Category Archives: PVT
VALIDACION DE DATOS PARA UN ESTUDIO DE PVT
El chequeo de la consistencia en la información reportada en un informe PVT y su validación resulta de extrema importancia durante la caracterización de reservorios. Esta afirmación se sustenta en que la gran mayoría de las herramientas de análisis, que emplea el reservorista a diario, requieren de información de fluidos coherente y representativa. Para realizar un buen chequeo de consistencia y validación de los datos informados en un ensayo PVT, se requiere seguir un orden lógico de los procesos de análisis. El siguiente es un posible esquema de trabajo:
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BALANCE DE MASA DE LA INFORMACION REPORTADA
Estos balances pueden ser aplicados a los diferentes procesos de separación empleados en el laboratorio.
Balance de masa global.
Este balance chequea que la densidad de la muestra medida sea consistente con la que se podría calcular empleando los valores reportados de densidad de tanque, gravedades específicas de los gases liberados en cada etapa de separación y relaciones de gas producido por etapa. La inconsistencia
EQUIPO DE PVT
en este análisis indicaría una anomalía en alguno o todos los datos del informe empleados en el cálculo.
Balance de masa por etapas.
Al igual que en el balance anterior, en este balance también se chequea que la densidad de la muestra medida sea consistente con la que se podría calcular empleando los valores reportados de densidad líquido, factor de volumen del líquido, gravedad específica del efluente y relación de gas producido en cada etapa del ensayo realizado en el laboratorio. A diferencia del balance anterior, en éste se va chequeando etapa a etapa el balance de materiales de la densidad. Los resultados inconsistentes de este análisis permiten identificar las etapas a partir de las cuales los valores reportados dejan de ser confiables.
Balance de masa composicional.
Este balance se determina, en cada etapa del ensayo, las fracciones de cada componente que se encuentran en estado líquido y gaseoso. Con las fracciones molares individuales se puede determinar el comportamiento de las constantes de equilibrio en el rango de presiones del ensayo. Este balance más complejo permite determinar la coherencia de la información calculada (fracciones de masa, y molares, salientes y remanentes en el sistema) y comparar las constantes de equilibrio así obtenidas con valores de referencia.
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GRAFICOS DE CONTROL
Entre los gráficos de control más empleados en esta etapa de análisis se encuentran el gráfico de Hoffman y el gráfico de constante de equilibrio versus temperatura de ebullición. Ambos gráficos son esencialmente similares ya que en el de Hoffman se grafica constante de equilibrio por presión versus una función especial que incluye las inversas de la temperatura y la temperatura de ebullición.
PVT ANALYSIS
En estos gráficos los puntos generados por los distintos componente deben presentar una tendencia rectilínea. La desviación de este comportamiento indica alguna anomalía cuya importancia, interpretación y corrección depende, entre otras cosas, del criterio del analista.
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CHEQUEO GENERAL DE PARAMETROS
Una de las partes más importante de cualquier chequeo de consistencia es la comparación de los parámetros globales del ensayo PVT con valores esperados a partir de correlaciones, ecuaciones semi-empíricas y base de datos locales. Este chequeo incluye una mezcla de criterio y experiencia del analista y da como resultado una validación parcial o total de los resultados obtenidos en el laboratorio. Es la parte en la cual el analista experimentado debe emitir juicios individuales acerca de los parámetros reportados. Debido a que este chequeo varía en gran medida dependiendo la información disponible, es imposible determinar a priori reglas generales de validacion.
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SIMULACION TERMODINAMICA
El objetivo de esta simulación es el de ajustar parámetros específicos del modelo de ecuaciones termodinámicas para así poder reproducir los resultados del ensayo PVT. Es importante remarcar que mediante este proceso de ajuste se puede corroborar la coherencia de los datos en su conjunto.
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PREPARACION ENTRADAS DE AMPLIO ESPECTRO PARA UN SIMULADOR NUMERICO
Una vez ajustado un modelo termodinámico del fluido de reservorio con datos apropiadamente validados se puede emplear el modelo para generar una variedad de condiciones operacionales presentes y futuras, sin necesidad de tener que ensayar en laboratorio cada una de estas situaciones. Esta herramienta permite evaluar condiciones tales como el inicio de la inyección de un fluido (agua, gas) posterior al comienzo de la depletación, existencia de viscosidades de superficie diferentes en cada pozo, cambio de condiciones óptimas de separación a lo largo de la historia del yacimiento, reconstrucción de historia de PVT, etc.
PROPERTIES OF FLUID RESERVOIRS – PVT TESTS
FLASH VAPORIZATION. A sample of the reservoir liquid is placed in a laboratory cell. Pressure is adjusted to a value equal to or greater than initial reservoir pressure. Temperature is set at reservoir temperature. Pressure is reduced by increasing the volume in increments.
This procedure is called FLASH VAPORIZATION, there is a graphic that represents the process of this porcedure where the plot reporduces part of an isotherm of a pressure-volume diagram. The shape is similar to that shown in below.
PRESSURE VS VOLUME
The pressure at which the slope changes is the bubble point pressure of the mixture. The volume at this point is the volume of the bubble -point liquid. Often it is given the symbol Vsat. The volume of the bubble point
FLASH VAPORIZATION
liquid can be divided by the mass of reservoir fluid in the cell to obtain a value of specific volume at the bubble point. Specific volume at the bubble point also is meaasured during other tests and is used as a check on the quality of the data.
All values of total volume, Vt, are divided by volume at the bubble point, and the data or information is reported as relative volume. Sometimes the symbol V/Vsat is used; however, we will use the sysmbol (Vt/Vb)f. The sysmbol (Vt/Vb)f means total volume divided by volume at the bubble point for a flash vaporization.
DIFFERENTIAL VAPORIZATION:
The sample of reservoir liquid in the laboratory cell is brought to bubble-point pressure, and the temperature is set at reservoir temperature. Pressure is reduced by increasing cell volume, and the cell is agitated to ensure equilibrium between the gas and liquid. Then, all the gas is expelled from the cell while pressure in the cell is held constant by reducing cell volume.
DIFFERENTIAL VAPORIZATION PROCEDURE
The gas is collected, and its quantity and specific gravity are measured. The volume of liquid remaining in the cell, V0, is measured. The process is repeated in steps until atmospheric pressure is reached. Then temperature is reduced to 60·F, and the volume of remaining liquid is measured. This is called residual oil from differential vaporization or residual oil.
Each of the values of volume of cell liquid, Vo, is divided by the volume of the residual oil. The result is called relative oil volume and is given the symbol BoD. In addition, the volume of gas removed during each step is measured both at cell conditions and at standard conditions.
The total volume of gas removed during the entire process is the amount of gas in solution at the bubble point. This total volume is divided by the volume of residual oil, and the untis are converted to standard cubic feet per barrrel of residual oil. The symbol RsDb represents atandard cubic feet of gas removed per barrel of residual oil. The gas remaining in solution at any lower pressure is calculated by subtracting the sum of the gas removed down to and including the pressure of interest from the total volume of gas removed. The result is divided by the volume of residual oil, converted to SCF/residual bbl, and reported as RsD.
SEPARATOR TESTS:
A sample of reservoir liquid is placed in the laboratory cell and brougt to reservoir temperature and bubble point pressure. Then the liquid is expelled from the cell through two stages of separation.The vessel representing the stock tank is a stage of separation
SEPARATOR TESTS
if it has lower pressure than the separator. Pressure in the cells held constant at the bubble point by reducing cell volume as the liquid is expelled.The temperatures of the laboratory separator and stock tank usually are set to represent average in the field. The stock tank is always at atmospheric pressure. The pressure in the separator is selected by the operator. The specific gravities of the separator gas and stock tank gas are measured. Often the composition of the separator gas is determined. Finally, a separator volume factor is calculated. It is the volume of separator liquid measured at separator conditions divided by the volume of stock-tank oil at standrad conditions, SP BBl/ STB.
SAMPLING FOR ANALYSIS OF OIL PVT WELLHEAD
Prior to sampling, we conducted a study and implementation scheme by Don Lucho who also guided us in using the tools and equipment should be used as we see:
Our goal was to obtain, as appropriate and taking into account the care of the case, a representative sample of crude store it in a pressure bottle of 400 cc for onward shipment and analysis at the Laboratory of the Faculty PVT (UNI-Lima ) which then could be obtained viscosity, compressibility, strength factor (Bo), oil density, solubility, bubble point, etc.
PVT CELL
It is designed to withstand high pressure and temperature changes (10000 psi and 350 ° F), the cell has a glass window with an opening of 4.4 cm in length and 0.64 cm wide, to observe the gas-oil interfaces mercury. The window glass has a thickness of about 2.54 cm and is housed in the main body settled with stamps and adjusted by internal screws.
The inlet valve to the cell is in the top together with a tube of 0.32cm diameter stainless steel that takes the form of a solenoid connection before entering the cell.
The cell is equipped with two arms to hold in your bathroom and can rotate fully without friction. The valve of the cell is needle that allows the appropriate section to control the formation of foam and come from the removal of any part of the liquid phase with gas in solution. The location of an exit visible on the top of the cell allows the rapid removal of the gas leaving a very small fraction of a cubic centimeter of gas without the danger of the gas-liquid equilibrium is altered by the output of liquid the cell.
The temperature control is done through a thermostat located in the control panel and using an oil bath.
Preparation of Cell PVT
If the cell is with mercury or other fluid is drained into a container that can hold a volume greater than 600 cm3, as is the ability of the cell. Then proceed to clean with gasoline or other fluids non-corrosive solvent. You should not release (remove) valves for cleaning. The solvent is injected or sucked air pushing to empty. Strongly stirred cell. It is enough two or three washings with 100-200 cc of solvent. It drains all
solvent and dry circulating air.
TYPES OF SEALS PVT CELL
Cell elastomer seals PVT
One of the most widely used seals and stamps are modern type “ring.” If a label like “ring” was used in a glass window, would experience little difficulty. Accommodation such a bad seal in the cell would result due to temperature change, a change in the reading volume in a quarter of a cubic centimeter or so.
Signet ring cell square section of the PVT
An improvement of the seal type “ring” seal ring is square. This acts similarly to the label “ring”, but because of their difference cross section, reduces the error caused by the accommodation of signet ring. ” This seal consists of two retaining steel square section placed in side by side and sized so that the space between is occupied by the retainer ring seal square.
Seal compound PVT cell
A composite seal is satisfactory compared with previous ones. Is a label consisting of a thin steel plate in a square interior which has been inserted a vulcanized elastomer resistant to chemical attack with trapezoidal section. Has properties of both seal against the pressure inside the cell, but not necessarily sealant if the cell is evacuated. The steel should be cleaned with a stream of wood that the elastomer can be cured in the settlement of the window.
These brands are safe, provided that they do not extend into the sealed area of the elastomer insert. The seal compound requires a torque of 5 lb-ft for external adjustment screws and 15 lb-ft internal screw adjusters. Any sealing method which is used in synthetic rubber or elastomer resistant to chemical attack, are always subject to attack over time. Your life depends mainly on the chemical composition of fluids in the analysis, pressure and operating temperature.
Phenolic resin seal the PVT cell
Is more durable and better seal the constant volume is obtained by a board of canvas reinforced phenolic resin, which is manufactured under the trade names Bakelite, Formica, TEXOLINE, Micarta, etc. This seal was used in cells built in the last twenty years. The seal consists of a board of 0.159 inch thick set with precision in the settlement of the window of the cell. Should take the greatest care in carving and this stamp set, so that its polished surface is not damaged, for example scratches, cracks, or bent.
The high temperature and pressure mounting to which the board is subject will not produce a significant effect on its compressibility and deformation. Its resin sealed most of the small marks, scratches on the hole in the window and onto the glass.
The material used for these meetings should be carefully selected. Can be used 2 sheets of Teflon of 50 microns (0.002 inch) thick, which are placed on each side of the resin. When the placement of the phenolic resin or phenolic resin, Teflon is used, “a torque of 10 lb-ft would be used for the adjustment screws and 50 Lb-ft screw adjusters.”
CONSTANT TEMPERATURE BATH
Rusk bath for visual cell has an internal diameter of 45.72 cm. The bathroom has a capacity of 18 gallons and a maximum current consumption of 3700 watts. In addition, a product that contains the heating elements and agitator, is equipped with an auxiliary camera. A wall-mounted pump is used to raise or lower the oil level in the bath and exposing the cell window completely in the oil when the equilibrium temperature is reached.
The bathroom consists of a controlled temperature oil circulating system. The bathroom should not be operated without oil circulation to prevent overheating of the heater elements. For this reason, the main switch is connected to the motor and agitator. The use of a switch could triple the warming unit inoperable, but the mixer will work as soon as the switch is on.
It is essential that the facility has a flat base to rest on wood for easy cleaning of the bottom, at its base there are 4 holes in each corner requiring an area of 61 x 61 cm.
The bathroom must be connected to a power line in compliance with the relevant specifications on the control panel and grounding connections are made on the main switch. When the cell is immersed in the bath is ready for testing.
Operation of the bathroom: The bathroom is filled with commercial oil very resistant to temperature (SAE 30) with the main switch is on, so the motor of the agitator went into operation and control of the heat off. Warming can now be operated. To get a quick bath temperature, triple switch is set to HIGH (high), and the thermostat is on, so it comes into operation the controller of heat. HIGH is used solely to achieve the desired temperature.
CONTROL PANEL CELL PVT
The control panel is responsible for certain cell functions such as shaking the cell, raise or lower the oil level, temperature control.
a) PILOT LIGHT.
Indicates the thermostat operation.
b) THERMOSTAT.
Controls the heat input from the heating system.
c) OIL LEVEL.
It’s a three position switch, with the top to raise the middle is neutral, lower is to lower it.
d) AGITATOR.
Drives the agitator that allows the circulation of hot oil within the cell.
e) THERMOSTAT.
Graduated scale to set the desired temperature, each division is 1 ° F.
f) TRIPLE SWITCH.
Provides heating rate to the system giving warmth and has 3 positions: high, low and medium.
g) MAIN SWITCH.
Provides the necessary energy and drives the agitator at a time.
PROPIEDADES DE LOS FLUIDOS DEL YACIMIENTO
- Tipos de fluidos del yacimiento (reservorio)
- Petróleos negros (black oils): diagrama de fase, identificación de campo, análisis de laboratorio
- Petróleos volátiles (volatile oils): diagrama de fase, identificación de campo, análisis de laboratorio
- Gases retrógrados (retrograde gases) : diagrama de fase, identificación de campo, análisis de laboratorio
- Gases húmedos (wet gases): diagrama de fase, identificación de campo, análisis de laboratorio
- Gases secos (dry gases): diagrama de fase, identificación de campo, análisis de laboratorio
- Propiedades de gases secos: condiciones estándar, factor volumen de formación del gas, coeficiente de compresibilidad isotermal de gases reales e ideales, compresibilidad pseudoreducida, coeficiente de viscosidad del gas, viscosidad de gases hidrocarburos puros, viscosidad de mezclas de gases viscosidad de un gas a alta presión, valor calorífico, efecto Jule-thompson
- Propiedades de gases húmedos: Factor volumen de formación de un gas húmedo, composición
- Propiedades de black oils (petróleos negros): Factor de volumen de formación del petróleo, radio de solución gas-petróleo, coeficiente de compresibilidad isotermal del petróleo, presión de burbuja, coeficiente de viscosidad del petróleo, coeficiente de expansión isobárica termal de un líquido, tensión interfacial, petróleos volátiles
- Propiedades de petróleos negros (black oils) – Datos de campo (field data): Yacimientos (reservorios) de petroleo negro – presión inicial del yacimiento (reservorio), tendencias de producción de gas, tendencias de presión, propiedades de los fluidos de producción – historia de presión, ajuste de los datos de gas en superficie, stock-tank correlación del radio gas-petróleo
- Estudio de los fluidos de yacimiento (reservorio): recolección de muestras de petróleo del yacimiento, procedimiento de vaporización flash en laboratorio, vaporización diferencial, ensayos del separador
- Selección de la condiciones del separador, relación gas-petróleo, etc
- Correlaciones: Presión punto burbuja, densidad de un líquido, etc
