Abstract
The mechanical properties of the high-temperature and high-pressure reservoirs in the southern margin of Junggar Basin have not been clearly understood, which correspondingly results in uncertainties when predicting the breakdown pressure. To address this issue, firstly, rock mechanical experiments under high temperature, high confining pressure, and high pore pressure were carried out. Secondly, empirical formulas related to the transformation of dynamic and static mechanical parameters in the regional strata were proposed. Finally, the existing prediction model for the formation breakdown pressure was improved by taking the wellbore seepage and thermal stress into consideration. Results show that under the reservoir condition of high temperature and high pressure, the rock sample tends to form closed shear cracks. High temperature causes thermal damages and the reduction of the compressive strength and elastic modulus, while the combined effects of high confining pressure and pore pressure enhance the compressive strength and plasticity of the rock sample simultaneously. Based on the correlation analysis, it is found that the static elastic modulus is linearly related to the dynamic value, while static Poisson’s ratio is a quadratic function of the dynamic value. These fitting functions can be used to obtain the profiles of static elastic modulus and Poisson’s ratio based on their dynamic values from the logging interpretation. Besides, the improved prediction model for the rock breakdown pressure can yield more accurate results indicated by the error less than 2%. Therefore, the proposed breakdown pressure prediction model in this study can provide theoretical guidance in the selection of fracturing truck groups and the design of the pumping schedule for high-temperature and high-pressure reservoirs.
Highlights
Breakthroughs have been made for oil and gas exploration of tight sandstones in Sikeshu sag in the southern margin of Junggar Basin [1,2,3], revealing that multiple sets of highquality sandstone reservoirs exist in the lower assemblage of the southern margin [4,5,6,7]
This study focuses on the rock mechanical properties and breakdown pressure prediction of ultradeep reservoirs in the southern margin, Junggar Basin
Aiming at the ultradeep reservoir in Sikeshu sag on the southern margin of Junggar Basin, the evaluation of rock mechanical properties was performed under a similar temperature-pressure condition to that of the ultradeep reservoir, with the consideration of the coupling effects of high temperature, high confining pressure, and high pore pressure
Summary
Breakthroughs have been made for oil and gas exploration of tight sandstones in Sikeshu sag in the southern margin of Junggar Basin [1,2,3], revealing that multiple sets of highquality sandstone reservoirs exist in the lower assemblage of the southern margin [4,5,6,7]. The average reservoir depth is approximately 5800-6000 m, and the pressure coefficient is 1.6-2.3 [8] It is a typical high-temperature and highpressure ultradeep reservoir [1, 8]. The rock deformation and failure modes may transit from elasticity and brittleness in shallow reservoirs to elastoplasticity and ductility in deep reservoirs [9]. Under this circumference, the rock mechanical properties of shallow formations obtained under conventional test conditions (i.e., high temperature and high pressure are not taken into account) are unable to represent the real in situ geomechanical properties in the deep reservoirs.
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