A Study of Horizontal-Wellbore Failure
- C. Hsiao (Halliburton Services)
- Document ID
- Society of Petroleum Engineers
- SPE Production Engineering
- Publication Date
- November 1988
- Document Type
- Journal Paper
- 489 - 494
- 1988. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 1.2.3 Rock properties, 5.1.2 Faults and Fracture Characterisation, 5.2 Reservoir Fluid Dynamics, 2.5.2 Fracturing Materials (Fluids, Proppant), 1.6 Drilling Operations, 4.1.2 Separation and Treating, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation
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Summary. A theoretical model of horizontal-wellbore failure has been developed based on maximum-normal-stress theory (for tensile fracturing) and Drucker-Prager failure theory (for compressive failure) under openhole conditions. This model may he used to help determine the permissible borehole operating-pressure range, providing a convenient scheme for selecting borehole pressure to minimize the risk of borehole failure during drilling and production.
Interest in increasing fracture area by creating multiple vertical fractures from a single horizontal wellbore has increased in recent years because increased effective drainage in a reservoir can result, providing better productivity conditions @ can be achieved in conventional vertical wells. However, borehole failure problems may become a major concern during drilling or hydraulic fracturing. While drilling, one has to control borehole pressure by selecting drilling fluid to prevent borehole failure, including borehole compressive failure and borehole tensile fracturing. On the other hand, hydraulic fracturing treatments are often performed to create a fracture in a reservoir rock intentionally by injecting fracturing fluid under pressure through a borehole. After a fracturing job, low borehole pressure, under drawdown conditions during production, may tend to cause borehole compressive failure.
By considering stress distribution in the wall of a wellbore, Daneshy presented a failure criterion for the tensile fracturing of an inclined wellbore in which the axis of the borehole was inclined with respect to the orientation of in-situ stresses. The stresses induced on the borehole wail because of fluid penetration into the permeable formation were considered, but the borehole compressive failure criterion was not included. His study was focused on the physical explanation of the fracture initiation and fracture trace of experimental observations. Bradley developed borehole failure criteria for the borehole instability problem of inclined wellbores by a semi-empirical approach. The effect of fluid flow into the formation was not involved in the evaluation of stress field on the borehole wall.
This paper presents a study of horizontal-wellbore failure. It gives a comprehensive stress analysis of the borehole wall in an isotropic, porous, elastic formation subjected to in-situ tectonic stresses. The chemical interaction between the injected fluid and the borehole wall is not considered. A theoretical model of mechanical failure phenomena has been developed from maximum-normal-stress theory (for tensile fracturing) and Drucker-Prager failure theory (for compressive failure) under openhole conditions. To apply such a failure model, it is necessary to know the material properties that are determined experimentally. Results shown in the last section of the paper have been obtained from idealized data. Field testing is required to demonstrate the validation of the developed failure model or to indicate what kind of modifications should be done.
Stress Distribution on the Borehole Wall
It is assumed that the rock in a permeable formation is brittle, linear elastic, homogeneous, isotropic, and porous, and that the fluid flow through the pores obeys Darcy's law. Fig. I shows the assumed possible distribution of pore pressure around the wellbore for penetrating and nonpenetrating fluids. Stresses are considered positive when tensile. The effective stresses, oij, can be expressed as (1)
sij = total stresses, p = pore fluid pressure, and bij = Kronecker delta.
Total stresses on the borehole wall can he obtained by superposing the three individual stress fields generated by (1) the three principal in-situ tectonic stresses, (2) the pressurization of the open hole of the wellbore, and (3) fluid penetration from the pressurized open hole into the permeable formation. When the treatment fluid has the same viscosity as the reservoir fluid, the formation has a uniform permeability, and the fluid flow is axisymmetric, the mathematical expressions of the effective stress distribution on the borehole wall with respect to cylindrical borehole coordinate system, rthetaz, defined in Fig. 2 are
(6) and (7)
= three effective normal stress components in cylindrical borehole coordinate system, = three effective shear stress components in cylindrical borehole coordinate system, = three normal in-situ stress components in
= three shear in-situ stress components in XYZ = borehole pressure,
u = Poisson's ratio of formation, and
= constant of porous elastic rock related to rock matrix and rock bulk compressibilities, cr and cb, by =1-cr/cb.
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