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       Tar Sand Basics          Tar Sand Math     Tar Sand Example    
 

TAR SAND BASICS
Tar sands (oil sands, bitumen sands) are mined or depleted by steam assisted gravity drainage (SAGD) or in-situ fire floods. In all these situations an adequate reservoir description is needed to assess the economics and progress of any project.

A conventional shaly sand analysis using the complex lithology porosity models is the foundation for the work. A shaly sand water saturation equation, such as the Simandoux model, is also needed. These topics are covered elsewhere in this Handbook.

The best tar sands are clean, medium to coarse grained, unconsolidated sands. However, they may be interbedded with finer, siltier, and shalier sands or overlain by lower quality reservoir rock. The log analysis needs to describe there variations, especially laterally continuous barriers to vertical flow of steam and oil movement.

Tar sands at 63 times magnification: shaly sand (left) with Vsh > 35%, clean sand (right) with Vsh < 5%.

The fluid column can be more complicated than conventional reservoirs. Here are some possibilities:
  1. bitumen with or without bottom water
  2. top water over bitumen with or without bottom water
  3. gas over bitumen with or without bottom water
  4. gas over top water over bitumen with or without bottom water
  5. any of the above with gas distributed unevenly in the main bitumen zone.

Since these sands are often shallow, the gas crossover on the density neutron log is quite large and can be used as a measure of the reservoir volume filled with gas. Comparison to (1 - Sw) gives the residual bitumen volume in the gassy zones.

Tar in carbonates is also extractable with SAGD, fire floods, or solvent floods. Gas is usually less of an issue because there is less likelihood of biogenic gas generation, but gas caps may exist in some plays.

TAR SAND MATH
Tar assay data from core analysis is often presented in terms of mass (weight) fraction (or percent) and sometimes also as volumes. Log analysis results are usually in volume fractions. Comparison between log and core results need some extra math compared to conventional oil and gas evaluations. Further, net pay is often determined by a tar mass fraction cutoff instead of porosity and water saturation.

GAS EFFECT
First lets look at the gas problem. If there is no gas crossover, you can skip this section. The conventional equation for porosity in a gas sand is:
     1: PHIe = ((PHInc^2 + PHIdc^2) / 2) ^ (1 / 2)

This equation is accurate enough for most gas zones, but in very shallow gas sands, it will underestimate porosity. The above equation must be replaced by:
     2: PHIe = ((PHInc^X + PHIdc^X) / 2) ^ (1 / X)

Where:
  X is in the range of 2.0 to 4.0, default = 3.0.
  PHIdc and PHInc are shale corrected values of density and neutron porosity respectively.

Density neutron crossover in a shallow gas sand with residual tar (shaded
area) and core analysis porosity (dots). The low neutron porosity indicates
 little hydrogen content; the effect on the density is much smaller. An X
of 3.0 or higher is needed to calculate effective porosity from logs.
Porosity scale is 0.60 to 0.00 ==>

The exponent X is adjusted by trial and error until a good match to core porosity is obtained.

PARTITIONING GAS and TAR VOLUMES
After shale volume and porosity have been calculated, water resistivity can be found in a bottom water zone below the tar, as these rarely has any residual tar. RW may vary somewhat in the tar sand interval and this can be adjusted if necessary by comparing calculated tar mass with core tar mass in non-gassy, relatively shale-free, intervals. Water saturation is then calculated from a shale corrected model such as Simandoux.

Many, but not all, gas zones related to tar sands have some residual tar. Hydrocarbon saturation is partitioned between bitumen and gas by the following method:

     3: Vwtr = PHIe * Sw
     4: Vhyd = PHIe * (1 – Sw)
     5: GasTarRatio = Max(0, Min((1 – TARmin), (PHIDc – PHINc) / MAX_XOVER))
     6: Vgas = GasTarRatio * Vhyd
     7: Vtar =  (1 – GasTarRatio) * Vhyd

Tar weight is calculated from log analysis as follows:
      8: WTtar  = Vtar * DENSHY
      9: WTshl   = Vsh * DENSSH
      10: WTsnd = (1 - Vsh - PHIe) * DENSMA
      11: WTwtr = Vwtr * DENSW
      12: WTrock = WTtar + WTshl + WTsnd + WTwtr

Tar mass fraction:
      13: Wtar = WTtar / WTrock
      14: WT%tar = 100 * Wtar

Where:
  TARmin = minimum tar volume in gas zone as seen on core analysis, could be zero.
  MAX_XOVER =  maximum density neutron crossover in a gas zone (fractional)
  Vxxx = volume fraction of a component
  WTxxx = weight of a component (grams or Kg)
  Wxxx = mass fraction of a component
  WT%xxx = weight percent of a component

<== Comparison of tar mass from log analysis (solid line) with tar mass from Dean-
Stark core analysis (dots)  Tar mass scale is 0.30 to 0.00. Zone opposite this
caption is gas with residual tar; above and below are tar with no gas.

Typical densities are  DENSMA = 2650, DENSW = DENSHY = 1000, DENSSH = 2300 kg/m3. This is the only way to rigourously calculate Tar Mass. Other equations have been used, such as the one shown below, but are less accurate, since shale volume is not explicitly enumerated:
     99: Wtar = ((1.0 - Sw) * Phie * DENStar) / (DENSrna * (1.0 - Phie))

Here, DENSma is a computed result from the log analysis, and is usually wrong when gas is present. It hides the shale correction term and individual rock and fluid parameters cannot be adjusted. I strongly recommend that this "simplified" version be avoided.

It should be noted that core data is usually derived from a summation of fluids process, such as Dean-Stark method, so the porosity from core matches total porosity better than effective porosity. Ditto water saturation. That's why we use tar mass and not porosity and saturation to calibrate log analysis to core data.

Tar mass from log analysis is plotted, as shown at the right, along with tar mass calculated from core analysis data, on the depth plots to show the match between log analysis and core data results.

The match between log analysis tar mass, porosity, and saturation with corresponding core data is usually excellent except in the very shaly, non-pay, intervals, mostly because the core data provided ignores shale and its effect on net grain density. The match in zones with high gas saturation varies in quality due to the inherent inaccuracy in the gas/tar partitioning calculation on the log analysis.

DEAN-STARK CORE ANALYSIS METHOD
This method is used in poorly consolidated rocks such as tar samds and involves disaggregating the samples and weighing their constituent components. Samples are usually frozen or wrapped in plastic to preserve the contents during transport. In the lab, the still frozen cores are slabbed for photography and description, then samples are selected and weighed.

Samples are then heated and crumbled to drive off water, and weighed again. The weight loss gives the water weight. Solvents are used to remove oil or tar. The sample is weighed again and the weight loss is the weight of oil. The matrix rock is separated into clay and mineral components by flotation, dried and weighed again, giving the weight of clay and weight of the mineral grains.
      15: WTwtr = WTsample - WTheated
      16: WTtar = WTheated - WTminerals&clay

<== Dean-Stark laboratory apparatus

By dividing each weight by its respective density and adjusting each result for the total weight of the sample, the volume fraction of each is obtained. Porosity is the sum of water plus oil volume fractions  Because the bound water in the clay is driven off by the drying sequences, this porosity is the total porosity.
      17: VOLwtr = WTwtr / DENSwtr / WTsample
      18: VOLtar = WTtar / DENStar / WTsample
      19: PHIcore = VOLwtr + VOLtar

Assuming clay bound water is driven off by heating and drying, then PHIcore equals total porosity. From comparison to log analysis results, it appears that some clay bound water remains in many cases, so PHIcore lies between total and effective porosity from log analysis.

Example of Dean-Stark porosity (dots) showing that it is less than total porosity from
logs (black curve) due to incomplete drying of clay. Trying to match log porosity
directly to core may be futile in many cases. Porosity scale is 0.50 to 0.00. ==>

TAR MASS FROM CORE LISTINGS
If not provided on the core listing, the equivalent value of tar mass from core analysis is derived from porosity, oil saturation, and an assumed oil density:
     20:  Wtar = PHIcore * Star * DENStar
     21:  Wwtr =  PHIcore * Swtr * DENSwtr
     22:  Wrock = (1 – PHIcore) * GR_DENScore

Where:
  Star = tar volume relative to pore volume
  Swtr = water volume relative to pore volume
  PHIcore = volume of water + valume of tar
  Wtar = tar mass fraction
  Wwtr = water mass fraction
  Wrockcore = rock mass fraction

Table 1 (above): When saturations and porosity are known (blue shading), all other terms can be calculated. GR_DENS must measured or assumed and DENSwtr and DENStar are usually assumed to be 1000 Kg/m3. Some core analysis reports do the math for you, some do not.

Since GR_DENScore represents a mixture of quartz and shale, this value should vary with shale volume. However  shale volume is never reported on core analysis, so the composite grain density from the rock sample is used. If grain density is not recorded in the core analysis, we must assume a constant of  2650 Kg/m3 or lower.

FLUID VOLUMES FROM CORE LISTINGS
If not provided on the core listing, the equivalent value of tar volumes from core analysis are derived from porosity, tar mass fraction, and an assumed oil density:
     23: Star = Wtar / (PHIcore * DENStar)
     24: Swtr = Wwtr / (PHIcore * DENSwtr)
OR 25: Swtr = 1.00 - Star

Where:
  Star = tar volume relative to pore volume
  Swtr = water volume relative to pore volume
  PHIcore = volume of water + valume of tar
  Wtar = tar mass fraction
  Wwtr = water mass fraction

Table 2 (above): If tar mass fraction and water mass fraction are known, as well as core porosity (blue shading), all other terms can be calculated. Some core analysis reports do the math for you, some do not.

PERMEABILITY

An example is shown on the right. Vertical permeability is especially important in SAGD operations, and similar equations can be developed by plotting core porosity against vertical permeability. Both horizontal and vertical perm can be generated from log analysis porosity and plotted versus depth. An alternate approach is to do a regression of Kv versus Kh.

TAR CUTOFFS and  PAY FLAG
A bitumen pay flag is calculated with a log analysis tar mass cutoff, usually between 0.050 and  0.085 tar mass fraction. A gas flag should also be shown on the depth plots where density neutron crossover occurs on the shale corrected log data.

TAR IN PLACE
Tar in place is calculated from:
      22: TAR = SUM (Wtar * DENSHY * THICK) * AREA

WHERE:
  AREA = reservoir area (m2)
  THICK = rock thickness (meters)
  TAR = tar in place (tonnes)
  Wtar  = tar mass fraction  (fractional)
  DENSHY = density of bitumen (g/cc)

If the oil equivalent in barrels or cubic meters is needed, the standard equation can be used:
      23: OOIP = KV3 * SUM(PHIe * Star * THICK) * AREA / Bo

Where:
  KV3 = 7758 bbl for English units
  KV3 = 1.0 m3 for Metric units
  AREA = spacing unit or pool area (acres or square meters)
  OOIP = oil in place as bitumen (bbl or m3)

Recovery factor for surface mining operations is very high, maybe 0.98 or better. For SAGD, RF = 0.35 to 0.50 are used. Since we can't keep the stream away from the shaly sands, recovery will vary with the average rock quality in a SAGD project. Since water has a very high latent heat, the volume of water to be steamed is as important to the economics as the volume of bitumen. High water saturation is bad news here, just as in conventional oil.

"META/TAR" SPREADSHEET -- Log Analysis in Tar Sands
This spreadsheet provides a tool for Log Analysis of Tar Sands, including tar mass, net pay, and reserves calculations. 

Log Analysis for Tar Sands. English and Metric Units.

Sample of input data and crossplots for "META/TAR" Spreadsheet, used to analyze tar sand zones.



Sample of "META/TAR" net pay summary table.
 

TAR SAND EXAMPLE

Tar sand analysis with top water, bottom water, top gas, and mid zone gas. Core and log data match - but tar mass is the critical measure of success. Core porosity matches total porosity from logs, due to the nature of the summation of fluids method used in these unconsolidated sands. Minor coal streaks occur in this particular area.

Water Satr'n           Statistical

Fluids Model

Fluids  Determinist

Tar Mass ic Model

ALTERNATE MODELS Comparison of petrophysical methods is often instructive. In the analysis shown at left, a probabilistic model (far left) is contrasted with a deterministic model (right). On the probab8listic model, tar is black, gas is red, water is blue, and clay bound water is gray. On the deterministic model, tar is red, gas is yellow, and water is white. Total porosity from core (black dots) and total porosity from log analysis are also shown on the deterministic model. There are differences in porosity, especially in the low porosity range, differences in gas content, and differences in bulk water volume. Core tar mass (right) was used to calibrate the deterministic model; the match is excellent in both gassy and non-gassy tar intervals. The statistical model was calibrated by comparing core water saturation to log analysis saturation (far left). The match is poor in some tar zones, reasonable in others, and of course is not a meaningful comparison in gassy zones. The statistical model was tweaked several times but was never completely satisfying because the calibration to core was based on saturation and not on tar mass. Tar mass comparison is the only correct way to match log analysis to core analysis in tar sand projects.

 

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