SPE 13LAAL-P-121-SPE

Automated Surface Gas Handling Through Expert Systems for Optimization of Artificial Lift Systems Miguel Ramírez and Carlos Brunings, PDVSA

Copyright 2013, S ociety of P etro leum E ngineers This paper was prepared for presentation at the S P E A rti ficia l L i ft Conference - A m eri ca s held in Cartagena, Colom bia 21-22 M ay 2013 .

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ABSTRACT

In reservoirs with a gas drive production mechanism, as the down hole pressure drops below the bubble point , gas breaks out of solution, and depending on the gas to oil ratio (GOR), the performance of oil wells with different

artificial lift pumping systems installed are directly affected by the presence of free gas in the fluid. As a result, pump volumetric efficiency decreases reducing production, affecting the life cycle of the pump, and increasing costs due to the need of rig intervention for installing gas separators, or different assemblies for gas handling

devices such as charge, tapered or two stage pumps in order to improve this volumetric efficiency.

Two newer systems for handling gas at the surface have been applied in high GOR wells in the heavy and extra heavy oil wells of Eastern Venezuela. The first system consists of implementing a closed loop device for controlling the pressure of the annular of a well to an optimal pressure value. The control algorithm based on techniques from

expert systems has been applied successfully in several wells. Basically, when the pressure of casing is being controlled by an automated pressure control valve set at an optimum value, it causes a variation of the dynamic fluids reducing the amount of free gas in the annular space and pump.

The second system consists of a PCP (progressive cavity pump) device that measures fluid load through a load cell installed at the drive head of the surface equipment and detect variation in dynamic fluid, torque and motor

load and surface pressure of the flow line are also recorded, and adjust the velocity of the pump with the variable frequency drive controlled by an expert system, which runs in a programmable logic controller developed for optimizing PCP. All these two systems have improved production and run life in more than 20% in different wells in

which have been evaluated.

1. INTRODUCTION

The development and implementation of applications based on expert systems for optimizing heavy and extra heavy oil wells started approximately in 1997 in the major Oil Company of Venezuela. First expert systems were

designed for specific artificial lift methods such as electrosubmersible pumps, progressive cavity pumps and sucker-rod pumps in order to adjust both the appropriate velocity of the pump and the diluent injection in an autonomous way. Taking into account an automation infrastructure based on intelligent instrumentation with

diagnosis capabilities and a completely open architecture that supports standard communications interfaces, a major characteristic of expert systems is its capacity of determining the real current state of the pump and the well.

When a reservoir reaches the bubble point pressure, there will be free gas bubbles within the oil phase. The reservoir pressure decreases while production goes on and it causes emerging and expansion of gas bubbles creating extra energy in the reservoir. These sorts of reservoirs are called Solution Gas Drive Reservoirs. The

current reality in PDVSA is that the progressive increment of free gas volume in most of our reservoirs is

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becoming a major concern for the productions operations because pumps simply lose volumetric efficiency and its

life cycle diminishes. Therefore, new challenges in the development of expert systems are focused on optimizing heavy and extrahaevy wells with relative high gas to oil ratio (GOR) no matter what sort of artificial lift method.

Two possible alternatives have been evaluated. The first alternative consists of installing a valve with its respective electric actuator and a pressure transmitter connected to the programmable logic controller (PLC) where the expert system is running. Other devices such as the variable speed drive, the downhole sensors, the head

pressure and temperature transmitters and the annular pressure transmitter are supposed to be integrated to the PLC. The control system essentially set an optimal annular pressure keeping in a specific values range the pump input pressure.

The other alternative consists of controlling the velocity of the pump according to the behavior of the current of the motor. The control system essentially keeps the current in a constant value. This alternative is useful when the

annular pressure control valve is not available or can not be installed because there are not facilities in the well. An additional instrument called axial load cell is important in progressive cavity pump for diagnostic purpose.

This paper is organized as follow. In section 2, the fundaments of expert systems and its application for the optimization of surface gas handling is described in details. Section 3 presents the effects of gas on down hole pumps and section 4 shows the benefits obtained from the implementation of these applications and finally

conclusions are given in section 5.

2. FUNDAMENTS OF EXPERT SYSTEMS AND ITS APPLICATION TO THE SURFACE GAS HANDLING

Formally, expert systems uses a repository of human knowledge captured and stored in a computer to solve

problems that ordinarily require human expertise. Its design covers and converges on a narrow area of expertise

called domain. Its basic structure is composed of a knowledge database and an inference engine that leads the determination of an appropriate answer for a specific problem. The knowledge base is the collection of domain knowledge given by an expert operator which is required for solving a certain problem, including fundamentals

facts, rules and other information.

In general, it is important to highlight that most applications for optimizing wells have two components: an expert

system and a program based on artificial neural network. An artificial neural network (ANN) is defined as a data processing system consisting of a large number of simple highly inter connected elements (artificial neurons) in an architecture inspired by the structure of the cerebral cortex of the brain. Applications based on ANN are commonly

used in order to detect and classify different behavior patterns and to predict the behavior of whatever process based on historical data. For instance some classical successful applications based on ANN are the generation of the surface dynacard graphs with its respective diagnosis in sucker-rod pump systems and the plotting of the

amperemeter chart in electrosubmersible pumps.

The first alternative mentioned before consist of controlling an optimal annular pressure in order to keep in a

constant range, the behavior of both the dynamic fluid level and input pressure pump. A careful selection of the well must be done before implementing this alternative. The expert system tries to increase the input pump pressure by closing the annular control valve while at the same time must avoid a significant decreasing of the

dynamic fluid level. A functional schematic is of this optimization system is showed in figure 1. The inputs of the expert system are the electric parameters, the down hole pressure and temperature and the

instruments at the head of the well such as head pressure and temperature and annular pressure. Furthermore, in parallel, a neural network is classifying the different operations regimes of the well and its output is the input of the expert system. The output of the expert system is an optimal value that is the input of a classical close loop control

system whose elements are: a controller which is an algorithm called PID (proportional, integral and derivative), the control valve of the annular pressure and the pressure transmitter. The manipulation of the control valve varies the value of the annular pressure. The aim of the PID algorithm is to try to equal the instant casing pressure to the

optimized casing pressure value given by the expert system. Figure 2 shows the schematic the second optimization system. The expert system is inside the close loop

controller and its output is the velocity of the pump. The idea is keep the behavior of the motor in a constant with a minimum variation.

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3. EFECT OF GAS ON DOWNHOLE PUMPS

Free gas reduces the liquid flow rates decreasing the pump efficiency , alters the pressure profile within the

pump concentrating cavity loading and internal friction and diminishes the ability of the fluid to cool the pump

resulting in localized internal heat build-up. Failures are characterized by hardening, cracking and chunking of the elastomer with damage concentrated at the discharge end of the pump. Furthermore, the effect of gas migration into the elastomer and explosive decompression is considered. This particular situation has been observed on

failure analysis for PCP pumps in the Orinoco Oil Belt. When producing high percentage of free gas, a lot of fluid slippage is required to compress gas reducing volumetric efficiency of pump. This fluid has little mass to absorb heat, then a significant amount of hot is transferred to the stator, increasing internal temperature, this effect

combined with high pressure loading at the discharge of the pump and high speed collaborates in an accelerated progressive failure from pump discharge to intake (1).

Gas affects the performance of ESP (Electrical Submersible Pumps) in several ways: It affects total volume of fluid by increasing the total of equivalent liquid to be handled through the pump. The gas decreases the bulk density of the fluid, pump power and motor amps decreases directly with the density which in turn causes a

reduction on output pressure reducing the flow into the equipment (gas surging). Also the pump can gas lock when the flow restriction on the impeller eye is so great that flow stops and the pump is locked, Gas interference due to cavitations through the formation of gas bubbles collapsing into the pump, and if the gas is concentrated in large

slugs instead of bubbles the pump cannot handle it (2) .

On sucker rods, gas affects the pump performance volumetric efficiency and displacement , producing gas pound,

due to improper valve spacing or even gas lock preventing fluid flow between standing and travelling valve.

4. FIELD RESULTS

Lab test experiments (3,) have demonstrated that extra-heavy viscous crude oils such as the Orinoco Oil Belt

have a foamy behavior (Figure 3), in which the gas bubbles are entrained in the oil due to high viscosity, low

diffusion coefficient and asphaltene content. As pressure is being lowered, reaching the pseudo bubble point large quantities of gas are produced thus an increment of the gas oil ratio is experienced.

This produced gas affect the volumetric efficiency of the bottom hole pump, hampering production. So the

annular pressurization technology focuses in keeping an optimum differential pressure in the annulus (Figure 4),

where favorable changes of relative permeability, GOR (gas oil ratio) and water cuts occur, with a change in fluids mobility as the gas either stays in, or enters into solution by pressurization reducing the foaming tendency in the annular space, improving pump performance (4).

Results of automatic gas pressuring of the casing are shown in the following graphs for different devices installed in heavy oil wells, evaluated throughout the years. Figure 5 shows production behavior for well MFB-656,

with an automatic load cell system installed (PCPOS) at the drive head and a control valve at the casing, it could stop the declining production experimented by this well from 1000 bpd (barrels per day) to 800 bpd, and increase well flow output back to original levels.

Other examples are shown, such as Figure 6 in which Well MFA-241, with a non stable (on-ff) production, due the big amount of gas handled through the casing, after putting well under pressure control together with a variable

frequency drive (SIAP), the well presented a continued production.

An example of improving pump performance down hole can be seen in the amperage charts for an electrical

submersible pump in well MFB-584, in which gas slugging and surging is present (Figure 7) before the installation of the pressure control device (Maxiprod), and after a smoothing out of the amperage chart, meaning that a higher density fluid is handled through the pump, and a lower intake pressure can be achieved, thus increasing the

production of the well.

Finally another example is shown in figure 8 of increment in production (February 2010) with installation of

another device (Awpa) that automatically measures fluid level and controls casing gas pressure A typical Surface flow Schematic can also been seen in figure 9 .

5. CONCLUSIONS

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1.- Artificial Lift equipment operation based on speed adjustment through intelligent expert system allows the

optimization system to take it’s own decision according to the behavior of the well permitting a better production 2.- Automated controlled valve system installed at the casing allows the optimal handling of the gas coming out

of the annular space , improving pump and production efficiency and reducing pump failure specially in pcp

devices

6. REFERENCES

1 Meetings Pdvsa- Weatherford, PCP failure analysis, November 2012, El Tigrito, Venezuela.

2 Wilson B "ESp's&Gas, SPE, Esp Pump Workshop, Houston 1993. 3 Otero, C. et al. Foamy Oil: Determination of Production mechanism and thermal effect over It. Paper UNITAR,

7th conference, 1098.040. 4 Brunings C . et al. Oil Production Increase through an Automated Annulus Pressure Control System applied

Extra-heavy Oil wells in PDVSA, San Tome District; World Heavy Oil Congress, 2009-128.

Figure 1: Functional schematic of a well optimization system (first alternative)

Figure 2: Functional schematic of a well optimization system (second alternative)

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Figure 3: Foamy oil behavior.

Figure 4: Effects of the annular pressurization

PSIA

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Figure 5: Production of well MfB-656

Figure 6: Well production MFA-241

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- Figure 7: Amperage chart Well MFB 584

Figure 8: Well production NZZ-177

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Figure 9 Surface well Schematic for automatic valve control