Optimizing the Most Common CT Operation in the US: Findings From More Than 60 Composite Frac Plug Underbalanced Milling Operations in the Piceance Basin

Abstract

Abstract The advent of multi-zone stimulation techniques has created an increasing need for zonal isolation devices, such as composite bridge and frac plugs. Bringing the well on to production requires that these devices be removed from the wellbore effectively and in a timely fashion. Milling composite frac plugs (CFP) using coiled tubing (CT) and positive displacement motors (PDM) is one of the most common and effective methods in the industry. Where formation properties allow, overbalanced CFP milling is routinely performed with high degrees of success. However, underbalanced milling operations that use nitrogen as a circulation aid pose additional challenges. Further, efforts to optimize the technique are often limited by the numerous parameters involved which reduce the repeatability of successful results. This paper presents the lessons learned from over sixty underbalanced CFP milling operations performed in the Piceance Basin. The results shown are uniquely relevant due to the fact that the operations involved in this study were performed within the same field, using the same type and size of PDM, mill, CFP, and CT. Very rarely can all these variables be kept as constants. Additionally, the same core field personnel were used over a one year period. This elevated level of consistency allows the author to focus on key controllable CT variables of the operation. The paper will review the optimization and results of: two-phase flow rates, PDM stalling, milling profiles and optimum wellhead pressures. Reviewing the field data has resulted in improved operational efficiencies by empirically developing two concepts: the "milling profile" and the "optimum milling wellhead pressure". These improvements will be discussed in the paper. Introduction The search for more economical means of exploitation of oil and gas reservoirs has instigated technological advances in the industry such as multizone fracturing. Popular variations of this technique commonly require the use of composite fracturing plugs in order to economically isolate the multiple fracturing treatments placed in a wellbore. In these cases, wells are perforated, fractured, and then a composite frac plug is set above the recently treated zone to provide mechanical isolation from the next treatments. The process is repeated for the next target zones above until all the prospective producing intervals are treated. Although some of these isolation plugs have been designed to allow some degree of hydrocarbon production through them, the full potential of the asset is often achieved only when all the CFPs have been removed from the wellbore. Milling CFPs with a CT-conveyed downhole mud motor (PDM) and a mill is one of the most widespread methods of choice for the removal of these devices due to numerous reasons. Compared to workover rigs - commonly referred to as CT's closest competitor for plug milling operations - CT provides a very flexible platform for plug removal since it can be reciprocated with ease. CT can travel up and down the wellbore at much higher speeds than workover rigs' jointed pipe, leading to shorter operations and quicker turnaround times for the well. In addition, CT can be rigged up on location in a shorter time than a workover rig reducing the overall operating time on location. Most importantly, CT can be safely deployed in the well under live conditions reducing the amount of personnel and equipment required on location to complete a milling operation (a wireline-set kill plug is not required). Since there is no need to pump heavy completion fluids for well control purposes, more favorable conditions for a healthy producing reservoir are fostered. This benefit is especially useful in underpressured or sensitive reservoirs.