Geothermal drilling time analysis : a case study of Menengai and Hengill

Abstract

Drilling operations are run on tight schedules and drilling time delays come at a high cost. A large part of drilling workdays is spent on making the wellbore, and activities that support drilling, contribute to productive time (PT), while a significant part of the time is spent on drilling problems, and activities aimed at finding solutions and solving these problems. This contributes to non-productive time (NPT). Problems occurring during drilling can be avoided sometimes but, on other occasions, they are beyond the drilling crew’s control; the causes are numerous and their effects are undesirable. This paper evaluates the extent of NPT associated with 15 wells drilled in Menengai, as well as identifying their causes and effects, and finally suggests recommendations aimed at increasing PT while reducing NPT to make the drilling process more effective. Data from 19 wells in Iceland were used for comparison. Workdays were analysed for 12 activities including: actual drilling, casing, cementing casings, cementing losses, equipment repair, wait on water, logging, changing the bit and bottom hole assembly (BHA), fishing, stuck pipe, reaming and ‘other’ activities. ‘Other’ activities were mostly waiting on materials, instructions and personnel. Blow out preventers (BOP) and wellhead installation and any other equipment installed after spading was included in this category. The analysed data was obtained from completion reports and drilling logs.

Because of the challenging geological conditions, drilling wells in Menengai took longer time than planned. In addition, the field was new and the crew did not know what to expect. The crew members were also new to each other and had to become accustomed to working together. Lost circulation was the major formation problem experienced, resulting in further problems of stuck pipe and drillstring failure. Drilling problems due to the formation increased with depth, with major challenges experienced at depths of about 2200 m, with a stuck pipe topping the list of drilling problems. It was concluded that productive time in Menengai could be increased through a change in technology, especially bit technology, while activities such as cementing could be accelerated with more knowledge of the subsurface conditions through logging and a thorough job of managing the loss zone. It is not possible to totally eliminate NPT, but minimizing it and increasing PT will result in a shorter project implementation time and reduce drilling costs.