Abstract

The advent of the CMR/NMR Log gave the geologist/engineer an important additional tool in the evaluation of unconventional reservoirs. Using a Permian Wolfcamp well from the Midland as an example, the following four applications will be illustrated: 1.) Determination of Total Organic Carbon [TOC] by the Porosity Deficit Method, 2.) Use of the CMR derived KSDR Permeability to define landing point, 3.) Use of T2 Times to define pore size cutoffs in the calculation of OOIPstb, 4.) Determination of Reservoir Producibility Index [PRI: Kausik, and others, 2015; and Reeder and others, 2016] to define zones with higher potentially producible oil, and 5.) Determination OGIPscf using NMR Log.

There are five methods to determine TOC: 1.) Schmoker, 2.) DeltaLogR (Passey), 3.) Uranium Content, 4.) Carbon Subtraction, and 5.) Porosity Deficit of these five methods only the last two are direct methods. TOC from the Porosity Deficit Method is calculated by the following equation:

Vke = FD - TNMRF

TOC = (Vke*RHOb)/(Kvr*RHOke)

Where:

Vke = volume of kerogen

FD = density porosity variable matrix analysis [includes kerogen]

TNNMRF = Total NMR porosity [insensitive kerogen]

RHOb = bulk density (g/cc)

RHOke = kerogen density (g/cc) (1.5 g/cc)

Kvr = kerogen maturity index (1.2)

The KSDR Permeability is calculated using the following equation:

KSDR = C*TCMRF^2*T2LOG^2)*10^6 (nD)

Where:

C = 0.35

TCMRF = Total CMR porosity

T2LOG = Logarithmic mean of the T2 distribution

KSDR(mD) = KSDR(nD)

*1000 KSDR in microdarcies

Using a permeability (KSDR) cutoff of 1.0mD as recommended by Brown (2015), a 600 foot interval with KSDR permeability greater than 1.0mD was delineated in the Wolfcamp B zone.

Origin Oil in Place in stock tank barrels (OOIPstb) was determine at the following T2 cutoffs: T2 3ms (76.5nm), T2 10ms (250nm), T2 33ms (827.5nm), and T2 100ms (2,500nm).

Drainage Area 960 acres [7,500 foot lateral]

Note in the above OOIPstb data that large amounts of oil are present using the T2 33ms and T2 100ms cutoffs which indicates that conventional pores are present in both the Wolfcamp A and B zones.

Kausik and others (2015); and Reeder and others (2016) have developed an additional method to define the superior zones in unconventional reservoirs called Reservoir Producibility Index [PRI] using a combination of CMR/NMR, GEOCHEM, and Triple Combo data. PRI is a ratio of carbon fraction of moveable light oil to total carbon fraction. Reeder and others (2016) recommend a PRI cutoff 0.1 to define zones with higher amounts of producible oil. In our Wolfcamp example there are two zones with PRI > 0.1 one is in the A zone [thickness 175 ft.] and the second in the B zone [thickness 575 ft.]. The geologist place a 7,500 ft. lateral right below the top of the Wolfcamp B zone inside the better zone defined by the KSDR Permeability, OOIPstb, and RPI. Initial production was 1,132bopd+1.0mmcfgpd and 1,965bwpd.

An additional application of CMR/NMR logs is the calculation of OGIPscf (Ravinath and others, 2016). Early work with this method indicates that calculated OGIPscf from conventional log analysis may be underestimating gas reserves by up to 30%, when compared to OGIPscf calculated from NMR Log.