Abstract

Interest is rapidly growing in obtaining reservoir geomechanical data suchas insitu stresses and rock strengths, since it is now realized that they areimportant to a wide range of applications; from bore hole stability, through towell stimulation and reservoir management issues such as optimal placement ofwater injectors and oil producers. Reservoir stress magnitudes can be inferredby continuous logs, for example by sonic based measurements. But these must becalibrated to direct measurements of stress. At present the only reliable, accurate measurements of insitu stress come from micro-fracturing. Following anintroduction to geo-mechanical effects on limestone reservoirs, the paperfocuses on a micro-fracturing technique, based on the Modular Dynamics Tester, MDT***, which efficiently and economically yields high quality stressinformation. The theory and application of micro-fracturing with the MDT***will be illustrated by an example in which reliable estimates of stressmagnitudes, and lower bounds of rock tensile strengths, were obtained in anonshore Abu Dhabi limestone reservoir. This is the first published example ofany stress test which has been performed by micro-fracturing in a limestonereservoir. To allow an understanding of the feasibility of the technique inlimestone reservoirs, current best practice guidelines for planning and runningMDT*** minifracs will be summarised. This will include recommendations on howthe MDT*** can be combined with sonic, imaging and caliper logs to gain othervaluable stress related data, such as principle horizontal stress directions, and continuous depth vs stress profiles (minifrac can only give point wisedata).

Introduction

This paper describes the successful application of a wireline conveyed tool, the MDT*** (figure 1 for tool diagram) to obtain accurate measurements of farfield stress magnitudes in an onshore Abu Dhabi field limestone oil field, using a microfracturing technique. The theory and application of this tool aredescribed later in the paper. The measurements were performed for furthercoupled flow and geomechanics reservoir modeling of the field. The generalapplications of reservoir stress data and some important equations involvingstress are briefly discussed below.

Knowledge of the stress distributions within a reservoir can have severalimportant field applications. For example:

  • Maintainence of well bore stability whilst drilling a well

  • Maximisation of the rate of penetration of the drill bit

  • Design of complex well trajectories

  • Optimisation of hydraulic fracturing stimulation

  • Minimisation of the tendency of a well producing fluids from a mechanicallyweak formation to induce collapse of that formation, and possibly, to produceparts of it to surface.

  • Effective management of reservoirs in which stress changes over time couldaffect production.

The above topics are discussed in more detail now.

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