The reservoir parameters obtained from any transient pressure analysis, reflect average values of the characteristics of the area around the well that has experienced a pressure disturbance due to the change of flow rate at the wellbore. This area is described by an associated radius called the radius of investigation. In the authors' knowledge, no work has been done to examine this radius in fractured or dual porosity reservoirs. This paper describes a new equation for evaluation of the radius of investigation for well tests in such reservoirs under pseudosteady state interporosity flow regime.

The results have shown that the radius of investigation in such reservoirs starts increasing in the fracture network proportional to the square root of the fracture conductivity. When the matrix contribution to the flow of fluids starts the rate of advance of the radius decreases until its magnitude reaches a maximum value and remains constant until the total system stabilizes. After this time the radius increases again with a lower rate dependent now on the total system conductivity.

The radius of investigation also called the radius of influence or radius of drainage is defined in many ways by several authors ^{1,2,3.4.5,6}. In most definitions this radius determines a circular system with a pseudo-steady state pressure istribution around the wellbore, and takes the form as follows:

Equation (1) Available In Full Paper

where A is a constant and r_{inv} is the radius of investigation. If the start of semi-steady state flow for a homogeneous and symmetrical bounded cylindrical reservoir at a time t_{De} of 0.3 is used, and the parameters are defined in oil field units where, r_{inv} is in feet, t is the time of flowing for a drawdown test or the time of shut-in when Δtp for a buildup test in hrs., K is the formation permeability in mds, φ is the reservoir porosity in fraction and c is the total system compressibility in psi^{−1}, the constant A becomes 0.029.

Odeh and Nabor^{7}, by using an RC analyzer obtained A to be 0.0257, and Kazemi^{8} from the numerical finite difference solution obtained it to be 0.035.

Hurst et al^{3}, Van Poolen^{5} and Slider^{9} separately used the concept of unsteady state radial flow to find out when to switch from infinite acting solution to finite solution of the homogeneous diffusivity equation. By taking the derivative of the difference between the above solutions with respect to time and putting it equal to zero the flowing equation for radius of investigation will be obtained:

Equation (2) Available In Full Paper

Matthews and Russell^{10} picked a time t_{De} of 0.25 intermediate to the two times corresponding to the end of infinite acting and the start of semi-steady state, and obtained the same Eq.2.

Muskat_{1}, Chatas_{11} and Craft and Hawkins_{12} by equating the volume of the fluid produced to the expansion of the fluid contained in the drainage area and by considering steady state conditions also obtained the same Eq. 2 for r_{inv}.

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