Revised Completion Procedures For S.W. Texas Low-Strength, Low Permeability, Geopressured Deep Wilcox Sands
Authors: Weeks S.G. 
Published: December 2007

Establishing and maintaining acceptable flow rates from deep, low permeability, geopressured lower Wilcox gas objectives in S.W. Texas has long been a challenge. A number of such tests have yielded only minimum flow potentials or have later been abandoned due to damaging sand production and/or casing failure-due in part to excessive pressure drawdowns.

Revised completion procedures have aided in initially establishing nominal flow rates. More recently, in the Rosita Field, Duval County, following an extensive core analysis and testing program, an initial daily open flow potential of program, an initial daily open flow potential of 1.7 MMCFG (48.1 × 10(-3) m) was increased to 80.0 MMCFG (2,265 × 10(-3) m) without any evidence of sand failure following ultimate stimulation and subsequent increased perforating density. Lack of sand production is attributed to minimum BHP drawdown during flow periods, as evidenced by a stabilized deliverability flow rate of 1.6 MMCFG/D (45.3 × 10(-3) m3/d) recorded with only a 10 psi (70 kPa) surface drawdown from a SITP of 8680 psi (59.8 MPa). This compares with an average of about 1000 psi (7 MPa) pressure drawdown per million for nine additional completions in the same field which were perforated for limited entry and initial ballout perforated for limited entry and initial ballout only.

Introduction

Major oil and gas reserves have been developed along the Eocene-Wilcox trend of the Texas Gulf Coast during the past several decades. However, for the most part, these reserves have been confined to hydropressured accumulations above 11,000 feet (3353 m). There has been only limited success with commercial exploitation of the deeper. Low-permeability, geopressured gas sands. These lower Wilcox objectives, herein referred to as the Deep Wilcox, have varied in depth from about 11,000 to 17,000 feet (3353 to 5182 m). Most of the completion intervals have had permeabilities between 0.1 and 3.0 md (0.1 and 3.0 × 10-3 um 2). Pressure gradients have ranged up to 0.93 psi/ft. (21. kpa/m).

More than 400 Deep Wilcox tests have been drilled during the past 35 years. Frequent "gas kicks" and encouraging log developments have been encountered. Unfortunately, most of these tests have not resulted in economical completions. This has been attributed to very low permeability, formation fluid sensitivity, sand production and/or associated casing collapse. However, results from some of these completions have been sufficiently encouraging, or otherwise a contributing factor, to merit continued efforts to commercially exploit this important gas objective.

Until recent activity in the Rosita Field, Duval County, the foremost Deep Wilcox producers of any consequence were at the northeastern end of the Wilcox trend in the North Milton Field, Harris County. This field has been a commercial success since initial development in 1963. While only minor stimulation has been necessary in North Milton, at other locations, stimulations, including fracture treatments, have failed to effect sustained productivity. With renewed interest, created productivity. With renewed interest, created by recent activity in Duval County, representative cores were obtained and a joint Shell and service company testing program undertaken. Concurrently, a comprehensive review of previous Deep Wilcox completions was made to further aid in evaluating more effective completion techniques.

GEOLOGICAL SUMMARY AND CORE STUDY, AS RELATED TO STIMULATION AND PRODUCTION PARAMETERS

The Wilcox Group consists of a variety of sand bodies interspersed in s hale. Typically, the sands are irregular with limited areal extent and thickness. The heterogeneous nature of these sands along the Texas Gulf Coast is well known.

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