A new integrated oil production enhancement technology based on water-flooding energy recovery is proposed. After providing an extensive review of the existing scientific and technical literature on this subject, the proposed integrated technology is described together with the related process flow diagram, the criteria used to select a target facility for its implementation and the outcomes of the laboratory studies conducted to analyze emulsion formation and separation kinetics. Moreover, the outcomes of numerical simulations performed using Ansys CFX software are also presented. According to these results, using the proposed approach the incremental oil production may reach 1.2 t/day (with a 13% increase) and more, even at low flow rates (less than 10 t/day), thereby providing evidence for the benefits associated with this integrated technology.

Oil and gas producers are currently focusing on improving oil production efficiency amid the slumping demand for hydrocarbons and relatively low hydrocarbon prices [

Many technologies known to improve oil production efficiency [

As previously noted, a high gas content at downhole pumping equipment intake results in a lower oil production efficiency [

Jet pumps are now widely used across the entire oil recovery, gathering, transportation, and treatment chain [

Field waterflooding allows retaining a high energy potential due to the high pressure in the piping system. The authors propose to efficiently use waterflooding system energy as an active medium of the surface jet pump; the passive medium here is the water-in-oil emulsion from the gathering line after the automatic well pad meter station. The proposed technology has several advantages, including less capital expenditures and the ability to use existing well pads and well infrastructure.

The process flow diagram is shown in

The technology is implemented as follows: a well pad is equipped with a high-pressure surface ejector, where the active medium is a chemical from the reservoir pressure maintenance system and the passive medium is a gas-liquid mixture from the producing well. A chemical feed unit mounted on the producing well flow line helps avoid the formation of a stable emulsion when implementing the technology due to intensive dispersion and demulsifier stirring in the high-pressure ejector. The described technology will allow reducing the linear pressure of the producing well, which provides conditions for annular pressure decline and an increase in submergence depth of the downhole pumping equipment below the dynamic fluid level. The downhole pumping equipment submergence under dynamic fluid level enhances the reliability of the submersible equipment, reduces failure rates, and drives up well fluid withdrawal.

A technology for annulus gas pump-out using a downhole jet pump [

A substantiated implementation of the developed enhanced oil recovery technology requires a comprehensive selection of a target for testing the development outcome. The best target is a producing well complying with the following requirements:

Operated with an electric submersible pump and electric screw pump, i.e., operating without a significant pressure surge;

Having a high linear and annular pressure;

Having a high gas/oil ratio (above 25 m^{3}/t);

In-field pipeline capacity potential at minimum 40% of the existing capacity;

Well pad located close to a free-water knockout unit or oil pre-treatment unit to avoid additional pumping through booster pump stations.

The essential requirement for operating producing wells with electric screw pumps and submersible pumps is maintaining consistent hydrocarbon recovery, i.e., a constant production rate and pumping head over long-term operation. Complying with this criterion will mitigate the risks associated with the accuracy in the choice and design of the jet pump, as well as boost the designed unit stability. The technology is designed to be implemented at well pads located relatively close to oil treatment gathering stations (oil pre-treatment unit and free-water knockout unit) to eliminate additional fluid transportation and pumping through the gathering and transportation system. The pressure maintenance system situated at the well pad can be implemented using fresh water as a working agent, which will allow additional oil desalting and improve its quality at field treatment. Preliminary calculations of the jet pump operation allowed determining an optimal range of well production water cut for the use of the given technology between 50% and 80%. The specified water cut range was selected to pass to the right-hand side of the emulsion viscosity-water cut dependency, i.e., to provide a water-in-oil emulsion to reduce the viscosity of the transported fluid and intensify system separation kinetics [

The development of criteria for efficient application of the integrated enhanced production technology based on pressure maintenance system energy recovery allows effectively choosing a target for its introduction and eliminating possible risks of failure to achieve the set objectives.

The objective of the research was determined in compliance with the developed criteria. Producing well with reference #1 was selected. Producing Well #1 is operated with the ESP-25-1500 electric submersible pump unit. The operating parameters are given in

No. | Parameter | Symbol | Unit of measurement | Value |
---|---|---|---|---|

1 | Well | 1 | ||

2 | Name of formation | Tl_{2−b} |
||

3 | Pump | ECP | ||

4 | Pump depth | H_{p} |
m | 1,541 |

5 | Current frequency | f | Hz | 45 |

6 | Wellhead pressure | P_{wh} |
MPa | 1.9 |

7 | Linear pressure | P_{lin} |
MPa | 1.9 |

8 | Fluid flow rate | Q_{liq} |
m^{3}/day |
19.3 |

9 | Water cut | W_{c} |
% | 59 |

10 | Oil flow rate | Q_{oil} |
t/day | 7.9 |

11 | Intake pressure (pump) | P_{int} |
MPa | 3.4 |

12 | Bottom-hole pressure | P_{bh} |
MPa | 4.9 |

13 | Dynamic fluid level | H_{fl} |
m | 1,491 |

14 | Annular pressure | P_{an} |
MPa | 2.3 |

15 | Formation pressure | P_{form} |
MPa | 11.1 |

16 | Static fluid level | H_{st} |
m | 207 |

17 | Pressure drawdown | P_{d} |
MPa | 6.2 |

18 | Productivity index (PI) | K_{prod} |
m^{3}/(MPa⋅day) |
3.7 |

The Table shows that the linear pressure equals the wellhead pressure and is 1.9 MPa. The annular pressure is 2.3 MPa, the intake pressure at the electric submersible pump is 3.4 MPa, the fluid flow rate is 17.9 m^{3}/day, and oil flow rate is 8.9 t/day. Analysis of well operating parameters shows that the submersible pump effective intake pressure (difference between the intake pressure and the annular pressure) is 1.1 MPa, which can lead to pump starvation and therefore cut time between overhaul. The water cut is 59%, which promotes water-in-oil emulsion formation.

The physical and chemical properties of Well #1’s water-in-oil emulsion are given in

No. | Parameter | Unit of measurement | Value |
---|---|---|---|

1 | Oil density under surface conditions at 20°С | kg/m^{3} |
864 |

2 | Produced water density under surface conditions at 20°С | kg/m^{3} |
1,177 |

3 | Water cut | % vol. | 59 |

4 | Salt content | mg/dm^{3} |
430 |

5 | Gas content in oil | m^{3}/t |
68 |

6 | Saturation pressure | MPa | 9.74 |

7 | Paraffin content in oil | % | 3.19 |

8 | Resin content in oil | % | 3.77 |

9 | Asphaltene content in oil | % | 15.31 |

Laboratory experiments were conducted on pre-sampled reservoir fluids of the implementation target. Water-in-oil emulsion models were prepared using a dedicated laboratory stirrer. The linear speed of emulsion stirring in the jet pump contractor was converted to the angular rotation speed of the stirrer using the formulas:

where

The average speed of emulsion flow in the jet pump contractor is 15–20 m/s, which is equal to 2,800–3,800 rpm of the stirrer. All laboratory tests were carried out at an emulsion temperature of 10°С, which corresponds to the average annual temperature of the emulsion in the target technology implementation area.

The intensity of stable water-in-oil emulsions formation of the research target was determined by dispersion analysis by varying the stirrer rpm. The average diameter was determined using a trinocular laboratory microscope. A fragment (up to 1 ml) of the emulsion was withdrawn from the prepared stirred sample for 30–60 s and transferred to a slide.

The water-in-oil emulsion separation kinetics were analysed using the bottle test technique, which essentially consists in determining the volume of the water separated from the water-in-oil emulsion prepared in a stirrer in graduated cylinders over time. The emulsion dehydration degree is defined as the ratio of the free separated water volume to the total volume of water in the sample [

The oil production increase potential resulting from the annular pressure drop is calculated using the formula [^{3}/(MPa⋅day); ^{3}.

The paper solves the problem of determining the characteristics of multiphase flow: including oil and water, formed in an oil jet pump. Oil and water are considered as continuous fluid, to describe the motion of which the Eulerian approach is used. Both fluids are incompressible. The simulated process is considered isothermal. The steady-state multiphase flow is determined in the research, transients are not considered.

The modeling uses an approach which means that each fluid is possessed its own flow field. The behavior of each fluid is described by its own system of the Navier-Stokes equations. The components of the velocity and pressure vectors are determined in the Navier-Stokes equations while the fluids interact with each other through interfacial forces.

Further, the α phase will mean water, and the β phase–oil, the volume fraction of the phases in the control volume will be denoted as

The mathematical formulation of the problem being solved is written in the form of the following Reynolds Averaged Navier-Stokes equations:

Momentum equations for

Continuity equations:

Volume conservation equation:

It is assumed that both phases have the same pressure field:

For the description inhomogeneous multiphase flow is used a mixture model that treats both phases

In the applied mixture model, it is assumed that the interfacial forces

The independent variables in

The eddy viscosity hypothesis is assumed to hold for each turbulent phase. Diffusion of momentum in phase

The research uses a k-ε turbulence model (k-ε model) with wall functions:

The transport equations for

where

For the

At the area boundary through which water is supplied to the domain, the bulk mass flow rate is set 1 kg/s, also volume fraction of water and oil

At the area boundary through which oil is supplied to the domain, the bulk mass flow rate is set 0.116 kg/s, also volume fraction of water and oil

A relative pressure of 1.9 MPa is set at the exit from the design area.

At the remaining boundaries of the domain, the conditions for complete adhesion to smooth walls are set.

The variation of the average diameter of water globules in the simulated emulsion and its specific surface area as a function of rpm is shown in

The diameter of the water globules in the emulsion expands to 156 μm at 1,500 rpm and then gradually decreases to 89 μm as the stirrer speeds up to 2,800 rpm, which indicates an intensification of dispersion processes and an increase in emulsion stability.

Due to the time available, the sampling technique, and emulsion fragment application onto the slide, the dispersed phase micro-droplets were able to coalesce, as illustrated by the results given in

Water-in-oil emulsion separation kinetics at different oil/water volume ratios were analysed using the above bottle test.

All emulsion types created are susceptible to self-breaking to a dehydration degree of 96–97% within a short period of time (6 min), which signifies the formation of an unstable emulsion when implementing the integrated technology. Thus, the stirring-derived product comprising well crude oil and an additional volume of water from the pressure maintenance system will be subject to natural in-pipe demulsification during transportation across the gathering system.

The authors performed a numerical simulation of the jet pump under given operating conditions using the pro-grade analytical complex Ansys CFX.

The numerical solution of the presented system of differential equations with given boundary conditions is carried out using the well-known finite volume method. A three-dimensional (3D) computational grid consisting of tetrahedral and prismatic elements is created inside the multiphase flow domain under consideration. During the numerical procedure, the characteristics of the multiphase flow, for example, velocity and pressure, are determined in the nodes of the volumetric calculation grid. The change of continuous fields of velocities, pressures, etc., inside the created elements is described by introducing shape functions that are approximations of fields through the desired values in the grid nodes.

In the finite volume method, a control volume is formed for each node from the surrounding parts of the elements. Next, the integration of

The Navier-Stokes equations include fluid pressure and velocities, which, in the case of incompressibility of the medium, can be determined by solving separate equations using segregated algorithms with an additional pressure-velocity coupling scheme (well-known numerical procedures as SIMPLE, SIMPLEC, PISO, etc., are used) and by jointly solving all equations simultaneously in a single numerical procedure with a common matrix. In this case, the associated solver is applied. This is the approach used in this study, a pressure-based solver from the ANSYS CFS software is involved.

Numerical procedure uses a co-located (non-staggered) grid layout such that the control volumes are identical for all transport equations. As discussed by Patankar [

The ANSYS CFX software package uses a coupled solver, which solves the hydrodynamic equations (for u, v, w, p) as a single system. This solution approach uses a fully implicit discretization of the equations at any given step). As a result, the solution of a system of partial differential equations is reduced to finding a solution to a system of algebraic equations. Directly in the problem being solved, the components of the velocity vector for water and oil, pressure, volume fractions of water and oil, parameters k and ε of the turbulence model for the water and oil phases were the unknowns determined in the nodes. Ansys CFX uses a Multigrid accelerated Incomplete Lower Upper factorization technique for solving the discrete system of linearized equations. It is an iterative solver. Ansys CFX uses a particular implementation of Algebraic Multigrid [

As it is mentioned earlier, the study determines the steady state solution of a multiphase flow, transients are not considered. The corresponding mathematical formulation with defining relations is presented in the form of

The mesh size sensitivity of the jet pump model was analyzed by modelling a water-oil mixing problem and through jet pump hydrodynamic tests using a series of models with the same geometry but different structured mesh sizes. The main criteria of the assessment sensitivity of the model and for proving mesh size was the jet pump outlet pressure.

Analysis of the mesh sensitivity shows that the finite mesh size affects the results of the numerical simulation. A mesh size of 2 mm or less does not lead to a change in the final result (linear pressure–as the resulting criterion in numerical simulation). An increasing mesh size of more than 2 mm leads to a pressure drop at the jet pump outlet, which leads to a change in the final result by more than 50%. Further calculations with a 2 mm mesh size are carried out. Successive calculations were carried out under a 2 mm mesh size.

To calculate the jet pump performance, a 3D model was created and its operation was computed for the selected target object (Well #1). The jet pump pressure and water superficial velocity distribution when implementing the integrated technology is shown in

The operating parameters of the jet pump and Well #1 are given in

No. | Parameter | Unit of measurement | Original (without jet pump) | Integrated technology |
---|---|---|---|---|

1 | Linear pressure | MPa | 1.9 | 0.98 |

2 | Water cut | % | 59 | 92 |

3 | Oil flow rate | t/day | 7.9 | 9.1 (potential) |

4 | Fluid flow rate | m^{3}/day |
19.3 | 107.3 |

The result of the jet pump performance simulation is a decline in linear pressure from 1.9 to 0.98 MPa, which inevitably leads to annular pressure drop in producing Well #1. The water cut then increases from 59% to 92%, and the fluid production rate grows to 107.3 m^{3}/day.

The theoretical flow rate increment due to the integrated technology will therefore be 1.2 t/day. This result is possible due to the annular pressure drop from 2.3 to 1.4 MPa.

The authors performed an extensive analysis of scientific and technical literature on the improvement of oil production and jet pump performance. The theoretical research resulted in a description and development of a process flow diagram of the waterflooding-based integrated enhanced oil recovery technology. Applicability criteria for the integrated technology were developed and the potential introduction target was selected. The integrated technology introduction target is a producing well with a 59% water cut.

The laboratory studies of the intensity of stable water-in-oil emulsions formation of the target object showed a growth in the emulsion dispersive capacity from 6.4 to 11.3 mm^{−1} as stirrer rotation speed increased from 1,500 to 3,000 rpm. This confirms the increase in emulsion stability. However, analysis of the water-in-oil emulsion separation kinetics showed the susceptibility of all emulsion types created to self-breaking up to the dehydration degree of 96–97% within a short time (6 min), which implies the impossibility of the formation of a stable emulsion when implementing the integrated technology.

The jet pump numerical simulation and calculation for the selected target showed a linear pressure decline from 1.9 to 0.98 MPa and an annular pressure drop from 2.3 to 1.4 MPa. Deep submergence of downhole pumping equipment under dynamic fluid level to a depth of 92 m allows for reliable operation of the submersible equipment, thus reducing the risk of downhole pumping equipment failure. The integrated technology ensures a 15% increase in the oil flow rate, i.e., additional daily growth of 1.2 tons for the selected target object.

Analysis of the mesh sensitivity shows that the finite mesh size affects the numerical simulation results. A mesh size of 2 mm or less leads to no change in the final result (linear pressure–as the resulting criterion in numerical simulation); however, a mesh size of more than 2 mm leads to a pressure drop at the jet pump outlet, leading to a more than 50% change in the final result.

Implementing the described technology will allow shifting the phase inversion point, i.e., increasing the water cut from 59% to 92%, which will promote hydrodynamic flow through the pipeline due to a decreased viscosity of the pumped fluid.

In view of the above, the authors successfully provided a rationale for the waterflooding-based integrated enhanced oil recovery technology, showed the main stages of scientific support of the integrated technology introduction, and proved the positive technological effect of its implementation.