Mini-Frac Tests and Bottomhole Treating Pressure Analysis Improve Design and Execution of Fracture Stimulations S.J. Tinker, SPE, Pennzoil Exploration & Production Co., P.D. Baycroft, SPE, BJ Services Company, and R.C. Ellis, SPE, Pennzoil Exploration & Production Co., and E. Fitzhugh, SPE, Pennzoil Exploration & Production Co.

Abstract

Observations from fracture stimulations of 25 infill wells in the Grayburg / San Andres Formations at the Waddell Field in Crane County, Texas are discussed and presented. Micro-frac stress profile testing and mini-frac testing were performed to help design propped fracture treatments. Dead string pressure or bottomhole gauges were used for mini-frac and micro-frac testing. Tip-screenout methods were used to achieve desired proppant concentration in the fracture.

Observations during treatments contributed to improved job design and execution. All but one well was equipped with a dead string during the frac job for observation of actual bottomhole treating pressure. Proppant transport problems detected in early treatments led to a change in fracturing fluid which allowed jobs to be pumped to completion. A wide range of leakoff characteristics was observed from well to well which made it necessary to include mini-frac testing as part of each stimulation. Large variations in measured leakoff occurred even in direct offset wells. Additionally, frac job fluid efficiency appeared higher than observed from mini-frac calibration work.

The micro-frac stress test well provided a unique opportunity during the frac job to observe actual pressure in the fracture at a point away from treatment perforations. The fracture treatment communicated with the annulus through stress test perforations located above the packer. Pressure behavior observed in the fracture measured from the annulus was significantly different from pressure inside the casing at the treatment perfs.

This paper presents data, observations, treatment improvements, discussion and conclusions concerning fluid selection, mini-frac and micro-frac testing, leakoff characteristics, and observations of pressure in the fracture.

Introduction

The E.N. Snodgrass lease is a 640-acre tract located in Waddell Field on the eastern margin of the Central Basin platform in Crane County, Texas. A map showing the field location is shown in Figure 1. The lease produces from an interval of about 300 feet in the Grayburg and Upper San Andres formations at a depth from about 3200 feet to 3500 feet. Production has been prolific and widespread from the Grayburg and San Andres dolomite formations in this area dating back to the 1920's.

First production began upon discovery in the E.N. Snodgrass No. 2 in 1936. Offset development did not occur until the 1950's when 30 wells were drilled on a 20-acre pattern. Early waterflood activity began in 1967 with tour wells being converted to injection. Five successful workovers which included additional perforating and fracture treating were performed in 1993. Success of the workover program led to a 10 acre infill drilling program and full scale waterflood conversion program. Typical wells produce at rates of 40 BTFPD to 100 BTFPD. The completion and stimulation work on the 10-acre infill wells are discussed in this paper.

The new 10-acre infill wells provided an opportunity to identify completion techniques that were most appropriate. A lease map with existing well locations and new infill wells is shown in Figure 2. One of the main challenges was to economically and efficiently fracture treat the 300 foot section of pay. Historically, this large interval had been fracture treated in three separate stages or by using limited entry technique with perforations throughout the entire pay interval. Mini-frac, step-rate, and micro-frac testing were all used to ensure the wells were completed effectively. Valuable and interesting observations were made during the completion program and are presented in this paper.

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