CHAPTER 3 WELL LOGGING

• Jurisdiction: United States

Basic Oil and Gas Technology for Lawyers and Landmen
(Apr 1979)
CHAPTER 3
WELL LOGGING
Dr. Douglas W. Hilchie
Colorado School Mines
Golden Colovado

A well log is a plot of one or more physical or chemical measurements versus depth. The measurements are made by an instrument lowered into the wellbore on an electrical cable which transmits the data to the surface and is recorded in the hoist and instrument (logging) truck. The logging tool is usually about 3.5 inches in diameter and up to 55 feet long. The armoured cable is typically 7/16 inches in diameter and has 7 electrical conductors. The logging truck typically carries 20,000 to 30,000 feet of cable and all the sophisticated instrumentation needed to process and record the data obtained. Today many of these logging trucks even carry minicomputors for processing the data.

Figure 1 A Portion of a Typical Rocky Mountain Well Log.

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Rock Types

It is difficult from well logs to identify the type of rocks penetrated by the wellbore. Geological information must be used in conjunction with well log data to solve reservoir evaluation problems. It is convenient in well logging to classify rocks into three groups:

1 Reservoir Rocks -those that usually can contain hydrocarbons

-Sandstones, congolmerates and silts

-Limestones and chaulks

-Dolomites

2 Shales -these are mostly clays and are seldom productive

-shales are almost impossible to evaluated with well logs

3 Others -generally non reservoir type rocks

-Anhydrite

-Gypsum

-Salt

-etc.

In this discussion we will only deal with reservoir rocks.

Reservoir Evaluation

Well logs are used for three basic purposes:

1. Detect the existense of hydrocarbons

2. Determine the amount of hydrocarbons present in barrels or in cubic feet.

3. Correlate geologically between wells.

Items 1 and 2 (above) require that the analyst be able to determine the following rock properties qualitatively or quantitatively. Sometimes we cannot determine rock properties quantitatively and thus cannot determine the amount or volume of hydrocarbons present. The following rock properties are those that must be determined to evaluate a reservoir:

a. Permeability — is the formation permeable enough (that is has sufficiently large holes in it) for the formation fluids to flow into the wellbore and be produced at commercial rates. This is a question, from well logs, that we can only answer after experience with a particular reservoir or reservoirs like the one in question. Well logs generally only allow you to say — yes the formation is permeable or no it is not permeable. They rarely let you say how permeable the formation is.

b. Porosity — this is the capacity of the formation to contain fluids. This we can determine very well if sufficient data are obtained. The porosity is the percentage of the formation that can contain fluids.

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c. Water Saturation -this is the percentage of the pore space (porosity) that is filled with water. The rest of the pore space is assumed to be filled with hydrocarbons (oil or gas). As the water saturation increases either you are approaching the oil (or gas) water contact, below which you will produce only water, or the permeability is decreasing and the formation may becoming impermeable. Good analysts can usually guess which is occurring.

d. Reservoir thickness -this can be determined from the well logs (at and just around the wellbore).

e. Reservoir area — this is necessary to determine the volume of the reservoir. This is impossible from just one well. Additional data in terms of more wells and seismic data can help with the estimate of this number. The more wells drilled, the more accurately we can determine the area of the reservoir. The fewer wells, the more inaccurate our estimate of the volume of the reservoir.

The volume of the reservoir is determined by:

Volume=Area×Thickness×Porosity×(1 — Water Saturation)×A

where: Area is in Acres

Thickness in feet

Porosity is in percent/100

Water Saturation is in percent/100

and A is 7758 for volume in barrels or 43560 for volume in cubic feet.

The volume determined above is the oil or gas in place. To determine the amount that can be produced the volume must be multiplied by a recovery factor. Discussion of recovery will be handled in the section on reservoir engineering. Recovery factors for Rocky Mountain reservoirs are typically in the 15 — 20% range although many factors may change this factor.

The Borehole Environment

The borehole, where the logging tool is located, is filled with drilling fluid...