Abstract

A heavy oil field, located in south-central Oman with unconsolidated sandstone reservoirs represents many drilling challenges. Past drilling with conventional mud motors in the field was associated with many drilling risks, including poor wellbore quality, difficulty steering, inadequate hole cleaning, and differential sticking. These risks resulted in extra correction trips, lost-in-hole bottomhole assemblies, sidetracking, and redrilling entire sections.

The field's current development plan is based on the factory drilling concept, which is driven by the number of wells that are required to be delivered in a given year. This requirement places a great amount of importance on drilling time. The average time from spud to spud in the field is 8 to 9 days; any additional trips have a large impact on the total number of wells delivered. In addition, due to the nature of the heavy oil sandstone reservoir, steam injection is required to enhance oil recovery. It is therefore critical to place the lateral hole section of the well in the center of the target corridor to avoid early breakthrough. To compound this, the fluvial nature of the reservoir, tight surface location constraints, and shallow reservoir true vertical depths (TVDs) have added to directional complexity.

To address the main technical challenges of directional assurance and greater drilling efficiency, the operator and service company explored alternative ways of drilling the 8.5-in. reservoir hole section, and this hole section was be the focus of our analysis.

The learning curve experienced in finding an answer for the technical challenges of directional control and drilling efficiency can be broken down to four distinct phases:

  • Phase 1 in 2008: Conventional motor

  • Phase 2 in 2009: Introduction of point the bit rotary steerable system (RSS)

  • Phase 3 in 2012: Introduction of high dogleg severity (DLS) rotary steerable system (RSS), a hybrid design (i.e., push-the-bit and point-the-bit system)

  • Phase 4 in 2013: Enhance drilling efficiency

Extensive data gathering and log analysis provided a better understanding of the main issues and ultimately led to the root cause. The analysis provided a clear solution, which was that a high-build-rate-capable RSS was needed to overcome the directional tendencies and deliver the planned well trajectory to the final total depth (TD) in one run to reduce the average well time.

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