Abstract

Hydraulic fracturing is the requisite methodology for completing nano-darcy matrix permeability tight gas shales. Commercial success in producing these reservoirs depends to a large extent on successful hydraulic fracturing. There is growing evidence that initiating hydraulic fractures from horizontal wellbores is often difficult, and requires abnormally high treating pressures. In this paper, we show that the combination of high stiffness, significant elastic anisotropy, and coupled elastic property and horizontal stress development, in tight gas shale reservoirs results in complex near-wellbore stress concentrations, not observed in isotropic rocks. Using finite element analysis (FEA) and numerical modeling, with continous mechanics and transverse anisotropic elasticity, we provide insights on the stress concentration resulting from various conditions of stress (normal and reverse faulting) and material properties. We also provide a simplified methodology for first-pass calculation of these stress concentrations, and thus for predicting the potential for problems during fracture initiation. Modeling near-wellbore effects in horizontal completions in anisotropic shales is straightforward. However, the calculations require a larger set of material properties (4 elastic constants), as well as in-situ stresses, and reservoir pressure.

Introduction

Successful production of hydrocarbons (gas and oil) from tight shale and sand reservoirs strongly depends on successfull hydraulic fracturing and the generation of a large fracture surface area. Maximizing fracture surface area and volume of impacted reservoir requires the existence of textural heterogeneity at a reservoir scale (e.g., the presence of mineralized fractures, preferential depositional fabric), and a favorable orientation of these with respect to the in-situ stresses(1). Field evidence from microseismic monitoring(2,3,4,5,6) indicates that in some tight gas fields a broad lateral spread of microseismic events is measured during fracturing. The spatial extent of these events defines the region affected by the stimulation treatment, and is often interpreted to represent multiple fracture branches as well as shear movement on pre-existing discontinuties. When the surface area created by the treatment is large, subsequent productivity is often prolific (2).

Unfortunately, other field experience also indicates problems initiating fractures. For example, the capacity of pumping equipment, or the safe pressure rating of the tubulars can be reached prior to fracturing. Moreover, even if or when fracture initiation occurs, the treating pressures can remain high, inhibiting the normal progress of the treatment (e.g., low pumping rates are insufficient for carrying proppant). Small stages of high viscosity fluids may then be used to facilitate fracture initiation and promore less tortuous (i.e., wider) fractures near the wellbore (along with sand slugs after initiation). This technique helps to lower the treating pressure after fracturing and facilitates subsequent pumping of the main treatment (commonly slickwater). However, a risk of high viscosity slugs is to leave residue and thus lower the fracture conductivity at its entry point with the wellbore.

In brittle, high modulus formations high fracturing initiation and treating pressures generally result from localized stress concentrations, and from the consequent tortuosity associated with small apertures and changing fracture orientations in the vicinity of the wellbore. Furthermore, because of the large inherent anisotropy in tight gas shales, near wellbore stress concentrations are considerably more variable than would be anticipated from traditional isotropic elastic modeling. Anisotrpy is a property that indicates that material properties are different in different directions. There are different degrees of anisotropy. If the material has the same properties in all directions, it is said to be isotropic. A transversely isotropic material (such as a stratified section) has properties that are symmetric about one axis (i.e., the vertical properties differ from properties that are the same in any horizontal direction. Orthotropy is a further derviation from isotropy (properties vary in three different orthogonal directions).8 In this paper, we analyze the near-wellbore stresses that develop in horizontal completions and in anisotropic tight shales, under varying stress regimes, pore pressure conditions, and material properties.

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