ABSTRACT:

Gas storage wells have been drilled in Halifax, Nova Scotia, Canada. One of these wells was cored from top to bottom penetrating about 500m of salt at 460 meters vertical depth. To obtain unconfined compressive strength (UCS), 18 intervals were tested with a total of 389 point load tests. Of the 61 tests, 18 intervals and 126 point load tests were done in intervals containing salt. The corresponding sonic log values from the same depth interval as the tests were used to calculate UCS values from existing correlations. The UCS values obtained from the available sonic log correlations matched well with the point load tests in most formations except for those containing salts. New correlations were developed from the point load tests and the corresponding sonic data for salt and are published in this paper. The new correlations show a reasonable match and provide a new tool for obtaining UCS values for salts from sonic logs. It should be noted that these new correlations require additional verification but should serve as a good starting point for subsequent wellbore stability analysis.

1. INTRODUCTION

1.1. Drilling Simulation

Well drilling represents a large share of field development costs. The use of drilling simulators has been used to plan and follow up the drilling process [1, 2, 3, 4, 5] and has been found to improve drilling operations. The use of drilling simulation software has helped to reduce costs substantially both offshore and onshore cases [3, 4]. Additionally, drilling simulator studies can improve the understanding of mature fields [5]. By performing drilling simulations, different drilling scenarios can be tested to find the best set of drilling operational parameters to optimize bit wear. The principles behind drilling simulation are based on the rock strength of the formations to be drilled. The rock strength can be determined through mechanical tests or can be estimated through drilling data by using correlations based on well log information [6, 7, 8]. Several correlations have been proposed using the information from sonic logs [6, 7, 8]. The computation of rock strength through correlations has the advantage that whole well sections can be modeled in as much detail as the well log data is recorded. Thus, the Apparent Rock Strength Log (ARSL) for the geological column of a new well can be constructed from available information of a closely related offset well. Field data is used as a reference in the first stage of the simulations to reproduce reported values of Rate of Penetration (ROP) and drill bit wear. In this way, future wells can be better planned by creating an ARSL for the new well and optimizing the calculated drill bit wear, bit runs, Weight on Bit (WOB), ROP, and related operational parameters.

1.2. Deformation Behavior of Salt

Polycrystalline salt exhibits deformation behavior that is similar to the deformation behavior of brittle rocks and ductile metals. Salt creeps under any deviatoric stress.

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