Introduction

Abstract

The BEI (Bureau d'Etudes Industrielles) is a six weeks project which allows last year engineering students to work as a team on a industrial challenge. The project which is presented here has been developped in collaboration with Schlumberger China Petroleum Institute.

The student team has joined the Simkarts Project, initiated in 2012 by Pr. Han Pingchou, Peking University PKU, and Bernard Montaron, Schlumberger China. This project has been created in order to develop a reservoir simulator which would simulate realistic multi-phase flows in the particular geologic context of the Tarim Basin, situated in West China. However, this simulator is not based on fluid dynamics laws and therefore needs to be validated.

In the BEI, three-phase flow (oil/water/gas) simulations are carried out, using  Jadim and Fluent, two CFD softwares, in order to simulate nitrogen injection in the specific fracture/cave configuration of the Tarim Basin.  The simulation method used is the Volum Of Fluid (VOF) method ,based on Navier-Stokes equations.

These simulations will give a good understanding of flow behavior in a case of gas injection and will help to optimized this oil recovery method. Moreover, results will permit to validate and optimized the reservoir simulator developped by Schlumberger.

The team

 

          

    Mohamed BELERRAJOUL

​    3rd year ENSEEIHT Student, Fluid Mechanics dpt                            

   belerrajoul

           

       Léa ROCHER

       3rd year ENSEEIHT student, Fluid Mechanics dpt

ROCHER

          

    Gaopan KONG

    International master student, Fluid Mechanics dpt

KONG

 

The supervising team

Mr Climent: ENSEEIHT professor and director of IMFT (Institut de Mécanique des Fluides de Toulouse)

Mme Pedrono: member of the IMFT (Institut de Mécanique des Fluides de Toulouse)

 

The industrial partner          

Schlumberger is the world’s leading supplier of technology, integrated project management and information solutions to customers working in the oil and gas industry worldwide. The company provides the industry’s widest range of products and services from exploration through production.

Present in China since 1980, Schlumberger is actually involved in project which take place in the Tarim Basin. Bernard Montaron has initiated the Simkarts Project in 2012 and will be our interlocutor inside Schlumberger China.

 

Industrial context

The Simkarts project has been initiated in 2012 by Pr. Han Pingchou, PKU, and Bernard Montaron, Schlumberger China, in order to understand  multi-phase flow dynamics in deep oil reservoirs located in the Tarim basin, West China.

Karst reservoirs of Tarim Basin in Xinjiang Province of People’s Republic of China have been determined as an area with large quantities of oil and gas and are exploited by Sinopec and Petrochina Tarim Oil Company​ since the 1990’s.

This carbonate formation of Ordovician age is buried at more than 6000 meters and represents a complex network of connected caves full of oil and water large of 20 x 20 km.                                                           

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                           The Tarim Basin                                                        A fracture/cave reservoir  

In order to optimize oil production, it is necessary to understand and predict oil flow inside the reservoir. Indeed, the fluid dynamic inside the reservoir is complex.  In the Tarim basin, an aquifer is situated below the reservoir. Therefore, as oil is pumped up, water goes up by capillarity effect. Moreover, in many reservoir, water or gas are injected in order to maintain pressure inside the reservoir.

If geologists can find oil deep in the ground by using variety of methods such as sismology, surface features, soil types, sniffers or satellites images, numerical simulators are needed in order to understand fluid dynamic inside reservoirs.

Very few oil reservoirs in the world produce oil from caves. In traditional reservoirs oil is contained in microporous rocks with typical pore size smaller than 30 microns. 

Existing reservoir simulators currently used by the oil and gas industry are designed to simulate fluid flow in porous media governed by Darcy's law.

These simulators are not appropriate for simulating fluid flow in cave networks where Darcy's law does not apply. Moreover, Navier-Stokes model is impracticable because of the complexity of the cave's geometry, the scale of the reservoir and massive calculation and data burden that it would involve.

Thus, the ultimate objective is to design a 3D reservoir simulator that can make realistic multi-phase flow simulations (gas, water, and oil) in large and complex cave networks made of thousands of caves distributed in 3D space and connected according to a given pattern. 

                     

Bernard Montaron, Schlumberger China, proposed to investigate the feasibility of using cellular automata technology for such a simulator, and therefore initiated the SimKARST Project in 2012 with Pr. Han Pingchou, Peking University PKU.

The first part of the project started in October 2012 with two PKU students who developed a prototype cellular automata software to simulate 2D flow of water and oil in a cell with a simple geometry.

A cellular automaton is a discrete model where particules are constrained to move on points of the lattice. Each particle is in a certain state, which is be in this case oil, water, gas or rock. Movement of particules between neighboring lattices points are governed by prescribed rules based on gravity for the vertical state and on probablility for the horizontal state. For instance, a rule stipulate that oil blocks will always move upwards when water blocks are on top of them.

If this method is a fast simlulation technique and give reasonable kinetic results, it is not based on any fluid dynamics and does not respect the physical properties of fluids. Moreover, velocities are limited by lattice spacing and time. Therefore, this method needs to be compared with experiments and simulations based on physics laws, in order to validate its accurency.

                        

                                 Representation of the reservoir by the automata simulator

It order to validate and to improve the automata simulator, two ENSEEIHT students, Nicolas Sobecki and Thibault Moreau, realized a summer intership in China  in order to conduct two-phase flow experiments on a specific geometry. The acquisition of this experimental data allowed to compare the experimental flow behaviour observed with  flow simulated with the cellular automata software.

At the same time, Tinting Han, another ENSEEIHT student, realized CFD studies with fluent in order to simulate  two-phase flow on the same geometry, in order to compare the results with experiments and data from the automata cellular. As CFD simulations are based on the Navier and Stokes equations, they respect physics law and give therefore accurate results.

               
          Two-phase flow experiment                                                     CFD Simulation with Fluent
 
As the simulator is still under development, a ENSEEIHT students team had, during their BEI project, carried out CFD simulations using complex geometries. These simulations, realized with two different CFD softwares and based on Navier and Stokes equations, helped Schlumberger to validate and to improve its simulator.
 
 

Gas Injection

Schlumberger came to us with another phenomenon to simualtion: the injection of gas in order to optimize oil recovery.

Indeed, oil extraction can be devided into different recovery stage. During the primary recovery stage, which corresponds to the first years of production, reservoir drive comes from a number of natural mechanisms: underground pressure in the oil reservoir is sufficient to force oil to the surface. However, over the lifetime of the well, pressure fall and is insufficient to force oil to the surface.

Therefore, after natural reservoir drive diminishes, secondary recovery methods are applied in order to increase reservoir pressure and thereby stimulate production.

Water or gas injection is a secondary recovery method used by oil and gas companies to increase oil recovery from an existing reservoir. Water or gas is injected to support pressure of the reservoir (also known as voidage replacement), and to sweep or displace oil from the reservoir, and push it towards a well.

Normally only 30% of the oil in a reservoir can be extracted, but this method increases that percentage (known as the recovery factor) and maintains the production rate of a reservoir over a longer period.

In the Tarim Basin, due to the complex geometry of the karst reservoir, a large amount of oil is wedged into the cap of the cavities:  38,000 m3 of oil produced (32%) for 80,000 m3 remaining (68%).

               

                                          Oil wedged into the cap of the cavities

The solution proposed by Schlumberger is to inject nitrogen, lighter than oil, in the reservoir to replace oil wedged  in the cavities. 

In this BEI, CFD simulations are therefore carried out in order to simulate this phenomenon. These CFD simulations will than help Sclumberger to understand flow behaviour in a case of gas injection and see how the technique can be optimized. Moreover, a comparison of CFD simulation and automata simulator results will permit to validate the reservoir simulator developped by Schlumberger for a three-phase flow.

Objectives

The objectives of the BEI is to:
 
1. Understand  three-phase flow dynamics when nitrogen is injected in a fracture/cave configured reservoir
  •  A bibliography of different methods to simulate multiphase flow is realized with a focus on the VOF method.
  • The main caracteristics of Jadim and Fluent, two CFD codes which permit three-phase flow simulation, are analyzed.
  •  CFD simulations of nitrogen injection in a simple cave geometry (three-phase flow) is carried out with Fluent and Jadim.
  • A parametic study is also carried out in order to observe the influence of the gas injection well position and of the outlet position.

2. Compare a commercial software (Fluent) and a research software (Jadim)

3. Compare CFD simulations based on physics Navier and Stokes equation with cellular automata simulations in order to validate the cellular automata simulator