Abstract

This paper presents the prejob engineering process and executional summary of the first North Sea application of a novel tubing-conveyed fracturing technique. The logistical challenges that were overcome during the completion of the project will also be discussed. This fracturing technique was implemented successfully to perform a large multistage acid treatment from a semisubmersible rig. Historically, wells in this field have been treated using dedicated stimulation vessels. Acid-fracture treatments have typically been pumped at high rates (50+ bbl/min), treating multiple intervals simultaneously using various methods for fluid diversion, including ball-sealer technology and limited-entry perforating, to ensure every target zone is contacted and adequately stimulated (Alastair et al. 1999). At present, the demand for stimulation vessels is extremely high because of the level of new well-development and general stimulation activity in the North Sea sector. This demand, coupled with a reduction in the number of available stimulation vessels operating in the area, led to the requirement for an alternative method of completing this well.

The overall success of this project demonstrates that there is a viable commercial and technical alternative to the traditional stimulation methodology associated with wells of this type. With further development to improve efficiency, these methods could become competitive with conventional vessel-based techniques.

Introduction

Well 23/26a-B1y is an operating company asset located in the Machar field, central North Sea. The Machar field was discovered in 1976 and is located in the U.K. Block 23/26a in circa 95-m water depth, approximately 150 miles east of Aberdeen. The field is a normally-pressured oil reservoir contained within a steeply dipping, fractured Cretaceous chalk and Paleocene sandstone in a high-relief structure over a salt diapir. It was planned for well 23/26a-B1y to be a producing sub-sea oilwell drilled on the east flank of the Machar field (Fig. 1). The reservoir target for the well was the Tor chalk (Fig. 2 and Table 1).

The Tor chalk is a highly naturally fractured reservoir caused by the uplift created by the rising salt. The fractured nature of the reservoir provides the primary production mechanism in the Machar field. Typical matrix permeability is in the region of <1 mD. The success of any well that is drilled in the Machar field is therefore largely dependant on the wellpath intersecting sufficient natural fracture swarms to allow the well to produce economically. Natural fracture targets are identified during the drilling phase by careful monitoring of the drilling returns to spot indicative spurt losses. These prospective loss-zones are then correlated against standard log data and the most attractive zones selected for perforation following the setting of the production liner. The primary components of the planned completion were a 5-½ in., 20-lb/ft, HP-2, Vam Top production tubing and a 5-in., 18-lb/ft, HP-2, Vam Top cemented liner. Refer to Fig. 3 for full details of the proposed completion for Well 23/26a-B1y.

Stimulation Design Selection

The fractures that show the largest potential for production are also typically the fractures that take the majority of mud losses during drilling and are consequently the most damaged. Previous stimulation treatments carried out in the Machar field have been designed to remove mud fluid and filter-cake damage from these fracture networks. A further objective of these treatments is to induce artificial fractures to improve productivity where the connectivity of the naturally existing fracture networks within the well is limited.

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