TECHNOLOGY INTERACTIVE SOFTWARE INTEGRATES GEOLOGICAL AND ENGINEERING DATA

Ghan S. Srivastava
Oxy USA Inc.
Tulsa

A comprehensive software package provides Oxy USA Inc. a set of interactive tools for rapid and easy integration of geological, geophysical, petrophysical, and reservoir engineering data for the purpose of reservoir characterization. The stacked curves system (SCPC), proprietary soft-ware of Oxy USA Inc., is used extensively within Occidental Petroleum Corp. to determine detailed knowledge of reservoir geometry and associated parameters crucial in infill drilling, field extension, and enhanced recovery projects.

As depicted in Fig. 1, SCPC has all the desktop management and mapping software tools necessary to fully address, analyze, and resolve three components of reservoir characterization:

1. Defining the geometry

2. Calculating reservoir properties

3. Making volumetric estimates

Judicious mapping of the reservoir structure and stratigraphy is key to understanding the reservoir shape and internal geometry. Reservoir properties such as porosity, permeability, saturation, and net and porosity feet provide clues about reservoir fluid storage and mobility. Pore volume, hydrocarbon pore volume, and hydrocarbons in place are essential data for evaluating the economic viability of a project.

SCPC is ideally suited for a reservoir management team concept. A geologist creates an SCPC project data base for correlating formation tops from well logs in association with core and test information to resolve the structural and stratigraphic relationship. The same SCPC data base is utilized by geophysicists to calibrate seismic horizons with formation tops and resolve time to depth relationships. Petrophysicists and engineers work the same data base for calculating reservoir properties and volumes.

In brief, SCPC provides exploration and production personnel a comprehensive software package for evaluating a prospect or reservoir. The software permits integration of analyses and interpretations by scientist of different disciplines in one project environment.

SCPC provides the tools to scrutinize input data, normalize log curves, relate seismic and well data, model surfaces, display cross sections, subdivide and correlate reservoir zones, incorporate faults, and subsequently make volumetric estimates and map calculated reservoir parameters. These parameters often are exported directly to reservoir simulators to evaluate the effects of water, steam, or CO2 floods planned for secondary and tertiary recovery projects.

BACKGROUND

The original concept, of the stacked curves system in the early 1980s was to display one type of log curve side-by-side for multiple wells at a constant or relative spacing. This display, as shown in Fig. 2, revealed correlation or normalization problems along with the local or regional variations (continuity, pinchout) of the log curve responses.

The stacked curves system, developed initially on the mainframe computer, provided limited interactivity with the mapping and modeling functions. Consequently, only a limited number of geoscientists and engineers used the system.

In 1990, the stacked curves system was redesigned and rewritten for the PC environment and became known as SCPC. Ease of use was the major contributing factor in company wide acceptance of SCPC.

SCPC is a dynamic system that has evolved to meet the requirements of different field studies. Currently, SCPC is being used to characterize reservoirs of practically all fields, domestic and international, operated by Occidental Petroleum Corp.

HARDWARE/SOFTWARE

SCPC is an interactive graphics system which is based on a PC platform using the OS/2 2.x operating system. It fully utilizes OS/2's multitasking features within the system and is written in the C language using OS/2 presentation manager tools. SCPC maximizes the use of the mouse, minimizes keyboard entries, and by providing default values ensures reasonable results.

An SCPC project data base can contain up to a maximum of 10,000 wells and 32,000 curve samples/log curve. There is a maximum of 200 log curves and 200 formation tops/well. These limits are arbitrarily set based on the largest project requirements to date. Available disk storage is the only limit for the number of seismic shotpoints and other parameters.

Displays within SCPC, such as maps and cross sections, are scalable. The user can specify a scale or have the system fit the display to a specific size paper. Plots are produced in HPGL/2 and PCL formats and can be sent to most Hewlett Packard plotters and printers. Color plots typically are sent to HP Designjet 650C (E size) and HP Paintjet XL 300 (A or B size) plotters, while page size black and white plots can be sent to HP Laserjet III or IV.

FUNCTIONS

The components of SCPC (Fig. 1) are grouped into three primary functions:

1. Data management

2. Transformation

3. Display

   These are organized logically to ensure a smooth transition through the various phases of a project. The multitasking capability of OS/2 operating system allows several functions to be activated at the same time. Related information is automatically transferred from one function to another. For example, as a new set of wells for a cross section is chosen in the base map mode, the cross section is automatically updated with the newly selected wells.

   DATA MANAGEMENT

   Data management functions are used to import and export, for quality control, and to edit well data. The data typically include a wide variety of subsurface, production, and historical data. Tables organize formation data, core descriptions, perforation intervals, formation and production tests, deviation survey, well logs, formation tops, seismic, production history, and other essential data.

   Well data typically are retrieved in ASCII format from data bases that reside on a mainframe computer, local or wide area network, and other media. After the data are loaded into SCPC, data management provides quality control tools that permit interactive editing of all data. These tools include customized spreadsheets, quick-view displays, and data tables.

   The spreadsheet functions provide tools for quick and easy editing of voluminous data. These data can be sorted using a variety of well identifiers such as operator, lease, and well name. Editing of data is quickly accomplished using the "find and change" option. Once edited, the data can be exported in file formats suitable for importing data into commercial spreadsheet programs. There are several quick-view displays in SCPC for graphically verifying the data. The "preview" option, for example, automatically scales on one display all available log curves (Fig. 3) associated with a well for a given depth range. This option consequently provides a quick and easy way to identify potential problems such as multiple copies of a curve, erroneous curves, and multiple runs without having to define parameters for building a cross section. Knowledge of these problems is used to make appropriate corrections to the curve data using delete, merge, or rename curves commands. Other quick-view displays include histograms of curve data or reservoir parameters, profiles of the deviation survey data, and production history displays.

   Data also can be edited or reviewed using a tabular data format. Data types appropriate for a tabular listing include log curves, deviation survey, production and injection fluids, and reservoir parameters.

   Data editing and quality control processes constitute a major part of any reservoir characterization project. These tedious processes can be greatly simplified and quickly accomplished using SCPC. The user can gain instant access to all well information from any display (i.e., map, cross section, or spreadsheet) by double clicking the mouse on the specific well identifier. If changes are made to the data, the data base is updated immediately.

   TRANSFORMATION

   Transformation functions are used to calculate new parameters for selected zones using current formation tops, well logs, Z-values, cores, and production data. These transformations can be confined to specific wells and zones through the use of data filters designed to search for wells meeting specific criteria. SCPC provides numerous data filters for selecting wells from the SCPC data base. These filters accelerate the process of selecting critical information needed for a specific analyses. Filters can be used to select, for example, only those wells with specific log curves or formation tops, wells in a assigned area, or wells with a specified range of zone parameter values.

   Transformations of parameters are defined in terms of upper and lower zone boundaries including formation tops, offset depth from a top, or actual measured or subsea depths. Multiple zones also can be selected for the transformation.

   Computations made using the formation tops include isopach and true stratigraphic thickness (TST). SCPC uses zone boundary definitions to calculate the isopach as the difference between the measured depth footage along the borehole. If multiple zones have been defined, an isopach can be calculated simultaneously for all selected zones and wells. For TST calculations, both a zone definition and dip data are required. If the borehole is deviated, deviation survey data can be incorporated in the TST and isopach calculations.

   Prior to any transformation of log data, the raw log curve data must be normalized. This is true especially when working with several generations of logging tools (e.g., gamma ray, spontaneous potential, and old neutron logs). SCPC provides tools to select high and low, such as sand/shale, curve amplitude picks from either curve data histograms or curves displayed in a cross section. A histogram of these picks is used to choose statistical mode values, which are utilized in the system or user-defined equations to normalize the curve data relative to the mode picks.

   SCPC provides tools to transform these normalized curves into reservoir property curves, such as porosity and saturation. The user can utilize either industry standard log analysis transformation equations or user defined equations. From these transformed curves, reservoir properties such as gross, net, average porosity, porosity feet, and hydrocarbon feet can be calculated using reservoir specific cutoffs for all the selected wells and zones.

   SCPC provides tools for developing porosity-permeability transforms and calculating statistical permeability parameters. With the cross plot module, core porosity is calibrated against the log porosity. This adjusted log porosity is used for calculating permeability if an adequate core porosity-permeability relationship can be established. Core permeability variations can be studied in statistical graphs, and calculated as Dykstra-Parson and Lorenz coefficients.

   Cumulative production, oil/water ratio, and gas/oil ratio for a given time period can be computed from production history data. These data can be displayed in geographical displays such as bubble and contour maps.

   Volumetric calculations, such as pore volume and hydrocarbon pore volume, can be performed for the entire pro*d area or restricted areas within several specified polygons. A volumetric report contains data pertaining to the calculated areas and volumes for the individual polygons and the total volume of the study area.

   Transformations generally are derived for the expressed purpose of building a geologic model. Once the transforms are completed, the calculated zone parameters can be gridded and then translated to the model grid. Sampled model grids can be exported with file formats used by various commercially available reservoir simulation systems.

   DISPLAYS

   Original or computed reservoir data within SCPC can be displayed in various forms. Maps and cross sections are the most frequent displays. Other displays include cross plots and histograms for showing statistical distribution of reservoir parameters and log curves.

   MAPS

   SCPC generates several types of maps for viewing, editing, and interpreting reservoir data. These include base maps, bubble maps, attribute maps, and contour maps.

   Base map displays show well and shotpoint locations, cartographic data, deviated borehole tracks, and other assorted data. SCPC comes with over 50 predefined symbols that can be used to post well and shotpoint locations. These symbols can be displayed in various sizes and colors for easy identification of production or completion patterns.

   Different cartographic information, such as township/range and lease boundaries, kept as individual layers in the system, are selectively displayed on the base map. The cartographic information can be imported from commercial cartographic data bases or added to the existing base map using drawing tools within SCPC. The system also plots a "worm track" on the base map to depict the path of a deviated well bore in the subsurface.

   The intersection between well bore path and depth interval or formation tops can be marked on the well bore path with a variety of descriptive symbols. In addition, other assorted data such as multiple lines of well information such as operator, lease, well name, formation tops, reservoir parameters, and cumulative production data can be posted in selected colors by the well.

   Seismic shotpoints from two or three dimensional seismic data can be displayed on the base map. Seismic Zs, such as amplitude and time, can be color coded based on their values. This color coding helps identify seismic anomalies and interpret seismic trends. Once a seismic anomaly is identified, the data can be examined in graphics profile or spreadsheet.

   The system provides several functions, graphical and nongraphical, for editing well location information. Graphically, the well location either can be moved by using the mouse to drag it to the new location or spotted by entering the exact location such as detail footage, latitude/longitude, or x/y data. A spreadsheet, specifically designed for well data, can be constructed for displaying, editing, sorting and printing the well identification and location data.

   A double click on a well symbol brings up a screen similar to a scout ticket report (Fig. 4). This screen provides access to all information pertaining to the well including location, well log, reservoir parameters, production, and well test data. This screen also contains the perforation, casing, cores, samples, shows, deviation survey, and other well information. At this point the information can be viewed or edited if desired. The scout ticket screen also can be accessed from the cross section and spreadsheet applications. A scout ticket report can be generated listing all or selected information for the wells.

   Beneath each well location, up to three log curves can be displayed. This option is helpful when studying the areal distribution of log curve patterns. These curves can be shaded using amplitude cutoffs to emphasize, for example, a pay zone or follow a particular facies.

   BUBBLE MAPS

   Bubble maps (Fig. 5a) display a circle at each well location which vary in size and color as a function of the selected parameters in the data base. Bubbles are used to indicate the presence and absence of a parameter or their variability. Circles with the same size but v g color often are used to indicate different data intervals. The radius of the bubbles commonly are scaled by a non-footage related parameter such as cumulative gas or by the scale of the map for footage related parameters such as drainage radius.

   Bubbles are the fastest way to display reservoir data either for quality control or for displaying their variability. They are also the most appropriate way to geographically display high variability data, such as cumulative production and average permeability because contouring such data is unreliable.

   ATTRIBUTE MAPS

   Attribute maps (Fig. 5b) graphically show the presence or absence of multiple information for a well. Attribute map displays are valuable for exhibiting alphanumeric information such as producing formations, type of available well logs, and completion dates.

   For numeric information, a range can be chosen for which a specific color can be assigned. The display looks similar to a pie diagram or a ring diagram with the well symbol in the middle. For each attribute (maximum of eight), a specific color is assigned.

   SCPC provides two types of gridding algorithms (triangular and rectangular) for contouring the Z data. The triangulation scheme of gridding connects all the wells to form triangles and the contours are traced using the interpolated values on the sides of the triangles. The rectangular scheme lays a grid over the wells and the contours are traced from the interpolated values at the grid nodes.

   There are various options available to enhance the gridding such as grid size, refinement, directional biasing, connectivity etc. Additional control points can be provided for guiding the contours to enhance interpretation. To contour net feet in a field, for example, a series of control points with a value of zero can be added to define the field boundary.

   The SCPC gridding algorithms quickly and easily produce contour maps. All the gridding parameters are queried in geological terms rather than numbers. There is practically no learning curve for producing a contour map suitable for quality control and identifying the geological or reservoir trend. If desired, the data can be exported as xyz or grid values to commonly available commercial contouring packages.

   SCPC provides several advanced surface modeling techniques including trend, residual, and derivative analyses. These techniques are ideal for identifying localized or subdued trends within the data.

   Faults can be digitized on any form of map. The degree to which these faults influence the contour gridding process is based on whether the fault throw diminishes at the end or not, thereby making the fault trace more translucent or less opaque to the gadding algorithm.

   SCPC supports a wide range of line types, widths, and colors used to make map displays. In addition, contour intervals can be color filled (Fig. 5c) for easier interpretation.

   CROSS SECTIONS

   Geological cross sections, either structural or stratigraphic, are easily built with log curves and formation data. Selection of the wells simply involves clicking on a series of wells in the desired sequence on the base map display. Cross section parameters include depth range, scale, datum, log curves, and formation tops.

   The curves can be displayed in four industry standard or user defined tracks using the specified colors, line type, curve scale, and track width. In addition, the depth track of the cross section, core, perforation, test, show, and other intervals can be displayed using unique symbols and colors.

   SCPC provides interactive editing of the log curve data while in the cross section display. By picking a series of "from-to" intervals using the mouse, one can perform curve editing such as depth shifting, amplitude adjustment, removal of erroneous curve segment, and straightening the curve drift.

   Log curves, original or computed, can be shaded using a palette of colors and a cut-off defined by a constant or curve statistics such as mean and standard deviation. SCPC also offers a special type of curve shading called Lithology (Fig. 6a), which shades a curve in different colors based on the value of another curve, such as a facies, porosity, saturation curve.

   Variability or continuity of a reservoir parameter can be displayed using color filled contours (Fig. 6b) of the log responses in the cross section plane. These contours are guided by the formation tops and account for facies-type variations of the reservoir parameter.

   Color cross sections offer an excellent way to evaluate the continuity of reservoir parameters such as porosity and the potential success of a water, steam or CO2 flood. The interwell color contour gridding process for most log data uses a linear interpolation technique. For log curves with highly variable amplitude responses, such as a core permeability curve, one can use a statistical interpolation algorithm based on the fractal dimension.

   Formation tops are interactively picked (digitized) on cross sections containing up to 200 wells. The tops are picked using the point and click method. Digitized tops are saved in the data base and are available immediately for contouring. Formation tops are connected from well to well using either straight lines or a spline that creates a smoother surface. Selected tops or specified depth offset from a top are used as a datum plane for displaying stratigraphic cross sections.

   Deviated borehole traces including horizontal wells, can be displayed on the cross section plane. For a different prospective view, the deviated survey data can be converted and displayed as an interpolated true vertical depth cross section. Faults are interactively added to the cross section of structurally complex areas. The system determines the extent of the fault zone and the amount of fault throw and accordingly processes the data, such as tops and color contouring, within each fault block independently.

   CROSS PLOTS

   SCPC creates two and three-dimensional cross plots of raw and computed log curves or reservoir parameter data. Options are available to simultaneously display a standard or user defined cross plot and corresponding log curves (Fig. 7). The cross plot and log curve displays are interactive, allowing the user to identify critical log intervals and simultaneously view the corresponding point on the cross plot. Sophisticated filters that search for wells meeting specific criteria can be created easily for the purpose of isolating critical information to plot.

   SCPC provides several industry standard or user defined cross plot overlays including Pickett and Hingle plots. Up to seven curves can be used to build the cross plot or ternary diagram, two for cross plot locations (three for ternary), one for color, and the rest as discriminators.

   Seismic variables, sampled to the wells, can be cross plotted against reservoir parameters for selected zones and wells. If a statistical relationship is established, the system can extrapolate the reservoir properties throughout the seismically covered area.

   Curve fitting algorithms are provided to quantify the mathematical relationships between the variables on the cross plot. Along with the regression (linear or quadratic) equation, the system provides the correlation coefficient and simple statistics that characterize the data. If a few questionable data points on the cross plot are impacting the numerical regression process, the user can fit (eye ball) a linear regression line to the data.

   Facies are defined on cross plots or ternary diagrams using polygons. Each polygon represents a unique data cluster that differentiates it from associated data. The system uses up to seven discriminators with the two-dimensional poly on to define a facies unit. SCPC incorporates these facies units to create a facies curve used to shade another curve, and calculate total footage, percentage and ratio of each facies. The areal variability of the calculated results can be displayed on bubble maps, attribute maps, or contour maps.

   HISTOGRAMS

   Histograms in SCPC display the frequency distribution and elementary statistics of log curves and reservoir parameter data. From the curve histogram, high and low (e.g. sand-shale) amplitude values can be graphically captured and stored for use in the log normalization procedure.

   ACKNOWLEDGMENT

   The author would like to acknowledge Ow USA Inc. for supporting the concept and development of SCPC and permitting the publication of this article. The concept of SCPC would not have been realized without the dedicated effort of Tom Whitwell in designing and writing the system. Also, thanks to Jim Landrum, Charles Scott, and Tom Whitwell for helpful comments and suggestions during preparation of this paper.

   REFERENCES

   1. Srivastava, G.S., Canfield, D.C. and Landrigan, E.V., "Stacked Log Curves," OGJ June 21, 1982, pp. 302-06.

   Copyright 1994 Oil & Gas Journal. All Rights Reserved.