Reservoirs Heterogeneity Reservoirs Heterogeneity Characterization of the Characterization of the

Shajara Member: Permo-Shajara Member: Permo-Carboniferous Unayzah Carboniferous Unayzah

Formation Formation *Al-Khidir, K. E., **Al-Laboun, A. *Al-Khidir, K. E., **Al-Laboun, A.

A., ***AlQuraishi, A. A. *M. S. A., ***AlQuraishi, A. A. *M. S. BenzagoutaBenzagouta

**College of Engineering, Al Amoudi Research Chair in College of Engineering, Al Amoudi Research Chair in EOR,EOR,

Department of Petroleum Engineering & Natural Gas Department of Petroleum Engineering & Natural Gas King Saud University,King Saud University,

***King Abdulaziz City for Science and Technology ***King Abdulaziz City for Science and Technology Petroleum and Natural Gas Research InstitutePetroleum and Natural Gas Research Institute **College of Science Department of Geology **College of Science Department of Geology

King Saud UniversityKing Saud University..

IntroductionIntroduction The Unayzah Formation is hydrocarbon bearing The Unayzah Formation is hydrocarbon bearing

in many oil and gas fields in Saudi Arabia. It is in many oil and gas fields in Saudi Arabia. It is composed of continental siliciclastic of Permo-composed of continental siliciclastic of Permo-Carboniferous age bounded by two regional Carboniferous age bounded by two regional unconformities, namely sub-unayzah and sub-unconformities, namely sub-unayzah and sub-Khuff. Khuff.

Based on Saudi Stratigraphic Code (1983), Based on Saudi Stratigraphic Code (1983), Unayzah Formation should be raised to a group Unayzah Formation should be raised to a group status and a new Shajara Formation is status and a new Shajara Formation is introduced. introduced.

The Shajara Formation is divided into three The Shajara Formation is divided into three members from base to top are: Lower Shajara, members from base to top are: Lower Shajara, Middle Shajara, and Upper Shajara Members. Middle Shajara, and Upper Shajara Members.

Local unconformities were interpreted as Local unconformities were interpreted as braided fluvial channels. braided fluvial channels.

ObjectivesObjectives

Determination of the thermodynamic Determination of the thermodynamic model based on Mercury Intrusion model based on Mercury Intrusion techniques .techniques .

Determination of fractal Dimension using 3-Determination of fractal Dimension using 3-D Fractal Model for characterizing pore D Fractal Model for characterizing pore structure and reservoir heterogeneity. structure and reservoir heterogeneity.

Application is on selected Samples from Application is on selected Samples from Shajara Reservoirs.Shajara Reservoirs.

Reservoir DescriptionReservoir Description

The Permo-Carboniferous Unayzah The Permo-Carboniferous Unayzah Formation consists of sandstone and Formation consists of sandstone and mudstone intervals bounded by two mudstone intervals bounded by two unconformities namely sub Unayzah unconformities namely sub Unayzah and Sub Khuff.and Sub Khuff.

Based on Saudi stratigraphic code, Based on Saudi stratigraphic code, Unayzah should be raised to a group Unayzah should be raised to a group status and a new Shajara Formation is status and a new Shajara Formation is introduced.introduced.

Lithologically, the Shajara Formation Lithologically, the Shajara Formation is classified into three members named is classified into three members named from bottom to top as Lower Shajara, from bottom to top as Lower Shajara, Middle Shajara and Upper Shajara Middle Shajara and Upper Shajara Members. Members.

Sedimentological Interpretation has Sedimentological Interpretation has revealed a braided fluvial channels. revealed a braided fluvial channels.

Type Section of The Shajara Type Section of The Shajara FormationFormation

N.26° 52 17.4 E. 43° 36 18N.26° 52 17.4 E. 43° 36 18

MethodologyMethodology The Autopore III was used for :The Autopore III was used for : Determination of Pc and pore throat Determination of Pc and pore throat

radius and intrusion volume (Hg).radius and intrusion volume (Hg). thus, The thermodynamic model thus, The thermodynamic model

equation is applied : equation is applied : Ln (Pc*Vi /r^2) = Df Ln (Vn^1/3 /r) + C Ln (Pc*Vi /r^2) = Df Ln (Vn^1/3 /r) + C Where, Vi = cumulative Intrusion Where, Vi = cumulative Intrusion

Volume Vn =Intrusion VolumeVolume Vn =Intrusion Volume This equation leads to the This equation leads to the

Determination of Df( Fractal Dimension) Determination of Df( Fractal Dimension) which is defined asSimilarity in which is defined asSimilarity in geometry and difference in Size geometry and difference in Size

Following the thermodynamic equation, Following the thermodynamic equation, the pore size radius was calculated the pore size radius was calculated (Washburn(Washburn,,s equation) as: s equation) as:

r = -2*σ*cosθ/Pc, wherer = -2*σ*cosθ/Pc, where r = pore radius in meter (m), r = pore radius in meter (m), σ = mercury interfacial tension in σ = mercury interfacial tension in

Newton per meter(0.485 N/m), Newton per meter(0.485 N/m),

θ = mercury contact angle measured θ = mercury contact angle measured in degrees(130°) and, in degrees(130°) and,

Pc = capillary pressure in Newton per Pc = capillary pressure in Newton per square meter (N/m^2).square meter (N/m^2).

Thermodynamic model for the Lower Thermodynamic model for the Lower Shajara Reervoir (3 samples). The Shajara Reervoir (3 samples). The

obtained straight line plot is assigned obtained straight line plot is assigned for fractal dimension systemfor fractal dimension system..

Affirmation of heterogeneity for 3 Affirmation of heterogeneity for 3 samples from the Lower Shajara samples from the Lower Shajara Reservoir. The non-uniformities Reservoir. The non-uniformities

delineate the heterogeneitiesdelineate the heterogeneities..

Thermodynamic model for the Middle Thermodynamic model for the Middle Shajara Reservoir (3 samples), the Shajara Reservoir (3 samples), the obtained straight line identifies the obtained straight line identifies the

fractal dimension of the systemfractal dimension of the system

Further confirmation of heterogeneity Further confirmation of heterogeneity for 3 samples from the Middle Shajara for 3 samples from the Middle Shajara

ReservoirReservoir

Thermodynamic model for the Upper Thermodynamic model for the Upper Shajara Reservoir of the Shajara Shajara Reservoir of the Shajara

Formation. The linearity displays the Formation. The linearity displays the fractal system of the 3 samplesfractal system of the 3 samples

Further confirmation of Further confirmation of heterogeneity for 3 samples from heterogeneity for 3 samples from

the Upper Shajara Reservoirthe Upper Shajara Reservoir

The 3-D Fractal ModelThe 3-D Fractal Model

This method is aimed to be This method is aimed to be correlated to the thermodynamic correlated to the thermodynamic model to confirm the Reservoir model to confirm the Reservoir

Heterogeneity based on Fractal Heterogeneity based on Fractal DimensionDimension

The 3 D model is Calculated based on The 3 D model is Calculated based on Mercury Saturation and Average Mercury Saturation and Average Capillary Pressure.Capillary Pressure.

33--D Fractal Model Equation and D Fractal Model Equation and Fractal Dimension (Df)Fractal Dimension (Df)

It is based on natural logarithm of It is based on natural logarithm of mercury saturationmercury saturation (Ln, Hg) divided by (Ln, Hg) divided by average average capillary pressurecapillary pressure. The latter . The latter one was plotted versus Ln(dSone was plotted versus Ln(dSHgHg / dP / dPcc).).

The slope was determined from the The slope was determined from the straight line and its value was added to straight line and its value was added to 4 to give the Fractal Dimension Df as 4 to give the Fractal Dimension Df as follows:follows:

Ln (dSLn (dSHgHg / dP / dPcc) = (Df -4) Ln ( P) = (Df -4) Ln ( Pcc))

33--D fractal model of the Lower D fractal model of the Lower Shajara Reservoir ( Lower Shajara Shajara Reservoir ( Lower Shajara

Member)Member)

33--D fractal model for the Middle D fractal model for the Middle Shajara Reservoir (Middle Shajara Shajara Reservoir (Middle Shajara

Member)Member)

33--D fractal model of the Upper D fractal model of the Upper Shajara Reservoir (Upper Shajara Shajara Reservoir (Upper Shajara

Member)Member)..

Table showing porosity and permeability results Table showing porosity and permeability results for the three Shajara Formation reservoirs for the three Shajara Formation reservoirs (Permo-Carboniferous Unayzah Group area, (Permo-Carboniferous Unayzah Group area, Al-Qasim district). Al-Qasim district).

ReservoirsReservoirsFaciFacieses

h h (feet)(feet)

Aver. Aver. r(r(μμm)m)

ΦΦ%%K K (mD)(mD)

UpperUpperShajaraShajara

ReservoirReservoir

SJ-SJ-1313

6.56.525252525973973

SJ-SJ-1212

13.113.16868282814401440

SJ-SJ-1111

4343363611971197

MiddleMiddleShajaraShajara

ReservoirReservoir

SJ-9SJ-90.90.97474323213951395SJ-8SJ-82929323213441344SJ-7SJ-713.413.44646353514721472

LowerLowerShajaraShajara

ReservoirReservoir

SJ-3SJ-31.61.6131334345656SJ-2SJ-23.93.92525353519551955SJ-1SJ-111.811.84141292916801680

ConclusionConclusion Based on Saudi stratigraphic code, the Permo-Based on Saudi stratigraphic code, the Permo-

Carboniferous Unayzah Formation is raised to a Carboniferous Unayzah Formation is raised to a group status and a new Shajara is introduced. group status and a new Shajara is introduced.

The Shajara Formation of the Permo-The Shajara Formation of the Permo-Carboniferous Unayzah group has been divided Carboniferous Unayzah group has been divided here, into three members, from base to top are; here, into three members, from base to top are; Lower, Middle and Upper Shajara Members.Lower, Middle and Upper Shajara Members.

Three fractal dimension units were identified Three fractal dimension units were identified on the basis of unconformities and mudstone of on the basis of unconformities and mudstone of the Middle Shajara Member.the Middle Shajara Member.

The thermodynamic model and 3-D fractal The thermodynamic model and 3-D fractal model of mercury intrusion show their model of mercury intrusion show their effectiveness, mathematical capabilities and effectiveness, mathematical capabilities and continuity in evaluating the fractal dimensions continuity in evaluating the fractal dimensions of the Shajara Reservoirs (Shajara Formation of the Shajara Reservoirs (Shajara Formation Permo-Carboniferous UnayzahPermo-Carboniferous Unayzah Group).Group).

The Shajara Reservoirs have show their The Shajara Reservoirs have show their heterogeneities, their bimodality and their heterogeneities, their bimodality and their megaporous classification. megaporous classification.

ConclusionConclusion

The results of the fractal dimensions in The results of the fractal dimensions in addition to pore size distribution addition to pore size distribution indicate the heterogeneity of the Shajara indicate the heterogeneity of the Shajara Reservoirs which accounts for a wide Reservoirs which accounts for a wide range of pore sizes.range of pore sizes.

A plot of incremental pore size A plot of incremental pore size distribution reveals bimodal behavior for distribution reveals bimodal behavior for the samples selected from three Shajara the samples selected from three Shajara Reservoirs of the Shajara Formation, Reservoirs of the Shajara Formation, except sample SJ12 which is except sample SJ12 which is characterized by monomodal pore size characterized by monomodal pore size distribution. distribution.

ConclusionConclusion

In our case, neither modality nor In our case, neither modality nor bimodality have great effect on bimodality have great effect on reservoir quality, but owing to sorting reservoir quality, but owing to sorting and fluid flow capacity (permeability). and fluid flow capacity (permeability).

The three Shajara Reservoirs are The three Shajara Reservoirs are classified as megaporous, with classified as megaporous, with average pore size of 26 μm for the average pore size of 26 μm for the Lower Shajara Reservoir, 49 μm for Lower Shajara Reservoir, 49 μm for the Middle Shajara Reservoir, and 45 the Middle Shajara Reservoir, and 45 μm for the Upper Shajara Reservoir.μm for the Upper Shajara Reservoir.