One often overlooked aspect of hydraulic fracturing is that the majority of reasonable permeability reservoirs (0.1 mD and above), on the North American continent, were in fact originally completed with straightforward hydraulically fractured vertical wells. However, as the average permeability of formations being developed deteriorated, this triggered a transition to multi-stage fractured horizontal wells and not unreasonably the fracture design and techniques that were developed to move from stage to stage were designed to be fit for purpose in these much lower permeability environments. These approaches, while suitable for lower permeability and unconventional formations, are not necessarily appropriate for higher permeabilities and conventional reservoirs.
The Khazzan development in the Sultanate of Oman Block 61 includes a multi-layered, reasonable permeability, gas reservoir, which may be categorized as a tight gas reservoir. In such tight gas developments, fractured vertical wells have historically been the preferred completion design, due to favourable economics. Following an extensive appraisal programme, the development of the Barik reservoir in Block 61 was approved in February 2014, and while successful appraisal had taken place with fractured vertical wells, not unexpectedly multi-stage fractured horizontal wells were subsequently proposed as an additional incremental improvement option for development. In order to successfully achieve this, a number of standard operational practices and assumptions associated with North American unconventional horizontals needed to be challenged, adapted and in some cases stopped.
The technical journey to deliver an effective multistage well design included an assessment of the impact of assumptions and considerations that drive unconventional practice, eventually leading to the road map to success that was developed. The learning includes three key pilot horizontal wells and clearly demonstrates incremental progression that was achieved, including technical obstacles faced, engagement with a complex stress-regime and how unconventional technology has to be adapted to be fit for purpose for the formation at hand. Not a static solution, the Khazzan development continues in the initial phase with fractured vertical wells achieving a rapid learning-curve and multi-stage fractured horizontal wells being optimized further.
The experiences and outcomes from the suite of wells in this project demonstrated that multistage fracturing of horizontal wells requires careful consideration, particularly in the selection of technologies and their application. The approach adopted in this project has led to developing the field with a healthy suite of competing completion techniques that offer best-fit solutions under different scenarios, and this set of complementary options will ensure that the development economics are maximised.