3D Petrophysical Model for Mansouriya Gas Field/Jeribe Formation by using Petrel

Mansuriya Gas field is an elongated anticlinal structure aligned from NW to SE about 25 km long and 5-6 km wide. The Jeribe formation considered the main reservoir, contains gas condensate fluid, has a uniform thickness about 60 m. The reservoir is significantly overpressured. In this study, the 3D geological model for Jeribe Formation in Mansuriya Gas Field is setup by using Petrel. Jeribe Formation represents the most important reservoir in Mansuriya Gas Field. Four vertical wells (Mn-1, Mn-2, Mn-3 and Mn-4) were drilled in Mansuriya Gas Field and used to set-up water saturation and porosity models represented by a 3D static geological model in three dimensions. The main reservoir, Jeribe Formation carbonate, is subdivided into 8 zones namely J1 to J8, according mainly to porosity log (RHOB and NPHI) trend, DT trend and saturation trend. Petrophysical model (porosity and water saturation) for Jeribe Formation was set-up from values of porosity and water saturation using Sequential Gaussian Simulation algorithm. According to data analyses and the results from modeling the units (J1, J3, J4, J5, J6 and J8) are considered as high-quality reservoir units due to the high PHIE, low water saturation and no shale content. Units (J2 and J7) are considered as non-pay units because of very high-water saturation. Cross sections of petrophysical model were conducted to show the vertical and horizontal distribution of porosity and water saturation between wells in the field.


Introduction:
Geological modeling and the static model are very important in the reservoir study. In two dimensions, a geological formation or unit is represented by a polygon which can be bounded by faults, unconformities or by its lateral extent, or outcrop. In geological models, a geological unit is bounded by 3-dimensional triangulated or gridded surfaces. The equivalent to the mapped polygon is the fully enclosed geological unit, using a triangulated mesh. For the purpose of property or fluid modeling these volumes can be separated further into an array of cells, often referred to as volumetric elements. These 3D grids are the equivalent to 2D grids used to express properties of single surfaces.
Identifying and recovering hydrocarbons require accurate, high-resolution geological modeling of the reservoir structure and stratigraphy [1]. The Mansuriya field is an elongated anticlinal structure aligned from NW to SE about 25 km long and 5-6 km wide. The hydrocarbon accumulation (gas condensate) is located in the Jeribe formation which is considered the main reservoir, has a uniform thickness about 60 m. The reservoir is significantly over-pressured [2]. Petrel, the modeling program, allows the user to interpret seismic data, perform well correlation, build reservoir models suitable for simulation, submit and visualize simulation results, calculate volumes, produce maps and design development strategies to maximize reservoir exploitation. [1].

Import Data
Data Prepared in files, one file for each data object, are imported by Petrel program. This data includes: Well heads: the position of the well in X, Y and Z directions.
Well tops: markers representing significant points.
Well logs and core data.

Structural Modelling:
It consists of three operations: fault modelling, pillar gridding, and vertical layering.
Eight contour maps have been built for the eight units of Jeribe formation as divided in the log interpretation shown in the CPI of the wells .These contour maps have been built depending on the contour map from the last study on the reservoir where it is corrected for seismic lines. Contour maps have been built from the tops of the Jeribe formation. In constructing these maps, old contour map direction is considered as a general direction in building the new contour maps [2].

Make horizons [2]
It is the first step used in defining the vertical layering of the 3D grid in Petrel.
The eight horizons that have been built up for the formation were built by entering main structural maps that had been built in (Structural modeling). Figures (6-A, 6

Layering
The layering allowed the definition of the final vertical resolution of the grid by setting the cell thickness or the number of desired cell layers [2]. The formation has been divided into many layers depending on petrophysical properties and these layers have been build depending along the vertical thickness of each layer which has been determined based on porosity log (RHOB and NPHI) trends, DT trend and saturation trend. Figures (7-A), (7-B) show the layering of Jeribe formation.

Scale up of Well Logs
For each grid cell, all log values that fall within the cell will be averaged according to the selected algorithm to produce one log value for that cell [2]. The porosity and water saturation values have been scaled up using the (arithmetic average). [10]

Petrophysical Modeling process
It is a process of assigning petrophysical property values to each cell of the 3D grid [2]. The petrophysics models include:

Porosity model
It was built depending on the final results of the porosity logs (density, neutron, and sonic logs) from CPI. Statistical sequential Gaussian simulation algorithm has been used. Some erroneous values appeared in the upscaled porosity model but the results in general are similar. Figure (8) shows the comparison between the porosity from log up scaled porosity.  Generally, it has been noticed that porosity values in Jeribe formation increases in the flanks better than on the crust and in the North West direction.

Water saturation model
It was built after the scale up of water saturation from CPI for each reservoir unit of the Jeribe formation in the Mansoriya field [1]. The same Geostatistical method was used as in the porosity, according to the available data. Figure (

Net to Gross reservoir estimation
Net to gross refers to the sum of productive intervals of a reservoir and is determined by the application of cutoffs of porosity, shale volume and water saturation which is a specified limit below which a formation would be unable to achieve or sustain commercial production [10].
No Vsh cutoff is applied for the Jeribe formation, as the petrophysical analysis found the Jeribe to be a clean formation. In order to designate the anhydrite as non-reservoir layers (layer 2 and 7), it is assumed to be (net to gross = 0).
The porosity cutoff has affects only zones 6 and 8 only and the average N/G for the other layers (2, 7 not included) is 0.95 as shown in in Table (1).  E49 pressure data could be divided into two groups according to their behaviour with gas gradient.

Gas Water Contact
The gas gradient was calculated from the specific gravity of the fluid which was calculated from the pvt model ≈ 0.11 psi/ft.This gas gradient line was passed through a weighted average of all the gas pressure points in both groups. Similarly, the water gradient was determined from the salinity of the water sample from test Well MN-4 to be 0.456 psi/ft [3]. The water gradient line has passed through the well test average reservoir pressure. The gas and water gradient lines intercept at depths of 1420 and 1380 mss. We have therefore taken two gas water contacts at 1420 and 1380 mss. Table (2) shows the raw test data that used to calculate GWC. Figure (12) shows gas gradient used to calculate GWC.

Gas Expansion Factor
The gas expansion factor at initial reservoir conditions is 270 scf/rcf, as derived from the pvt reservoir fluid composition model [1].

Volumetric Calculation
The volume of Initial gas in place of Jeribe formation which was calculated by Petrel at a GWC at 1420 mss is 2,330 bcf and at 1380 mss is equal 1,723 bcf. Figure (13) shows the equations which has been used for volume calculations in Petrel. Tables (3) and (4) show the volumetric calculations summary for Contacts at 1380 mss and at 1420 mss respectively. [2] No.26- (   1. Structural model shows that Mansuriya Gas Field represents an elongated anticlinal structure aligned between NW and SE, about 25 km long and 5-6 km wide.

No
Vsh cut-off is applied for the Jeribe formation, as the petrophysical analysis found the Jeribe to be a clean formation. In order to designate the anhydrite as nonreservoir layers (layer 2 and 7), it is assumed to be (net to gross = 0) .The porosity cutoff has affected zones 6 and 8 only and the average N/G for the other layers (2, 7 not included) is 0.95.