The design of apparata for testing of reservoir core samples with fluids varies with specific objectives and applications. Typical testing systems include core holders, fluid reservoirs, pumps, flow meters, sample collectors, control systems for temperature, pressure or flow, and data acquisition systems. The degree of sophistication of the design of the core testing apparatus depends on the requirements of particular testing conditions and expectations. High quality and specific purpose laboratory core testing facilities can be designed, constructed, and operated for various research, development, and service activities. Ready-made systems are also available in the market.

The schematic drawing given in this article indicates that primitive core testing systems consist of a core holder, a pressure transducer controlling the pressure difference across the core, an annulus pump to apply an overburden pressure over the rubber slieve containing the core plug, a reservoir containing the testing fluid such as a drilling mud or filtrate, a displacement pump to pump the testing fluid into the core plug, and an effluent fluid collection container, such as a test tube. There is no temperature control on this system. It operates at ambient laboratory conditions.

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The schematic drawing given in the image and shows the elements of a typical overbalanced core testing apparatus. This system has been designed for core testing at a near-in-situ temperature and stress conditions, although other features are similar to that of the primitive system shown in this article. The elements of a typical underbalanced core testing apparatus, which also operates at near-in situ temperature and stress conditions.

Special Purpose Core Holders

Core flood tests can be conducted in one-dimensional linear and radial modes show a schematic of typical radial flow models. Radial models

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better represent the effect of the converging or diverging flows in the near-wellbore formation. However, linear models are preferred for convenience in testing and preparation of core samples.

The majority of the core flow tests facilitate horizontal core plugs because the application of Darcy's law for horizontal flow does not include the gravity term and the analytical derivations used for interpretation of the experimental data is simplified. This approach provides reasonable accuracy for single-phase fluids flowing through small diameter core plugs. However, when multi-phase fluid systems with significantly different properties and paniculate suspensions are flown through the core plugs, an uneven distribution of fluids and/or suspended particles can occur over the cross-sectional areas of cores. This phenomenon complicates the solution of the equations necessary for interpretation of the experimental data. In particular, errors arise because, frequently, the transport phenomena occurring in core plugs are described as being onedimensional along the cores. In order to alleviate this problem, it is more convenient to conduct core flow tests using vertical core plugs.

Consequently, the gravity term is included in Darcy's law, but errors associated with uneven distribution of fluid properties over the cross-sectional area of the core plugs are avoided. Therefore, Cernansky and Siroky (1985) used a vertical core holder. The dimensions of the core plugs are mportant parameters and should be carefully selected to extract meaningful data. Typically 1 to 2 in (2.54 to 5.08 cm) diameter and 1 to 4 in (2.54 to 10.58 cm) long cores are used. The aspect ratio of a core plug is defined by the diameter-to-length ratio. Small diameter cores introduce more boundary effects near the cylindrical surface covered by the rubber slieve. This, in turn, introduces errors in model-assisted data interpretation and analysis when onedimensional models are used, as frequently practiced in many applications for computational convenience and simplification purposes.

On the other hand, short cores do not allow for sufficient distance for investigation of the effect of the precipitation and dissolution processes and depth of invasion (Fambrough and Newhouse, 1993; Gadiyar and Civan, 1992; Doane et al., 1999). Longer cores are required for measurement of sectional or spatial porosity and permeability alteration. As described by Doane et al. (1999), a number of special purpose core holders have been designed.

This type of system is usually used with small core plugs. It only yields core response, integrated over the core length. on sectional permeability alteration over the core length. Especially, core holders designed for tomographic analysis using sophisticated techniques, such as NMR, Cat-scan, etc., may provide additional internal data. However, it is not always possible to obtain sufficiently long core plugs. In this case, several core plugs of the same diameter can be placed into a long core holder to construct a long core (20-40 cm long) and capillary contact membranes are placed between the core plugs to maintain capillary continuity (Doane et al., 1999).

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As emphasized by Doane et al. (1999), small diameter core plugs are not sufficient for testing of heterogeneous porous rocks. Therefore, full

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diameter core plugs have been used to alleviate this problem. But, Doane et al. (1999) warn, full diameter core plugs would not be representative when significant anisotropy exists between the horizontal and vertical permeability, such as in typical carbonate formations. For the latter case, they recommend the core holder arrangement shown in this article. In this system, the two opposing side surfaces of the core plugs are flattened by facing off and the fluid is flown over the side surface by means of a specially designed sleeve. This provides larger surfaces exposed to fluid to include the effect of the heterogeneous features of the core plugs.

Obtaining and testing representative samples of fractured formations are difficult (Doane et al., 1999). Actual core samples containing natural fractures are preferred, but they are often difficult to obtain because core samples are usually poorly consolidated and may not include natural fractures (Doane et al., 1999). Then, a hydraulic fracturing apparatus can be used to prepare artificially fractured core plugs.


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