Petrophysical Analysis of an Iraqi Gas Field ( Mansuriya Gas Field )

*Corresponding author  Peer review under the responsibility of University of Baghdad.  https://doi.org/10.31026/j.eng.2020.03.09  2520-3339 © 2019 University of Baghdad. Production and hosting by Journal of Engineering.  ). http://creativecommons.org/licenses/by /4.0/ license  4 e CC BY This is an open access article under th  Article received:21/7/2019  Article accepted:24/9/2019  Article published:1/3/2020


INTRODUCTION
Petrophysics is the study of rock properties and their interactions with fluids (gases, liquid hydrocarbons, and aqueous solutions). The geologic material forming a reservoir for the accumulation of hydrocarbons in the subsurface must contain a three-dimensional network of interconnected pores in order to store the fluids and allow for their movement within the reservoir. Accurate knowledge of these properties required for efficient development, management, and prediction of future performance of the oilfield, (Schlumberger, 1989).

The Area under study
The Mansuriya Gas Field is located in block 45 in Diyala governorate about 45 km north east of Ba'quba. It is also located about 100 km north east of Baghdad

METHODOLOGY:
The available logs data were given as LAS file format for all the wells .The environmental corrections to the log readings were done before starting the interpretation. The corrections and interpretation of well logs were achieved by using Interactive Petophysics software (IP) version (3.5). It was found that there is no big difference between the corrected logs and the raw log readings because the logs were run in a good borehole with a good mud system. Accordingly, the input data used where considered the corrected logs to be used evaluate Jeribe formation in Mansuriya Gas Field. The interpretation also includes the determination of porosity, saturation and all the required parameters such as (m and n). Mansuriya Gas Field has four drilled wells and all of them penetrate the Jeribe formation. Logs and core data were available from all four wells.

THE ENVIRONMENT CORRECTION OF WELL LOGS
The environmental corrections are carried out to minimize the effect of the bad hole conditions, (Asquith , et al.,1982). IP software was used for the environmental corrections.

RESISTIVITY LOGS
Deep resistivity is a basic measurement of reservoirs fluid saturation and is a function of porosity, (Schlumberger, 1989).

Density log
One of the porosity logs that measure the bulk density for the required depth then equation (1) is used to calculate the porosity as follow, (Pirson, 1993). ∅ = − − Where, ΦD: porosity from density log (fraction). ρma: the density of the matrix, [whose value is 2.85 (gm/cc) according to core analysis] (TPOC,2014). ρb: the bulk density of the formation, (gm/cc) and, ρf: the density of fluid.

Neutron logs
It is used to determine the porosity directly while its value relative to the amount of hydrogen in the formation which is either from the hydrocarbons or from water in the pores of the formation, Pirson,1993.

Sonic log
One of the porosity logs that measure the capacity of the formation to transmit compressional sound wave for the required depth. Equation (2) is used to calculate the porosity from sonic log value as follow, (Pirson, 1993): Where, Фs: porosity from sonic log (fraction). (1) (2) Journal of Engineering Volume 26 March 2020 Number 3 104 Δtma: the matrix interval transit time [whose value is 43.5 (μsec/ft) according to core analysis] (TPOC, 2014), Δtlog: interval transit time in the formation, μsec/ft; and Δtf: interval transit time in the fluid within the formation [For fresh water mud = 189 (μsec/ft); for salt-water mud = 185(μsec/ft).

Gamma ray log
Gamma ray log is used to detect the radioactive elements such as uranium thorium and potassium, (Pirson, 1993).

Shale Volume Determination
The behavior of GR log in carbonate rocks as compared to the spectrally corrected gamma ray, GRC curves, where the uranium component (often associated with dolomitisationl) is removed and therefore only comprises the thorium and potassium-40 contributions, showed the gamma response to be low compared to the total gamma ray response that also contains the uranium contribution. For this reason the Jeribe was taken to be petrophisicaly "clean" in terms of shale content, (Crain, 2015). Also, the SP log in well MN-1 indicates that there is no shale or very small amount in the Jeribe formation as shown in Fig. 3. The gamma ray responses from Well MN-2 are presented in Fig. 4, where the total gamma ray GR is the red curve and the spectrally corrected CGR is the green curve. Where, ρmaa : apparent density of matrix (gm/cc), Δtmaa : apparent transit time in rock matrix (μsec/ft), and, Фta : apparent total porosity (fraction) , (Schlumberger,1989).
The matrix was determined by the matrix identifiers (MID) crossplot (ρmaa and Δtmaa) for the Jeribe formation as shown in Fig. 5 for well MN-1. While the most of data were plotted around dolomite region and calculated mineral composition show high content of dolomite mineral with some calcite, it is concluded that the Jeribe formation consist of dolomite mineral with some calcite also gas-trend is clear in the Jeribe formation. These crossplots show that the matrix is mainly dolomite and some limestone.

M-N CROSS PLOT FOR MINERAL IDENTIFICATION.
The demonstration procedure of this type of cross plots for mineral identification was presented by Schlumberger.
It is a two-dimensional display of all three porosity log responses in complex reservoir rocks, (Schlumberger, 1989).

DENSITY -NEUTRON CROSS PLOT FOR LITHOLOGY:
It is used to determine the lithology of a formation and also the total porosity, (Asquith, et al.), 1982, as in Fig. 7 for well MN-1. Jeribe lithology is interpreted as mainly dolomite from the above models, limestone or dolomitic limestone is reported as dominant lithology from cuttings and core description.

Determination of Porosity
There are three types of porosity: 9.1Total porosity: It is the ration of the all pores within the rock and the bulk volume using the following equation,

Effective porosity:
It represents all the pores within the rock without the pores occupied with clay, (Schlumberger, 1989). Ф = Ф -(1 − ℎ) (8) Where: Фe = is the effective porosity. While the the Jeribe formation is considered to be clean so the total porosity is equal to the effective porosity.

Secondary porosity
It is the porosity formed after the deposition process such as the porosity of fractures. SPI is the secondary porosity index and can be calculated as follows, (Asquith, et al., 1982).

CALIBRATION TO CORE.
Core analyses were available from all wells and the core porosity data were compared to the log derived values. Although the comparison showed some scatter, log and core data were in broad agreement, (Servet, 1998). Core porosity with log porosity is plotted for each well and then the porosity from log is corrected according to the equation derived from the plot, (Schlumberger, 1989). Fig. 9 shows the plot of core porosity and porosity derived from log for all wells. The resulted porosity has a name of (Phi_ND_corrected).

DETERMINATION OF FORMATION WATER RESISTIVITY RW
A water sample from the Jeribe formation was obtained in Well Mn-4. The well was tested and found to be water bearing. A salinity of 91,000 ppm was determined from chemical analysis, which is equivalent to 0.048 Ohm.m at 131 °F using Shlumberger chart (Gen 6), ( Schlumberger, 2007).

DETERMINATION OF ARCHIE PARAMETERS USING PICKETT'S METHOD Pickett plot method is used to calculate (m) and (a) from well logs by the following equation:
= − Ф + (10) By plotting Rt vs. Фe on log-log scale, m is the slope of the line (which represents the points with Sw =100%) and aRw is the intersection point with y-axis at Фe=1. With known Rw from other sources (a) can be easily found. Fig. 11 shows the picket plot for well MN-4 which shows the values of the tortousity factor (a), saturation exponent (n) and the cementation factor (m) which is obtained by the above technique, Asquith, (et al., 1982). In order to determine the cementation factor (m), Fig. 10 log-log plot between the core formation factor and core porosity to get the slope which represent (m) for well MN-4, (TPOC, 2014). There is a good agreement between Rw from pickett plot and from water analysis and Rw is considered to be (0.048 ohm.m) at formation temperature (138 F).

FLUID AND FORMATION ANALYSIS
This step considered the final and the most important step in the static formation evaluation to get more accurate water and hydrocarbon saturations for each level.

Water Saturation.
Archie equation is used to calculate water saturation. This is an appropriate calculation for a conductive 91,000 ppm equivalent sodium chloride brine, shale free rock and total porosity. In the case of Well MN-1, laterologs were used and the deep laterolog response is used as Rt. Wells Mn-2, -3 & -4 were drilled using an oil based mud (OBM) over the Jeribe Formation, and here the deep induction response is taken as Rt, (Asquith, et al., 1982)

CUTOFF CALCULATIONS
The cutoff value is applied to specific reservoir parameter (porosity, permeability and water saturation) in order to split the formation into pay and non-pay sections, (Asquith, et al., 1982). From the available core data and well logs data for all the four wells, cutoffs for the formation units have been determined as follow: 15.1 Porosity cutoff, (Asquith, et al., 1982).
The available core data analyses have been used to determine the cutoff of core porosity, a plot of permeability (log scale) versus porosity (linear scale) with the intersection of straight line at permeability value of (0.01,0.1 md) with the best fit line.
No porosity cut off is found if cutoff permeability 0.01 md is applied while the porosity cut off approximately equal to 0.1 only for unit J6 & J8 if cutoff permeability 0.1 md is applied .
To identify water saturation cutoff values, cross plot between water saturation and log porosity for the reservoir units of Jeribe formation has been made, taking log porosity cutoff value and intersecting it with the drawn curve to find water saturation cutoff. Fig. 16 shows the water saturation cut off for J6 unit. It can be seen that there is no saturation cut off for the units of Jeribe formation.

NET TO GROSS
Net to gross defined as the thickness of productive reservoir rocks (net) within the total reservoir (gross) thickness, (Baker, et al., 2007). For Jeribe formation net to gross has been determined by using porosity and water saturation cutoffs, Table 1 shows the net to gross values for formation units with log calculations summary of well MN-1.

CONCLUSION :
-The Matrix Identification (MID), M-N and Density-Neutron crossplots indicates that the lithology of Jeribe formation consists of dolomite, limestone with some anhydrite also gas-trend is clear in the Jeribe formation. -The main reservoir, Jeribe Formation carbonate, is subdivided into 8 zones namely J1 to J8, based on mainly porosity log (RHOB and NPHI) trend, DT trend and saturation trend. -The Jeribe formation was considered to be clean in terms of shale content .The higher gamma ray because of the uranium component which is often associated with dolomitisationl and when it is removed and only comprises the thorium and potassium-40 contributions, showed the gamma response to be low compared to the total gamma ray response that also contains the uranium contribution.
-While the Jeribe formation is considered to be clean in terms of shale content so the total porosity is equal to the effective porosity. -No porosity cut off is found if cutoff permeability 0.01 md is applied while the porosity cut off approximately equal to 0.1 only for unit J6 & J8 if cutoff permeability 0.1 md is applied . -It can be concluded that no saturation cutoff for the units of Jeribe formation is found after a cross plot between water saturation and log porosity for the reservoir units of Jeribe formation and applied the calculated cut off porosity. -The permeability is predicted using two methods the FZI and the Classical .The two methods yields approximately the same results for all wells.