Composite: Is Multi-Plug Milling That Fast?

Abstract

Multi-stage stimulation by use of composite plugs (CP) for isolation has been in practice for over 15 years in North America. It continues to be a cost effective solution for shale plays requiring hydraulic stimulation of horizontal wells. Currently, there are dozens of plug venders in North America that supplied hundreds of thousands of plugs to the market. In order to perform effectively, plugs must convey down long horizontal wells, deploy at the appropriate depth in a well, hold pressure during a frac, and be removed utilizing conventional wellbore cleanout techniques, which in most areas involve milling with coiled tubing. The milling aspect of designing a plug can be particularly challenging, as it can be difficult to simulate without conducting a full scale test. Additionally, there are many different types of bits or mills to choose from. Thus, testing is required when designing a plug to optimize milling performance. Failure to do so can result in increased milling time and cost, something unacceptable in these tight-economic plays. Mill out testing during the qualification phase is key to the plug performance in real bottomhole conditions. Multiple plugs are used in a single well that ideally will be milled out sequentially. Thus performing a mill test with only one plug may result in an optimistic view of plug milling. There are several important parameters in evaluating milling performance or millability in a test, including: overall millout time, penetration rate, debris size, and stump size. The millout time per plug in a well is defined as the average time taken to mill through a single plug, starting when the mill contacts the top of the plug and ending when the plug‟s anchoring mechanism gives way. This leaves the bottom part, or stump, un-milled. The shape and size of the stump may significantly affect how the debris is conveyed to the next plug as well as the milling of the next plug. Millout time must calculate as an average of no less than two plugs milled in sequence. This paper will describe the best practices for determining the millability of a CP in a multiple plug scenario, and show how milling tests can be used to improve the design of a plug and optimize mill selection. This in turn maximizes the chances of successfully and efficiently removing CPs in the multi-zone treatment application, with the overall goal of improving the economics of these wells.

Bridge Plug Drillouts Cleaning Practices—An Overview

“…They added that during the experiments, real bottomhole conditions should be simulated, and millout time, penetration rate, debris size, and stump size should be measured. The authors demonstrated howmilling tests could improve the design of a plug, enhance mill selection, and improve the economics of the completion operation [2].…”

“…Wiper trip speeds range from 35 to 50 ft/min. [2] Highlighted the importance of millability tests. The authors highly recommended milling at least two plugs to allow for a more realistic evaluation of a composite plug milling performance.…”

Effects of Downhole Parameters on Horizontal Milling Applications

“…After the plug was partially drilled the remainder of the stump would hold together. When drilled with large diameter CT string and extended reach tools, the recommended WOB would be exceeded causing the lower slips to fail and allowing the plug to move farther down the well (Comer, 2014;Craig, 2012). The movement in the plug down hole was often mistaken back at surface as a very short drill time, often under 5 minutes.…”