Production Optimization for Natural Flow and ESP Well A Case Study on Well NS-5 Mishrif Formation-Nasriya Oil Field

As the reservoir conditions are in continuous changing during its life, well production rateand its performance will change and it needs to re-model according to the current situationsand to keep the production rate as high as possible.Well productivity is affected by changing in reservoir pressure, water cut, tubing size andwellhead pressure. For electrical submersible pump (ESP), it will also affected by numberof stages and operating frequency.In general, the production rate increases when reservoir pressure increases and/or water cutdecreases. Also the flow rate increase when tubing size increases and/or wellhead pressuredecreases. For ESP well, production rate increases when number of stages is increasedand/or pump frequency is increased.In this study, a nodal analysis software was used to design one well with natural flow andother with ESP. Reservoir, fluid and well information are taken from actual data of Mishrifformation-Nasriya oil field/ NS-5 well. Well design steps and data required in the modelwill be displayed and the optimization sensitivity keys will be applied on the model todetermine the effect of each individual parameter or when it combined with another one.

of stages and operating frequency.
In general, the production rate increases when reservoir pressure increases and/or water cut decreases. Also the flow rate increase when tubing size increases and/or wellhead pressure decreases. For ESP well, production rate increases when number of stages is increased and/or pump frequency is increased.
In this study, a nodal analysis software was used to design one well with natural flow and other with ESP. Reservoir, fluid and well information are taken from actual data of Mishrif formation-Nasriya oil field/ NS-5 well. Well design steps and data required in the model will be displayed and the optimization sensitivity keys will be applied on the model to determine the effect of each individual parameter or when it combined with another one.

2-Introduction:
There are two parameters controlling the well performance which are inflow performance relationship (IPR) and Vertical Lift Performance (VLP). IPR is known as the relationship between well flowing bottom-hole pressure (Pwf) and production rate so it represent the flow from reservoir to inside wellbore.
There are many correlations and methods can be used to describe the reservoir performance. Each correlation has its own conditions to be applied according to reservoir and flow type. The most important methods which could be used for black oil reservoir are Vogle, Darcy and Fetkovich. In this work, the productivity index (PI) is already calculated from PLT data of well NS-5, therefore it can be used directly in nodal analysis.
The VLP depends on many parameters such as fluid PVT properties, tubing inside diameter, surface pressure, well depth, water cut and gas oil ratio. The total pressure loss from well bottom to surface is the magnitude of the three terms, gravity, friction and No.27-(6) 2020 Journal of Petroleum Research & Studies (JPRS) E19 acceleration. In oil well completion design, the gravity component should be comprised around 75% of the total pressure gradient [1].
Electrical Submersible Pump (ESP) components are key parameters in ESP design and any change in one or more of it will affect overall ESP performance. ESP components are; motor which is the system prime mover and electric motor with different type and size of ESP motors that give a different amount horsepower required. Gas separator, the presence of free gas in produced fluid decreases the ESP efficiency, so that a gas separator is used to remove the gas from produced fluid to the annulus. Pump, used to lift the fluid from down hole. To improve ESP capacity several pump stages could be used. Power Cable: used to supply the electric power to the motor down hole [2].
The objectives of well modeling & analysis are as follows [3]: 1. To calculate the optimum flow rate at which the well will flow with a known wellbore conditions and completion.
2. To evaluate the well and when it might be ceased to produce. This could be due to time when the reservoir pressure depletes.

3.1-IPR Generation:
The inflow performance relationship is modeled based on production log data for this well. The PLT data for well NS-5 are listed in Table (1) [4]. the IPR results are generated using PLT as shown in Figure (1).

3.2-Matching Pressure Gradient:
The actual measured data of PLT test is used to obtain the best fit vertical flow correlation which described the test rate, well head pressure, well head temperature and flowing bottom hole pressure. As shown in Figure ( line was the closest one to the measured point from all used correlations, so that may use after making this correlation fully matches with measured point.
In spite of being Hegedron & Brown correlation didn't distinguish between flow regime, but it gives the nearest calculated results to the measured results also the liquid hold up starts to decrease at a value very close to bubble point which indicate gas liberation and changing in flow regime from one phase liquid to two phase bubble, so it will be used to describe the well lifting performance.
Hagedron & Brown correlation is considered as most widely applied of oil wells as VLP correlation. It works well for bubble flow regime and slug flow regime in many applications. It could be used in wells for slug flow at moderate to high production rates also it use pipe roughness to describe two phase friction factor [5].
Ansari et al (1994a & 1994b) prepared eight different two-phase flow correlations and its relative errors, the smaller the relative performance factor, the more accurate correlation. According to Ansari's result, Hegdorn and Brown was found best correlation for current case [6].

Fig. (2) Matching VLP Correlations with PLT
To fully match Hagedorn & Brown correlation with the test point, it should be multiply be parameter 1 & 2 which will be define later. Parameter 1: is the multiplier for the gravity term in the pressure drop correlation.
Parameter 2: is the multiplier for the friction term.

3.3-Matching VLP/IPR with Measured Data:
Match VLP correlation and IPR with test point to obtain the difference in liquid rate and bottom hole pressure for measured and calculated data as shown in the Figure (3) below:

3.4-Well design sensitivity:
The most important part of building a well physical model is to evaluate well performance under different reservoir and operation conditions such as: decline in reservoir pressure, increase in water cut, changing in well head pressure according to De-Gas Station circumstances and changing in production tubing size due to design requirements.
The production sensitivities will be applied to the designed well to determine well state under different situations. Liquid flow rate and bottom hole pressure will be calculated as they are the main production parameters.  (4) show the effect of reservoir pressure decline and water cut increase on production rate (Assumed WHP= 975 psi which is the same wellhead pressure as PLT data).  As shown in Table (3) and Figure (4) that the increasing in water cut can cause the reduction in the production rate. When reservoir pressure and WHP are 3366 psi and 975 psi respectively, the well produced 2633 STB/Day and 1589 STB/Day for WC equal to 0% and 20% respectively. The well was ceased when water cut equal and more than 40%.

Fig. (5) VLP/IPR relationship for different WHP and Tubing size.
As shown in Table ( inch ID, the well produce d 2633 STB/Day, while the production rate increase to 3204 STB/Day when tubing ID increase to 3.83 inch for same wellhead pressure.

4-ESP Well Design and performance sensitivity:
The electrical submersible pump is consider as one of the most important artificial lift method used in the oil industry because it required very little surface space, can be installed in vertical or highly deviated well either onshore or offshore. Also, it can be used in casing size (4.5 inch) and larger. ESP can be used in wells up to +13,000 ft in depth and it can handle fluid rate reach to 60,000 BPD depending on its size, design and operation conditions. If the ESP not operated at recommended operation parameters, the ESP efficiency will decrease and it may get failure [7]. Figure (6) shows the common ESP components.

4.3-ESP Design Parameters:
The main parameters were used in ESP design are: 1-Pump depth: this depth is the depth at which the pump of ESP set and it is represent the depth of intake pressure. Pump depth should be above the perforation interval and far enough from ESP erosion factors such as sand production and also should be below the depth at which the bubble point pressure is reached.  (4). So that the design rate will be consider as the midpoint between minimum and maximum rate which is about 6000 BPD.

3-Gas separator:
if free gas enters the pump, ESP efficiency can be decreased because gas separator is needed to take the free gas out from ESP pump and direct it to the annulus. The decision of putting gas separator depending on Dunbar plot which is a relationship between the intake pressure, gas liquid ratio and the intercept with gas separator efficiency curves. If the test point above the red line, then no need for gas separator as shown in Figure (7). The final ESP design parameters chosen to design the ESP are listed in Table (5).     As shown in Table (   As shown in Table ( increasing the water cut (WC) to well production rate (975 psi WHP and 60 Hz).

Conclusions:
1-The production rate increases when tubing size increases and/or wellhead pressure decreases for both of natural flow well and ESP well.
2-For ESP well, the production rate increases when ESP frequency increases and/or number of stages increases.
3-As the production conditions are changing during reservoir life, thus it is very important to re-design the well from time to time according to new situations.
4-ESPs are very useful to increase well productivity when the well is not able to produce under natural flow condition or the production rate is low.
5-Some well ceased while production under natural flow, but it could be putted again in production by using ESP.