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Estimation of free gas at the pump inlet

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Today the optimization of oil wells remains of great importance and will continue to be for many years. At UPC Global, our mission is to provide advanced technical support to our clients in the field of artificial lift systems, thus optimizing oil production, improving practices and promoting safe working conditions backed by our team of specialists.

Also, providing relevant information on artificial lift is part of what we do and for this reason the point to be discussed in this note is the handling of the gas at the pump inlet through the use of static bottom gas separators; and a simple way to estimate the volume of free gas at the level of its suction from the properties of the fluids.

The relevance of this topic is that a frequent reason for the inefficiency in the operation of the bottom pumps is the incomplete filling of liquid caused by gas interference.

In the case of wells with high gas production, we recommend the use of:

  • Double stage pumps (SIS or circle A).
  • Gas unlockers.
  • Deeper pump settling.
  • Long races and low speed.
  • The use of gaskets is not recommended since the pump would handle all the gas.
  • Static bottom gas separators (gas anchors).

A practical way of handling high volumes of gas at the pump suction level is by using the difference in densities between the fluids, which leads to separation due to the segregation by the gravity of the Liquid and Gas phases, where the Acceleration rate of gas is higher than that of oil.

How fast is the separation of gas bubbles, it is a function of the difference between the rate of descent of the liquid and the rate of ascent of the gas relative to the liquid (Sliding speed).

This phenomenon is complex and takes into account several parameters such as speed and direction of the different fluids, viscosity of the liquid (oil, emulsion), the difference in densities, interfacial tension, size of the gas bubble.

It is worth noting that Echometer has a performance simulator of the Bottom Gas Separators (Reference 1) which allows estimating the “Pump Filling Factor (Separator Pump Fillage Factor).

This factor represents the percentage of liquid that a bottom gas separator, with given specifications, delivers to the pump at different travel rates (Reference 1). This factor is a direct indicator of the performance of the background separator.

For the development of the simulator, a series of laboratory experiments were carried out where different production rates, different background gas separator configurations, different SPM and different ascent rates of gas bubbles were simulated. And in this way, the performance of the separators was evaluated.

Added to this in (Reference 1) Cutoff values (limits) are presented for flow velocities in the gas separator ring and gas bubble sizes.

Estimation of free gas volume at pump inlet level.

Steps:

  1. Estimate the fluid supply capacity of the producer training

 

  • Calculate Well Inflow Behavior (IPR) based on the well production test.
  • Estimate the maximum allowable rate according to the IPR and possible production problems (sandblasting, among others).

 

  1. Determine the energy supply at the pump level (Pump Intake Pressure)

 

  • Determination submergence level.

 

This variable is established with the equipment, applications, and methodology developed by Echometer. The principle in the detection of the submergence level is based on the detection of echoes. For this, there is a piece of equipment capable of generating a pressure pulse that generates an acoustic wave, and the echoes are reflected from the connection of the pipes (tubing collars) and the liquid level. This acoustic trace allows us to determine the level of liquid and therefore the submergence.

 

  • PIP Determination (Pump Intake Pressure).

 

Once the submergence level is obtained, the pressure at the inlet of the pump is determined.

Calculations:

These are simple and are developed in-depth in References 4. The information required is: Well configuration, fluid data, liquid level (acoustic data).

The available programs (TWM/TAM) make estimates of the pressure generated by the gas column from the relative density of the gas, additionally, the pressure generated by the degassed liquid column is calculated (dissolved gas correction)

In summary, the flowing bottom pressure (PBHP) is calculated as the sum of the head pressure (CHP), plus the pressure generated by the gas column and the degassed liquid column.

  1. Calculation of the percentage of free gas at the pump inlet from the properties of the fluid.

 

Data:

 

The production test includes Liquid flow (ql), Tubing pressure (THP), Casing pressure (CHP), RGP (%) and AyS.

 

PVT test: ºAPI, Gas in solution (Rsi), relative density of gas (gg), relative oil density (gp), Petroleum Volumetric Factor (Bo @ PIP), Volumetric Factor of (Bg @ PIP), Deviation Factor of Gas (Z), Gas in solution at PIP conditions (Rs @ PIP)

 

Conditions:

 

Efficiency percentage of the Gas bottom separator.

 

The maximum percentage of free gas allowed to pump suction.

 

Calculations:

 

  1. Gas Oil Ratio once the gas anchor (RGP anchor) is installed:

 

  1. Ratio of Free Petroleum Gas to Pump conditions

 

  1. Oil (qo), Water (qw), Gas (qg) rates, from the P.V.T properties to pumping conditions (@ PIP).

 

  1. Total rate (qT).

 

  1. Fraction of each component.

 

Basic Example:

 

Data:

 

Data from the PVT Well Test Data                                              Well Test Data

 

gp

ºAPI

RSi

gg

Rs                        @ PIP

RGP            

ql

PIP's

Temp.

%       AyS

Adim

º API

(pcn/bn)

Adim

(pcn/bn)

(pcn/bn)

(bn/dia)

(lpcm)

(ºF)

adim

0,9218

22,00

540

0,7241

75

540

100

700

234

0

* Subatured deposit

 

Conditions:

% Gas Separation Efficiency                                                                       % max free gas in the Pump

% Efi

%

80

30

Calculations:

 

RGP           Gas Anchor (pcn/bn)

RGP                  Free    gas (pcn/bn)

Bo                  (by/bn)

Z                Adim.

Bg                   (by/pcn)

qo @ PIP                          (bpd)

qw @ PIP                         (bpd)

qg  @ PIP           (bpd)

q  total        (bpd)

fo

fw

fg           libre

168

93

1,12

0,939

0,0046

112

0

43

155

72%

0%

28%

* All calculations and data are referred to pump level (@ PIP)

Discussion:

The placement of a bottom gas separator turns out to be a practical and efficient way to handle the free gas at the pump inlet. In various technical articles (Ref 1, 5) various ways of simulating performance and optimizing the operation of these types of equipment are presented.

It is important for the optimization engineer to know an estimate of the free gas at pump conditions, which is why it is presented in a simple way, based on properties of the fluids obtained from PVT analysis, to estimate the volume of free gas (bls/day) at pump inlet conditions (PIP) and be able to see if it exceeds the maximum gas volume value allowed in the pump.

References:

  • N. McCoy, Ken Skinner and O. Lynn Rowlan, Echometer Company, Kyle Marshall, Capsher Technology, Tony Podio, Down-Hole Gas Separator Performance Simulation Software
  • Walker, C.P.:”Determination of Fluid Level in Oil Wells by the Pressure-wave Echo Method” AIME Transactions April 1937.
  • McCoy et al.:”Acoustic Determination of Producing Bottom Hole Pressure” paper SPE 14254 presented at the 1985 SPE Annual Technical Conference and Exhibition, Las Vegas NV.
  • Robles, J. and A. L. Podio, “Effect of Free Gas and Downhole Gas Separation Efficiency on the Volumetric Efficiency of Sucker Rod Pumps and Progressing Cavity Pumps,” Proceedings of the 43rd Annual Meeting of the SWPSC, 1996.
  • Bohorquez, R., Ananaba, V., Alabi, O., Podio, A. L., Lisigurski, O. and Guzman, M.: 2009, “Laboratory Testing of Downhole Gas Separators”, SPE Production and Operations, Volume 24, Number 4, pp.499-509.
  • Campbell, J. H., Brimhall, R.M., 1989. “An Engineering Approach to Gas Anchor Design,” SPE Production Operations Symposium.
  • Clegg, J.D., “Another Look at Gas Anchors”. 1989, Southwestern Petroleum Short Course Proceedings pp.293-307.
  • Guzman, M., 2005. “Downhole Gas Separator Performance in Sucker Rod Pumping System”, M. S. Thesis, The University of Texas at Austin, Austin.
  • Lisigurski, O., 2004. “The Effect of Geometry on the Efficiency of Downhole Gas Separators”, M. S. Thesis, The University of Texas at Austin, Austin.
  • McCoy, J.N., Podio, A.L., Lisigurski, O., Patterson, J., and Rowlan, L.: “A Laboratory Study with Field Data of Downhole Gas Separators,” SPE 96619, Presented at the 2005 SPE Annual Technical Conference held in Dallas, Texas, 9-12 October.
  • McCoy, J.N., Podio, A.L.: “Improved Downhole Gas Separator,” 1998, Presented at the Annual Southwestern Petroleum Short Course.

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