Optimizations of an Oil Well in Amal Field under a Continuous Gas Lift by Using Graphical Method, Mathematical Method, and PROSPER Software

Abstract

The production of any oil reservoir is a result of the natural forces around the reservoir. These forces are called driving mechanism, which stars strong in the early well life, and then becomes weak to deliver fluids from the wellbore into the surface. Here, an artificial lift method should be ready to be signed directly in order to continue producing fluids. There are several types of artificial lift methods used in the petroleum industry such as Gas Lift. In this chapter, an oil well under a continuous gas lift process located in the Amal oil field is analyzed using three different methods. These methods are graphical method, mathematical method, and PROSPER software. The aim of this chapter is to determine the gas injection point and valve distribution of the gas lift system correctly. From the well history, the well had been put under a gas lift process with the inaccurate design of the gas injection point and valve distribution that result in a high water cut. In the end of this chapter, the steps of redesigning of the gas lift process by using the three mentioned methods showed great results that will be discussed in more details.

1. Introduction

Most oil wells will flow naturally for some period of time after they begin producing. This period of time might take several months to several years depending on the pressure supported. The reservoir pressure provides enough energy to bring fluid to the surface in a flowing well. As the well produces this energy is consumed and at some point, there is no longer enough energy available to bring the fluid to the surface, and the well will be put under one of artificial lift methods. The commonly used artificial lift methods in the petroleum industry are Sucker Rod Pumping, Hydraulic Pumping, Electrical Submersible Pumping, Gas Lift, Plunger Lift, and Progressing Cavity Pumping. Each one of the mentioned methods that are shown in Figure 1 has its own explanations, this chapter will only cover the gas lift.

Figure 1.

Artificial lift systems [1].

Moreover, approximately 50% of wells worldwide need artificial lift systems. There are varieties in the using methods depending on some factors. The pie chart in Figure 2 shows the estimated worldwide usage of different lift methods in recent years.

Figure 2.

Percentage of usage of artificial lift in the world for around 900,000 wells [1].

2. Gas lift systems

Gas Lift is one of the artificial lift methods that is widely used in oil wells. This method utilizes an external source of high-pressure gas for supplementing formation gas to reduce the bottom-hole pressure and lift the well fluids. Usually in the gas lift process, there is plenty of gas available in the area. The basic component of gas lift consists of gas compression which is composed of a compressor and dehydration plant, manifolds, gas lines, meters, and rate control devices [1]. The surface and subsurface components of the gas lift system are shown in Figure 3.

Figure 3.

Basic components for a gas lift system [1].

3. Types of gas lift supplements

There are two basic types of gas lift supplements. They are called continuous-flow gas lift and intermittent gas lift. The principle of the two types is described as: Gas lifting fluids from a well by the continuous injection of high-pressure gas to supplement the reservoir energy (continuous flow), or by injecting gas beneath an accumulated liquid slug for a short time to move the slug to the surface (intermittent lift) [2]. In this chapter, the well had been put under a continuous gas lift since there is plenty of gas available in the Amal oil field.

4. Gas lift valves

Continuous gas lift usually consists of a number of unloading valves with an orifice valve at the operating point. The role of the unloading gas lift valve is to allow smooth, positive, and reliable unloading of the well to the orifice over many years with continuously changing conditions. There are different types of unloading valves, namely casing pressure-operated valve (usually called a pressure valve), throttling pressure valve (also called a proportional valve or continuous flow valve), fluid-operated valve (also called a fluid valve), and combination valve (also called a fluid open-pressure closed valve). Different gas lift design methods have been developed and used in the oil industry for applications of these valves [3]. The selection of the gas lift valve depends on the gas injection and well condition. Moreover, the valve spacing is not only the distance between the valves but a range where API recommends to meet the following desired [3]:

1. To be able to open unloading valves with kickoff and injection operates pressures.

2. To ensure single-point injection during unloading and normal operating conditions.

3. To inject gas as deep as possible.

5. Design methods

Three methods have been used in this chapter to detect the gas injection point and valve locations. These methods are: the graphical method, mathematical method, and PROSPER software. Each method has a different technique to detect the gas injection but they are same in the principle. The procedure of all the mentioned methods will be explained in the coming subtitle of this chapter.

5.1 Graphical method

This method depends on some data to draw a flow chart as shown in Figure 4. It is an old method and needs the best fit of drawing to have accurate results of gas injection point and valve location. The needed data are: well head pressure, bottom hole pressure, reservoir pressure, kick off pressure, operating pressure, static gradient pressure, unloading gradient pressure, kick off gradient pressure, oil flow rate, water flow rate, average specific gravity (oil specific gravity, water specific gravity), and total depth. The steps will be explained in the calculations and results section of this chapter.

Figure 4.

Example of the procedure of the graphical method [4].

5.2 Mathematical method

This method depends on calculations and does not need any drawing to detect the gas injection and valve locations. All the calculations will be done by using the below equations. However, it depends on some data that are taken from the graphical method or might be available from the well information such as static gradient and unloading gradient. The equations used in the mathematical methods to determine the gas injection point and the valve locations are listed below [5]:

$$\mathrm{T}=\frac{100F^o+\left[70+\frac{{1.6}^{\ast }\mathit{\text{Depth}}}{100}\right]}{2}$$E1

$${\mathrm{\gamma }}_{\mathrm{avg}}=\left[\left({{\mathrm{\gamma }}_{\mathrm{o}}}^{\ast }\frac{\text{oil\hspace{0.17em}\hspace{0.17em}rate}}{\text{total\hspace{0.17em}\hspace{0.17em}rate}}\right)+\left({{\mathrm{\gamma }}_{\mathrm{w}}}^{\ast }\frac{\text{water\hspace{0.17em}rate}}{\text{total\hspace{0.17em}\hspace{0.17em}rate}}\right)\right]$$E2

$$\mathrm{Gs}={{\mathrm{\gamma }}_{\mathrm{avg}}}^{\ast }0.433$$E3

$$\mathrm{Dv}1=\frac{{\mathrm{P}}_{\mathrm{so}}-{\mathrm{P}}_{\mathrm{wh}}}{{\mathrm{G}}_{\mathrm{s}}}$$E4

$$\mathrm{Dv}2=\mathrm{Dv}1+\left[\frac{{\mathrm{P}}_{\mathrm{so}}-{\mathrm{P}}_{\mathrm{wh}}-\left(\mathrm{Dv}1\ast {\mathrm{G}}_{\mathrm{u}}\right)}{{\mathrm{G}}_{\mathrm{s}}}\right]$$E5

From Eq. (5), engineers can continue deriving equations to calculate the next valve locations until reaching the gas injection point or depth of the static oil level.

5.3 PROSPER

PROSPER is a well performance, design, and optimization program that is part of the Integrated Production Modeling Toolkit (IPM). It is a tool that is introduced by the industry to be used for the artificial lift application. This software provides more flexibility and accurate results for production engineering just needs more data to be inserted in it. Figure 5 shows the opened window of PROSPER software.

Figure 5.

PROSPER software first page.

6. Case study

The case that has been studied in this chapter is an oil well in the Amal field which belongs to Harouge Oil Company. The well that chosen is located in the N area, and named as X. The N area is located northeastern part of Amal Field, which encloses an area of 48,000 acres. The average reservoir pressure is maintained at around 3180 psia by an active edge-bottom water drive. Figure 6 shows the location of Amal wells [6].

Figure 6.

Amal field well location map [6].

6.1 Field data for N_Area

The field data from the N area have been brought for analysis in this study. The data that need to be used for gas lift optimization are listed in Table 1. These data are arranged separately based on the PROSPER simulator input requirement, since the software needs more data, and also for other methods. Tables 1–5 show the obtained data from Harouge Oil Company will give a better that will be used in this study.

Parameters	Quantity	Unites
Reservoir Pressure	4420	Psia
GOR	800	SCF/STB
Oil Gravity API^o	35	API
Gas Gravity	0.83	Fraction
Water Salinity	190,000	PPM
Bubble Point Pressure	2180	Psia
Oil FVF	1.393	STB/BBL
Oil Viscosity	0.635	CP

Table 1.

PVT $ IPR data for well (X) [6].

Measured depth (MD), ft	True vertical depth (TVD), ft
10,000	10,000

Table 2.

Trajectory description data for well (X) [6].

Casing information				Tubing information			Pressures
length	Weight	OD	ID	weight	OD	ID	Reservoir	Bottomhole	Wellhead
feet	lb/feet	inch	inch	lb/feet	inch	Inch	Psi	Psi	Psi
10,000	47	9 ⅝	8.681	9.3	3½	2.992	4420	2600	150

Table 3.

Include the casing information tubing information, and pressures for well (X) [6].

	Temperatures			Production test
	Reservoir	Bottomhole	Wellhead	Oil	Water	Liquid	Water cut	Produced Gas
F^°		F^°	F^°	STB/day	STB/day	STB/day	%	M Scf/day
240		220	140	2000	850	2850	30	1600

Table 4.

Include the reservoir temperature, bottom hole temperature well head temperature and production test for well (X) [6].

Well name	Productivity index (J)	Kill fluid
		Gravity	Gradient
	STB/day/Psi	ppg	Psi/feet
WELL (X)	1.6	8.5	0.44

Table 5.

Include the productivity index (J) and kill fluid properties for well (X) [6].

7. Calculations and results

The three mentioned methods will be applied for well X to detect gas injection point and valve distributions respectively. Short Expansions of each method will be shown in the coming sections of this chapter.

7.1 Graphical method results

In this method, there are some steps that need to be followed in order to draw a flow chart such as in Figure 4. These steps are: First, calculate the average specific gravity by using Eq. (3), second, plot P_wh P_so at zero depth as shown in Figure 7, after plotting P_wh and P_so, calculate P_so equivalent to total depth with plus 100 Psia and minus 100 Psia as a safety factor and plot them at the total depth. Third, plot P_wf at total depth in order to determine the flowing level, then choose any depth to calculate P_wf equivalent at that depth. After having all this information, the gas injection point can be detected and the first valve location is also determined. Finally by using the ruler (rule of thumb) other valve locations can be determined easily. From Figure 7 the valve location are determined and listed in Table 6.

Figure 7.

Design by hand of gas lift for well X.

Valves	Depth
First valve	2900 ft
Second valve	4500 ft
Third valve	5500 ft
Fourth valve	6100 ft
Fifth valve (GIP)	6470 ft

Table 6.

Shows valve locations for well X by using a graphical method.

7.2 Mathematical method results

In this method, only calculation will be done to determine the valve locations and compare them with the graphical method, the gas injection point cannot be detected from this method. The last valve location will be the gas injection point. The procedures of the mathematical method for well X are: First, the static gradient has been calculated in the graphical method which is 0.367, while the unloading gradient needs to be estimated, which is the slope of the line that is used to detect gas injection point in Figure 7 which is 0.141 Psia/ft. Second, after having the unloading and static gradient first valve location can be estimated by using Eq. (4) which is 2785 ft. Third, from now on all the coming valve locations can be easily estimated by using Eq. (5). For further valve locations, equations can be generated just one point should be taken into production engineering consideration, which is the space between valves that is not less than 200 ft. and does not pass the gas injection point as well. The results of the mathematical method are listed in Table 7.

Valves	Depth
First valve	2785 ft
Second valve	4423 ft
Third valve	5387 ft
Fourth valve	5954 ft
Fifth valve(GIP)	6484 ft

Table 7.

Valves locations for well X by using mathematical method.

7.3 PROSPER software results

The PROSPER software is used in this study to get better optimization of the gas lift design and to compare its results with other methods. All the needed data are listed in Tables 1–5. The coming software windows show input data with their matching and the valves location (Figures 8–12).

Figure 8.

Gas lift design mean menu and well information.

Figure 9.

Gas lift design rate with valve calculations, PROSPER software result.

Figure 10.

Valve calculations with tubing, opening, and closing pressure.

Figure 11.

Gas lift design report results.

Figure 12.

Complete result of gas lift design for well X.

7.4 Result’s comparison

The results of the gas lift process using the graphical method, mathematic method, and PROSPER Software are listed in the Table 8. The result shows a difference in the values between the mathematical method and the graphical method both results are acceptable as long as the distance between two valves is more than 200 ft. The mathematical methods resulted more valves than graphical method. The PROSPER Software results are closer to the graphical method than the mathematic method, which indicates that software might be more accurate in scaling drawing than hand.

	Methods
Valves Name	Graphical	Mathematical	PROSPER
First Valve	2900 ft	2785 ft	2968 ft
Second Valve	4500 ft	4423 ft	4965 ft
Third Valve	5500 ft	5387 ft	6222 ft
Fourth Valve	6100 ft	5954 ft	6531 ft. GIP
Fifth Valve	6470 ft. GIP	6484 ft. GIP

Table 8.

Comparison between the graphical method, mathematical method, and PROSPER.

7.5 Casing and tubing pressures

After having all the gas injection points and the depth of valves by the used methods, the calculation of pressure in casing and tubing at each valve depth for well X can be done by using the following equation [4]:

1. Determine the weight of the gas column from Figure 13 by having depth and gas gradient.

2. The pressure in the casing at the depth of each gas lift valve: P_c = P_so + (W_g) (depth).

3. Determination of the desired tubing pressure at each depth of the gas lift valve: P_t = (G_u) (Depth) + P_wh (Table 9).

Figure 13.

Weight of gas column [4].

Valves Location	Depth,ft	Pc,Psia	Pt,Psia
First Valve	2900 ft	1097	566
Second Valve	4500 ft	1154	810
Third Valve	5500 ft	1188	953
Fourth Valve	6100 ft	1208	1037
Gas Injection Point	6470 ft.	1226	1116

Table 9.

Casing and tubing pressure for each valve location of graphical method for well X.

8. Conclusions

1. The gas lift process is one of the most used methods as artificial lift methods in the petroleum industry. The supplements of gas injection can be by continuous flow gas lift or intermittent flow gas lift.

2. The gas injection point and the valve locations have been determined by using the three methods. The results show different values of the gas injection point and the valves locations, the differences between them are accepted.

3. The results of graphical method usually have different results of mathematical method and PROSPER software, because it depends on the accuracy of the engineering drawing. PROSPER software provider quick and more results matching.

4. The obtained results from the three methods show the optimum gas injection point should be about 6500 ft. to maximize gas production and minimize water cuts.

5. In gas lift designs an extra value should be installed in case oil level decreases, as a result, the authors would recommend to have an additional valve at a depth of 6800 ft. for future use.

Acknowledgments

The authors would like to express their gratitude to the Harouge Oil Company for supplying the data used in this chapter, and also for providing the software that was used in the study.

Nomenclature

Symbol	capture
Pwf	bottom hole pressure
Pwh	well head pressure
Pt	tubing pressure
Pc	casing pressure
Pvo	tubing pressure
Pk	kick off pressure,
Pso	operating surface pressure
GOR	gas oil ration
GIP	gas injection point
FVF	formation volume factor
Yw	water specific gravity
Yg	gas specific gravity
Yo	oil specific gravity
Gs	static loading gradient
Gu	unloading gradient
Gk	gradient kill fluid
Dt	total depth
Dv	depth of valve