Insulated Ice Pad Technology Enables Extended Season Drilling on Alaska's North Slope

Abstract

Abstract In the 1993/94 winter exploration season, BP Exploration (Alaska) Inc., (BPXA), drilled two wells in the Pt. Thomson area of the North Slope. In March 1993, pre-fabricated, insulated panels had been transported to the North Slope and installed at the first exploration location. Their purpose was to insulate an underlying ice pad from thawing during the Arctic summer. The survival of the ice pad through the summer made possible a very early mobilization of the drilling rig the following October. This enabled sufficient time to be available that winter to drill two exploration wells in the same season, greatly reducing rig and equipment remobilization costs. Outline Exploration drilling on Alaska's North Slope is typically conducted in the winter months when locations remote from existing gravel road infrastructure can be accessed by the construction of temporary ice roads, or alternately, by the movement of equipment and supplies across the frozen tundra by all-terrain vehicles. In late 1992, BPXA began planning an exploration drilling program to be conducted in the winter of 1993/94. The area of interest lay approximately 10 miles south of Point Thomson on the Beaufort Sea coastline, and 50 miles east of the closest infrastructure at Prudhoe Bay (Fig. 1). Exploration wells in the 1970s and 1980s utilized gravel pads to support the drilling operation. It was in this time-period that the original exploration and appraisal drilling in the Pt. Thomson area took place. Operations frequently continued into the Arctic summer using helicopters as the only method of resupply. Alternately, operations were suspended at the end of the winter, for continuation after tundra freeze-up occurred the following fall. Historic exploration well costs reflected these expensive measures required. Over the past decade, the use of gravel pads for exploration wells has become an unacceptable and unnecessary option on the North Slope. This has been due to the long-term environmental impact imposed (vs. the temporary, one-season ice pads that are now utilized) and the prohibitive costs of gravel pad construction in such a remote environment. No wells had been drilled in the Pt. Thomson area since 1985. A review of the available historic well data showed that most wells had taken in excess of 150 days from spud to rig-release and on some occasions over 220 days. This far exceeded the winter season available for exploration drilling operations of about four months using the now-conventional ice pad location. Although drilling performance on these old wells seemed poor by today's standards, a review of offset data and an understanding of the sub-surface geology encountered in the area, showed this to be by far the most technically difficult drilling environment existing on the North Slope. The challenge for BPXA a decade later was two-fold:to use drilling technology and practices developed in the intervening period to safely drill new exploration wells in the area faster, and,devise a cost-effective method of extending the ice pad based drilling season. The goals were to firstly ensure that a single well could be successfully completed in one winter season, and secondly, if operations did indeed progress with a step-change improvement from previous Pt. Thomson area performance, to provide the opportunity of drilling a second well in the same season. This would greatly reduce equipment mobilization costs and obtain invaluable sub-surface information a full year earlier than would otherwise be possible. Insulation of thin gravel pads had been performed in the past as a method of retaining structural integrity of the gravel over a summer period by keeping its bound water frozen. A design study was carried out at the end of 1992 to determine the feasibility of insulating an ice pad over an Arctic summer using readily available insulation materials. A finite element analysis model was used to demonstrate the concept's merit and provide figures on insulation thickness required at the specific well location. A plan was quickly developed to design, construct and ship the pre-fabricated insulation panels. In March 1993, an ice pad was constructed on the drilling location. P. 359