Review of the Completion Practices in the Wilcox Formation in South and South Central Texas

Abstract

SPE Members Abstract The Wilcox formation is a primary target for exploration and development of relatively low permeability tight gas reservoirs. Many operators permeability tight gas reservoirs. Many operators have and will continue to drill for the Wilcox in South and South Central Texas. This formation continues to make a major economic contribution to the area's oil and gas industries. This paper will present background information on the geology of the present background information on the geology of the Wilcox formation. The data presented will include lithology data derived from x-ray diffraction, and Scanning Electron Micrograph (S.E.M.) studies of whole core and/or sidewall core samples. The formation rock characteristics, such as permeability, porosity, youngs modulus, and formation water porosity, youngs modulus, and formation water analysis are also discussed. Some of the latest completion techniques, including cementing, casing and perforating programs, are discussed in detail. Bottom hole pressure, thermal and fracture gradients are reviewed due to their important influence on completion practices. A variety of stimulation procedures have been used very successfully on the Wilcox formations. Stimulation fluid selections, volumes and even injection rates have their influences on completion practices and resultant production. Finally, the practices and resultant production. Finally, the types and size proppant selected and the volumes used are presented to maximize the production and to provide an optimum completion program. provide an optimum completion program Introduction The Wilcox sand of the Eocene era were deposited along the South Texas Gulf Coast, primarily in a 10county area (See Figure 1) The most prolific zones were discovered in: Webb, Zapata, Duval and Live Oak counties. Reserves for the area are estimated in excess of 600 billion cu. ft. (18billion cubic meters)/yr. with some additional associated condensate. Early Wilcox exploration was sporadic due to lack of pipeline availability as well as unattractive gas pricing. A second obstacle, abnormally high pressured zones between 9,000 and 13,000 ft(2743 and 3962m), also slowed development of the area. Drilling in the Wilcox trend increased dramatically after gas prices rose under the Natural Gas Policy Act of 1978. Since 1978, drilling activity has been steady throughout the trend. In general, the Wilcox is a tight, poorly consolidated, fine grained sand that is somewhat sensitive to contact by foreign fluids. A wide variety of drilling and completion practices have been attempted in an effort to obtain commercially productive zones. Most wells drilled require some stimulation to yield commercial production rates. A well boreclean-up with a weak (less than 7.5%) acid or 2%KCl based fluid may dramatically improve production, however, hydraulic fracturing is usually production, however, hydraulic fracturing is usually required. In some instances where the Wilcox contains 150 - 500 ft (45.7 - 152.4m) of zone, Massive Hydraulic Fracturing (MHF) is required. This paper's discussion will focus on the fracturing and MHF treatment applications, specifically applicable to the Wilcox formation. FORMATION DEPOSITIONAL AND ROCK CHARACTERISTICS The Wilcox and Midway groups of the lower Eocene series constitutes the oldest of the thick sandstone/shale sequences within the Gulf Coast system (See Figure 2). Sediments within the updip section were deposited primarily by fluvial processes. Downdip sediments were transported across processes. Downdip sediments were transported across the Wilcox fluvial plain and were deposited in huge deltaic systems. Some deltaic sediments were reworked and transported along the shore by marine processes and then redeposited on barrier bars and processes and then redeposited on barrier bars and strand plains. Growth faults developed near the shorelines of several of the larger deltaiclobes where thick layers of sand were redeposited on previous sediments. P. 497