Large-Scale Experiments Show Proper Hole Conditioning: A Critical Requirement for Successful Cementing Operations

Abstract

Abstract Proper hole conditioning means to establish a hole that is free of cuttings, debris, etc., and that has a mud in a fully "displaceable" or "circulatable" condition to allow the spacer and cement slurries to effectively displace the mud out of the hole. During this research project, several parameters that influence hole conditioning were investigated using large scale well models. Resistivity probes were used to detect the travel time of "tagged" slugs as they traveled through the models. This allowed measurements of the circulation efficiency or "circulatable hole" (portion of the mud circulating at a given time) to be made during the tests. Resistivity probes were used since currently available "tagging" techniques (liquid calipers normally used in the field) lacked the accuracy that was needed for the research experiments. To simulate hole conditioning across areas of good permeability, for example across pay zones and even partially depleted zones, circulatable hole measurements were made across manmade sandstone cores. Measurements were also made in pipe-in-pipe (impermeable) sections. Different types of mud were used. This paper discusses the impact of hole conditioning on the outcome of a cementing operation. The paper also includes, based on the data collected, a compilation of practical recommendations for improving hole conditioning and therefore cement job displacement efficiency in actual field application. Discussion Field experience has suggested that perhaps the most important parameter controlling the success of a cement job is the condition of the hole before the cementing operation. If the hole has not been properly prepared (conditioned) to receive cement, the cement job will likely not be a good one even if other generally accepted practices such as use of spacers, turbulent flow, etc., are followed during the actual execution of the cement job. Actually, many cementing practices known to achieve good cement jobs including the ones just described plus others such as pipe movement, use of centralizers, etc., are effective to the extent that they contribute to help condition the hole to receive cement. Hole conditioning means to establish a hole that is free of cuttings, debris, etc., and that has a mud in a fully "displaceable" or "circulatable" condition to allow the spacer and cement slurries to effectively displace the mud out of the hole. For best results, this condition of "circulatable" hole must be established before the first barrel of cement-mud spacer is pumped down the casing. Field experience has also shown that in general, it is not possible to determine ahead of time how much conditioning will be necessary for a given well to prepare it for a good cement job. The reason is that each well is different and therefore, reacts on an individual basis as it is conditioned. The old belief that one or two "bottoms up" is enough conditioning prior to a cement job is unquestionably incorrect based on many field testimonies throughout the industry. It is the well and not the engineer or foreman that decides when it (the well) is ready (properly conditioned) to receive cement. The role of the engineer or foremen should be one of monitoring the situation and detecting when the well is telling him it is ready for cementing. In many cases, it will take substantially more conditioning than one or two "bottoms up" to properly complete the process of conditioning the hole. On the other hand, a properly conditioned hole has the best chance for a good cement job and the least chance of requiring costly squeezing later on. One way to make quantitative measurements of "circulatable hole" in a well is to "tag" the mud as it is pumped down the casing during the circulation period, and to measure the total time that it takes for the "tagged" slug to make a complete circulation 1 (use of fluid calipers). Several "tagging" means have been used by the industry to monitor hole conditioning: carbide pills, walnut hulls, oats, paint, etc. These different "tagging" techniques have been partially successful depending on the given situation, sometimes exhibiting a wide range of unreliability. P. 135^