Abstract

This research examines stress variations with depth in the Permian Basin Spraberry/Dean/Wolfcamp Shale to understand why many microseismic events occur at upper formations when we hydraulically fracture the lower formations. An interesting phenomenon of drilling-induced tensile fractures is observed in the image log. That is, there are drilling-induced tensile fractures in the Spraberry and Dean formations, but there are no drilling-induced tensile fractures in the Wolfcamp formation. This brings out a question: how variable is the stress state with depth? We estimate the pore pressure and the three principal stresses with depth to answer this question. The pore pressures and minimum principal stresses are analyzed from the Diagnostic Fracture Injection Test, which are consistent with literatures. Maximum horizontal stress in the Spraberry formation is constrained by observation of drilling-induced tensile fractures, while maximum horizontal stress in the Wolfcamp formation is constrained through the estimation of Uniaxial Compressive Strength. We find the stresses in the Wolfcamp formation are more isotropic than those in the Spraberry formation. More importantly, the decreasing of the pore pressure gradient and the frac gradient when going from the lower formation to the upper formation leads to many out-of-zone microseismic events.

1. INTRODUCTION

The Permian Basin represents one of the nation’s oldest and most widely recognized hydrocarbon bearing regions. Unlike other plays such as the Bakken and Eagle Ford, the Permian shows much greater geologic complexity, consisting of several unique sub-basins (Fig. 1), each with its own unique characteristics [1].

The specific area studied in this research is in Midland Basin, which is east of the Central Basin Platform, west of the Eastern Shelf, and north of the Val Verde Basin. The formations discussed in this paper are the Spraberry, Dean, and Wolfcamp series, which are at the bottom of the Permian system. The three formations are mainly shale facies, but with high mechanical complexity. Fig. 2 shows the wells we studied, which are drilled in the Spraberry/Dean/Wolfcamp formations. Most data, including sonic log data, Formation Microresistivity Image (FMI) data, and core test data, come from the monitor well M. Besides, we analyze the Diagnostic Fracture Injection Test (DFIT) in four horizontal wells, A, B, C, and D.

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