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Computing Petrophysical Cutoffs

Many times, reservoir studies use legacy or former petrophysical cutoffs already computed. However, you may want to validate those VSH, PHIE and SW cut-offs, or even estimate or compute suitable cutoffs for a new formation or members under study.

There are several techniques or criteria to define cutoffs from a production point of view. This section illustrates and provides a free GLS script program to compute or estimate the cutoffs from interpreted LAS petrophysical logs (that is, with VSH, PHIE and SW curves), using the Cumulative Hydrocarbon Column technique.

The concept of the hydrocarbon column in a formation is simple: HCOL = PHIE * (1-SW) * Delta_H, is the equivalent height of pure hydrocarbon column contained in a zone of thickness Delta_H > HCOL, when poured into a recipient.

Clean rocks with low Volume of Shale VSH usually have few problems or capability to store hydrocarbons. As a rock becomes more shaly, it will be more difficult either to store hydrocarbons, or the hydrocarbon to migrate from the source rock to be trapped into the reservoir. There is an elbow point of VSH beyond which there are no more significant contributions to store hydrocarbons. That point could be taken as a VSH cutoff for pay rocks. The same concept applies for effective porosity PHIE (and also permeability). There is a threshold point of tight porosity with a low capability to store hydrocarbons.

The sequential algorithm can be summarized as follow:

1. `STEP 1: Seek the elbow point for VSH`

Define the top and base of the interest zone. Compute the total hydrocarbon column for all log steps without any restrictions. Then apply regular decreasing values of VSH_cutoff (that is, scan from right to left, like VSH_cutoff = 100%, 95%, 90%, ..., 0%) and compute the total hydrocarbon column, rejecting rocks for which VSH > VSH_cutoff. A plot of pairs {(VSH_cutoff, HCOL)} would show the elbow point to pick a VSH cutoff value.

2. `STEP 2: Seek the elbow point for PHIE (or even permeability)`

Keep and freeze the VSH elbow cutoff value VSH_cutoff picked in Step 1. Then compute the total hydrocarbon column while scanning effective porosity cutoffs, accepting only those rocks for which (VSH < VSH_cutoff) and (PHIE > PHIE_cutoff). A plot of pairs {(PHIE_cutoff, HCOL)} would show the elbow point to pick a PHIE cutoff value.

3. `STEP 3: Seek the elbow point for SW`

Keep and freeze the VSH and PHIE elbow cutoff values picked in Steps 1 and 2. Then compute the total hydrocarbon column while scanning water saturation cutoffs, accepting only those rocks for which (VSH < VSH_cutoff) and (PHIE > PHIE_cutoff) and (SW < SW_cutoff). A plot of pairs {SW_cutoff, HCOL)} would show the elbow point to pick a SW cutoff value.

 Step 1: This real plot shows a typical curve behaviour to pick the elbow point for a VSH cutoff. Not in all cases the elbow point shows clearly like in this example. Usual values of VSH cutoffs for clastic oil reservoirs ranges from 20% to 35%. In those cases where the VSH cutoff was computed as high as 40% or more, odds are that the VSH model was too pessimist, perhaps when a linear Index of Gamma Ray VSH was used. Step 2: This real plot shows a typical curve behaviour to pick the elbow point for a PHIE cutoff. Not in all cases the elbow point shows clearly like in this example. Usual values of PHIE cutoffs for clastic and carbonates oil reservoirs ranges from 12% to 25%. Step 3: This real plot shows a typical curve behaviour to pick the elbow point for a SW cutoff. Not in all cases the elbow point shows clearly like in this example. Usual values of SW cutoffs for clastic and carbonates oil reservoirs are around 50%.

REMARKS:

1. `Cutoffs for gas reservoirs will be completely different`

The computed cutoffs depend on rock-fluid interactions (and even pressure and temperatures). While a (VSH > 40%) and a (PHIE < 15%) could be a barrier for medium and heavy oils, gas might flow easily under those conditions due to its lower viscosity and wettability behaviour.

2. `Never apply or use a SW cutoff into a reservoir simulation grid model`

You must declare to the simulator all the existing fluids, even those zones bearing 100% of water, like aquifers. Use the full ternary VSH, PHIE and SW cutoffs only for mapping, economic screening, and volumetric purposes. For numerical reservoir simulation, use only VSH_cutoff and PHIE_cutoff to compute the Net-To-Gross ratio NTG include files. However, if you are lucky enough and have access to several relative permeability curves, you could arrange several sets for different combinations of VSH and PHIE intervals.

3. `Different wells, even in the same formation's member could show different cutoffs`

Since rock properties vary for different locations, the petrophysical cutoffs could be easily different for several wells. If a single cutoff system is required for a reservoir, there are two approaches:
1) Take a compromised average, typical or representative cutoffs for all the wells.
2) Extend the algorithm to compute the whole Original Hydrocarbon in Place, instead of the Hydrocarbon Column per well. The latter approach is the preferred one, but requires a lot of computational effort, resources, software, and time.

Finally we present below a sample GLS script program to estimate petrophysical cut-offs. It looks like a large program, but it's not, because it is step-wise heavily commented to guide you. You do not need neither to type the source code nor load a special interpreted LAS log file. The GeolOil license contains in its demo data-set both files. Don't hesitate to try the code: GLS is free, simple and specific for log petrophysics (Matlab is not). If you think it is complex, try to achieve the same results using either Petrel, an Excel spreadsheet or a Matlab script program to handle depth loops with missed -999.25 values. The program displays in its Message Window, the pair values that can be copied and pasted into other software to make the cutoffs plots and pick the elbow threshold points.

This example runs "out of the box" with your GeolOil license On a typical Windows installation just follow: [File] -> [Open_GLS_Script] -> [Open] -> [UserHomeDir] -> [GeolOil_DB] -> [GLS_examples] -> [cutoffs_example.gls] -> [Run]. The script will display on the right window a set of pair values that can be plotted to pick a cutoff value.

The figure below shows a GLS script program to estimate VSH, PHIE and SW Cutoffs

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