Este coeficiente puede calcularse a partir de situaciones reales de perforación para una geometría de pozo específica, utilizando programas informáticos que emplean las cargas de superficie de la sarta como datos de entrada. Estos datos incluyen:
Peso de recogida (Pickup weight).
Peso de bajada (Slack-off weight).
Lecturas de torque.
Cada uno de estos valores puede generar un coeficiente de fricción independiente. Cuando los tres coeficientes coinciden en un mismo pozo, se le otorga credibilidad al modelo y se genera confianza para predecir el torque y el arrastre en pozos posteriores.
Cálculo del Coeficiente de Fricción durante la salida del pozo (Tripping Out)
1. Mediante el Arrastre (Drag) de la Sarta
La carga de recogida (Pickup load) se lee directamente del indicador de peso. Esta lectura incluye el peso del bloque viajero, el gancho y el kelly (si se usa). Para calcular las fuerzas de arrastre, se requiere conocer la tensión en el tope de la tubería de perforación (por debajo del equipo de superficie). Por lo tanto, es necesario restar el peso del equipo viajero de la lectura del indicador de peso.
Procedimiento:
Se ingresan en el programa informático la tensión en el tope de la tubería, los datos de la sarta y la geometría del pozo.
El programa calcula la tensión necesaria para levantar la tubería utilizando diferentes coeficientes de fricción hasta que la tensión calculada coincida con la medida por el indicador.
El coeficiente con el que coinciden ambos valores es el coeficiente correcto para ese pozo.
Nota: Para resultados precisos, el indicador de peso debe estar bien calibrado y se debe conocer con exactitud el peso del equipo viajero.
2. Mediante el Torque de la Sarta
El coeficiente de fricción también puede determinarse midiendo el torque mientras se rota la sarta fuera del fondo (off-bottom).
El torque debe medirse en pies-libras (ft-lb) y no en amperios.
Al igual que con el arrastre, el coeficiente donde el torque medido es igual al calculado por el modelo es el correcto.
Recomendación: Dado que muchos taladros no tienen indicadores de torque calibrados con precisión, se recomienda priorizar la medición mediante la tensión (peso).
Factores que afectan el Coeficiente de Fricción
Tipo de lodo.
Condición del pozo: Si el agujero es abierto (open hole) o está revestido (cased hole).
Rangos típicos de Coeficientes de Fricción:
El "Coeficiente de Arrastre" (Drag Coefficient)
Es importante notar que el Torque y el Arrastre no surgen solo de la fricción pura, sino también de:
Tortuosidad del pozo (micro-desviaciones).
Acumulación de recortes (cuttings).
Hinchamiento de arcillas (swelling shale).
Pegadura por presión diferencial (differential sticking).
Otros impedimentos mecánicos.
Por ello, en los programas de simulación, el coeficiente de fricción se considera más bien un "Coeficiente de Arrastre" (Drag Coefficient). Este es un valor compuesto que incluye todos los factores que afectan el movimiento.
Además, este coeficiente suele ser diferente para el movimiento rotacional, el movimiento axial o una combinación de ambos, y puede variar según la dirección del movimiento (subiendo o bajando) y las condiciones cambiantes del pozo.
Tabla de Contenidos
- Coeficiente de fricción Definición
- Cálculo del coeficiente de fricción de la sarta de perforación durante el disparo fuera del agujero:
- Factores que afectan el coeficiente de fricción de la cadena de perforación
- Rangos típicos de coeficientes de fricción de cadena de perforación:
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