Laboratory Measurement of Hydraulic-Fracture Conductivities in the Barnett Shale

Abstract

Summary Horizontal wells that intersect multistage transverse fractures created by low-viscosity fracturing fluid with low proppant loadings are the key to revitalizing production from the Mississippian Barnett shale in the Fort Worth basin in Texas. However, direct laboratory measurements of both natural- and induced-fracture conductivities under realistic experimental-design conditions are needed for reliable well-performance analysis and fracture-design optimization. In this work, a series of experiments was conducted to measure the conductivity of unpropped natural fractures, propped natural fractures, unpropped induced fractures, and propped induced fractures with a modified American Petroleum Institute (API) conductivity cell at room temperature. Fractures were induced along the natural bedding planes, preserving fracture-surface asperities. Natural-fracture infill was taken into consideration during conductivity measurements. Proppants of various sizes were placed manually between rough fracture surfaces at realistic concentrations. The two sides of the rough fractures either were aligned or were displaced with a 0.1-in. offset. After pressure testing on the system integrity, nitrogen was flowed through the proppant pack or unpropped fracture to measure the conductivity. Results from 88 experiments show that the conductivity of hydraulic fractures in shale can be measured accurately in a laboratory with appropriate experimental procedures and good control over experimental errors. It is proved that unpropped, aligned fractures can provide a conductive path after removal of free particles and debris because of the brittleness and lamination of shale. Moreover, poorly cemented natural fractures and unpropped displaced fractures can create conductivities of up to 0.5 md-ft at formation-closure stress, which is one to two orders of magnitude greater than the conductivity provided by cemented natural fractures and unpropped aligned fractures. This study shows that propped-fracture conductivity increases with larger proppant size and higher proppant concentration. Longer-term fracture-conductivity measurements indicate that, within 20 hours, the fracture conductivity could be reduced by as much as 20%.

Influence of Normal and Shear Stress on the Hydraulic Transmissivity of Thin Cracks in a Tight Quartz Sandstone, a Granite, and a Shale

“…These effects are well known from previous experimental studies (e.g., Chen et al, 2015;Cuss et al, 2011;Engelder & Scholz, 1981;Guo et al, 2013;Kranz et al, 1990;McKernan et al, 2017;Tanikawa et al, 2010;Zhang et al, 2013Zhang et al, , 2015Zimmerman & Bodvarsson, 1996). In experiments designed to investigate the effect of low amplitude fluid pressure oscillations of seismogenic origin on permeability of Berea sandstone, Elkhoury et al (2011) andCandela et al (2015) reported permeability enhancements by up to 50% following fluid pressure oscillations of 20 s period for 3 min.…”

“…For example, Zhang et al (2013Zhang et al ( , 2015 used a standardized hydraulic flow cell to measure the normal stress dependence of fracture transmissivity over the range of reservoir pressures and temperatures and demonstrated the transmissivity-enhancing role of sand proppants (American Petroleum Institute industry-standard; API RP 61, 1989;API RP 60, 1995).…”

Creep of Posidonia Shale at Elevated Pressure and Temperature

“…Experimental and modelling results usually show enhanced fracture closure rates at high closure stress and temperature, low Young's modulus and high amount of soft minerals, low fraction of proppants with small size, low fracture roughness and minor shear displacement (minor self-propping). The presence of water also accelerates the rate of fracture permeability reduction, in particular for clay-rich shales (Akrad et al, 2011;Alramahi and Sundberg, 2012;Guo and Liu, 2012;Kassis and Sondergeld, 2010;LaFollette and Carman, 2010;Liu and Sharma, 2005;Morales p [4] et Pedlow and Sharma, 2014;Reinicke et al, 2010;Stegent et al, 2010;Volk et al, 1981;Zhang et al, 2013;Zhang et al, 2015;Zhang et al, 2016b). The brittleness of shales, which correlates to some extent with their composition and elastic properties (Rybacki et al, 2016;Zhang et al, 2016a), may be also relevant for the long-term fracture healing rate.…”

Effect of Permeability Anisotropy on the Production of Multi-Scale Shale Gas Reservoirs

“…According to the experimental results of Zhang et al [35], the relationship between the effective permeability and gas pressure can be written as:…”