Preliminary Results on Multi-Stage Creep Experiments of the Wolfcamp Shale at Elevated Temperature

Rassouli, F. S. (Stanford University) | Zoback, M. D. (Stanford University)


ABSTRACT In this paper, we report multi-stage creep experiments in shales at three different temperatures. We used two samples from the Wolfcamp formation in the Permian Basin with different mineralogy and bedding orientations. In these experiments, we increased the confining pressure to 40 MPa followed by an increase in differential stress to 40 MPa at room temperature. The differential stress was then kept constant several hours. We repeated these loading steps at 50 °C and 80 °C to study the effect of temperature on viscoplastic properties. The results from this study showed that the viscoplastic deformation of the horizontally drilled sample with bedding planes is more affected by the elevation of temperature than the vertically drilled sample with no distinguished bedding planes, although the latter sample has a higher percentage of clay and organic matter. 1. INTRODUCTION Propagation of hydraulic fractures requires the pressure inside the fractures to exceed the magnitude of the least principal stress. In this context, vertical propagation of hydraulic fractures in unconventional shale formations is controlled by variations of the magnitude of the least principal stress with depth. In previous papers from our group, we showed that the magnitude of stress variation is a function of the relative degree of viscoplastic stress relaxation. This time-dependent viscoplastic behavior is shown to be affected by the mechanical and mineralogical properties of the rocks, especially clay plus kerogen content [1–4] as well as the reservoir stress and thermal conditions [5–7]. So far, we have used the following simple power-law model, a simple model that fits the collected creep data over different time periods reasonably well [2,8]: (equation) In this model ε is creep strain, σ is the applied differential stress, t is time, J is the creep compliance factor and B and n are creep constants. In these publications [6, 10], we argued that the B and n values represent recoverable and inelastic deformation of shale rocks, respectively. We conducted all these creep tests at room temperature. Here, we try to initiate expanding the power-law model for creep date at intermediate reservoir temperatures.

  Geologic Time: Phanerozoic > Paleozoic > Permian (1.00)
  Industry: Energy > Oil & Gas > Upstream (1.00)

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