Experimental Study of Fracture Conductivity for Water-Fracturing and Conventional Fracturing Applications

Abstract

Summary Hydraulic fracturing treatments that use treated water and very low proppant concentrations (commonly referred to as water-fracturing treatments or "waterfracs") have been successful in stimulating low-permeability reservoirs. However, the mechanism by which these treatments provide sufficient conductivity is not well understood. To understand the effects of fracture properties on conductivity, a series of laboratory conductivity experiments was performed with fractured cores from the east Texas Cotton Valley sandstone formation. The results of this study demonstrate that fracture displacement is required for surface asperities to provide residual fracture width and sufficient conductivity in the absence of proppants. However, the conductivity may vary by at least two orders of magnitude, depending on formation properties such as the degree of fracture displacement, the size and distribution of asperities, and rock mechanical properties. In the presence of proppants, the conductivity can be proppant- or asperity-dominated, depending on the proppant concentration, proppant strength, and formation properties. Under asperity-dominated conditions, the conductivity varies significantly and is difficult to predict. Low concentrations of high-strength proppant overcome the uncertainty associated with formation properties and provide proppant-dominated conductivity. The implication of these results is that the success of a water-fracturing treatment is difficult to predict because it will depend significantly on formation properties. This dependence can be overcome by using high-strength proppants or proppants at conventional field concentrations. Introduction Although proppants are routinely used to achieve conductivity during hydraulic fracturing treatments, recent fracturing treatments using treated water and very low proppant concentrations (commonly referred to as water-fracturing treatments or "waterfracs") have been successful in low-permeability reservoirs.1–4 The mechanism by which these treatments provide sufficient conductivity is not well understood. The presence of residual fracture width caused, for example, by surface asperities and proppant bridging, and the lack of damage associated with the use of gels in conventional proppant treatments, are possible explanations.2,5 Residual fracture width has been observed during laboratory experiments6 and field tests7 and can be attributed to the combined effects of surface roughness and fracture displacement.8 The surface asperities are thought to withstand high formation-closure stresses and create sufficient conductivity for wells completed in very low-permeability formations. The low concentrations of proppant are added to supplement the asperities and improve overall fracture conductivity. Factors affecting fracture conductivity and proppant-pack permeability have been reported in the literature. The importance of parameters such as fracture displacement, fracture roughness, mechanical properties, and closure stress on fracture conductivity have been demonstrated in the absence of proppants.9–12 When proppants are present, parameters such as proppant strength, proppant concentration, and closure stress have been shown to be important.13,14 However, these studies were performed with fractured cores in the absence of proppants or with proppant and flat, parallel core faces. No study has addressed the effects of fracture properties on conductivity in the presence of low concentrations of proppant (i.e., conditions that may exist during water-fracturing treatments). This paper investigates the effects of fracture properties on conductivity for a variety of conditions ranging from fractured systems to water-fracturing conditions to conventional proppant fracturing conditions. A series of laboratory conductivity experiments was performed with fractured cores from the east Texas Cotton Valley sandstone formation. Jordan sand and sintered bauxite proppants were used at concentrations of 0, 0.1, and 1.0 lbm/ft2, and the conductivity was measured at effective closure stresses ranging from 1,000 to 7,000 psi. The work investigates the relative influence of proppants and asperities on conductivity and demonstrates the benefits of using proppants. Water-Fracturing Treatments. Water-fracturing treatments discussed in the literature use a water-based fluid containing friction reducer (usually a manmade synthetic polymer, but a low concentration of natural guar polymer is sometimes used as a substitute for the friction reducer), clay stabilizers, and surfactants as necessary. This fluid is intended to serve as the pad fluid and to provide proppant transport. Common water-fracturing treatments involve pumping a pad fluid for the first 50% of the job, followed by a proppant stage where the proppant concentration is held constant at 0.5 lbm/gal. At the end of the job (usually the last 5%, based on fluid volume), the proppant concentration is increased to 2 lbm/gal. The higher proppant concentration is intended to improve connection between the wellbore and the fracture. Some potential problems with water-fracturing treatments include low conductivity and poor proppant transport. In low-permeability formations, low fracture conductivity is not a major limitation to production, provided the conductivity is not too low. The poor proppant transport is caused by the low viscosities of the water-fracturing fluids and results in rapid settling of the proppant particles. This inability to carry proppant a significant distance away from the wellbore can severely limit the effective fracture length. Fracture length is the key variable for initial production potential and ultimate recovery from very low-permeability formations. Therefore, if the proppant does not get transported toward the tip of the fracture, the success of the water-fracturing treatment will depend entirely on the conductivity created by surface asperities or some other mechanism. Experimental Methods Fractured Cores. Sandstone cores from the east Texas Cotton Valley formation were used in this study. The cores (which were obtained from depths ranging from 8,500 to 10,000 ft) had porosities of approximately 12% and permeabilities of approximately 0.05 md. Rock mechanical properties were determined from static triaxial compressive strength tests conducted at ambient temperature and 4,500 psi confining pressure. Young's modulus ranged from 3.6×106 to 7.0×106 psi, and Poisson's ratio was approximately 0.32.

Development of an Efficient Embedded Discrete Fracture Model for 3D Compositional Reservoir Simulation in Fractured Reservoirs

“…Maloney et al (1989) showed that the proppant packs may crush under stress leading to reduction in the fracture porosity. Likewise, Fredd et al (2001) presented laboratory data showing the effect of closure stress on propped fracture conductivity for a rock with relatively high Young's modulus. Therefore, recent studies such as Rubin (2010) and Cipolla et al (2010) used specific closure-stressconductivity tables in the simulation of unconventional reservoirs to account for the dynamic behavior of hydraulic fractures.…”

Measurement of realistic fracture conductivity in the Barnett shale

“…The displaced fractures have irregular surfaces and therefore do not completely match after closure. Laboratory and field tests have reported residual fracture widths that can be attributed to the combined effects of surface roughness and fracture displacement of the rock (Branagan et al, 1996;Van Dam and de Pater, 1999;Fredd et al, 2001). There are multiple mechanisms of propped fracture conductivity.…”

A new theoretical method to calculate shale fracture conductivity based on the population balance equation

“…The second dataset is collected from the published literature by Fredd et al (2001). In their work, they measured the proppant size distribution after the conductivity experiment at 7000 psi closure stress.…”