CARACTERISTICAS DE LOS PETROLEOS VOLATILES
CARACTERISTICAS DE LOS PETROLEOS VOLATILES. La forma “clásica” de diferenciar Petróleos Negros y Volátiles se basa en valores límite de Relación Gas-Petróleo o de Factores de Volumen de Petróleo. Diferentes autores coinciden en asignar los siguientes límites:
VALORES DE GOR Y Bo
Los líquidos con valores inferiores a los indicados se consideran Petróleos Negros, en tanto que los que superan estos límites se catalogan como Petróleos Volátiles. Moses, empleando un criterio consistente y haciendo notar que todos los petróleos son volátiles en mayor o menor medida, prefiere emplear los términos Petróleos Comunes (“Ordinary Oils“) y Petróleos Cuasi-Críticos (“Near-Critical Oils“) para hacer referencia a ambas clases de fluidos.
Quizás la manera más simple de señalar las complejidades que caracterizan el comportamiento de los Petróleos Volátiles es la de comparar la aplicabilidad de algunos parámetros clásicos en la evaluación de reservorios.
En los Petróleos Negros el Factor de Volumen (Bo) es un dato de importancia primariapara la evaluación del sistema. El Bo establece la relación entre el volumen de petróleo extraído, en condiciones de reservorio
DIAGRAMA DE FASE PETROLEO NEGRO
y el volumen de petróleo obtenido en condiciones de tanque. El Bo (diferencial, flash o compuesto) es un valor relativamente fácil de trasladar desde la medición de Laboratorio a la escala de Reservorio.
En los sistemas de Gas y Condensado el Bo es un dato carente de significado físico pues, en condiciones normales, ni un pequeño porcentaje del líquido de tanque proviene de líquido presente en el reservorio. En los sistemas de Gas y Condensado cobra interés una propiedad diferente: La Producción Acumulada. Esta última expresa la fracción (en moles o su equivalente en Volumen STD) ya producida en cada etapa de la depletación. Nuevamente se trata de una propiedad fácilmente medible en el Laboratorio y directamente escalable al Reservorio.
De modo que en resumen, el Bo es una propiedad fundamental para caracterizar Petróleos Negros, pero carece de significado para los sistemas de Gas y Condensado. La Producción Acumulada describe
DIAGRAMA DE FASE DE PETROLEO VOLATIL
el comportamiento de los sistemas de Gas y Condensado, pero carece de aplicación en el caso de los Petróleos Negros (los moles y volúmenes de gas y de petróleo son lo suficientemente diferentes como para que carezca de sentido hablar de los moles producidos en cada etapa de la liberación diferencial). Bien, los Petróleos Volátiles están a mitad de camino entre los Petróleos Negros y los sistemas de Gas y Condensado. Y esto se traduce en que ni el Bo ni la Producción Acumulada describen adecuadamente sus propiedades. La razón es simple:
- Una fracción importante del líquido de Tanque proviene de la condensación de componentes presentes en el Gas libre que acompaña la producción de líquido.
- Una fracción apreciable de los moles presentes en el líquido, al comienzo de la explotación pasan a la fase Gas durante la depletación. Y una vez en fase gaseosa estos componentes pueden producirse como Gas y acumularse como Líquido gracias a la condensación en condiciones de superficie.
De este modo, la correcta descripción de la evolución de un sistema de Petróleo Volátil implica una adecuada evaluación de las curvas de Permeabilidad relativa del sistema, dado que a una misma presión de reservorio pueden corresponder producciones de líquido (y gas) muy diferentes, en función de la proporción entre gas y petróleo que alcanza los pozos productores.
TYPES OF GAS COMPRESSION
GAS COMPRESSION. There are many cases in gas production operations in which the pressure of a gas must be raised to a higher value. As the pressure in a gas reservoir depletes, it will eventually reach a point where it will no longer overcome all the pressure losses in the system and the pressure of the line into which the gas in being delivered. It is then necessary to add a compressor to the system to supplement the reservoir energy.
TYPES OF GAS COMPRESSOR
In this type of application, the suction or intake pressure, and possibly the volume compressed, will change with time even though the discharge pressure may remain constant. Compressors have been used to lower the wellhead pressure below atmospheric, that is, to pull a vacuum on the well in order to obtain maximum rates. Compressors are also used to overcome the losses incurred in the long distance transportation of natural gas through transmission lines. This may require large capacity machines operating at essentially constant conditions.
The reinjection of gas for pressure maintenance or cycling requires compression of produced gas to a high pressure to move sufficient volumes into the reservoir. A compressor capable of operating under a wide range of conditions.
The engineer is concerned with essentially two types of compressor design problems:
- Determination of the power required to compress a certain volume of gas from some given intake pressure to a given discharge pressure.
- Estimation of the capacity of an existing compressor under required pressure increase conditions. It is also frequently necessary to calculate the temperature increase occurring in the gas as it is compressed.
TYPES OF COMPRESSORS
The principal types of compressors are positive-displacement, or intermittent flow, units and continuous flow units.
- Positive-displacement units are those in which successive volumes of gas are confined within a closed space and elevated to a higher pressure.
- Continuous flow units are those in which a rapidly rotating element accelerates the gas as it passes through the element, converting the velocity head into pressure, partially in the rotating element and partially in stationary diffuser or blades.
Reciprocating compressors are positive-displacement machines in which the compressing and displacing element is a piston having a reciprocating motion within a cylinder.
Rotary positive-displacement compressors are machines in which compression and displacement are effected by the positive action of rotating elements.
Sliding-vane compressorsare rotary positive-displacement machines in which axial vanes radially in a rotor eccentrically mounted in a cylindrical casing. Gas trapped between vanes is compressed and displaced.
RECIPROCATING GAS COMPRESSOR
Liquid-piston compressors are rotary positive-displacement machines in which water or other liquid is used as the piston to compress and displace the gas-lobs handled.
Two-impeller straight-lobe compressors are rotary positive-displacement machines in which two straight mating lobed impellers trap gas and carry it from intake to discharge. There is no internal compression.
Helical-or spiral-lobe compressors are rotary positive-displacement machines in which two intermeshing rotors, each with a helical form. Compress and displace the gas.
Centrifugal Compressors are dynamic machines in which one or more rotating impellers, usually shrouded on the sides, accelerate the gas. Main gas flow is radial
Axial compressors are dynamic machines in which gas acceleration is obtained by the action of the bladed rotor shrouded on the blade ends. Main gas flow is axial.
Mixed-flow compressors are dynamic machines with an impeller form combining some characteristics of both the centrifugal and axial types.
Ejector are devices that use a high velocity gas or steam jet to entrain the inflowing gas, then convert the velocity of the mixture to pressure in a diffuser.
Every compressor is made up of one or more basic elements. A single element, or a group of elements in parallel, comprises a single-stage compressor. Many compression problems involve conditions beyond the practical capability of a single compression stage.
STRIPPING – A METHOD OF CONTROLLING WELLS
Stripping is moving pipe into or out of the well against well pressure when the force of that pressure is less than the weight of pipe being stripped. Remember additional influx and/or excessive pressures can occur if pressure is not monitored and corrected for the displacement of the pipe being stripped and gas expansion.
Exercise care when stripping. If necessary pipe weight (tripping in or out under pressure) is not maintained, pipe can be blown from well. Stripping complications can occur due to some preventers being wellbore pressure assisted to various degrees. Also, the wear factor on sealing elements may lead to element failure and pressure venting to rig floor.
STRIPPING
If preventer develops a leak, this may lead to rapid failure of a sealing element and/or preventer and may jeopardize the operation. There is also the possibility of the wrong preventer being opened if speed exceeds caution. All stripping operations should be performed carefully, with all personnel briefed and familiar with their responsibilities.
Stripping policies and procedures vary. Depending on pressure, pipe, collars and tool joints may not strip down of their own weight, but require a pull-down (snub) force. String weight must be greater than computed force or pull down force (snubbing) will be required. The equation shows why it might be necessary to start pipe with a few stands of ram to ram stripping rather than with the annular preventer. When using ram to ram stripping, tool joint is never in the preventer so the termD is smaller.
Travelling blocks have been used to push pipe down. This is dangerous because pipe might slip back up and start to unload out of the hole. Be careful about the beginning of stripping operations. If pipe is not heavy enough to go into the hole against the well pressure, it needs to be kept under restraint at all times while stripping, until it is heavy enough to overcome upward forces.
When stripping in or out of the hole it is necessary to have a float or inside BOP in the string. Also, a safety valve should be on the open box as a joint or stand is pulled/lowered. Two safety valves may be used. One is on the string and another is either taken off the last joint pulled or made up on the next to be run. These valves must be in place in the event the float or inside BOP fails, so the string can be shut in. Safety valves should be left open so pipe will not pressurize without warning Stripping operations require excellent communications between the choke operator and driller. As tool joint nears the floor, driller must inform choke operator that he will be slowing and stopping pipe. Choke operator must dictate the overall rate of pipe movement, as it will be his responsibility to maintain pressures as close as possible according to calculations.
Some operators close off the accumulator bank and strip using accumulator pumps for pressure. This is a bad technique as pumps are used too erratically. A better procedure would be to close off one-half of the bank and keep it for reserve or to turn off either electric or air pumps and keep one type of pump for reserve.
TECNICAS UTILIZADAS PARA LA CEMENTACION DE POZOS
TECNICAS UTILIZADAS PARA LA CEMENTACION DE POZOS
BRADENHEAD METHOD:
Fue el primer método utilizado y que se cumple a través del tubing o barra sin el uso del packer. La presión obtenida cerrando el BOP o válvula de control después que el cemento ha sido bombeada cerca de la zona a tratar. Una determinada cantidad de lechada se bombea a una cierta altura por afuera del tubing o barras. Los tubings son luego levantados fuera de la lechada y se cierran las válvulas de superficie.
STAGES OF CEMENTING
A continuación se bombea el fluido de desplazamiento por los tubings hasta que se consigue la presión que se desea o hasta una cantidad determinada de fluido haya sido bombeado. Este método se usa extensamente en pozos poco profundos, en obturaciones y zonas de perdida de circulación durante la perforación.
La aplicación del método está restringida porque hay que presurizar el casing y por lo tanto las máximas presiones están limitadas por las especificaciones de resistencia del mismo.
Además es difícil ubicar el cemento exactamente en el lugar deseado sin usar un packer.
METODO UTILIZANDO PACKER:
El método usa packer o tapones recuperables o no, ubicados en una posición cercana a la parte superior de la zona a ser intervenida. Esta técnica se considera superior a la anterior ya que confina la presión a puntos específicos.
Antes de ubicar el cemento se hace una prueba de presión para determinar la presión de rotura, en algunos casos la sección inferior de la zona a ser tratada puede ser aislada con un tapón puente. Cuando la presión deseada se ha obtenido la lechada remanente debe ser reversada. Los objetivos y las condiciones de las zonas son las que gobernara la elección del método a ALTA PRESION O BAJA PRESION.
REQUERIMIENTOS DE UNA CEMENTACION A PRESION:
Muchos trabajos se definen por la presión requerida para obtener un sello “SELLO HIDRAULICO”. La técnica de ALTA PRESION utiliza agua salada para determinar la presión de rotura de la formación. No se debe usar para esta finalidad la inyección porque puede tapar o dañar la formación. Después de la rotura la lechada de cemento es colocada cerca de la formación y bombeada a bajos caudales; a medida que el bombeo continua la presión de inyección comienza a subir hasta que la presión de superficie indica que se ha producido ya sea deshidratación del cemento o penetración de la lechada. La presión es momentáneamente mantenida para verificar las condiciones estáticas y luego es liberada para determinar si el cemento se mantiene en el lugar. El exceso de lechada por afuera de las perforaciones debe ser reversado.
SQUEZZE CEMENTING-CASED HOLE
Si la presión deseada se obtiene se aplica el METODO DE HESITACION O NUEVAS ETAPAS. Ello involucra a preparar “BATCH” de 30 a 100 sacos, colocarlos frente a la formación, esperar hasta que este cercano al punto de fragüe y repetir la operación tantas veces como sea necesario.
LAS TECNICAS DE BAJA PRESION están consideradas como el método más eficiente en base al desarrollo de los aditivos de control de filtración y de packer recuperable. Con esta técnica se evita la fractura. Se usa Hesitación, el cemento es colocado en una sola etapa pero con periodos alternados de bombeo y espera. El control de las propiedades de perdida de filtrado determina que se forme un revoque contra la formación o dentro de las perforaciones, mientras que el cemento remanente permanece en estado fluido en el interior del casing.
La pérdida por filtración de las lechadas de cemento puro es muy elevada y se produce una deshidratación en el casing antes que la lechada haya cubierto completamente un área de la formación.
HESITATION SQUEZZE
El resultado puede ser un tapón de cemento a través de las perforaciones abiertas en la zona superior y no cubre con cemento las perforaciones inferiores.
El control de filtrado evita la perdida de fluido prematuro como la rápida formación de un bloque solido en el casing. Los cementos que contienen aditivos para perdida por filtrado pierden agua frente a la formación mucho más lentamente que con un cemento puro.
SQUEEZE CEMENTING PROCEDURE
Remedial, or secondary, cementing is performed to exclude water or gas from a well, improve primary cementing job, recomplete in a new zone or repair corroded/damaged casing. Good primary cement jobs eliminate problems when drilling, completing and producing a well. If primary cement job is inadequate, and cement bond log (CBL) did not show deficiency, a great deal of money may be spent trying to repair it by squeeze cementing.
Squeeze cementing displaces cement to a desired point; it is controlled by a packer(s) or a permanent packer already in casing is used as a squeeze tool. Once at the desired depth, cement is circulated down to the squeeze point. The tool is set to isolate/protect casing from high pressure. Cement is pumped into area to be sealed off. Hydraulic pressure is applied, squeezing cement slurry against the formation. This may be done in open hole or through perforations in casing or liner. Excess cement can be reversed out of well or drilled out at a later date.
Jobs are successful if cement is left in casing opposite perforations or damaged area, and not drilled out after squeeze operation.
HIGH PRESSURE SQUEZZE AND CEMENT OPERATIONS MAY REQUIRE ADDITIONAL PUMPS
So plug back jobs have been the most successful. There have been poor results with block squeezes to shut off water, especially in gas wells where at common depths fractures are vertical, not horizontal, (cement layers radiating from wellbore in a circle) as was once thought.
Vertical fractures have vertical wings which make it difficult when trying to shut off water. In almost all cement squeezes, cement goes up the hole between the formation and casing. Once annular channel is shut off, producing zone can be squeezed. Whole cement does not enter formation pores, but rather the water in the cement. Water is forced into the formation under pressure, leaving cement to plate out across formation face. The water loss, coupled with a chemical reaction, sets up, or hardens, cement. If enough pressure is applied to fracture the formation, cement could enter fracture.
Important prerequisites for a good squeeze cement job are clean perforations, channels and a cement slurry designed to meet the downhole conditions and type of squeeze to be performed. Minimal blockage and clean surfaces assure a better and more thorough bond; sometimes an acid job may be used to ensure this. A wide selection of oil well cement is used for squeeze cementing, varying from heavy to light slurries. Additives adjust water/cement ratios, viscosity, set strength, pumpability time, temperature tolerance and other factors. There are many methods of applying cement under pressure. Terms used in squeeze cementing are:
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Bradenhead Squeeze – There is no packer in the hole. Casing valves are closed and the well is pressured up on the casing and the workstring during the operation.
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Bullhead Squeeze – The packer is set when the job starts and all fluid in the workstring is pumped into the formation ahead of the cement. The casing may be pressured, if necessary, to reduce the differential pressure across the packer.
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Hesitation Squeeze – Cement is pumped out through the casing perforations and into the annular area between the casing and formation. Then the pumps are stopped for a few minutes. Pumping is started and stopped until the desired pressure is obtained.
- HESITATION SQUEZZE
