Oil initially in place calculation by geologic and dynamic methods in Nahr Umar formation of Nasiriya oil field

Original oil in place is most critical stages of reservoir management, where the economic advantage of the reservoir is evaluated by estimation of the petrophysical properties and oil reserves. This work was carried out in five wells of Nasiriya oilfield, which is one of the Iraqi oil fields in the southern region. The aim of this study is to calculate oil in place from available data in Nahr Umar formation, having a complex lithology by two methods (static and simulation). It was found that the static model used for computing the petrophysical distribution oil in place was equal to (114 MM or 716 MM STB) and 117 MM or 734 MM STB for the dynamic one.

and saturation function model as well rock physics model to estimate OOIP by the simulation model. The most important phase of a reservoir study is probably the definition of a static model of the reservoir rock. Generally, the production capacity of a reservoir depends on its geometrical/ structural and petrophysical characteristics. A static reservoir study typically involves five main stages [4]:

Determination of Formation Tops Unit:
The formation tops determination is the first task for the reservoir engineer to build the static model. It can be done by determination of similar rock properties where the wells are correlated together. The main purpose is to determine the horizons of formation units and its sequence for accurate calculation of oil in place [4].
No.28-(9) 2020 Journal of Petroleum Research & Studies (JPRS) E5 Due to the variation of top formation depth in the final geological report (FGR), and final well report, the top of Nahr Umar is distinguished by the electro faces of the formation, the data used are:-depth of the invasion, Gamma-ray corresponding log, density log, sonic log, neutron log. Figures (3) and (4) show the correlation of wells and units and

Modeling of Petrophysical Properties:
Defining the quantity description of well logs data to estimate the main characteristics of reservoir rocks which are porosity, permeability, water saturation and volume of clay.
The petrophysical characteristics results are merged to build the geological model. [2] Sequential Gaussian simulation is a new algorithm of geo-statistics that recommended to use with continuous property for reservoir modeling because of its simplicity, flexibility and it is reasonably efficient [5]. The geological model is represented as a reference to estimate the amount of oil in the reservoir; it forms a basis for the initialization of the dynamic model. In the following paragraphs the next stages are described in more details.
The geological model is subdivided to a high number of grids. The properties of the grid estimate the amount of oil present. The quantitative study of a porous and permeable space in reservoir rock forms a part of petrophysics, a discipline which plays a fundamental role in reservoir studies. Petrophysical model is divided into three sub-models.

Porosity Model:
The values of Porosity were the output of well-log results (CPI) of Nahr Umar  Table (2).

Water Saturation Model:
Water saturation model is made from the results of (CPI) of Nahr Umar Formation.
The method which is used to build the saturation model is "sequential Gaussian simulation." This model is developed for each unit of the reservoir as shown in Figure (7).
The Table (3) illustrates water saturation values for each unit:  The volume of clay model: The modeling of clay is made from clay volume by gamma-ray that is presented in (CPI) of Nahr Umar Formation. The method that is used to build the clay volume model is "sequential Gaussian simulation." Figure (8) shows the developed Clay volume model for each unit, where volume of clay values for each unit was given in Table 4.  The permeability values are distributing over the permeability model according to "Sequential Gaussian simulation" method. Permeability was estimated by margining neural network technique and hydraulic flow units method [6,7]. Figure (9) shows the permeability distribution and Table (5) illustrates the permeability of each unit in Nahr Umar formation.

Oil Water Contact:
Water contact is the lowest level of producible oil, oil and water are produced above this reservoir height until the relative permeability to water becomes extremely low and only oil will flow [8]. After studying well logs for Nahr Umar formation it has been noticed that the (O.W.C) level at (2440 m). Figure (10) shows the oil-water contact for Nahr Umar formation.

Calculation of Hydrocarbons in Place by Static Model
Estimation of oil originally in Place (OOIP) represents the last stage of the static model. The geological provides all necessary information needed to use volumetric equation [9]. Original oil in place (OOIP) in an oil reservoir is given by:

Oil in Place by Dynamic Model:
Fluid Model: The required data to build fluid model is provided from PVT report of Nahr Umar formation in Nasiriya field, which is consisted of mean characteristic of the reservoir. The main fluid properties are illustrated in Figures (11 to 13).

Relative Permeability Estimation:
Because of relative permeability data is not available. Corey model is used to estimate the relative permeability's of two fluids which are presented by oil and water critical water saturation should be determined clearly because that value of water saturation at which the fluid begins in movement [10]. Relative permeability curves are shown in Figure (14). Hawkins-Luffel and Harris's approach are used to estimate capillary pressure [11] [112].
The result of capillary pressure is shown in Figure ( Figure   (16) shows the matching in water saturation by static and dynamic model [13]. Figure (17

Conclusion:
Depending on the petrophysical properties distribution (effective porosity (∅e), permeability (k), the volume of clay (VCL), water saturation (Sw)), the pay zone is NU-1 with high of porosity and permeability and low water saturation. NU-2 has a high volume of clay and low porosity, NU-3 has medium to high water saturation, NU-4 low permeability and porosity. NU-5 has high water saturation and porosity and permeability. Special core analysis TVD: Total vertical depth