The Use of Logs And Correlations For Rock Mechanical Predictions

Des etudes de la correlation entre la force mecanique et les lochs de cable, pourrait être un outil très valuable pour l'elaboration et l'analyse supplementaire des operations de forage. Les faits mentionnes ci-dessous montrent comment les correlations de ce genre sont executees et appliquees par la societe Saga Petroleum. Il s'est trouve que ces correlations seront valables sur une grande gamme de lithologies, et elles ont egalement prouvees d'être un outil efficaee dans l'elaboration et la poursuite de la stabilite des puits fores.

Die Korrelation zwischen der mechanischen Starke und dem Bohrprofil können ein nuetzliches Mittel fuer die Planung und die nachtragliche Analyse von Bohroperationen sein. Diese Arbeit beschreibt, wie man solche Korrelationen ausfuehrt und wie sie bei der Saga Petroleum angewandt werden. Die Korrelationen haben sich als gueltig erwiesen fuer eine weite Bandbreite von Lithologien, und sie sind ein wirksames Mittel, urn die Integritat des Bohrloches zu planen und weiter zu verfolgen.

Geomechanical methodology is now becoming used more frequently to investigate different and costly challenges m petroleum exploration and exploitation. Wellbore integrity (collapse and fracturing) during drilling and production, reservoir subsidence and caprock integrity are just a few of these challenges. To apply geomechanical methodology, it is necessary to have a good indication of the material parameters of the rock to be drilled or produced from. It is however, a costly operation to take a core and determine its material parameters, and consequently this is not always done. The core will in addition only represent a point measure. In applied geomechanics it is often important to analyse a larger portion of the well, and consequently a continuous "strength log" would be desirable. Strength logs have been proposed by several investigators, among others Avedissian and Wood (1968), Deere et. al. (1969) and Tokle et, al. (1986). Sonic properties were the basis for Avedissian and Wood (1968) and for Kazi et. al. (1983), while engineering classification and index properties were the basis for Deere et. al. (1969). Finally, Tokle et. al. (1986) based their work on a multiple regression analysis of several logs. All investigators claimed that their approach had some merit, but the practical applicability has not been encouraging. Lately IKU Petroleum Research (Trondheim, Norway) (1994) has revisited the strength log concept, based on correlations between sonic (acoustic) measurements during triaxial testing, and strength data. This correlation gives the uniaxial strength parameters, C° and m (slope of failure criterion). This paper discusses the practical approach taken in Saga Petroleum to follow up this work by using measured strength parameters from cores to make correlations with wireline logs. Hopefully we will be able to determine linear correlations between strength data and wireline logs. By applying these correlations to the wireline logs, a continuous record of strength could be made for a new well from previous recorded strength measurements.

In Saga Petroleum geomechanical testing is done systematically for most of the wells operated by Saga Petroleum. The results from these tests are stored in a geomechanical database. In the database interpretations can be made at several levels. At the plug level an interpretation of the elastic properties, Young's Modulus and Poisson's ratio (E, v) can be made. If failure-results from triaxial tests are available, performed at minimum three different effective confining stress levels (σ¹3), a linear Mohr-Coulomb failure criterion can be determined.