Evaluating Shale Volume Estimation Techniques in Gelama Merah Field

Abstract

Determination of the distribution of shale volume is one of the most significant factors to be considered in the assessment of formation, as the presence of shale decreases the reservoir's active porosity and permeability. In this paper, shale volume has been calculated using three different methods. We have been given Gelama Merah field been requested by the management to refine the work according to the experience on reserve calculation specifically shale volume. If development is possible, how do you calculate the volume of shale? Is relevant the volume of shale obtained from the linear equation? If linear equation does not matter, what equation should we use to correct it? How do these equations affect the effective calculation of porosity, Sw and reserve? In this paper we calculate the shale volume by using linear and non-linear equation, the importance of linear equation of shale volume versus non-linear equation and the impact of these equations on effective porosity, Sw and reserve calculation,

Introduction

Gelama Merah field is located Sabah Basin South China Sea.

They have average water depth of 42,8 m.

The location of Gelama Merah is in Block SB-18-12 offshore Sabah in Malaysia with the latitude of 5° 33′ 49.98′′ N and longitude of 114° 59′ 6.34′′ E. This field is located and 43 km northwest of Labuan, 130 km southwest of Kota Kinabalu and approximately 10.5 km east of the Samarang Complex.

In Gelama Merah, there are 2 wells that were drilled which are Gelama Merah-1 and Gelama Merah-1 ST1. The Gelama Merah-1. The presence of hydrocarbon reservoir at stage IVC middle unconformity sand have been discovered in Gelama Merah - 1.

Gelama Merah was drilled vertically from 70.1 metres to 1636 metres from the Kelly Bushing TVDDF.

Problem Statement

We have been given Gelama Merah field with a total productive reservoir area of 100,000 acres (approximately 404.7 sq.Km) and recovery factor of 40%. We have been requested by the management to refine the work according to the experience on reserve calculation specifically shale volume. If the development is possible, how to calculate shale volume? Is the shale volume obtained from the linear equation is relevant? If linear equation is not relevant, what equation should we use to correct them? How do these equations affecting the effective porosity, Sw and reserve calculation?

This project have been carried out to:

1. Calculate the shale volume by using linear and non-linear equations.
2. Determine the relevance of linear equation of shale volume versus non-linear equation.
3. Determine the effects of these equations to the effective porosity, Sw and reserve calculation.

Literature Review

From the studies that we have done to gather the information, we have learnt that shale volume calculation are used to discriminate between reservoir and non-reservoir rock. The data that are used to calculate the shale volume can be obtained from the gamma ray log. It is common to use the gamma ray log as an indicator of shale volume. Throughout the studies, geoscientist has created many formula to calculate the shale volume . Those formula can be divided into linear equation and non-linear equations. The linear equation of shale volume is the first approach to estimate the rock’s shale volume from the gamma ray log data. The data measured by the gamma ray probe is the most frequently method used to estimate the shale volume. Calculation of the gamma ray index is needed to determine the volume of shale from gamma ray log.The gamma ray index is calculated using formula:

IGR = GR − GRmin GRmax − GRmin

where GR denotes the gamma ray reading of the given depth-point, GRmin and GRmax are the gamma ray values of the clean formation and shale, respectively (Asquith and Krygowski, 2004). To calculate the first order approximation of shale volume is used.

Vsh = IGR

It is usually overestimating the shale content of the rocks. To obtain a realistic estimation, non-linear equations are used. There are various non-linear equation such as Larionov (1969) for older rocks, Larionov (1969) for tertiary rocks, Steiber (1970) and Clavier (1971). These formulae are used to reduce the uncertainty in the rock shale volume estimation.The non-linear gamma ray will predict much less Vsh than Linear technique which might also right the calculation from the linear equation. The non-linearity used to be used in unconsolidated rocks because they have a tendency to be greater chemically immature and can also incorporate radioactive minerals such as feldspars that may want to contribute to gamma however are unrelated to shale volume.

Methodology

Step 1: Determine the zonation and depth interval We zonate the data by using butterfly effect and pay zone in track 3. From this method, we obtained 15 zones. The reason why we zonate the log into zones is to differentiate between shale and shaly-sand.

Step 2: Read the reading from track 1 - GR (GAPI) Track 1 display the GR reading in GAPI unit. GRmin, GRmax and GRlog was read from the data. For the GRmin, we took the least reading throughout the data which was 47. Furthermore, we also got GRmax from the data which was 98.

Step 3: Identify Porosity Neutron and Bulk Density Porosity Neutron can be read from NEUT (dec) log while Bulk Density can be read from RHOB (g/cc) log. These two logs were read according to the zonation located at Track 5.

Step 4: Identify Rt can be read from the Rdeep (OHMM) log which is located at the Track 4. The Rt was identified for each zonation.

Step 5: Calculate Porosity Density Porosity was estimated by using the equation

∅ = ρma− ρf

ρma − ρb

Where; maρ = matrix density

bρ = bulk density

fρ = density of fluid

Step 6: Calculate Porosity Density Neutron

∅DN =( )^1/22

∅D +∅N 2 2

Step 7: Calculate the IGR by using linear formula. The gamma ray index is calculated using formula:

IGR = GR − GRmin GRmax − GRmin

where;

GR = gamma ray reading of the given depth-point,

GRmin & GRmax= the gamma ray values of the clean formation and shale to calculate the first order approximation of shale volume;

Vsh = IGR

Step 8: Calculate the shale volume using other relevant formula (non-linear equation).

In this report, we used Steiber (1970) and Clavier (1971) as non-linear equation.

a) Steiber (1970)

Vs h= IGR / ( 3 - 2 × IGR )

b) Steiber (1971)

Vsh = 1.7 [(3.38 – (IGR + 0.7)²]^½

Step 9: Identify thickness to identify the thickness of the shaly sand, there are several steps which are:

1. Shale volume cutoff. We need to know the shale volume cutoff that can be obtained from the report of Gelama Merah which is 65%.
2. Find the GRlog for the cutoff by using the IGR formula.
3. Draw a line for the GRlog at the GR (GAPI) log at Track 1.
4. Shade the shaly sand that are located at the left side of the GRlog line.
5. Read the thickness for each zonation.

Step 10: Calculate Effective Porosity. Effective porosity was calculated by using the equation:

∅E = ∅DN x (1-Vsh)

Step 11: Calculate Water Saturation. Water Saturation was calculated by using Archie Water Saturation equation:

Swa =[ ]∅ − Rtm

a − Rw

Step 12: Calculate h x ∅ x (1-Sw)

h x ∅E x (1 - Sw)

∅E = Effective porosity

Sw = Water saturation

Step 13: Calculate Average Effective Porosity & Average Water Saturation

Average ∅E = Total ∅E / 15

Average Sw = Total Sw / 15

Step 14: Calculate NTG

TNG = Total h / Gross

Gross = 951.44 ft (290 m)

H = thickness

Step 15: Calculate GIIP

GIP = 43560 x area x total h x ∅ x (1-Sw)

BG

Step 16: Calculate Reserve Calculation

Reserve Calculation = GIIP x FR

Flow Chart

5.0 RESULT

GRmin 48 Rho ma 2.65

GRmax 98 Rho f 1

Rw 0.265 Recovery factor 40%

m 1.64 Area (acres) 100,000

n 2 Vsh cutoff 65%

a 1 BG 0.01

Conclusion

From this study, several outcomes were conclude. By calculating using three volume of shale equations which comprise of linear equation and supported by two non linear equations which are Steiber and Clavier equations, we obtain by using linear equation the effective porosity shown is the lowest. Thus, it has the lowest value for reserve compared to Steiber and Clavier equations. It may affected by highest saturation of water. It also has lowest value of Gas In Place (GIIP). Meanwhile, Steiber equation shows the highest value for reserve which affected by effective porosity. It also may affected by lowest saturation of water. Moreover, it also has high value of Gas In Place (GIIP). Clavier equation has intermediate values for effective porosity thus has moderate value for reserve with intermediate saturation of water. It also has intermediate value of Gas In Place (GIIP).