Milling Casing Exit Windows from a Hydraulic Workover Unit at Small Footprint Platform

Abstract

Abstract When first discovered, the gas field located in New Zealand was one of the largest gas fields in the world. After more than 30 years of production however, the reserves have declined drastically. To manage the decline, infill drilling campaigns have recently been initiated to access previously bypassed gas reservoirs in the field. A development campaign to boost the field recovery factor was kicked off with the objective of boosting recoverable reserves and extending the life of the field by drilling four sidetracks from existing depleted wells. A lightweight, compact and modular hydraulic workover unit (HWU) was installed on the previously unmanned, small footprint production platform with a small crew size of 32 people. While slimhole drilling and completion with a HWU had proven cost effective and a successful concept during previous workover campaigns, sidetracking from larger casing sizes at 2000-2400 metres depth with a HWU, including one dual casing exit design in this campaign, was the first of its kind for the operator. The four sidetracks were completed and the operation was a technical success, in large part due to the pre-job planning, project management and identification of unique challenges to this particular project. This paper will discuss the operational challenges encountered by the HWU including BHA handling and tripping in of the whipstock assembly when working with only a 12-foot jack stroke and minimal deck space. Additionally, the minimal rig capabilities of the HWU created challenges with regards to optimum whipstock and milling configurations/operations resulting from limitations on rotary torque capacity, rotary speed, pumping capacity and drill string racking capability, all of which were resolved by creative applications of the simple, robust yet versatile design of the casing exit technology.