Doug Bentley

The Effect of Mechanical Properties Anisotropy in the Generation of Hydraulic Fractures in Organic Shales

Organic shale reservoirs have very low matrix permeabilities. An extensive conductive hydraulic fracture network is necessary to impose a pressure drop in the formation to produce hydrocarbons at an economic rate. In addition, horizontal wells permit the initiation of multiple hydraulic fractures within the reservoir section of the organic shale. The location of the lateral landing point can have a significant impact on hydraulic fracture geometry. The stimulated fracture system is influenced by the extensive horizontal laminations that are pervasive in shale reservoirs. The laminations will strongly influence the hydraulic fracture height because of the difference in rock mechanical properties measured normal and parallel to the bedding planes. In order to accurately predict fracturing height from logs in this environment, these mechanical property differences must be taken into account. A series of sonic logs have been run in organic shales and the stress profile generated from these logs has been estimated, accounting for the difference in mechanical properties in the vertical and horizontal directions. This stress profile has been calibrated to measured closure stresses acquired in-situ via micro-fracturing of multiple intervals in vertical, openhole environments. The results show that ignoring the impact of mechanical property anisotropy can lead to significant errors in the estimation of hydraulic fracture height. Correspondingly, the optimal landing point of a horizontal wellbore may not be selected when ignoring this effect. This can result in excessively high fracture initiation pressures, difficulty achieving injection rate or proppant placement, and unexpected fracture height growth. Simulations of hydraulic fracture width indicate that thin high-stress intervals can create pinch points that limit vertical fracture conductivity. Each of these factors can result in un-optimized hydrocarbon productivity. Mechanical Properties Anisotropy A fundamental property of shale is textural anisotropy due to the platy nature of the abundant clay minerals within its matrix. As the clay minerals are deposited they are aligned by gravity with the surface of the earth. This fine scale alignment, along with subtle differences in clay content and other minerals, creates fine-scale layering or laminations. The presence of these laminations leads to differences, or anisotropy, in many fundamental rock properties including permeability, electrical resistivity, acoustic velocity, moduli, and Poisson’s ratio. The anisotropic nature of shale should be taken into account when attempting to predict its behavior. This paper will focus on the effects of anisotropy in organic shale to hydraulic fracturing and the resulting fracture geometry. Hornby et al. (1999) demonstrated acoustic anisotropy by comparing compressional slowness recorded in a series of deviated boreholes drilled in the North Slope of Alaska. They demonstrated that the compressional slowness of shale decreased significantly as borehole deviation increased. Shale is much slower normal to laminations than parallel to laminations. Sonic anisotropy was minimal in the underlying sandstone (Fig 1). If the shale laminations are horizontal and the formation has no dip, the formation is defined as transverse isotropic with a vertical axis of symmetry (TIV). The sonic velocities measured in the two vertical orthogonal directions are assumed equal. They are different from the horizontal velocity measured parallel to the laminations. Fig. 2 demonstrates TIV anisotropy. Sonic velocity is the same in all directions for an isotropic formation.