Abstract

An efficient system of running gelled diesel stimulation fluid has been developed which allows continuous treatment and requires no pre-gelling or batch mixing. Continuous gelling while pumping is possible with this unique system, which is a combination of phosphate ester chemistry and computer-controlled metering incorporated in a mobile trailer or pre-blender. Operational and environmental advantages have been reported after field applications of this system.

Stimulation treatments using gelled oil as the fracturing fluid have been routine for many years. Most of these treatments have been performed by batch mixing gelled all in storage tanks before pumping it into the formation. This, however, can result in costly waste of premixed gel should the job be cancelled or terminated prematurely. The system discussed in this paper gels the fracturing fluid while it is being pumped downhole, to eliminate the disposal problems associated with batch mixing.

The pre-blender is capable of mixing at 10 to 40 bbl/min and metering two liquid additives (phosphate ester gellant and activator) into the diesel. The automated equipment can be used to vary the concentrations of the additives, gel viscosity, or job volume in real-time at any point during the job without wasting gel.

Environmental and operational advantages of running gelled diesel continuously include (1) no left-over gel in tank bottoms to dispose of after the job. (2) pre-gelling time saved. (3) operators pay only for materials actually used during the job, and (4) elimination of the difficult task of withdrawing highly viscous, batch-mixed gels from storage tanks.

In Alaska, several wells have been hydraulically fractured using the technique. The diesel-based gel is found to be preferable to water-based gel systems in areas where temperatures can reach well below the freezing point. The equipment of this system is also designed to withstand arctic conditions.

This paper describes the fluid system as well as the equipment design and metering criteria for the continuous gelled oil system. Field application cases of this system will also be presented.

Formation Description

The Kuparuk River Field is located 25mi (40 km) west of Prudhoe Bay on the North Slope of Alaska and covers an area of approximately 200 sq mi (518 sq km). It was discovered in 1969 by BP Alaska and Sinclair Oil Company, but was considered uneconomical.1–4

In the 1970's, a small portion of the Kuparuk River Field was developed, however, full-scale production and development did not begin until 1981.3

The producing interval of this formation consists of upper and lower zones of Cretaceous sand. The upper zone is denoted as the "C" sand while the lower zone is labeled the "A" sand. These zones are fine-to very fine-grained marine sands, silt, and mudstone. The sand is approximately 300 ft (91 m) thick, but the average net pat thickness for the two producing intervals ranges from 35 to 80 ft (11 to 24 m).4–7 The upper zone ("C" sand) is usually not hydraulically fractured.

The lower zone of the Kuparuk River formation has an average porosity of 23% and as average permeability of 60 md.4–7

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