Abstract

The Wilcox Lobo Trend of south Texas contains vast quantities of natural gas in low permeability formations. Economic production from low permeability gas sands is possible through the use of hydraulic fracturing. Through the years, varying hydraulic fracturing techniques have been used to recover the gas from this trend. The success of fracturing techniques varies and depends on many factors. Several authors have demonstrated the importance of propped fracture conductivity in the production enhancement process. Without sufficient conductivity in the proppant pack, fracture fluid cleanup and adequate hydrocarbon production are difficult to achieve. The work presented in this paper will show the evaluation of these different completion techniques and products used in the Wilcox Lobo Trend by the authors. The work demonstrates the importance of fracture conductivity, supports previous work in other producing regions and presents some additional methods for achieving increased conductivity. The techniques, products and evaluation methods used in this paper can be used to benefit other operators in the Wilcox Lobo Trend as well as those working in other low permeability gas formations.

Introduction

The Wilcox Lobo Trend is located in south Texas, primarily in Webb and Zapata counties. The productive intervals are a series of geopressured, low permeability sands. The Wilcox (Lobo) formations are a sequence of stacked sands and shales of Paleocene age, overlain by the Wilcox shale of Eocene age. Extensive faulting, present in the Lobo section, has resulted in a slump complex of rotated fault blocks. The size of these fault blocks adds to the complexity of the trend. The permeability of the productive sands ranges from well below 0.1 md to over 1 md. Massive Hydraulic Fracturing (MHF) treatments have played an instrumental role in the development of this trend as many wells will not produce economically without these treatments.

Several papers have discussed the use of MHF treatments in the Wilcox trend. These discussions range from fracture treatment optimization studies¹ to a case study of early production results and MHF treatment success² to how to improve fracture conductivity through the use of multiple fracturing fluids³. All of these papers agree that improving the conductivity of the propped fractures in the Lobo Trend leads to increased productivity.

Understanding formation permeability before pumping a MHF treatment can have a significant impact on the economic success of MHF treatments pumped in the Lobo Trend. This was suggested by Kerchner, et al.⁴ in a recent paper presenting a methodology for evaluating the permeability of each Lobo Trend sand before fracturing. Determining an independent measurement of permeability allows for the evaluation of the success or failure of the MHF treatment process in each well. This permeability methodology was employed to evaluate the reservoir quality of each well in this study.

The wells presented in this study are all nearby offsets from Conoco's Vaquillas Ranch leases. They were completed between October 1997 and September 2000. They were all treated with similar fracturing fluids, low pH zirconium crosslinked CMHPG, high conductivity proppants and sufficient fracturing fluid breakers. There are a total of twenty-one wells being examined. These wells will be divided into three datasets depending on completion complexity (the number of intervals completed and commingled for production in a single wellbore). Each of the datasets contains one or more wells which was treated using a surface modification agent (SMA), that will be discussed in the next section, to potentially enhance the conductivity of the proppant pack.