Kick-Tolerance Misconceptions and Consequences for Well Design

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 140113, ’Kick-Tolerance Misconceptions and Consequences for Well Design,’ by Helio Santos, SPE, Erdem Catak, SPE, and Sandeep Valluri, Safekick, prepared for the 2011 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 1-3 March. The paper has not been peer reviewed. Kick tolerance defines the appropriate number and setting depths of casing strings required to achieve drilling objectives. It also is 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. Introduction Even though kick tolerance is a critical and fundamental concept for the drilling industry, there is no standard used by operators, drilling contractors, or training institutions. Hence, there are several definitions of kick tolerance and different ways of calculating it. This lack of consistency may be why the subject is not well understood and, there-fore, is sometimes used dangerously. Definitions of kick tolerance may be in terms of pit gain, mud-weight increase, or underbalance pressure. Another point of disagreement is on how the predicted pore pressure should be used in calculations. Some companies use a value greater than the mud weight, while others use a value greater than the predicted pore pressure. Despite the variations, the goal is consistent: to use a procedure that ensures safe drilling of a well. Often, this lack of a standard and of under-standing the topic leads to uncertainty and discussions during drilling. Questions often arise regarding whether it is safe to continue drilling. Because wells are now drilled in more-challenging environments, such as high-pressure/high-temperature and deep and ultradeep water, a small variation in the way that kick tolerance is calculated can lead to premature abandonment of the well or, worse, to a hazardous drilling situation. Kick-Tolerance Calculation—Current Approach The first step of a simplified kick-tolerance calculation (i.e., constant temperature, constant density, and no compressibility) is to define the maximum vertical height of a gas influx Hmax at the casing shoe (assumed to be the weakest point in the open hole). Hmax is deter-mined on the basis of fracture gradient; mud weight; kick-fluid density; predicted pore pressure; and adjusted maximum allowable annular surface pressure (MAASP), which is reduced by a safety margin. What is conceptually wrong is that if the bottomhole-assembly (BHA) length is greater than Hmax, the kick cannot be circulated out of the wellbore because it will reach the top of the drill collars with a kick height greater than Hmax, which would induce losses at the shoe.