Abstract
Fracture geometry is an important concern in the design of a massive hydraulic fracture for improved natural gas recovery from low-permeability reservoirs. Determination of the extent of vertical fracture growth and containment in layered rock, a priori, requires an improved understanding of the parameters that may control fracture growth across layer interfaces. We have conducted laboratory hydraulic fracture experiments and elastic finite element studies that show that at least two distinct geologic conditions can inhibit or contain the vertical growth of hydraulic fractures in layered rock:a weak interfacial shear strength of the layers andan increase in the minimum horizontal compressive stress in the bounding layers.
The second condition is more important and more likely to occur at depth. Differences in elastic properties within a layered rock mass may be important-not as a containment barrier perse, but in the manner in which variations in elastic properties affect the vertical distribution of the minimum horizontal stress magnitude. These results suggest that improved fracture treatment designs and an assessment of the potential success of stimulations in low-permeability reservoirs can be made by determining the in-situ stress st ate in the producing interval and bounding formations before stimulation. If the bounding formations have a higher minimum horizontal stress, then one can optimize the fracture treatment and maximize the ratio of productive formation fracture area to volume of fluid pumped by limiting bottomhole pressures to that of the bounding formation.
Introduction
In 1949, Clark introduced the concept of hydraulic fracturing to the petroleum industry. Since then, hydraulic fracture treatment to enhance oil and gas recovery in tight reservoir rocks has become standard practice. More recently, as a result of an increased need for better recovery techniques, massive hydraulic fracturing (MHF) has been used in low-permeability, gas-bearing sandstones in the Rock Mountain region and in Devonian shales of the Appalachian region, where it is uneconomical to retrieve gas in the conventional manner. Massive hydraulic fractures are designed to extend as much as 1000 m (3,281 ft) radially from the wellbore and generally require up to 1000 m3 (6,293 bbl) of fracture fluid.
MHF has been developed by trial and error, and the results are uncertain in many situations. Some of these large-scale stimulation efforts have been successful, but others have been extremely disappointing failures. The reasons for these failures are not clear, but it seems likely that improved understanding of the fundamental mechanisms of hydraulic fracturing should suggest ways of improving the efficiency and reliability of the MHF stimulation technique or at least indicate where this technique can be applied successfully.
Among the many technological problems encountered in MHF, one of the most important questions that must be answered properly to design a hydraulic fracture treatment for optimal gas recovery concerns the shape and overall geometry of the fracture. The question of fracture height and whether the hydraulic fracture will propagate into formations lying above and below the producing zone. When a fracture treatment is designed, the height of the fracture is the parameter about which the least is known, yet this influences all aspects of the design.
A hydraulic fracture usually grows outward in a vertical plane and propagates above and below the packers as well as laterally away from the wellbore. Vertical propagation is undesirable whenever the fracturing is to be contained within a single stratigraphic interval. If the hydraulic fracture is not contained within the producing formation and propagates in both the vertical and lateral directions (an elliptical fracture), failure of the treatment can occur because the fracture fails to contact a sufficiently large area of the reservoir. Moreover, there is an effective loss of the expensive fracture fluid and proppant used to fracture the unproductive formations. An extreme example where the containment of a hydraulic fracture is essential is the case of developing a fracture in a gas-producing sandstone without fracturing through the underlying shale into another sandstone that is water-bearing. Therefore, it is of great economic importance to the gas industry to understand the parameters that can restrict the vertical propagation of massive hydraulic fractures.
There are several parameters that are considered to have some effect on the vertical growth and possible containment of hydraulic fractures.
SPEJ
P. 19^
Probability-Statistical Models to Forecast Hydraulic Fracturing Efficiency
