An Original Technique to Characterize Naturally Fractured Reservoirs Utilizing Pressure Transient Analysis

Elkewidy, Tarek I.¹
(1)Petroleum and Energy Engineering, American University in Cairo, Cairo, Egypt.

This new technique characterizes all kinds of naturally fractured reservoirs (carbonates, basements and clastics), on the Megascopic scale of well testing delineation. The technique is based on an original view of pressure transient data.

Conventional well testing analysis techniques, do not often work for naturally fractured reservoirs since they are based on a homogeneous reservoir model. In addition, all available techniques to characterize naturally fractured reservoirs from pressure transient analysis are very much theoretical models based on unrealistic geometrical assumptions.

The technique has the merit of working on real reservoir data. It utilizes pressure buildup through the fact that formation fluids travel across different systems in heterogeneous naturally fractured reservoirs; matrix, fractures and the damaged area. A unique graphical characterization of shut in well pressure versus time will illustrate the effect of fluid movements from the matrix system (or the tiny fractured system) through the main fracture system and across the damage area (if any) into the well. Fluid movements through each system are represented graphically. The technique is further optimized through application of pressure derivative methods to yield a very characteristic graphical representation (triangle) of each hydraulic “flow” unit in the reservoir.

The presence of the triangle sides confirm the existence of a secondary porosity system (fractures) and/or damaged area. The slopes and intersection values of the straight lines are utilized into exclusive formulas to yield the most important petrophysical and engineering information about the heterogeneous naturally fractured reservoir (and in many cases other kinds of reservoirs) including; effective fractures, matrix and skin systems volumes, partitioning coefficient, fracture intensity index, formation resistivity factor, formation tortuosity, effective drainage radius, damage radius, effective cementation exponent, fracture porosity, matrix porosity, storativity ratio, fracture permeability, matrix permeability, damaged (skin) permeability, average permeability, pressure drop across the damage area, skin factor, damage permeability, average/dimensionless diffusivity factor, flow efficiency, damage ratio/factor, economic implication of formation damage, average hydraulic “flow” unit quality index. This presentation and document will touch on the theory and present actual field application examples.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.