Direct Estimation of API Oil Gravity from standard wireline logs: An Innovative Approach Ramalingam P, Director Petrophysics ramalingam@telestoenergy.com

ABSTRACT Traditionally, three types of porosity logs are recorded in the open hole: density, neutron, and sonic. They are used to determine the porosity separately, but also porosity and lithology combined from their cross plots. Next to this, three types of ratios, namely M, N, and P, can be computed from these logs. The M and N ratios are used in the classic M-N cross-plot to determine the lithology of the formation. The present study, a novel approach has been considered by a new cross plot using the Density log vs. N to estimate the oil gravity. It is observed that clustering of points occurred on these cross plots. It is also observed that there exists a correlation between the cluster separation, and the API gravity of the oil produced from these intervals. The study was carried out using the data of wells from various basins of the world. The results show that this method can be used reliably to get a first estimate of API gravity of oil directly from the logs, even before testing a formation. Introduction: The American Petroleum Institute gravity, or the API gravity, is a measure of how heavy or light a petroleum liquid is in comparison to water. The API gravity is used to compare the relative densities of the petroleum liquids. If one petroleum liquid floats over another then that liquid is less dense, or said to have higher API gravity. Physically API gravity has no units, but it is often referred to as “degrees”. The API gravity of oil is graduated in degrees on a hydrometer instrument. The API gravity can be estimated by the formula given below in equation 1.

------------------------(1) at 60 °F. Where SG is the specific gravity. The formula can be rearranged to give equation 2.

------------------(2) The Specific gravity of oil is the ratio of the density of oil and density of water, which is given in the equation 3.

-------------------------------------------------(3) Where: SG is specific gravity at 60 degrees Fahrenheit. ρoil Density of oil ρH2O Density of water. In the oil industry, quantities of crude oil are often measured in metric tons. One can calculate the approximate number of barrels per metric ton for a given crude oil based on its API gravity. The formula is as given below in equation 4.

------------(4) Table-1: The table below shows the classification of crude oil with respect to API gravity of oil and its corresponding oil density in grams /cc.

There are three types of porosity logs available for the evaluation of the porosity of the formation. These are the Neutron porosity, the Density log, and the sonic log. The lithology indicators ratios M, N and P values are computed using these porosity logs through the formulae in equations 5,6 and 7. A cross plot is constructed using density log value taken on the Y axis and the N computed from the neutron and density logs in X axis. Clustering of points is observed with small separation for different APIs of oil in the formation. The regions of these clusters of the point directly to the API of the oil. The calibration of the clusters is achieved using the production testing results and analyzed oil sample data from the wells. The thus calibrated cross plot was tested with the data from various basins spread across the globe. Using this cross plot, the API gravity of oil from 15 to 40 can be predicted with reasonable certainty.

API Gravity Fluid Type Specific Gravity

-8 Heavy oil & Brine 1.1460-4 Heavy oil & Brine 1.10980 Heavy oil & Brine 1.07605 Heavy oil & Brine 1.036610 Heavy oil & fresh water 1.000015 Heavy oil 0.965920 Heavy oil 0.934030 Light oil 0.876240 Light oil 0.825150 Condensate Fluids 0.7796

METHODOLOGY: M, N, & P values (1) are computed from the sonic, density and Neutron porosity log data (Δt, ρb & ФN) : M= {(Δtf –Δt)/(ρb – ρf) } X C ….... (5) N = {(ФNf-ФN)}/{ρb- ρf} …………….(6)

P = {(Δtf –Δt)/(ФNf - ФN)} X C....... (7) Where: Δt – compressional sonic log data. Δtf - sonic fluid velocity value. ρb - density log data. ρf - density fluid value. ФN - neutron log value. ФNf - neutron fluid porosity value.

C=0.01 multiplication factor is used for English unit system C=0.03 multiplication factor is used for Metric unit system

Table-1: Matrix values of various well logs.

Mineral or Rock

Symbol as a

fraction of bulk volume

Sonic Δt(sec/

ft)

Density(D)

Electron density

g/cc

Neutron (H) (indicated porosity as H.I), GNT tool

Lime stone L 45 2.71 0 Dolomite D 42 2.87 0 Sand Sd 55.5 2.65 0-4

A cross plot is constructed using the values of density log and lithology ratio N for 2 different wells. The data of the entire well is plotted

Well - A Well -B

Figure-1: The density log is plotted in the Y axis and the N value is plotted in the X axis. The two cross plot are presented here as an example in figure-1 are from 2 different wells, in separate fields. The Gamma Ray log is used for the Z color axis. The low GR is indicated with red and the high GR is indicated with blue. The red regions relate to coal, the green regions are the reservoir rocks, and the blue regions are the shale and/or other heavy minerals. The above plots in figure-1 can be used for a rough mineral identification too. The different types of coal in the cross plot can be identified using the density values of coal, like Lignite-(1.47 gm/cc), Bituminous-(1.24 gm/cc), and Anthracite-(1.19 gm/cc) in a RHOB vs. N cross plots. The region pertaining to the various minerals are identified using the laboratory determined density and N values. A slight distortion in the spread of the green region is observed due to the presence of oil. This compressed scale plots show the effect of the hydrocarbon in the reservoir rocks. The region with the green points has been zoomed in with expanded scales to estimate the API gravity of oil.

Calibration Procedure for determination of API: The log data of fields A, B, C, D, E and F has been collected, and the Density Vs N plot has been constructed. The data of the wells in the figure -1 is plotted with N on a scale from 0-5. The same cross plot has been repeated in Figure 2 with the X axis scale running from 0 to 1. The data of the tested oil bearing zones have been plotted, color-coded per well. There is a distinct separation visible of points clustered per API gravity of oil. The secondary Y-grid lines (showing porosity) have been drawn using a matrix density of 2.65 grams/cc. The trend line is drawn for each and every cluster of points and the corresponding intersection of the trend line with the zero porosity line is calibrated to the flowed oil API from that well. This cross plot can thus be used as a reference for establishing the API gravity of oil

Figure-2: The density log is plotted in the Y axis and the N plotted in the X axis.

Table-2: Actual field data (API gravity)

Field Name Average Depth API OIL in Feet from sample

F 3450 16 E 3500 17 D 7500 26 C 1950 33 B 7000 33 A 11000 41

The interpretation of API gravity of oil is verified using the testing results of the wells. CONCLUSION: With the help of this new cross plot technique of RHOB vs. N, one can predict with reasonable certainty the in-situ API gravity of oil from standard well logs. This will be extremely useful for identifying blocks with different charges in a field and it will be very valuable in generating a first estimate of the volume of the oil (Bo factor). REFERENCES:

1. Burke J.A., Camphell R. L., and Schmidt A. W., “The Litho=Porosity Cross Plot” SPWLA 1969,p 271

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