Abstract

The Haynesville Shale in northwest Louisiana and east Texas is a geologically unique gas play in which many petrophysical, engineering, and mechanical properties are close to optimal. With high geopressure gradients ranging from 0.8 to >0.95 psi/ft and reservoir pressures ranging from 8000 to 17,000 psi, it is one of the most prolific shale-gas plays in North America. Through the use of horizontal wells and multiple-stage fracturing, gas production reached >7 Bcf/d in August 2011, and the play has surpassed the Barnett Shale in north Texas as one of the highest gas-producing plays in the United States. The objectives of this study are to investigate the effects of petrophysical, geochemical, geologic, mechanical, and engineering properties, as well as completion practices, on Haynesville Shale production.

Core data show that connate water saturations range from 15 to 40% in the Haynesville. Low connate water saturation is attributed to water expulsion by oil and gas during hydrocarbon generation from organic matter within the shale. Nevertheless, slow fluid escape and gas generation at high temperatures resulted in an abnormally high reservoir pressure and pressure gradient, even in this relatively high porosity rock. The effects of the high geopressure gradient have been to increase reservoir pore pressure, to preserve porosity and permeability, and to enhance free gas content and the brittle nature of the gas shales. The average porosity of the Haynesville Shale is high, ~11%, and the free gas content is enhanced by high porosity and gas density. Because of the high formation pressures, effective stresses of the Haynesville are low, and laboratory compression tests show that the rocks are highly brittle at these low effective stresses.

Production from the Haynesville is a complex function of geopressure gradient, effective stress, reservoir quality, and completion practices. A wide range of completion parameters, such as length of horizontal well, choke size, number of stages, and proppant volume, have been tested to find optimal production strategies. Large choke sizes, which increase initial potential, can have a detrimental effect on long-term production and smaller choke sizes lower the decline rates and increase long-term well productions. Initial potential and production are higher in the east and south regions with higher pressure, carbonate/silica content and total organic carbon than the northwest region in Texas with lower total organic carbon but higher clay content.