A Case Study of the Wilcox (Lobo) Trend in Webb and Zapata Counties, TX
Authors: Robinson B.M. , Holditch S.A. , Lee W.J. 
Published: December 2007

Summary
The Wilcox (Lobo) trend of Webb and Zapata counties, TX, is a series of geopressured, low-permeability sands with average depth from 5,000 to 12,000 ft [1525 to 3660 m]. More than 1,000 wells have been drilled in this prolific trend during the last 10 to 12 years. Although actively developed earlier, the trend became even more attractive after its classification by the Federal Energy Regulatory Commission (FERC) as a "tight" gas formation. Essentially, development of the Wilcox (Lobo) has been successful because of modern technological advances. This paper presents the results of several years of study involving the geologic history, completion methods, massive-hydraulic-fracture (MHF) stimulation treatments, reservoir evaluation, and numerical analysis of hydraulically fractured wells in this trend, all of which illustrate the application of this modern technology.

Introduction
Low-permeability gas reservoirs have been developed actively in the U.S. during the past 10 to 15 years. Three of the most important criteria for the successful development of any area are (1) gas market availability, (2) gas prices adequate to justify the risks involved, and (3) proper prices adequate to justify the risks involved, and (3) proper application of modern technology. A key element is the development and transfer of the technology necessary to develop low-permeability gas sands economically. Many authors have contributed to the transfer of technology during the past few years by presenting papers concerning fracture fluids, fracture design equations, pressure-transient analysis, and other specific topics, pressure-transient analysis, and other specific topics, Papers that describe a field case history are normally Papers that describe a field case history are normally quite useful to most petroleum engineers. This paper presents the results of several years of work involving the geopressured Wilcox (Lobo) formation in Webb and Zapata counties, TX, including the geologic history, completion techniques, hydraulic-fracture stimulation techniques, and reservoir evaluation of the Lobo formation. In the 1960's, several wells were drilled and tested in the Wilcox (Lobo) formation. Because of low gas prices and a lack of pipelines, however, the area was considered uneconomical and remained inactive for several years. Successful production from the Wilcox (Lobo) sands in Mexico renewed interest in this trend in the early 1970's. Consolidated Oil and Gas Co. drilled its N.H. Clark Well 1 directly across the Rio Grande from established production in Mexico. This well produced at rates of up to 7.0 MMcf/D [198 × 10(3) m3/d] and officially discovered the Wilcox (Lobo) trend in the U.S. Early exploratory drilling, which was concentrated near the U.S./Mexico border, soon established the Wilcox (Lobo) trend as a major gas-producing horizon. As drilling progressed farther from the border, the Lobo was encountered at greater depths and the productive extent of the Lobo was expanded to include an area of at least 1,800 sq miles [4660 km2]. Even though the shallower sands near the border seem to produce at higher initial rates than the deeper sands, virtually every well drilled in the Lobo trend is fracture-stimulated to increase gas flow rates and recoverable reserves. As a result, MHF technology has been used quite extensively, particularly in the deeper areas. Because fracture stimulation costs can amount to a significant fraction of the total well costs (10 to 20%), it is important to optimize this phase of the completion. State-of-the-art analysis techniques, such as numerical history-matching simulation, have been used by many operators in an effort to optimize and to ensure effective fracture stimulation treatments. Even though more than 1,000 wells have been drilled, the Lobo sands are far from being completely developed. The productive limits of the trend are still being explored, with wells being drilled farther to the north, east, and south of the U.S./Mexico border. In these directions, the sands dip to depths of approximately 10,000 to 12,000 ft [3050 to 3660 m]. Because of the abnormally high pore-pressure gradients associated with these depths, the pore-pressure gradients associated with these depths, the drilling and completion techniques must be well integrated.