Introduction to Well Logging (Formation Evaluation Overview)

Introduction to Well Logging

Formation Evaluation Overview

The Scope of Formation Evaluation

Formation evaluation covers a large variety of measurement and analytic techniques. Although the emphasis is on wireline logging techniques and log analysis methods, these are far from the only tools available to the formation evaluator. Well logs are central only in the sense that they are recorded in practically all wellbores and are directly relatable to all the other parameters available from the associated sciences. For example, a geophysicist needs borehole measurements to determine a time-depth relationship, and a petrophysicist needs core analysis to properly define log response, but a thin section or scanning electron microscope (SEM) photo of a rock sample are of no direct help to the interpretation of a seismic section, nor is a vertical seismic profile (VSP) of any help in deter-mining relative permeability. However, all the measurements are pertinent to the complete task of defining a reservoir's limits, storage capacity, hydrocarbon content, productivity and economic value.

To place the various disciplines in perspective, it is valuable to consider the overall problem of formation evaluation in terms of orders of magnitude. If one meter is taken as a unit of measurement, then each formation evaluation technique can be placed in order, as shown in Table 1 .

 

Thus, formation evaluation techniques cover at least twelve orders of magnitude. Equally far-ranging are the physical principles employed to make the basic measurements. An enlightening way of viewing the vast spread is to consider the frequency employed by the measuring processes available, as illustrated in Table 2 . Few other sciences require, or use, such a wide range of measurement techniques over such a wide range of physical dimensions.

 

Formation Evaluation Objectives

The primary objective of formation evaluation is to determine the size of a reservoir, the quantity of hydrocarbons in place, and the reservoir's producing capabilities. The initial discovery of a reservoir lies squarely in the hands of the exploration geologist using seismic, gravity and magnetics studies, and other geologic tools. Formation evaluation presupposes that a reservoir has been located and is to be defined by drilling as few wells as possible. Enough data should be gathered from those wells to extrapolate reservoir parameters fieldwide and arrive at realistic figures for both the economic evaluation of the reservoir and the planning of the optimum recovery method. Formation evaluation offers a way of gathering the data needed for both economic analysis and production planning.

What, then, are the parameters that the manager, the geologist, the geophysicist, and the reservoir and production engineers need? Which of these can be provided by seismics, by coring, by mud logging, by testing, or by conventional wireline logging?

The geophysicist needs to know the time-depth relationship in order to calibrate conventional seismic and VSP surveys. The geologist needs to know the stratigraphy, the structural and sedimentary features, and the mineralogy of the formations through which the well was drilled. The reservoir engineer needs to know the vertical and lateral extent of the reservoir, its porosity (the nature of the porosity) and permeability, fluid content, and recoverability.

The production engineer needs to know the rock properties, be aware of overpressure if it exists, be able to assess sanding and associated problems, and recognize the need for secondary recovery efforts or pressure maintenance. Once the well is in production, he/she also needs to know the dynamic behavior of the well under production conditions and be able to diagnose problems as the well ages.

Engineers also need to know formation injectivity and residual water saturation to plan waterflooding and monitor waterflood progress when it is operational.

The manager needs to know the vital inputs to an economic study-the original petroleum hydrocarbons in place, recoverability, cost of development and, based on those factors, the profitability of producing the reservoir.

Log measurements, when properly calibrated, can give the majority of the parameters required by all these professionals. Specifically, logs can provide either a direct measurement or a good indication of

• porosity, both primary and secondary (fractures and vugs)
  

permeability

water saturation and hydrocarbon movability

hydrocarbon type (oil, gas, or condensate)

lithology

formation (bed) dip and strike

sedimentary environment

travel times of elastic waves in a formation

From this data, good estimates may be made of the reservoir size and the petroleum hydrocarbons in place.

Logging techniques in cased holes can provide much of the data needed to monitor primary production and also to gauge the applicability of waterflooding and monitor its progress when activated.

In producing wells, logging can provide measurements of

flow rates

fluid type

pressure

residual oil saturation

From these measurements, dynamic well behavior can be understood better, remedial work planned, and secondary or tertiary recovery proposals evaluated and monitored.

In summary, logging, when properly applied, can answer a great many questions from a wide spectrum of special interest groups on topics ranging from basic geology to economics.

Of equal importance, however, is the fact that logging by itself cannot provide answers to all formation evaluation questions. Coring, core analysis, and formation testing are integral parts of any formation evaluation effort.

Objectives

The objective of interpretation of wireline well logs depends very much on the user. Quantitative analysis of well logs provides the analyst with values for a variety of primary parameters, such as:

• porosity
  

water saturation, fluid type (oil/gas/water)

lithology

permeability

From these, many corollary parameters can be derived by integration (and other means) to arrive at values for:

• hydrocarbons-in-place
  

reserves (the recoverable fraction of hydrocarbons-in-place)

mapping reservoir parameters

But not all users of wireline logs have quantitative analysis as their objective. Many of them are more concerned with the geological and geophysical aspects. These users are interested in interpretation logs for:

• well-to-well correlation
  

facies analysis

synthetic seismograms

regional structural and sedimentary history

In quantitative log analysis, the objective is to define

• the type of reservoir (lithology)
  

its storage capacity (porosity)

its hydrocarbon type and content (saturation)

its producibility (permeability)

As a preliminary to discussing methods of log analysis it is worthwhile to define the terms used.

Formation Evaluation Methods

In practice, the order in which formation evaluation methods are used tends to follow the order-of-magnitude table, i.e., from the macroscopic to the microscopic. Thus a prospective structure will first be defined by seismic, gravity, and/or magnetics studies. Most wellbores drilled through such a structure are mud-logged and/or measured while drilled, from which cores may be cut or sidewall samples taken. Once the well has reached a prescribed depth, logs are run. Subsequent to logging, an initial analysis of mud log shows, together with initial log analysis, may indicate zones that merit examination either by the wireline formation tester or by drillstem testing. Should such tests prove the formation to be productive, more exhaustive analyses will be made of all available data, including core analysis. The whole process is summarized in Table 1 .

 

(Mechanical) Mud Logging

Mud logging, more precisely referred to as hydrocarbon mud logging, is a process whereby the circulating mud and cuttings in a drilling well are continuously monitored by a variety of sensors. The combined analysis of all the measurements provides an indication of the rock type and its fluid content. The sundry measurements are displayed on a log as curves or notations as a function of depth. Not all wells are logged in this manner. Development wells, for example, normally are drilled and logged by wireline logging tools only. In contrast, wildcat wells nearly always are monitored by the mud-logging process. The advantages of mud logging include the availability on a semi-continuous basis of actual formation cuttings analysis (which, in turn, gives immediate indications of rock type and hydrocarbon presence) and the ability to predict drilling problems (such as overpressure) before they become unmanageable.

Coring

A number of methods are in use to cut cores in a wellbore. Conventional cores are cut using a special core bit and retrieved in a long core barrel. The recovered core sample may undergo physical changes on its journey from the core depth to the surface, where it can be analyzed. More sophisticated coring mechanisms now in use conserve either the orientation, the pressure, or the original fluid saturations of the rock sample gathered. An awareness of these changes and sampling methods is essential to an understanding of core analysis results.

Other coring methods are available where additional rock samples are required after the well has been drilled and before it has been cased. These methods require wireline tools that cut core plugs from the sides of the wellbore.

Many parameters needed to correctly interpret openhole wireline logs can only be determined from accurate core analysis that presupposes cores have been cut. Thus, coring plays a major part in field development.

Measurements While Drilling (MWD)

Increasingly, formation properties are being measured by use of special drill collars housing measuring devices at the time the formation is drilled. These MWD tools are particularly valuable in deviated offshore wells where wellbore path control is critical and where an immediate knowledge of formation properties is vital for decision making on such matters as choosing logging and casing points. Although not as complete as openhole logs, the measurements obtained by MWD are rapidly becoming just as accurate and usable in log analysis procedures.

Formation Testing

Formation testing is the "proof of the pudding." If the well flows petroleum (oil or gas or both) on a drillstem test (DST), no amount of logging data or core analysis can deny that a productive zone has been found. However, a drillstem test provides not only proof that hydrocarbons exist in the formation and will flow, but also supplies vital data about both the capacity of the reservoir and its ability to produce in the long term. Correct interpretation of pressure records from drillstem tests help the overall formation evaluation task immensely.

Wireline formation testers (RFT)
complement drillstem tests by their ability to sample the fluid in many different horizons in the well and also gather detailed formation pressure data that it is almost impossible to obtain from a DST
alone. This detailed pressure information can be used to calculate fluid contacts, such as the free water level.

Openhole Logging

Openhole logging provides the great meeting place for all the formation evaluation methods. Only through openhole logging can a continuous record of such formation properties as porosity, water saturation, and rock type be made, versus depth. In particular, wireline logs can record formation self potential, electrical resistivity, bulk density, natural and induced radioactivity, hydrogen content, and elastic properties. Almost without exception, every well drilled for hydrocarbons is logged with wireline instruments. Unfortunately, the full potential of the logs is not always utilized, or the logs are incorrectly analyzed, because of a lack of training on the part of the analyst or a lack of understanding of where wireline logs fit in relation to the other formation evaluation tools.

All too often logs are seen as an end in themselves and are considered in isolation. It is hoped that this module will encourage the reader to take a broader view of log analysis in the context of overall formation evaluation.

Figure 1 illustrates the formation evaluation picture and the central role of openhole logging and log analysis.