Petrophysical Properties and Well Log Interpretations of Tertiary Reservoir in Khabaz Oil Field / Northern Iraq

The aim of this study is interpretation well logs to determine Petrophysical properties of tertiary reservoir in Khabaz oil field using IP software (V.3.5). The study consisted of seven wells which distributed in Khabaz oilfield. Tertiary reservoir composed from mainly several reservoir units. These units are : Jeribe, Unit (A), Unit (A'), Unit (B), Unit (BE), Unit (E),the Unit (B) considers best reservoir unit because it has good Petrophysical properties (low water saturation and high porous media ) with high existence of hydrocarbon in this unit. Several well logging tools such as Neutron, Density, and Sonic log were used to identify total porosity, secondary porosity, and effective porosity in tertiary reservoir. For Lithological identification for tertiary reservoir units using (NPHI-RHOB) cross plot composed of dolomitic-limestone and mineralogical identification using (M/N) cross plot consist of calcite and dolomite. Shale content was estimated less than (8%) for all wells in Khabaz field. CPI results were applied for all wells in Khabaz field which be clarified movable oil concentration in specific units are: Unit (B), Unit (A') , small interval of Jeribe formation , and upper part of Unit (EB). Keyword: Tertiary reservoir, Petrophysical properties, Khabaz field. قارعلا لامش / يطفنلا زابخ لقح يف يثلاثلا نمكمل يرئبلا سجلاو ةيئايزيفورتبلا صاوخلا تاريسفت دومحم ريبج دمحا دعاسم ذاتسا طفنلا ةسدنه مسق , ةعماجلا يبارافلا ةيلك ناضمر اللهدبع دمحا ذاتسا دعاسم طفنلا ايجولنكت مسق , ةيجولنكتلا ةعماجلا ديجملادبع بيجن فسوي * ريتسجام بلاط طفنلا ايجولنكت مسق , ةيجولنكتلا ةعماجلا ةصلاخلا ةطساوب يطفنلا زابخ لقح يف يثلاثلا نمكملل ةيئايزيفورتبلا صاوخلا ديدحتل رابلاا تاسجم ريسفت وه ةساردلا هذه نم فدهلا ( جمانرب (IP ( رادصا 3.5 ةينمكم تادحو ةدع نم نوكتي يثلاثلا نمكملا . رابخ لقح يف هعزوم رابا ةعبس تنمضت ةساردلا .) يه تادحولا هذه . ةيساسا : Jeribe,UnitA,UnitA',UnitB, UnitBE ,and UnitE ( نا ثيح UnitB لضفا ربتعت ) Journal of Engineering Volume 26 June 2020 Number 6 19 ةينمكم تادحولا تايمك دوجو عم ةيلاعلا ةيماسملاو ضفخنملا يئاملا عبشتلا ثيح نم ةديج ةيئايزيفورتب تافصارمب زيمتت اهنلا ةدحولا هذه يف نوبراكورديهلا نم همخض ( اهنم ةريثك رابا تاسجم. Neutron , Density ديدحتل ةساردلا يف تمدختسا يتلا ) كم يف ةلاعفلا كلذكو ةيوناثلاو ةيلكلا ةيماسملا .يثلاثلا نم ( مسترم مادختسا ةطساوب يثلاثلا نمكملا ةيراخصلا ديدحتل (NPHIRHOB ( مسترم ةطسارب كلذ متي نداعملا ديدحتلو تيامولودلاو يريجلا رجحلا نم يساسا لكشب نوكتي ثيح N/M ثيح ) %( نم لقا يثلاثلا نمكملا مومع يف ليشلا مجح رًدقت . تيامولودلاو تياسلاك نم نوكتي 8 بت .) ( جئاتن ني (CPI لقح رابلاا لكل : يهو رابخ لقحلل يثلاثلا نمكملا تادحو نم هددحم تادحو يف زكرمتت ةكرحتملا طوفن هنأب زابح Unit B) ( , ) Unit A' , ) ( ل يولعلا ءزجلا و يبريج نيوكت نم هددحم قطانم Unit EB .) :ةيسيئرلا تاملكلا قح , ةيئايزيفورتبلا صاوخلا , يثلاثلا نمكملا زابخ ل .


INTRODUCTION
Petrophysical interpretation represents one of most essential process to understand reservoir properties in subsurface structures, geological formation, and physical measurement. This process was used to identify Petrophysical properties which related to interpretations and corrections of well log records (Cannon, 2016). The well log related to samples which can be obtained in to subsurface. Reservoirs are subdivided in to several units or zones depends on various Petrophysical properties (primary and secondary porosity, lithology, permeability, fluid saturation, mineralogy (Ellis and Singer, 2008).

MAIN OBJECTIVES
1. Select units in tertiary reservoir which have good porosity (total, primary, secondary, and effective).

Mineralogical and lithological identification for units of tertiary reservoir.
3. Introduce percentage of shale content in all units of tertiary reservoir. 4. Identify concentration of movable and residual oil in tertiary reservoir units. 5. Quantify hydrocarbon saturation (oil and gas) in tertiary reservoir units.

AREA OF CASE STUDY
Khabaz oil field is represent one of the more importantly north Iraqi oil fields which characterized by multiple pay zones which produces from tertiary (Jeribe reservoir) and cretaceous (upper and lower Qamchuqe reservoirs). Its located North West (NW) of Kirkuk city and (12 km) far away from center of Kirkuk city. It's encircled by three oil fields Bai Hassan from North West and Baba dome exist in Kirkuk field from north east and Jumbour field from South east as shown in

METHODOLOGY
Digder software (V.3.03) used to digitize the scanning copies for wells to provide LAS files for seven wells in Khabaz field (Kz-1, Kz-2, Kz-3, Kz-4, Kz-9, Kz-14, Kz-15).These LAS files for well log are imported to interactive petrophysics software (IP V.3.5), then readings of log are taken as one reading per (0.15m). The log curves are checked to be in depth with each other. All log curves, then depth-matched, the available gamma ray readings taken as a reference guide for depth matching, true corresponding between gamma ray readings and other logging tools was clear at formations tops.. Environmental corrections were made using the current Schlumberger charts which are applied to (IP) software as environmental corrections module, caliper logs, mud properties, and temperature gradient were supplied more accurate for corrections. LAS files were imported to (IP) software which used to determine and correct reservoir properties, resistivity logs, density logs, neutron log, m and n as shown in flow work diagram in Fig. 2.

BOREHOLE ENVIROMENT
Through drilling operations, normally can be noted pressure of mud column larger than pore pressure in the formation, in order to prevent Blowout into the borehole. The differential pressure between mud filtrate and mud column in the permeable zone and some sediments in the mud which deposited on borehole formed mud cake. Mud cake has very low permeability about (4-10 md). The rate of mud filtrate invasion, whenever move away from borehole shown in Fig. 3. Flushed zone represents a zone which have most of hydrocarbon and water are invaded also called invaded zone (Schlumberger, 1989).

Figure 3.
Borehole environmental and symbols of resistivity, thickness, water saturation, and diameter of borehole.

ENVIROMENTAL CORRECTION OF WELL LOGS
The borehole environmental can be made by measurement of well logging. A lot of well log records should be modified to stander condition of well log operations because of there are big difference between condition of log tool and condition of bore hole (Bowen, 2003). Basically, there was characterized several log measurement leads to various corrections such as. 1. Resistivity logs corrections for borehole effect, invasion of wellbore effects, and bed thickness effects.

Density logs corrects for only borehole size.
3. Neutron logs require correction for formation, temperature, borehole diameter, mud salinity, and formation pressure.

Resistivity Log Corrections
Apparent resistivity is measured by resistivity log. It's consider a resistivity of homogeneous medium. Apparent resistivity consider true resistivity if measurement condition be known. Before this step must be correction various kinds of resistivity log such as, LLS to LLSC, DLL to DLLC, MSFL to MSFLC and so on which illustrated in Fig. 4. These corrections were done by IP software (Imad, 2012).

Density and Neutron Logs Corrections
Density and neutron must be corrected in order to obtain more accurate for calculations porosity determination shown in Fig. 5. Corrections have been applied by using the following equations For density porosity log applying the following equations (Schlumberger, 1997):- For neutron porosity log applying the following equations (Debrandes, 1985):-

Gamma Ray Log Correction 6.3
The GR is used to identify shale beds when SP log reading curved. As well as used to evaluation and quantify radioactive minerals (Schlumberger, 2013). IP software used to environmental corrections for gamma ray log for (Kz-2) shown in Fig. 6

Shale Volume Estimation
Shales are not permeable zone but usually porous and have hydrocarbon. The existence of shale in the reservoir effects on value of porosity and water saturation which are derived from well log.
Well logging tools such as (Neutron and sonic) logs will record too high, while Resistivity logs records too low when shale exists in formation. If density of shale will read equal or greater than reservoir index matrix leads to density log records too high (Aljawad and Tariq, 2019). To get more accurate of shale volume calculations must be make corrections for Gamma Ray log shown in Fig. 7.
Tertiary rocks can be classified as young rocks depending on geologic period of rocks .So best model to consider for young rocks is Larionvo equation (Schlumberger, 1997). The first step to estimate (Vsh) by using gamma ray log as follow: Then for tertiary reservoir young rock method was used (Vsh) as formula (Asquith and Gibson, 1982):

Porosity Determination
The total porosity in Jeribe formation can be calculated by integration of Neutron -Density logs readings (Bowen, 2003).

Total Porosity
Generally total porosity can be introduced as a ratio pore space of rocks to bulk volume (Ezeke, 2010). = + 2 (5)

Effective Porosity
Effective porosity consider total porosity smaller than fraction of pore spaces which are fill by clay and shale. In clean formation, total porosity is similar to effective porosity shown in Fig. 9.

DETERMINATION OF LITHOLOGY AND MINERLOGY
It's necessary to determine lithology of formation to describe sediments, solid, rocks, matrix and reservoir characterization, and that have been done by using two cross plot.

The Neutron-Density Cross Plot for Lithology Identifications
This cross plot is much important in well log analysis to determine porosity and lithology of formation by cross plot neutron porosity verses bulk density (ΦN Vs. ρb), for producing fresh water base mud, clean matrix rocks and for full structured liquid. This cross plot explained many lithology such as, sandstone, limestone, dolomite and good lithological resolution for calcite, quartz, and anhydrite are easily selected (Schlumberger, 2008). Neutron-density cross plot for selected wells in khabaz field are clarify that most of points fell between limestone and dolomite lithology curve that illustrated in Fig. 10 for well (kz-2). The M-N cross plot generally can be used to facilities lithology interpretation and identify more complex minerals (Ross, 2015). These plots integrated of three mainly porosity logs in order to describe a particular minerals quantities by plotting M and N values in the chart. The M and N values are slopes of individual minerals, calcite, dolomite, sandstone, Gypsum, sulfur, gas and secondary porosity area on the sonic-density and density-neutron cross plot charts in Fig. 11. M and N are introduced by following equations.
The M-N cross plot for tertiary reservoir shows that points focused between lines calcite and dolomite with observed tendency approach to secondary porosity area that illustrated in Fig. 11 for well (kz-2).

Determination of Archie's parameter (a,n, and m)
Application of Pickett's plot is consider a method that's give a graphical solution to Archie's equation to predict water saturation and predict Archie parameter (a,m and n) by plotting resistivity verses porosity (on logarithmic scales), data arrangement as a straight line which can be used in log interpretation by the following equation.
For water saturated zone (Sw = 100%), so the equation becomes After plotting (logRt) vs. (log∅e) , (m) is represent the slop of line and (aRw) is intercept point on y-axis at (∅e = 1). So (a) can be easily found if known value of (Rw). The (n) saturation exponent can be determined depends on irreducible water saturation (Swi) levels by using the following equation.
Above equation (n-m) is represent slope of line and (m) can found it from Pickett's plot (previous step).So the saturation exponent (n) can be easily determined (Schlumberger and Houston, 1989). Table 1. Contains values of Archie's parameter (a, m, and n) for tertiary reservoir by using Pickett's plot. Fig. 12 is plot of true formation resistivity (Rt) verses effective porosity by (IP V3.5) for well (Kz-2).

Fluid saturation
Fluid saturation and porosity are represent most essential reservoir properties to obtained more accurate estimate of oil in reservoir. Because of heterogeneity of reservoir is continues related to these properties verses depth of formation, to get best estimation. Several well log analysis includes resistivity, acoustic are used to get of these properties (Schlumberger.1972 Water saturation in flashed zone is (Sxo) are expressed as:- Shallow resistivity tools is used to determined water saturation (Sxo) and deep reading resistivity tools used to computed water saturation (sw).The residual oil saturation and movable oil saturation can be determined by using the following equation .

Bulk Volume Analyses and CPI
The bulk volume of hydrocarbon (BVHC) is represent volume of hydrocarbon which is existed in pore space and rock reservoir, and the bulk volume of water (BVW) is referred to unit volume of pore space filled by water (Tonietto, et al.,2014).
The area between water saturation in flashed zone (Sxo) and water saturation in un-invaded zone (Sw) represents bulk volume of movable hydrocarbons.

Net to Gross reservoir calculation
Net pay is important parameter in reservoir calculation because it's used to identify geological section which penetrated formation and identify interval of reservoir which have amount of hydrocarbon is a function of production interval. Net pay is quantified value which can be used in Cutoff evaluation are performed to log interpretation data shown in Fig. 14. Cutoff is represent specific value used to remove non-reservoir interval (Baker, 2017). Figure 14. Net to Gross in (Kz-2) well.

CONCLUSIONS
1-Secondary porosity index was highest value in unit (B) which represents an essential unit with storage capacity and transmissibility includes tertiary reservoir, total porosity still greatest without any variation as compared with effective porosity kept lowest due to effect shale of volume. 2-Volume of shale estimate less than (18 %) in Jeribe formation includes tertiary reservoir. 3-From Neutron-density cross plot illustrates tertiary reservoir composed mixture of dolomiticlimestone, and From M-N cross plot explain the mineralogy of tertiary reservoir consist of calcite and dolomite. 4-CPI results gives highest percentage of movable hydrocarbon are focused on (unit B) of tertiary reservoir and (unit A') and small interval of Jeribe formation and top part of (unit EB). 5-Hydrocarbon saturation composed of oil and gas, where production in khabaz field includes oil and gas. Hydrocarbon saturation ranging from poor to moderate as a compared with water saturation. NOMENCLATURE a, n, and m= Archie's Parameters, dimensionless . PHINC= Corrected Neutron porosity. RHOBC= Corrected bulk density. gm/cc Vsh= Shale volume, fraction. Sxo= Water saturation in the invaded zone. Rtirr= True formation resistivity at irreducible zones, Ω.m. Rmf= Resistivity of mud filtrate, Ω.m. Δtfl= interval transit time in the fluid.
Δtma=interval transit time in the matrix.