Case Study - ECD Management Strategy Solves Lost Circulation Issues On Complex Salt Diapirs/Paleocene Reservoir

Abstract

Drilling horizontal infill wells in the Pierce field in the UK Central North Sea is challenging due to the narrow drilling window caused by depletion in a highly fractured reservoir, which is further complicated by water injection and the proximity of other production wells. Wellbore strengthening was attempted in the reservoir section of Pierce B5, although, when a pre-existing fracture further weakened by depletion was encountered, losses occurred. During the next ten days over 5,000 barrels of non-aqueous fluid (NAF) were lost downhole. During the recovery from the losses event, 15 individual treatments with loss circulation materials (LCM) were made. More than 50MT of LCM was used without success, however, the well was stabilised enough to secure the drilled footage and complete the well. Two additional horizontal wells in the same field were successfully drilled using a low-equivalent circulating density (ECD)-management strategy combined with the development of a low-ECD drilling fluid system and the use of a high-fluid-loss, high-strength (HFHS) pill. The dual problems of barite sag and poor hole cleaning, that can be problematic when reducing ECD, were successfully managed. This low-ECD technique can be applied in other subsea depleted wells where narrow drilling windows exist. This paper describes how a detailed analysis of the losses event on Pierce B5 provided an improved understanding of the loss mechanism resulting in a revised ECD management strategy. An engineered low-ECD NAF with a HFHS pill combined with learnings on ECD awareness in the planning and execution phases mitigated the drilling risks on subsequent wells. This has implications when encountering very large fractures that are difficult to overcome, not only on future Central North Sea wells, but also has wider global significance for wells drilled in similar geological settings.

Utilization of Vermiculite for Solving Hematite Sagging in Water-Based Drilling Fluids

“…Several factors contribute to solids settlement including the drilling fluid properties, temperature, solid particle size, well inclination, and drilling parameters such as the pipe annular velocity [12][13][14]. This sagging phenomenon causes density variation and fluid instability which may result in pressure control issues in upper sections, elevated equivalent circulation density at the bottom, pipe sticking, and loss of circulation [15,16].…”

Impact of Perlite on the Properties and Stability of Water-Based Mud in Elevated-Temperature Applications

“…Consequently, the mud weight becomes less than the formation pressure in the upper parts of the drilled section, which may cause a severe well control issue. , The solid particles, accumulated in the lower part of the well, increase the mud weight and equivalent circulating density (ECD). This increase may induce fractures in the formation leading to partial or total loss circulation, especially when the mud window is narrow. , Moreover, these accumulated solids interfere with drilling and completion operations and cause pipe sticking. − …”

“…This increase may induce fractures in the formation leading to partial or total loss circulation, especially when the mud window is narrow. 29,30 Moreover, these accumulated solids interfere with drilling and completion operations and cause pipe sticking. 31−33 Many techniques were introduced in previous studies to mitigate the solids sag and other fluid stability issues.…”

A Comprehensive Experimental Study on Wellbore Strengthening

“…Wellbore strengthening includes chemistry methods such as settable gels or resins (Aston et al, 2007), high-fluid loss pills (Friedheim et al 2012, Murray et al 2013, and engineered particulate lost circulation materials (LCM), see e.g., Aston et al (2004), Fuh et al (2007), van Oort et al (2009). This paper only deals with wellbore strengthening techniques using particulate LCM.…”