Abstract

This study was undertaken to develop a correlation curve to show the variation of fracture pressure gradient with depth in the Samaan Field Offshore Trinidad. The method used to predict the Samaan formation fracture gradients is that of Ben Eaton. The results indicate that Samaan fracture gradients vary from 0.6 psi/ft at 1000 feet to 0.99 psi/ft at 12,000 feet.

Introduction

Fracture pressure gradients are necessary in designing casing and mud programs and help to avoid lost circulation problems while drilling. Fracture gradient has been shown to be a function of three independent variables, namely formation pore-pressure, overburden stress gradient and Poisson's ratio. In this study, each of the above three variables has been determined for the Samaan area and the resulting fracture pressure gradient curve has been constructed. pressure gradient curve has been constructed.

Prediction Methods

Several studies have been carried out to arrive at a method of predicting formation fracture gradients. Hubbert and Willis in 1957 and Matthews and Kelly in 1967 published papers outlining two different methods of predicting fracture gradients. Ben Eaton in 1969 developed a method which is modification of the Hubbert and Willis approach.

Hubbert and Willis Method

Hubbert and Willis used the following equation to determine fracture gradient:

$Pw=(S−P)(u)+P$
(1) D D D 1-u D
(A) (B) (C)

Term (A) is the effective stress gradient, (B) is the horizontal to vertical stress ratio and (C) is the formation pressure gradient. Hubbert and Willis assumed that the over-burden stress gradient S/D was equal to 1.0 psi/ft and Poisson's ratio (u) equal to 0.25. Substitution of these values in equation (1)results in the following:

$PW=13(1+2P)PW=13(1+2P)$
(2) D

However, these assumptions of Hubbert and Willis have been shown to be in error in many areas.

Matthews and Kelly Method

Matthews and Kelly presented a fracture gradient equation similar to equation.

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