The purpose of this paper is to investigate rock failure by applied hydraulic forces. This investigation will be approached through a review of basic rock mechanics concepts.

Rock mechanics is the one most important physical consideration from the time a well is spudded until abandonment. Rock mechanics is defined as the area of knowledge which deals with how rocks that comprise the outer part of the earth's crust behave and respond to applied forces. We are interested in three basic modes of rock behavior - failure, failure prevention, and yielding.


Rock behavior influences several phases of well life including drilling, completing, and operating. Specifically it influences

  1. formation drillability,

  2. lost returns,

  3. hole sloughing,

  4. hole closure,

  5. abnormal pressure,

  6. perforating,

  7. fracturing,

  8. additional recovery injection pressures and pattern efficiency, and

  9. squeeze cementing.

This paper will be limited to rock behavior and response due to applied hydraulic forces. Other important man-induced forces that affect rock behavior are mechanical, such as the drill bit, packer, and fracture proppant, and explosive forces, which include the conventional jet perforating charge, proposed jet drilling charge, and the proposed atomic fracturing charge.

Before we can determine rock behavior due to applied forces, we need to briefly consider in-situ forces and rock properties.

In-Situ Rock Stresses

Materials that make up the earth's crust can behave as an elastic, plastic, or elastic-plastic material depending upon material nature, applied force, and temperature. Materials that have a tendency to behave plastically at low stress levels (depths less than 20,000 feet) are shale and salt and among the materials that normally behave elastically are sandstone and limestone.

Normally, when we study a material, we like to consider it homogeneous and isotropic. The rock that we are concerned with is porous and laminated; therefore, rigorously speaking, the rock in question is neither homogeneous or isotropic since homogeneous and porous are contradictory adjectives and likewise isotropic and laminated are opposites. However, initially in our stress analysis, we will consider the rock to be an elastic, homogeneous, isotropic, porous material.

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