Abstract

This paper presents results of an extensive laboratory and field program to optimize the fracturing fluid system used in the ARCO-operated Eastern Operating Area of the Prudhoe Bay Unit on the north slope of Alaska. This study focuses on the fracturing fluid rheology, fluid loss additives and retained proppant conductivity for this high permeability fracturing application. Results of 60 static fluid loss tests performed on filter paper led to optimization of degradable starch loadings and identification of 5% diesel as an effective fluid loss additive. Subsequent dynamic fluid loss tests on Prudhoe core discounted benefit of the diesel, but proppant conductivity testing showed the diesel did improve proppant pack clean-up. Results of mini-fracs performed showed diesel provided identical performance to the significantly more costly degradable starch additive. Laboratory tests of fluid rheology found that orders-of-magnitude increases in viscosity could be obtained by increasing the pH of the delayed-borate crosslinked guar systems used for the 200F BHT. Despite recent changes in strategy due to smaller treatment sizes (as described by Martins et al.), the results of these laboratory tests have been used to dramatically lower fluid loss additive costs and improve fluid system performance in the high permeability Prudhoe fracturing program.

Introduction

Over the last 8 years there has been considerable activity in the fracturing of two medium to high permeability oil reservoirs in Alaska: the Kuparuk field and the Prudhoe Bay field. The evolution of fracture treatments from the original objective of small, short fractures to overcome near-wellbore formation, to the current state of high conductivity tip-screen-out design at Kuparuk has been presented by Niemeyer and Reinart, Pearson et al., and Pospisil et al. Hydraulic fracturing at the Prudhoe field began in earnest in 1989, and the evolution of this program has been presented recently by Reimers et al., Martins et al. and Martins et al. All of these papers provide excellent reviews of the reservoir characteristics, summaries of fracture treatment design strategies, and results of treatment implementation in these two fields.

The focus of this paper is on a laboratory and field study to optimize the fracturing fluid system used in the ARCO Eastern Operating Area of the Prudhoe Bay Field. The work presented in this paper was performed in 1990 and 1991, and led to significant modifications to the fluids and fluid loss additives as they existed in late 1989 and early 1990. Due to evolving strategies and decreasing treatment sizes since 1991, some of the results from this paper are superseded by techniques presented in Martins et al. The results of this work, however, assisted stimulation engineers and operations personnel in their efforts to substantially reduce costs and dramatically improve well productivity in this very successful hydraulic fracturing program.

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