It is said that unconventional gas shales are easy to find but difficult to fracture. This is so because of the very large surface area (1 to 10 million square feet) required in these ultra low permeability systems for gas production at economic rates, and the problems associated to create it and maintain it. Successful hydraulic fracturing in these reservoirs requires understanding of near-wellbore conditions of perforation, fracture initiation, and near-wellbore fracture tortuosity. It also requires understanding of far-field conditions of fracture containment, fracture complexity, proppant transport, and rock-fluid interactions. Unfortunately maintaining the created surface area bounded within the reservoir section is typically problematic. More importantly, the low proppant carrying capacity of slick water and the tortuosity of the fracture geometry results in poor proppant transport and placement. This leads to considerable loss of surface area and fracture conductivity during production, thus the challenge. In this paper we concentrate on the development of hoop stresses in horizontal wells as a function of the type of completion (cased hole or open hole), and time (creep). We show that the wellbore hoop stresses are significantly affected by the quality of the cement job (cased and cemented completions). Mud channeling during displacement results in uneven support at the formation/casing interface, and leads to considerable changes in the hoop stresses, and the associated breakdown pressures for fracture initiation. We conclude by stating that understanding these effects will result in better decisions and better near-wellbore conditions for fracture initiation.
Economic production from unconventional gas shales requires of adequate conditions of reservoir quality and completion quality. Reservoir quality defines the potential of the reservoir in terms of gas in place, pore fluid saturations, permeability, organic content and maturity, and the effect of the latter on reservoir pressure. Completion quality defines the reservoir production potential, as defined by conditions that facilitate reservoir contact with hydraulically created, surface area, fracture complexity (increasing the surface area per unit reservoir volume), and its preservation during production. In addition, completion quality is also controlled by the connectivity between the near-wellbore fracture region, often dominated by stress concentrations around the near-wellbore, and the far-field fracture region, dominated by the far-field stresses. Finally, there is the question of fracture initiation and the potential of some fracturing stages along the horizontal wellbore being more difficult to breakdown than others. In this paper we study the near-wellbore stress concentrations, as affected by rock creep, the type of completion (cased hole or open hole), and the resulting conditions of breakdown (tensile failure), during wellbore pressurization for hydraulic fracturing. We show that the wellbore hoop stresses are significantly affected by time-dependent creep and by the type of completion (open hole or cased and cemented completions). Results show significant changes in the stress concentrations with increasing elastic anisotropy. The wellbore pressure and the pore pressure are balanced for all cases. It is apparent that in all cases, the top and bottom orientations (90°) of the horizontal wellbore are the most prone to hydraulic fracturing.
