Modeling of Minifracture Pressure Behavior for a Compressible Fluid

Date

1992-08

Authors

Zhu, Ding

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Abstract

To meet the requirement of using high quality foams or other compressible fluids in minifrac test, this research presents a comprehensive model to simulate pressure and temperature behavior and their interference in a minifrac test. The model has extensive applicability compared with the current industry standard analysis. Two major fracture geometry concepts, a linear fracture and a penny-shaped fracture, have been included in the model. A slightly compressible fluid such as a water-based gel, and a highly compressible fluid such as a foam or a gas, can be simulated. Energy and mass conservation are the mathematical bases of the model. A rock mechanics theory providing the relation between a crack opening and pressure and the equations describing the relation of temperature, pressure and specific volume have been used to complete the model. The solution of the mathematical model is approached by numerical simulation. The results of the model show that for slightly compressible fluids and a normal leakoff coefficient, the effect of fluid warming in the fracture after shut-in on pressure decline is not significant, and thus, the standard meyhod can be applied safely. For highly compressible fluids, the effect of fluid warming in the fracture changes the pressure behavior, and the leakoff coefficient will be underestimated using the standard analysis. Fluid warming in the wellbore will sometimes be important, depending primarily on the well configuration. The procedure recommended in this work should be followed in minifrac pressure analysis to assure an accurate result. Abnormal pressure behavior could occur at typical minifrac conditions for a highly compressible fluid, and at extreme conditions (low leakoff coefficient) for slightly compressible fluid in minifrac tests. The model developed in the work can predict and explain abnormal pressure behavior.

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