Integration of Fracturing Dynamics and Pressure Transient Analysis for Hydraulic Fracture Evaluation
Authors: Arihara N. , Abbaszadeh M. , Wright C.A. , Hyodo M. 
Published: December 2007

This paper presents pre- and post-fracture pressure transient analysis, combined with net fracture pressure interpretation, for a well in a naturally fractured geothermal reservoir. Integrated analysis was performed to achieve a consistent interpretation of the created fracture geometry, propagation, conductivity, shrinkage, reservoir flow behavior, and formation permeability characteristics. The interpreted data includes two-rate pre-frac injection tests, step-rate injection tests, a series of pressure falloff tests, and the net fracturing pressure from a massive fracture treatment. Pressure transient analyses were performed utilizing advanced well test interpretation techniques and a thermal reservoir simulator with fracture propagation option. Hydraulic fracture propagation analysis was also performed with a generalized 3-D dynamic fracture growth model simulator.

Three major conclusions resulted from the combined analysis:that an increasing number of hydraulic fractures were being simultaneously propagated during the fracture treatment,that the reservoir behaved as a composite reservoir with the outer region permeability being greater than the permeability of the region immediately surrounding the wellbore andthat the created fractures extended into the outer region during the fracture treatment but retreated to the inner region several days after stimulation had ceased.

These conclusions were apparent from independent pressure transient analysis and from independent hydraulic fracture propagation analysis. Integrated interpretation, however, increased the confidence in these conclusions and greatly aided the quantification of the created hydraulic fracture geometry and characterization of the reservoir permeability.

Introduction

Hydraulic fracturing is an effective way of well stimulation in tight oil and gas reservoirs. Development of geothermal systems including hot dry rock reservoirs also employs this technology. Hydrothermal energy extraction is typically controlled by the conductivity of the natural fracture system intersected by a wellbore. Hydraulic fracture stimulation is often applied to less prolific producers to enhance productivity by establishing the communication with nearby natural fracture systems.

Effective hydraulic fracture stimulation primarily depends on the modeling and diagnostic capability required to optimally design field operations and to reliably estimate the created geometry and dimensions of the induced hydraulic fracture systems. Realistic three-dimensional fracture models are necessary tools for these purposes. The required functions to be possessed by models are such that formation is characterised by rock and fluid parameters including stress, modulus, permeability, pressure, fluid saturation, etc., that physical mechanisms of fracture initiation, fluid leakoff fracture propagation and closing are modeled, and that observed well pressures can be reproduced by simulating a single fracture or multiple fractures.

Well testing is another indispensable tool which is normally conducted before and after hydraulic fracturing in order to obtain data for fracture evaluation. Analysis of injection and falloff tests in a fractured well can possibly be complicated by several effects including the multiphase effect and temperature effect. Another complexity in injection and falloff tests is caused by dynamic behaviors of the fractured well. Dynamic opening and closing of fractures are amplified in the case of an unpropped fracturing treatment. Because injection rate is usually very high in geothermal well testing, dynamic fractures can be easily initiated from natural fractures.

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