Using 3D Fracture Simulation Alone May Result in Incorrect Fracture-Geometry Determination and Unreliable Real-Time Fracture Analysis
Authors: Blauer Roland E. , Holcomb David L. 
Published: December 2007

A sophisticated, commercially available, pseudo three-dimensional fracture simulator is used to demonstrate that during hydraulic fracturing a single net-treating pressure plot and fracture-fluid leak-off curve can be achieved by radically different fracture geometries and a large range of reservoir properties. The simulated fracture geometry for each reservoir description and combination are used to predict estimated future production performance using reservoir simulation. Production performance is shown to be a combination of reservoir quality and fracture geometry. However, essentially identical long-term production performance can be expected from different combinations of reservoir quality and fracture geometry. Consequently, decisions made from fracture simulations using only real-time fracture treatment data may yield unreliable correlations between hydraulic fracture design and reservoir performance.

Introduction

Modern pseudo three-dimensional and full three-dimensional fracture simulators have evolved sufficiently that when used carefully and in conjunction with pressure transient testing, radioactive tracer logging, production monitoring and reservoir engineering can provide an engineer with a reasonable understanding of the fracturing process, a reliable estimate of fracture geometry, an estimate of proppant placement and sufficient fracture description to augment reservoir engineering studies. A simulation study can provide methods to modify and optimize the fracture treatments of future wells and can be used to maximize the exploitation of hydraulically fractured reservoirs if a single unique fracture geometry can be determined. However, the interactions between the fracture fluid, the rock mechanical properties, the proppant and the reservoir properties are sufficiently complex that multiple fracture geometries may be created from a single set of data collected during the fracture treatment. Multiple fracture geometries are most likely when performing fracture simulations using only real-time pressure data. Decisions made based upon such real-time analysis need to be treated as informed estimates only.

Early-time fracture geometry estimates may be improved using fracture-fluid leak-off monitoring, pressure transient testing and radioactive tracer logging immediately after fracturing. These technologies permit an early determination of closure pressure, average reservoir transmissivity, skin damage, created fracture height and proppant distribution, each of which can be correlated with the calculated fracture simulation and the reservoir description used in the model. However, estimated future production performance based upon this data will not be reliable unless consideration is given to sufficient historical production data of offset wells which were stimulated with essentially the identical fracture treatment.

Using initial production data to prove the validity of the fracture geometry and reservoir description may decrease the number of simulated fracture geometries which satisfy both the fracture simulator and the reservoir response to the simulated fracture geometry. Yet, a single, unique fracture geometry is typically not determined and validated with only the early-time reservoir performance.

Further, data obtained during the fracture treatment may be significantly in error. Measured pressure fluctuations may have several causes, fracture fluid properties may vary from design specifications, reservoir properties may be different than those used in the original design and field measuring equipment may be improperly calibrated. Despite these potential data and analysis problems, several simulated fracture geometries can be obtained which apparently match the treating data.

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