Najurieta, Humberto L.,* SPE, Inst. Mexicano del Petroleo A method to calculate the unsteady-state pressure behavior within the fractures of a homogeneously fractured reservoir is presented. The technique allows the analysis of pressure buildup and drawdown tests for layer- like and block-shaped fractured reservoirs. The fractures pressure is shown to depend on four parameters, and a method for its pressure is shown to depend on four parameters, and a method for its determination is proposed. Introduction The importance of the pressure behavior as a source of information on reservoir characteristics is evidenced by the extensive theoretical work developed in this field. Homogeneous reservoirs can be described theoretically for a great number of boundary and production conditions.The basic theory for the analysis of homogeneous isotropic reservoirs is based on the line-source solution to the radial diffusivity equation: (1) The solution is (2) Eq. 2 shows that homogeneous isotropic reservoirs can be described using at least two parameters: the reservoir transmissivity T=k h/mu and the reservoir diffusivity eta=k/phi muc. Several techniques are used widely to estimate these parameters; from them, we can calculated two unknowns (e.g., k and phi provided we know the remaining (h, mu, and c). The provided we know the remaining (h, mu, and c). The definition of the skin effect and the use of the superposition principle and image wells widen the practical applications of Eq. 2. practical applications of Eq. 2. Several authors have dealt with nonhomogeneous and nonisotropic reservoirs. The solutions proposed are complex and generally require computer handling.Among the heterogeneities in a reservoir, there is an important one that is due to the presence of natural matrix fractures. In this case the productive pay is fragmented by a spatial fracture network as a pay is fragmented by a spatial fracture network as a result of the natural geologic factors. Few authors have suggested theories to aid in calculating the characteristics of naturally fractured reservoirs. A detailed theory, developed by Warren and Root, based on the theoretical work by Barenblatt and Zheltov, assumes a network of orthogonal, equally spaced features. Thus, the reservoir is made up of blocks that are able to exchange fluids with the fractures.Barenblatt and Zheltov suggested that the flow from the matrix could be considered as a first approach in a semisteady-state regime. With this assumption Warren and Root developed the differential equations and obtained analytical solutions for well test analysis. When these solutions are plotted in a conventional way (e.g., a Horner plot), plotted in a conventional way (e.g., a Horner plot), they show two parallel straight lines connected by a transition zone of variable slope. The vertical distance between them is related to the relative storage capacity of the fractures and the slopes with the flow capacity of the reservoir. JPT p. 1241

Introduction Interference tests are one of the most valuable techniques to analyze reservoirs. Due to its characteristics, it is the only procedure that can measure transmissivity (T = kh/mu) and storage values (S = phi ch) among wells and provide, moreover, information about the anisotropy of the productive formation. The technique consists of the generation of pressure signals in the reservoir by opening and pressure signals in the reservoir by opening and closing a well. The signals spread through the field and are measured in other wells. The analysis of these pressure signals provides information about the dynamic and productive characteristics of the reservoir. The disturbance produced by the production changes propagates through the reservoir and can be measured in other wells of the reservoir with highly sensitive instruments. The shape of the measured curve depends mainly on the transmissivity and on the reservoir storage in the area affected by the pressure pulse. pulse. Fig. 1 shows the pressure signal that is measured in the observation well due to the changes in flow rate produced in the pulsing well. At a first approach, we can consider the field as homogeneous, isotropous, and of constant thickness in the tested area. We also can suppose that the reservoir limits are far enough that they do not affect the pulse propagation. Hence, the pressure pulse is described mathematically by the solution of pulse is described mathematically by the solution of the differential equation: ............…(1) where Delta p is the pressure signal measured in the observation well, and eta is the diffusivity of the reservoir: .............(2) The simplest case presented is that of the signal produced by closing the pulsing well; in that case, produced by closing the pulsing well; in that case, the solution of Eq. 1 with the corresponding boundary and initial conditions is .............(3) The analysis of interference tests consists of finding a pair of values of T and q (or T and 3) so that Eq. describes in the most accurate way the experimentally measured curve. then the pressure pulses are generated by a succession of flow pulses are generated by a succession of flow changes, it is necessary to use more complex expressions than Eq. 3, obtained by applying the superposition principle. Different analysis techniques have been developed for the interpretation of interference tests in homogeneous and isotropous reservoirs. There also exist techniques that allow the determination of the anisotropy of the reservoir, thus enabling the detection of preferential orientations of transmissivity. In the last years, successful applications have been carried out in secondary recovery projects to study waterflooding and to detect tight projects to study waterflooding and to detect tight barriers. The availability of highly sensitive electronic pressure gauges has allowed the extension of the pressure gauges has allowed the extension of the application fields of interference tests, being these techniques in an increasing stage of application. Some papers on the application of these techniques in naturally fractured reservoirs have been published. The Warren and Root theory was extended by Kazemi et al. to interference tests. Gringarten also has presented a set of carefully conducted field tests and proposes the use of the heterogeneous equivalent concept in interference tests interpretation.

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Secondary recovery projects in dual porosity reservoirs with high transmissivity fractures must be carefully managed to avoid early water break-through into producing wells. If water production does begin in an injector/ producer system, the origin of the water channelling must producer system, the origin of the water channelling must be identified so that remedial changes can be made in the injection-production schedule. This paper shows that pressure interference testing between injection and producing wells, is a better method for locating problem injectors than the alternative technique using radioactive tracers. Furthermore, information derived from interference tests can be used to prevent water break-through in waterflood projects. Calculations of the interference response before and after break-through are shown in an example. A simple graphical method is used to illustrate water advancement. Introduction Secondary recovery in naturally fractured reservoirs is more difficult to achieve than in non-fractured reservoirs. The traditional concepts of water-flooding in single porosity reservoirs must be replaced by more advanced porosity reservoirs must be replaced by more advanced methods designed to impede the rapid advance of water towards the producing wells. Techniques currently used in fractured reservoirs are based on the phenomenon of imbibition in which water displaces oil from the matrix by means or capillary forces. It is only possible to accomplish a satisfactory conclureion of a water flood project, in a commercially reasonable time, if specifically determined petrophysical and capillary conditions prevail. Controlling the advance of the water through the fracture network is critical. Slowing down water break-through into the production wells can be achieved by regulating the flow rate of both injected and produced fluids. The most frequently used techniques for observing water advance in conventional reservoirs employ either radioactive tracers in the injected water, or the analysis of fall-off, build-up and interference tests. The use of radioactive tracers has also been applied in fractured reservoirs. However, it is impossible with the tracer method to anticipate where the water will break through. It can be therefore too late to take steps to control and optimize the water flood process. N matter which method is used, the main problem is the lack of reliable data, especially concerning the distribution of transmissibility or the fracture network. In this paper we demonstrate that interference tests have an advantage over tracer method. They can be effectively used for data acquisition and supervision of water advance through the fracture network. Experience has shown that the channelling of water in non-fractured reservoirs can be identified by means of interference tests of short duration, that is, usually less than three days for well-spacings of the order of three hundred metres.

We present a method to obtain a 2D description of a reservoir in terms of transmissivity and diffusivity estimated from well testing. This technique is based on the averaging of the parameters in a proposed interference-test measurement area. This area depends on time, diffusivity, and pressure-gauge sensitivity. We discuss a field application in a heterogeneous, anisotropic reservoir, and we use a simulator that uses oiVwater production data from a pilot water-injection test to validate the application's results. 2132200 2130100 • 4 ( I J \ / I / • 12 r...--, / \ / / J J / / /