Kick Tolerance Misconceptions and Consequences to Well Design

Abstract

Kick tolerance is a key and fundamental concept used in well design. It defines the appropriate number and setting depth of casing strings that are required to achieve the drilling objectives. It is also used during drilling to determine whether it is safe to continue drilling or if there is a need to run a casing string. Alternatively, it is used to indicate whether it is safe to circulate a kick out of the well or whether bullheading is necessary. During development of a new well control system, a thorough review of the fundamental concepts involved was carried out, and, in relation to kick tolerance, a few misconceptions were identified. Among those misconceptions are issues related to the calculation of the kick volume on bottom when a BHA is present, the effects of friction losses on the wellbore annulus and choke line, and confusion between kick tolerance and maximum allowable pit gain, derived from the additional formation flow into the wellbore after the well is shut in (afterflow). Another important misconception is the assumption that an approach utilizing a single bubble model and ignoring effects of temperature, influx density and gas compressibility (z) factor in the final calculation will always result in a conservative solution. This paper utilizes four example wells to demonstrate the effect of each component on the final kick tolerance calculation. In order to make it easier to understand, each effect is calculated individually using the same single bubble model that has been historically used. The results show that some components have more impact than others, and the consequence is not always in the same direction. Additionally, the effects on deepwater wells are more dramatic due to the long choke line. Therefore, generalization and simplification of the kick tolerance calculation in well design is not possible.

Advancing Deepwater Kick Detection

“…Although the industry is working to develop systems which take a holistic approach, very few systems currently offer an automated choke which can properly respond when an influx/loss is detected because how to respond is not a trivial question. For example, work has been done to better estimate kick tolerance (Santos et al 2011) which highlights how the single bubble, homogenous wellbore model fails to take important variables into account. Without all wellbore and fluid effects accounted for, flow and pressure data can only be said to be tightly correlated.…”

Distributed Fiber Optic Sensing for Real-Time Monitoring of Gas in Riser during Offshore Drilling

“…It occurs when the pressure in the wellbore is lower than that of the formation fluids, thus causing flow. Factors such as inadequate equivalent circulating density (ECD), higher than expected formation pressures, mud losses during drilling, and swabbing can all trigger a gas kick [1]. Usually, the influx is removed by circulating the well by means of an adjustable surface choke [2].…”