CRAIN'S
PETROPHYSICAL
HANDBOOK
c. 1978 - 2008 E. R. (Ross) Crain, P.Eng.
Rocky Mountain House, Alberta Canada T4T 2A2
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Updated 15 Jan 2008
CHAPTER EIGHT: CALCULATING HYDROCARBON CONTENT Table
Of Contents
8.00 Introduction To This Chapter TABLE 8.01: Summary Water Saturation Method Click here to go to NEXT CHAPTER
CHAPTER EIGHT: CALCULATING HYDROCARBON CONTENT 8.00
Introduction to This Chapter Water saturation is the ratio of water volume to pore volume. Water bound to the shale is not included, so shale corrections must be performed if shale is present. We calculate water saturation from the effective porosity and the resistivity log. Hydrocarbon saturation is 1 (one) minus the water saturation. Most oil and gas reservoirs are water wet; water coats the surface of each rock grain. The water is held in place by surface tension. The surface water does not move while the oil or gas is being produced. This situation is shown in Figure 8.00 (left).
FIGURE 8.00: Water wet formation with hydrocarbon before invasion (left) and after invasion (right) When a reservoir is drilled, some of the fluids near the wellbore are pushed away and the zone is invaded by the drilling fluid. If hydrocarbons were present, the water saturation after invasion will be higher than the original reservoir conditions (Figure 8.00 right). A shallow resistivity log will see the invaded zone water saturation. A deep resistivity log should see the original formation water saturation as long as invasion was not too deep.
He also found that F varied with porosity: 2: F = A / (PHIt ^ M) For a tank of water, R0 = RW. Therefore F = 1. Since PHIt = 1, then A must also be 1.0 and M can have any value. If porosity is zero, F is infinite and both A and M can have any value. However, for real rocks, both A and M vary with grain size, sorting, and rock texture. The normal range for A is 0.5 to 1.5 and for M is 1.7 to about 3.2. Archie used A = 1 and M = 2. In fine vuggy rock, M can be as high as 7.0 with a correspondingly low value for A. In fractures, M can be as low as 1.1. Note that R0 is also spelled Ro in the literature. For shale free rocks with both hydrocarbon and water in the pores, he also defined the term Formation Resistivity Index (I) as: 3:
I = Rt / R0 Archie used an N of 2 and the usual range is from 1.3 to 2.6, depending on rock texture. It is often taken to equal M, but this is not supported by core data in all cases. Rearrangement of these four equations gives the more usual Archie water saturation: 5: Swa = (A * RW@FT / (PHIe ^ M) / RESD) ^ (1 / N) True resistivity (Rt) is estimated from the deep resistivity log (RESD) or RESD corrected for borehole effects and invasion (RESDc). RESD can come from many different logging tools, such as the Electrical Survey (64 inch Normal), induction log, laterolog, and all their modern siblings. The actual curve name and abbreviation must be determined from the log heading Porosity is determined as in Chapter Seven. A, M, and N are called the saturation exponents or the electrical properties of the rock. These are determined from log or core data as described later in Section 8.18. A more thorough explanation of Archie’s Laws is contained in Chapter Eighteen. Shale corrections are applied by adding a shale conductivity term with an associated shale porosity and shale formation factor relationship. Numerous authors have explored this approach, leading to numerous potential solutions for water saturation, some of which are described in this Chapter.
8.01
Definition of Saturation Porosity is the capacity of the rock to hold fluids. Saturation is the fraction of this capacity that actually holds any particular fluid. Porosity, hydrocarbon saturation, the thickness of the reservoir rock and the real extent of the reservoir determine the total hydrocarbon volume in place. Hydrocarbon volume, recovery factor, and production rate establish the economic potential of the reservoir. Irreducible water saturation is the minimum water saturation obtainable in a rock. Water is usually the wetting fluid in oil or gas reservoirs, so a film of water covers each pore surface. The surface area thus defines the irreducible water saturation. Formations at irreducible water saturation cannot produce water until water encroaches into the reservoir after some oil or gas has been withdrawn. Small pores have larger surface area relative to their volume so the irreducible water saturation is higher. If pores are small enough, the irreducible water saturation may be 1.0, leaving no room for oil or gas to accumulate. Of the total amount of oil or gas present in a reservoir, only a fraction of it can be produced, depending on the recovery efficiency. This recovery factor, normally determined by experience, is typically in the 20% to 50% range for oil, and may be as high as 95% for gas zones, or as low as 5% in heavy oil. Recovery factor can sometimes be estimated from log data by observing the moveable hydrocarbon volume. Here are the standard definitions needed to understand this Chapter. Note that definitions 1 through 10 are in Chapter Seven.
This is the standard definition of “water saturation”. Older books use this term to define total water saturation. Since all interpretation methods described here correct for the effects of shale, we are not normally interested in the total water saturation, except as a mathematical by-product. As effective porosity approaches zero, the water saturation approaches one (by edict, if not by calculus)
The amount of free water in the invaded zone is usually higher than in the un-invaded zone, when oil or gas is present. Thus Sxo >= Swe. The water saturation in the invaded zone between the flushed and un-invaded zone is seldom used.
All volumes defined above are in fractional units. In tables or reports, log analysis results are often converted to percentages by multiplying fractional units by 100.
8.02
Visual Overlays of Resistivity and Porosity In a fairly porous, clean sand or carbonate sequence the hydrocarbon zone may stand out because of its increased resistivity relative to the same zone in another well or a deeper zone which is water bearing. High resistivity with moderate to high porosity is either hydrocarbon or fresh (or brackish) water.
The overlay approach is less effective in shaly sands, due to the conductivity of the clay minerals. When the zone is shaly, a resistivity log will often track the porosity log whether hydrocarbons are present or not. Figure 8.02 shows a set of logs from two offsetting wells, and it is evident that the curves track each other reasonably well. In addition, there is no obvious "Mae West" effect. It is thus difficult to observe by visual means alone that the well on the left will produce only water, and the well on the right will produce only gas.
These overlays can be produced on computerized logging trucks or at the computer center. Scales may be in porosity units (compatible scale overlays), or in units of formation factor, usually called Fr/Fs overlays. Another computed product which can be useful is the Rwa log, discussed later in this Chapter.
8.03
Density Neutron Crossover
CAUTION: If the logs had been recorded on a limestone scale, there would always be some crossover in a clean sandstone, whether there was gas or not. Conversely, a gas filled limestone will infrequently show crossover if it is recorded on a sandstone scale, but it will (usually) if recorded on a limestone scale. Similarly, a dolomite logged on a limestone scale (a common occurrence) will show no crossover because of the lithology effect. The same well logged on a dolomite scale (not so common) will show crossover if the zone is dolomite and filled with gas, but also if the zone is limestone or sandstone without gas. Care should be taken to account for logging scales and lithology when using the crossover technique. Because of the large matrix effect due to dolomite, crossover can seldom be seen in dolomitic limestone, dolomitic sandstone, or pure dolomite unless an appropriate scale shift is made to the density and neutron logs. In the absence of a computer, this may be accomplished by overlaying the density and neutron curves in known oil or water zones, and looking for crossover that might indicate gas or limestone above or below the water or oil zones.
8.04
Other Indicators of Hydrocarbons Other indicators of hydrocarbons such as gas or oil in the mud, a gas or mud log at the well site with shows of oil and gas, drill stem test results, production from offset wells, and sample or core staining or fluorescence, are often relied upon to narrow down the possible zones which may contain hydrocarbons. All the data in the well history is therefore very important to the log analyst. Sonic log skipping may be an indicator of gas in the formation or in the mud, or a fractured formation, but may be due only to poor logging instruments or poor quality control. It is hoped that logs are run to minimize skipping and the log analyst should not rely on the presence of skips to indicate gas.
8.05
Calculating Environmental Corrections for Resistivity Logs Borehole corrections for mud salinity and hole diameter should be applied first, if needed. Most computer aided log analysis software has this capability. Fortunately, borehole corrections can often be safely ignored when the log is run in a good borehole with a good mud system. The newest array induction logs attempt to produce Rt with all corrections applied. Do not over correct your data. 1. Borehole Environment Corrections to Resistivity For those who insist on superfluous detail, formulae are provided here for the deep induction log (Figure 8.04) for no standoff and in Figure 8.05 for 1.5 inches standoff (the usual case).
Note that the abbreviations shown above are those used in this FORTRAN program and do not conform to abbreviations used in this book. Hole size is in inches and correction to the code for metric dimensions is required. Charts for these formulae are given in Figures 8.06 to 8.10 for various tool types.
It is instructive to determine the borehole corrections for some typical cases and run a sensitivity study with one of the saturation equations to see if the corrections have a measurable impact.
The
borehole signal for induction logs is subtracted from the induction
conductivity measurement and reciprocated to obtain corrected
resistivity. WHERE: COMMENTS:
The
borehole correction for laterologs is a correction factor which
is divided into the original log reading to obtain the corrected
value. WHERE: COMMENTS: 2. Invasion Corrections The second correction is for the effects of invasion of mud filtrate into the formation. Knowledge of the invasion profile can be used to correct the deep resistivity log for this effect. The profile knowledge comes from the medium and shallow resistivity data when compared to the deep resistivity.
The
invasion corrections for dual induction logs are computed as follows: WHERE: COMMENTS: RESDc is often called Rt, the "true" resistivity - see warning below Figure 8.11.
Below is a sample sensitivity analysis that shows the correction factor Rt/RESD is greater than 1.0 for many real situations. The same factor (Rt/Rild) on Figure 8.11 is never greater than 1.0.
NUMERICAL
EXAMPLE: Thus, invasion is so deep that the dual induction reads nearly three times too high. If this is a water zone, the correction is reasonable. If it is hydrocarbon bearing, the correction makes no sense.
The
invasion corrections for dual laterologs are computed as follows: WHERE: COMMENTS:
This chart can raise or lower the Rt. Use the correction only if the correction raises Rt. The reader is encouraged to run a sensitivity analysis, similar to the one shown earlier for induction logs, for the laterolog in a salt mud case and a fresh mud case.
NUMERICAL
EXAMPLE:
8.06
Calculating Diameter of Invasion
1:
IF RESTYPE$ = "DIL" WHERE: COMMENTS: Solutions to these formulae are also shown in Figure 8.11 and 8.12. While diameter of invasion is not used to correct other data, it is a useful quality control indicator. NUMERICAL
EXAMPLE: C
= (1.5 / 0.35) * (1.0 - 0.35) / (1.5 - 1.0) = 5.57
8.07
Formation Water Resistivity l. Drill stem test recoveries of water in your well or nearby wells, analyzed for chemical content and water resistivity in the laboratory. 2. The water catalogue published by your local well logging society or similar catalogues created by searching in-house data bases. 3. Back calculation of RW@FT from log data in a clean (non shaly) zone - usually called the Rwa method, or the water zone (Ro) method. 4. Calculation from knowledge of the SP value in a clean zone. A sample of the type of data found in a water catalogue is shown in Figure 8.13. Note that data is tabulated and also posted on a map, and is based on a standard temperature of 25 degrees Celcius (77 degrees Fahrenheit). Thus, catalogue or DST values must be converted to an equivalent value at the formation temperature.
The following relationships are needed to manipulate water resistivity data prior to calculations of water saturation.
1:
FT = SUFT + (BHT - SUFT) / BHTDEP * DEPTH WHERE: COMMENTS:
NUMERICAL
EXAMPLE: Metric
units example:
1:
FT1 = SUFT + (BHT - SUFT) / BHTDEP * DEPTH) WHERE: COMMENTS:
NUMERICAL
EXAMPLE: 2.
Water resistivity to salinity.
1: WSa = Cc1 * 1.645 WHERE: COMMENTS: | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Almost
all saturation computation methods rely on work originally done
by Gus Archie in 1940-41. He found from laboratory studies that,
in a shale free, water filled rock, the Formation Factor (F) was
a constant defined by:
Visual
identification of such zones is based on the so called "Mae
West" curve shape; that is, the porosity and shale indicators
deflect to the left and resistivity deflects to the right. One
of the easiest ways to see this effect is to overlay the porosity
and resistivity logs on a light-table or a computer screen. In
water zones, overlay the porosity and deep resistivity curves
so as to track each other. They will have the same general shape
and deflect in the same direction throughout all the beds. In
a hydrocarbon zone, the resistivity log should deflect to the
right when compared to the porosity log, This is shown in Figure
8.01.
FIGURE
8.03: Density Neutron Crossover may show gas
