Kill Rate and Kill Rate Pressure:

Most well-control situations require preventing the formation fluid from flowing and circulating out the kick fluid from the well. While killing a well, pump speed plays a critical role. Pump pressure depends on this speed and a slight change in speed may affect circulating pressure.

The fluid circulation rate used for killing a well is called ‘Kill-rate'. Kill rate, also known as 'Slow Pump Rate' (SPR) or 'Slow Circulating Rate' (SCR) is a predetermined fluid circulating rate, expressed in fluid volume per unit time, planned to be used to circulate under kick conditions. The kill rate is usually some selected fraction of the normal circulating rate used while drilling.

‘Kill rate pressure’ (KRP) is the pressure required to overcome the friction in the circulating system at a given pup rate. Since the fluid properties and other well parameters can affect kill rate pressures, it is important that kill rates and kill rate pressures are taken regularly.

The kill rate pressures ideally should be recorded by circulating through the choke manifold to reflect the actual pressures while killing a well. The pressures recorded by circulation through the bell nipple with an open BOP may not provide an accurate representation. Although choke friction pressure is usually negligible in surface stacks, it should be considered in critical wells where narrow pressure margins are expected. Accurate gauges on the drilling console and the choke manifold should be used to record kill rate pressure.

It is important to document kill rate pressures under the following circumstances:

·        Changes in fluid density or flow properties

·        Modifications to bit nozzle or BHA configuration

·        After drilling 500 feet or more of the new hole

·        During each tour

·        Following any pump repairs.

There are different methods for recording kill rate pressure. In typical scenarios, the pressures are taken approximately at one-sixth to one-half of the normal circulating rate while drilling. Alternatively, the idle speed of the pump can be used, progressing up from there. Some drilling rigs use specific stroke rates like 20, 30, and 40 strokes per minute. Some operators require the driller to determine the circulating rate at specific pressures such as 200, 300, 400 psi, etc.

The benefits of pumping at slow rates include:

·        It allows for a smooth increase in mud weight when pumping at slow rates

·        more time for choke reaction

·        Lower likelihood of exceeding pump pressure rating

·        Reduced likelihood of high-pressure surges

·        decreased chance of formation fracture and hence subsequent complications in a well control situation

·        Lower likelihood of differential sticking

·        Slower kill rate reduces in high-pressure fluctuations when gas reaches the surface. This helps maintain the pressure schedule due to more choke reaction time than at high pump rates.