Summary

Net fracturing pressure calculated from measured bottom hole pressure verifies the effectiveness of the alternating phase technique for controlling acid fluid loss. Results of before and after job simulation cases modelling the process are shown. Subsequent pressure buildup data are also presented. presented

Introduction

Evaluating alternating phase fracture acidizing treatments is complex because of the changing fluid loss rate and the varying rheological properties of the different phases. This paper shows data acquired during an alternating phase (abbreviated "alpha") fracture acidizing treatment. Measured values included temperature profiles, surface pressure, and bottom hole treating pressure and pressure, and bottom hole treating pressure and temperature. The data were used to calculate the fluid efficiency and net fracturing pressure, and to interpret the wellbore fracture height. Friction pressures calculated from the two measured pressures are compared to values calculated using pressures are compared to values calculated using friction pressure charts. The actual fluid efficiency was used to revise the effective leakoff coefficient for the final simulation case. The calculated dissolved rock profile from the final simulation is compared with the fracture wing length obtained from a pressure buildup test.

The treatment described in this paper was performed on the C. A. Goldsmith et al #1420 in the Goldsmith (5600) Field, Ector County, Texas. The Holt and Upper Clearfork Formations treated are highly stratified dolomites. The bottom hole temperature for the treated well is 105 degrees F The field produces oil and associated gas.

FORMATION CHARACTERISTICS AND STIMULATION PRACTICES

The Holt and Clearfork zones are Permian Period formations deposited in the Guadalupian and Leonardian Ages, respectively. In Goldsmith, these zones are cool, heterogeneous dolomites. Principal production is from the Upper Clearfork, with 10 to production is from the Upper Clearfork, with 10 to 20% of the production from the Holt. The Upper Clearfork has an average gross thickness of 400 feet, with the top of the zone ranging in depth from 5200 to 5600 feet. The Holt has an average thickness of about 350 feet. It directly overlies the Upper Clearfork. There are distinct sublayers within the Holt and Upper Clearfork Formations which can be correlated across the Goldsmith Field. Average air permeabilities range from greater than 5 md to less than 0.5 md. Core studies have shown these formations to be greater than 95% acid soluble. Slabbed core reveals many small layers one or two inches thick with visually different compositions. significant natural fracturing was observed during the core studies.

At original reservoir pressure, the Holt and Upper Clearfork zones have an apparent fracture gradient of 0.59 psi/ft. They are relatively brittle formations. A published compilation of rock properties for many fields shows Young's Moduli for properties for many fields shows Young's Moduli for the Clearfork ranging from 5.81 × 10–6 to 9.25 × 10, and Poisson's Ratios ranging from 0.226 to 0.340. There are no thick shale layers within either the Holt or the Upper Clearfork. A dense anhydrite layer with a thickness of up to 50 feet occurs near the top of the Holt across most of the field.

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