ABSTRACT

Impact rock mechanics have in deep, complex and non conventional well derilling as well as reservoirs management within the oil industry. The use and application of new geomechanic technologies have shown big technical and economic benefits in the last few years. This paper presents adjustment for Kuster and Tokzos's model in oil fields in Mexico, to forecast rock elastic properties, such as, (K) volumetric moduli and (μ) shear moduli, starting from field available information (geophysical logs), such as, sonic and density logs. This model is an original work, showing semi-empirical equation adjusted to Mexican fields, these equations are widely displayed, with the aim of making usage and analysis easier. Leading to a substantial enhancement in the forecasting of elastic properties when comparing to the existing models of effective media for studies in Mexico. The model was validated, with two naturally fractured carbonated wells, and one with special data logs.

INTRODUCTION

Geological characteristics from material could in most cases depend on pressure, time and the environmental conditions; reservoirs represent vast areas at high depths so, they can only be evaluated on the basis of Small samples, obtained from specific points. Due to these extreme depths, obtaining rock mechanics characteristics turns into comes to be a to difficult and expensive task.

For solving this problem in a small part, methodologies to forecast rock mechanics properties can be used, It is known that formation does not count on a unique strain-stress and linear ratio. The same material could present differences at different confine pressures.

The methodology used to achieve this purpose, in other words, forecasting poro-clasto-dynamics properties in Mexican fields, starts with the fact of compiling geophysics data logs, both with this information and the effective media theory, mainly from Kuster and Toksoz, rock elastic properties can be forecasted (elasto-dynamics moduli).

It is of utmost importance to perform this study, as Mexico counts on over 50 oil fields and about 35 fields are naturally fractured carbonated reservoirs, thus they are more susceptible to change stress and strain, meaning that this methodology not only can be used for naturally fractured reservoirs, but for others.

EFFECTIVE MEDIA THEORY

(Mavko, 1998)

To forecast elastic properties the research of a theory leading to mathematical equation was needed, to relate rock elastic properties with each of their components. This theory is called The Effective Media Theory (Watts 1976, Norris 1985, Zimmerman 199 I, Berryman 1992).

To forecast effective elastic properties of a grain and pore mixture theorically, it is needed to specify the following: (Budiansky 1965, Berryman 1995)

  • The material volume fraction

  • The material elastic model

  • Detail of geometry inclusions

The effective media theory considers rock as a solid material, containing inclusions, pore or fractures, this theory is the base for calculating elastic properties.

The stress strain behavior in an isotropic material can be characterized by: volumetric moduli (K), shear (μ), Young (E), Poisson relation (v) and the Lame constant (λ).

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