging or co-kriging techniques, and now understanding application of the kriging or co-kriging techniques to the situation of updating a geocellular model would understand how to apply the kriging or co-kriging techniques to the geocellular model <NUM>.

<FIG> shows an overhead view of a depiction of a distributed rock property among a layer of cells within the geocellular model <NUM> after interpolation. In particular, <FIG> shows a visual representation <NUM> of one distributed rock property (e.g., porosity). Also visible, and for reference, are the locations of the wells <NUM>-<NUM> where each well corresponds to the location of each respective cell in the geocellular model. The distance between each of the lines may indicate either a measured rock property or an interpolated rock property. For example, if the rock property shown in <FIG> is porosity, the farther apart each adjacent line is on the representation <NUM>, the higher the porosity. In contrast, the closer each line is to adjacent line, the lower the porosity.

More specifically, the distance <NUM> 'd' between lines <NUM> and <NUM> is constant within the representation, indicating that the porosity between lines <NUM> and <NUM> is approximately the same. In contrast, the lines grow closer together near the representation of well <NUM>, such that the distance <NUM> 's' between lines <NUM> and <NUM> indicates a lower porosity than between the lines <NUM> and <NUM>.

Multiple rock properties may be "mapped" into individual visual representations such as the one shown in <FIG>. In addition, it may be possible to create a representation with more than one rock property depicted. Although representation <NUM> is shown to be similar to a topographic map, any suitable representation is possible, including a heat map; a shaded variant map; or other depiction of a distributed rock property in the geocellular model.

Additional data may be obtained by way of a seismic survey. That is, a seismic survey (e.g., land-based survey, marine survey) may have been conducted to provide seismic mapping of the subsurface rock deposits associated with the underground hydrocarbon formation <NUM>, resulting in seismic data. Thus, seismic data and/or well log data may be used to provide more accurate interpolations of rock properties including, in some cases, better estimations of the depositional facies of the rock deposits.

Obtaining a seismic survey may involve placing long strands of cable across the surface <NUM> of the earth, the cable having periodically spaced seismic receiving devices. The seismic receiving devices are placed in a grid pattern over or proximate to the formation of interest. After the seismic receiving devices are placed, a seismic event is triggered, for example by detonation of dynamite or through the use of vibrator trucks which contact the surface of the earth and impart energy. The energy, whether created by dynamite or by trucks, propagates through the various earth layers to the formation of interest, and portions of the signal reflect back to the surface receivers.

For example, seismic waves may be propagated through the formation <NUM>, where a portion of the formation <NUM> may be comprised of different rock type layers, each layer having different acoustic impedance. For each boundary the seismic wave encounters, some of the energy in the wave will be reflected at the boundary, while some of the energy will be transmitted through the boundary. Because, at the time of the seismic survey, the geologist does not know the composition of the formation <NUM>, the seismic survey will provide some data regarding the formation <NUM>, but will not provide enough data to give a clear indication of the depositional facies underground.

<FIG> shows an overhead view of a representation of a seismic map of rock deposits <NUM> and <NUM>. Seismic survey data is coarse in the vertical domain; in other words, while it may be possible to know the depositional facies of the rock deposits at a well location where there are measurements, the geometric mapping away from the well may only be accurate within, for example, <NUM> to <NUM> feet. In the horizontal domain, however, seismic data has better resolution, and thus can be used to more accurately predict coarse rock types located farther away from the wellbore in the horizontal plane.

<FIG> shows an example of the depositional facies of rock deposit <NUM> obtained by a seismic survey, shown by hashed line <NUM>. In addition, the actual shape of the depositional facies of rock deposit <NUM> (shown by solid line <NUM>) for comparative reference. The seismic survey may provide the general shape of the rock deposit <NUM>, including a general outline of the channel and the two splays, but seismic data alone cannot refine the geometry to the actual shape (i.e., solid line <NUM>) of rock deposit <NUM>.

Seismic data, therefore, can be used as a co-variable to further refine the geocellular model <NUM>. For example, to the extent seismic data relates to a property to be distributed into the cells of the geocellular model <NUM> (e.g., acoustic impedance as it relates to porosity), seismic data can provide an additional variable to refine the accuracy of the model.

As with well log data, distribution of rock properties among the cells of the geocellular model may be made by any suitable interpolation technique taking into consideration seismic data. For example, a rock property may be assigned to a cell in the geocellular model by co-kriging a rock property datum and seismic data obtained from a seismic survey. While co-kriging is one possible technique for the distribution of rock properties, other suitable analytical modeling techniques may be used such as kriging, simulation, and/or another method of interpolation.

The discussion has focused on distributing an example rock property within a geocellular model. In the explanations above, one property (e.g., porosity) was distributed among the cells; however, in embodiments and implementations of the invention, multiple rock properties are, distributed such that each cell of the single geocellular model comprises multiple properties. For example, cell <NUM> might have multiple values corresponding to the following rock properties: porosity, permeability, gamma radiation, resistivity, and acoustic impedance.

Once rock properties have been distributed in the geocellular model, it becomes possible to make a determination as the rock type at each location in the geocellular model. In other words, using the rock properties associated with a cell corresponding to the location of the unknown rock type in the formation, by application of associations of rock properties with rock types, a determination can be made as the rock type in each cell of the geocellular model. In principle. determination of rock type after distributing of the rock properties can be made by a geologist or geophysicist, but in embodiments and implementations of the claimed invention the rock type determinations are made programmatically. The end result is a geocellular model where each cell contains a rock type, but the rock type is determined based on the distributed rock properties. Stated otherwise, a formula or algorithm is applied to each cell of the model in order to ascertain a rock type for the cell based on the plurality of associated and distributed rock properties. The application of the formula may occur on a cell-by-cell basis; for example, for each cell which is populated with respective rock properties, the rock type may be calculated as each cell has been fully filled with the desired information.

It should be noted that although there may be some initial user input (e.g., initial quality control, calibration, specification of which analytical model to run), the creation of the geocellular model, reading a rock property from a well log, associating the value of the rock property with a cell in a geocellular model, assigning the value of the rock properties to the cells in the geocellular model, and then determining rock type from the rock parameters are executed without user input.

<FIG> shows a method useful for understanding the present invention. In particular, the method starts (block <NUM>) by creating, by a computer system, a geocellular model of an underground formation (block <NUM>), the creating by: reading a first value of a first rock property associated with a first well log (block <NUM>); associating the first value of the first rock property with a first cell of a plurality of cells of the geocellular model (block <NUM>); assigning a value of the first rock property to each cell of the plurality cells based on the first value and a datum of information, the datum of information distinct from the first value (block <NUM>). In addition to data obtained from one well log, data may also be obtained or ascertained from, but not limited to, a second well log, multiple well logs, variograms, and/or seismic survey data. Thereafter, the method ends (block <NUM>).

<FIG> shows a computer system <NUM>, which is illustrative of a computer system useful for understanding the present invention. In particular, computer system <NUM> comprises a processor <NUM>, and the processor couples to a main memory <NUM> by way of a bridge device <NUM>. Moreover, the processor <NUM> may couple to a long term storage device <NUM> (e.g., a hard drive, solid state disk, memory stick, optical disc) by way of the bridge device <NUM>. Programs executable by the processor <NUM> may be stored on the storage device <NUM>, and accessed when needed by the processor <NUM>. The program stored on the storage device <NUM> may comprise programs to implement the various embodiments of the present specification. In some cases, the programs are copied from the storage device <NUM> to the main memory <NUM>, and the programs are executed from the main memory <NUM>. Thus, the main memory <NUM>, and storage device <NUM> shall be considered computer-readable storage mediums. In addition, a display device <NUM>, which may comprise any suitable electronic display device upon which any image or text can be displayed, may be coupled to the processor <NUM> by way of bridge <NUM>. Furthermore, computer system <NUM> may comprise a network interface <NUM>, coupled to the processor <NUM> by way of bridge <NUM>, and coupled to storage device <NUM>, the network interface acting to couple the computer system to a communication network.

In the specification and claims, certain components may be described in terms of algorithms and/or steps performed by a software application that may be provided on a non-transitory storage medium (i.e., other than a carrier wave or a signal propagating along a conductor). The various embodiments also relate to a system for performing various steps and operations as described herein. This system may be a specially-constructed device such as an electronic device, or it may include one or more general-purpose computers that can follow software instructions to perform the steps described herein. Multiple computers can be networked to perform such functions. Software instructions may be stored in any computer readable storage medium, such as for example, magnetic or optical disks, cards, memory, and the like.

References to "one embodiment", "an embodiment", and "a particular embodiment" indicate that a particular element or characteristic is included in at least one embodiment of the invention. Although the phrases "in one embodiment", "an embodiment", and "a particular embodiment" may appear in various places, these do not necessarily refer to the same embodiment.

From the description provided herein, those skilled in the art are readily able to combine software created as described with appropriate general-purpose or special-purpose computer hardware to create a computer system and/or computer sub-components in accordance with the various embodiments, to create a computer system and/or computer sub-components for carrying out the methods of the various embodiments and/or to create a computer-readable media that stores a software program to implement the method aspects of the various embodiments.

Claim 1:
A method comprising:
measuring, with measuring tools within each of a plurality of wellbores, a respective plurality of well logs each comprising data regarding a plurality of rock properties of rock surrounding the respective wellbore;
creating, by a computer system, a geocellular model (<NUM>) of an underground formation having the plurality of wellbores, the creating by:
for each wellbore of the plurality of wellbores, reading a measured value of the plurality of rock properties associated with a first well log, and
associating the values of the plurality of rock properties with a respective cell (<NUM>) of a plurality of cells (<NUM>-<NUM>) of the geocellular model;
assigning a value of the plurality of rock properties to each cell of the plurality of cells based on the measured values and a datum of information, the datum of information distinct from the measured values and derived from a variogram regarding the rock property; and
determining a rock type for each cell based on the values of the plurality of rock properties assigned to that cell.