Pore Pressures Control Drilling Economics
Authors: Howell J.N. 
Published: December 2007

Rock strength and mode of failure are functions of the rock pore pressures. Consequently, drilling rates should be relatable to the pore pressure and might be used to map the formation pressure profile while drilling. This paper summarizes a literature survey which was made to determine how pore pressure influences drilling response, and presents suggestions for considering these effects to minimize unnecessary well costs.

Introduction

Blowouts and lost circulation due to abnormal and subnormal pore pressures have plagued the drilling industry since its inception. The monetary losses are unknown but they must be astronomical considering the expensive precautions the drilling industry now takes to prevent their occurrence.

Mud programs are designed and maintained to provide hydrostatic pressures in excess of assumed formation pore pressures. If the . actual pore pressures are much greater than the design assumptions, the flow of formation fluids into the wellbore is possible and a blowout may result. Conversely, if the actual pore pressure is considerably lower than the assumed pressures, drilling fluid is lost to the permeable zones. Fracturing of shallow, unprotected formations, with complete loss of returns, has occurred when using weighted muds.

In addition to causing blowouts and lost circulation, formation pore pressures affect rates of penetration. Chip formation under a rotary rock bit and the hydraulic removal of these chips from their fracture craters are related to the formation pore pressure.

The normal formation pressure gradient in the Gulf Coast is considered to be 0.465 psi/ft of dept)" however, pressure gradients as high as 0.85 Psi/ft have been encountered. The high pressure gradients were always encountered below zones of normal pressure gradients, and it was noted that the change in pressure gradients usually occurred across thick shale sections such as the Hackberry and the Discorbis D Shale Wedge.

The changing pressure gradients in these thick, essentially homogeneous shales should produce measurable changes in the physical properties of the shale.

This paper is a simulation of a literature survey which was made to determine the effects of pore pressure on the drilling characteristics of rocks, specifically shale. The effects of formation pore pressure on drilling response chip formation, and removal are considered in detail, and means of detecting changes in formation pressure gradients are postulated.

BASIC ROCK MECHANICS

Structural failure is induced in sedimentary rocks by applying stresses which exceed the shear strength of the rock frame. The type of failure produced and the ultimate strength of the rock are variables which are functions of the pressure conditions surrounding the rock at the time of failure.

Mode of Failure

Rocks, like any other solid, will deform elastically with increasing stress until the yield strength of the rock is reached; at this point the rock fails in shear. If the shearing failure is accompanied by a complete loss of cohesion along the shear plane the rock is said to have failed by brittle fracturing. When forces of adhesion existing along the shear plane after fracturing exceed the shear strength of the rock frame before failure, the mode of failure is said to be ductile.

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