Improving Drilling Performance in Troublesome Clay Formations in the Heidrun Field

Abstract

Abstract Despite the use of inhibitive water-based mud (KCl), high pump rates to obtain good hole cleaning and relatively high mudweight, problems with cavings, pack-offs and lost circulation persisted when drilling Tertiary clay formations in the Heidrun field offshore Mid-Norway. A study was therefore initiated, including extensive data collection (logging and coring of the shale) and subsequent core testing, data analysis and evaluation of field experience. The study showed that the major problem zone was not the low-density zone as anticipated. However, the core revealed a fractured and crushed zone, which was eventually found to be the main source of the problem. The study has resulted in a new strategy. An improved borehole stability model has been established, and the mud and operational strategy has been revised. The mudweight has been reduced, the salt content of the mud has been reduced, and good hole cleaning is obtained through a low-viscosity mud which is kept close to turbulent flow. Furthermore, wells are now planned without steering in these formations, if possible. An economic evaluation of the study was also performed, showing a substantial benefit/cost ratio. So far, the cost reduction for an average well is close to 20 MNOK (ca. 2.5 million USD). With more than 50 wells left to drill in the field, the potential for cost-saving is large. Additionally, there is a huge and non-quantified effect from accelerated production. Introduction General field information. The Heidrun field was discovered in 1985 and is located in the Haltenbanken area (Fig. 1) offshore Mid-Norway. The field is developed with a Tension Leg Platform (TLP). The platform is anchored to the seabed at 350 m depth using 16 tension legs made of steel. The production and injection of excess gas is performed using wells drilled through a template located on the seabed directly underneath the platform, while water injection is performed through two separate satellite templates in addition to platform injection wells. At production start in October 1995, ten wells were pre-drilled nine producers and one was gas injection well. According to the current well plan, continuous drilling of new wells will continue until the end of 2014. At present there are 20 oil-production-, 8 water injection- and one gas injection well completed on the field. The Heidrum field is situated in an enviromentally vulnerable area, and drilling was accepted only with water based mud. A KCI-mud has been utilitzed throughout the drilling phase. The inhibitive KCI-mud systems used have been governed by possible reactive clay formations above the reservoir. As the field is developing, present and future wells are predominantly extended-reach (ERD) wells with sail angles of typically 55–77° (in the 17 1/2" and 12 1/4" sections). Background and scope. During drilling of the first wells a high viscosity mud with a relatively high KCI-content was used. Experience from the pre-drilled wells showed large problems with pack-offs and lost circulation during drilling, typically in the Lower Tertiary (Brygge, Tare and Tang) formations. Thyis was believed to be caused by hole instability, possible thief-zones and poor hole cleaning, despite a pump rate which was typically some 6,000 l/min in the 17 1/2" and 12 1/4" hold sections. A typical response during drilling was to increase the mud weight due to large amounts of cavings reported over the shakers. As the TLP production drilling started in 1995 the same experience was gathered with troublesome drilling in the Tertifary clay formations. In early 1997 a study was therefore initiated by the drilling department in order to improve hold stability predictions for future wells. General field information. The Heidrun field was discovered in 1985 and is located in the Haltenbanken area (Fig. 1) offshore Mid-Norway. The field is developed with a Tension Leg Platform (TLP). The platform is anchored to the seabed at 350 m depth using 16 tension legs made of steel. The production and injection of excess gas is performed using wells drilled through a template located on the seabed directly underneath the platform, while water injection is performed through two separate satellite templates in addition to platform injection wells. At production start in October 1995, ten wells were pre-drilled nine producers and one was gas injection well. According to the current well plan, continuous drilling of new wells will continue until the end of 2014. At present there are 20 oil-production-, 8 water injection- and one gas injection well completed on the field. The Heidrum field is situated in an enviromentally vulnerable area, and drilling was accepted only with water based mud. A KCI-mud has been utilitzed throughout the drilling phase. The inhibitive KCI-mud systems used have been governed by possible reactive clay formations above the reservoir. As the field is developing, present and future wells are predominantly extended-reach (ERD) wells with sail angles of typically 55–77° (in the 17 1/2" and 12 1/4" sections). Background and scope. During drilling of the first wells a high viscosity mud with a relatively high KCI-content was used. Experience from the pre-drilled wells showed large problems with pack-offs and lost circulation during drilling, typically in the Lower Tertiary (Brygge, Tare and Tang) formations. Thyis was believed to be caused by hole instability, possible thief-zones and poor hole cleaning, despite a pump rate which was typically some 6,000 l/min in the 17 1/2" and 12 1/4" hold sections. A typical response during drilling was to increase the mudweight due to large amounts of cavings reported over the shakers. As the TLP production drilling started in 1995 the same experience was gathered with troublesome drilling in the Tertifary clay formations. In early 1997 a study was therefore initiated by the drilling department in order to improve hold stability predictions for future wells.