Wells Reference guide

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Contents

Key Business Processes

Planning

Once a team of geoscientists has conceived a hydrocarbon prospect and proposed a well location, planning the well begins.

During this stage, geoscientists on the team will evaluate seismic and geological data and contribute a well prognosis, which spells out the anticipated stratigraphic units and their estimated depths. The engineer will be responsible for design of the well, from selecting the number and type of drill bits to installing completion strings to produce the formation fluids. As well, the land department will assess the land rights and regulations involved and determine who the stakeholders are.

Drilling

The drilling stage involves many operations, including (but not limited to) surveying the site; selecting a rig contractor and rig; spudding the well and recording spud date; filling out drilling reports; and coring, testing, and logging.

Evaluating

Once the target formation is reached, the team of oil and gas specialists begins evaluating logs, examining cores, and analyzing tests.

Sampling

It is also useful to track where fluid and rock samples come from in the wellbore, the methods used for collecting and analyzing the samples, the results of analysis, and sample storage.

Completing

A well that encounters a viable source of hydrocarbons must then be completed and put on production. This process involves opening the producing interval to the wellbore and isolating the reservoir from contamination by water-producing intervals, etc.

Producing or Injecting

You may find that the successful well has encountered a new pool in an existing field. You will likely need to report this information to your regulatory agency. In addition, during this stage of the life of the well, you will conduct periodic tests and determine volumes of produced fluids, composition, and allocation.

Alternatively, you may be using this well to inject water and improve production from surrounding wells.

Selling

At some point, as production declines with time, you may decide that producing the well is no longer profitable or that you want to sell your interests in the well and funnel money into a higher priority project. In any case, you contemplate farming out your interest in the well.

In determining its worth, you will have assessed remaining reserves, contacted other stakeholders, completed the legal agreements governing its sale, etc.

Abandoning

Eventually, your new well may begin producing at such high water-oil ratios that you want to abandon it. Operations associated with abandonment include obtaining the appropriate permits for abandoning the well, contracting a rig to perform the operation, plugging the well, recovering the wellhead and other equipment, and reclaiming the well site and disposing of it.

Model Overview

Model Design Discussion

Well

A well is a set of holes drilled in the ground for the purpose of exploring for, or obtaining production of, hydrocarbons. The hole is drilled by a rock bit. Most wells have only one hole (wellbore), although some are a complex set of branching holes with a common surface point.

In the Well Module, all the information is referenced to a particular well. The primary key is the Unique Well Identifier. The UWI is therefore part of the primary key in every related table throughout this module. WELL_VERSION is an almost identical table, with the addition of SOURCE to the primary key. This allows for storage of multiple versions of the same information about a well, and the promotion of one set of information to the WELL table for routine use.

The WELL table contains several types of information:

  • general: name, class, type, depth, operator, regulator, elevations, general location
  • summary indicators (flags): discovery, faulted
  • information copied from other parts of the Well Module: location, status, dates, etc.
  • connections to other tables in the Well Module: node, source (for WELL_VERSION)
  • connections to other parts of the Model: lease, tax code

The WELL table was the first table designed in PPDM, about 1989. It has undergone changes over time, but still retains some of the original requirements. These may be important to support older implementations, even if there is now a better way. For example, as database systems have evolved, the need to denormalize for query performance has diminished, and so some columns in this table are best left unpopulated if the data can be efficiently retrieved from the primary table. Any attempt to store the same information in more than one table is an invitation to quality problems and complicates data loading procedures.

Directional Surveys

The position of the wellbore may be surveyed by a downhole device, mounted in the drill string or operated on a wireline. The data may be recorded and held within the device, or transmitted in real time to the surface. The simplest survey device records only the inclination of the wellbore at selected depths. More advanced devices allow for accurate three-dimensional mapping of the entire well.

Directional survey information is captured in the PPDM in three related tables:

  • general information: type of tool, dates and depth range, computation method, coordinate system
  • survey stations: measured and recalculated X, Y, Z position, and related data.
  • geometry: a table specifically for spatially enabling the PPDM.

Logs

A log is an organized record of measurements. In the Well Module, the log data is ordered by depth. The measurements deal with the rocks and fluids, the materials placed in the wellbore, and the drilling processes. The data may be raw or observed properties, but are more usually computed, adjusted, or interpreted. Most well logs are obtained by a device suspended in the wellbore on a wire line. These wireline (or “mechanical”) logs are captured in the PPDM model, in various related tables:

  • general information: contractor, dates, data about the well at the time of logging
  • run (trip): dates, depths, tool, related mud samples, formation interval logged
  • curves: properties measured, curve scales, digital sampling interval
  • mud (drilling fluid): sampling times, density, resistivity, temperature, fluid loss

Properties such as the composition of gases entrained in the drilling fluid are measured by devices placed in the stream of the circulating fluid when it reaches the surface. The data is recorded with computed depth values, so that the values represent events at the drill bit. The Log Module was not designed for this type of “mud log” information. However, the data can be captured in the table WELL_LOG_CURVE. For example, C1, C2, etc. can be defined as curve types.

Lithology logs describe the observed or interpreted geological features of the rock intersected by the well. This type of log is captured in the Lithology module of PPDM.

Some of the common log-related terms used in the model are job, run, pass, tool string, tool, bottom and top depths, and curve. These terms are defined in a separate Glossary, available from PPDM.

Cores

A core is a cylinder of rock obtained by a hollow drill bit. It is the only undisturbed sample of the subsurface that is available for direct examination. Cores are obtained only for intervals of special interest in the well, because of the high cost.

Sidewall cores are a special category. They are small cylinders of rock obtained at selected depths from the side of the open wellbore, usually after the well has been logged. The coring device uses a small drill or explosive projectile to cut the core. Sidewall cores are not necessarily collected in depth order.

Information is obtained from the cores by direct examination (description and geological interpretation), by laboratory analysis, and by mechanical measurement (e.g., spectrometry).

Core information is captured in the PPDM model in various related tables:

  • general information: contractor, dates, data about the well at the time of coring
  • coring operation: dates, depths, diameter, length, recovery, formations cored
  • descriptions: textual remarks on the entire core, and about samples taken for analysis
  • analysis procedures: laboratory company, dates, methods, sample type, and size
  • analysis results: porosity, permeability, density, etc.
  • core shift: adjustments to match coring depths to logging depths

Lithology logs are textual and graphic descriptions of the observed or interpreted geological features of the core. This type of log is captured in the Lithology module of PPDM.

Well Tests

A well test is a set of procedures, performed under specified conditions, to measure the factors affecting the capability of a well for production or injection of fluids.

The well test portion of PPDM is large and complicated. It has a long history in PPDM; was included in the early versions of the model; and is used, with modifications, in many implementations. Some of the complexities are:

  • There are several types of tests, each creating special kinds of data.
  • Some tests are not assigned a unique number in the field, so the numbers required in the primary key must be created prior to loading the data for these tests.
  • Some column names, inherited from earlier versions, do not conform to current naming principles. Because of numerous implementations, the destructive impact was deemed too great to change these names without a strong business purpose.
  • Some tables, especially the parent table WELL_TEST, have many columns of denormalized values. These were created for the convenience and efficiency of queries, but are not mandatory. We recommend that they not be used without a special purpose.
  • Many columns have a foreign key to a reference table, to another table in the well test subject, or to another table in a different part of the model. This integration is powerful within the database, but requires extra attention to data loading.

The five-part primary key in the WELL_TEST parent table is carried through all the child tables. This key consists of:

  • UWI: identifies the well and wellbore in which the test was performed
  • SOURCE: identifies the source of the data
  • TEST_TYPE: describes the general type of test, such as drill stem or initial productivity. Allowable values are in R_WELL_TEST_TYPE.
  • RUN_NUM: identifies the run (trip) into the well for the purpose of testing. With DSTs and WLTs, it is possible to conduct several tests during a single run.
  • TEST_NUM: identifies the individual test generating a set of data such as depth, time interval, pressures, and recoveries

Because SOURCE is part of the primary key, the model does not allow for some of the test data to be supplied from a different source. For example, you might purchase most of the test data from your primary data supplier, VENDOR A. But you buy test pressure data from VENDOR B, who offers higher quality or more comprehensive coverage for this data type. In the model, you must keep VENDOR A as the source throughout the well test tables. Data from VENDOR B could be identified in the remarks column, or by extending the model by adding your own column such as XTND_SOURCE.

See Appendix A: Sample Queries for an illustration of the primary key for the test tables.

Completions and Related Operations

A completion is a set of operations to prepare a well for the production or injection of fluids. Several other tables are closely related to completions. They provide information about:

  • Perforation is the operation of making holes in the casing to allow fluid exchange between the formation and the well.
  • Tests are the operations and measurements to determine fluids and pressures in a reservoir. A completion is not essential for some kinds of well tests (e.g., drill stem test, formation pressure test).
  • Treatment is the operation and materials applied to “stimulate” the reservoir to enhance productivity (e.g., acid wash).
  • Tubular is the casing and production tubing placed in the well during drilling and completion.
  • Cement is the operation and materials used to secure the casing in the wellbore and to prevent the flow of fluids in the annulus (space between casing and rock).
  • Production String is the conduit within a producing well for fluid exchange between the formation and the surface.
  • Production String Formation is the name of the rock unit connected to the production string.

Analysis of Reservoir Fluids

Gas, oil, and water obtained from a subsurface reservoir may be analyzed for compositional and physical properties. The information is often important for evaluating the potential of a reservoir, for determining the components of production and their commercial value, and for modeling the performance of the reservoir.

Fluid analysis is captured in the PPDM in the following pair of tables for each of gas, oil, and water:

  • sample: where, when, and how the analyzed sample was obtained from the well, plus some analyzed properties
  • analysis results: property (e.g., C3, H2S), measured or computed value, and units.

The WELL_TEST_ANALYSIS table is less comprehensive than the fluid analysis tables, but is intended to record information commonly reported for a well test (e.g., GOR, H2S, oil gravity). The source of this information may be the test report, whereas data in the fluid analysis tables is usually derived from the laboratory reports.

Well Positions

The three-dimensional path of the wellbore is valuable information in exploration, drilling operations, production planning, and lease administration. This path must be expressed in numeric values that allow for accurate mapping. If the well is not positioned correctly, significant errors are likely in relation to seismic lines, pipelines, surface facilities, lease boundaries, cultural and topographic features, other wells, and subsurface targets.

Survey information is captured in the PPDM in numerous tables, many of which are shared by other subject modules. These general tables can be considered as several types:

  • reference values for coordinate systems, computation methods, etc.
  • coordinate system and parameters
  • survey methods
  • survey points: X Y, Z field data, and computed values
  • methods of expressing the position for mapping purposes: geodetic or relative

In the Well Module, the most important positional information is stored as latitude and longitude sets for the surface location and certain key subsurface points, such as the bottom of the hole, kickoff points, and target intersections. Some of these values may be denormalized to the WELL table for more efficient retrieval. The entire path of a wellbore may be described or plotted using computed survey data in the directional survey tables.

Any set of coordinates is potentially confusing or useless without the related information about the coordinate system and reference points. An organization could declare that only one coordinate system is used for all its spatial information, and then not bother to populate some of these tables in the PPDM. This is probably a short-sighted decision, degrading the quality and integrity of the database.

Tables and Columns: Wells

General Information

Well Positions

Well Legal Locations

Directional Surveys

Horizontal Drilling

Geological Interpretation

Cores

Drilling Results

Tests

Completions and Related Operations

Pressures

Checkshot and Velocity Surveys

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