The Application of the Repeat Formation Tester to the Analysis of Naturally Fractured Reservoirs

Abstract

ABSTRACT The RFT* has important applications in the study of naturally fractured reservoirs in which low permeability matrix blocks are contained within a system of connected fractures of high permeability. The pressure inside an individual matrix block, which is observable with the RFT, is determined by the boundary pressure in the fractures delineating the block. Hence RFT data plotted on a pressure-depth diagram follow gradients corresponding to the fluids in the fracture system with changes in slope at the gas-oil and oil-water contacts. In an unproduced reservoir the actual OGOC and OOWC in the fracture system may be determined whereas logs measuring fluid saturations in the blocks indicate quite different distributions because of capillary pressure effects. RFT data from new wells in a developed reservoir allow monitoring of the movement of the GOC and OWC in the fracture network as a secondary gas cap forms and aquifer influx occurs. Oil recovery from a fractured reservoir is controlled by the permeability of the fracture system and by the permeability and size of the blocks. Fracture permeability is readily available from the analysis of well pressure transient tests but the matrix properties are more difficult to determine. The test pressure response of the RFT, with the probe set in a block, is affected both by the matrix permeability and the block size and a simplified model of the process based on a spherical matrix block with a constant pressure outer boundary and a central point sink has been developed. It can be demonstrated that with a pressure gauge of 1 psi resolution matrix permeabilities of less than 5 md and block sizes of less than 2 m can be detected. The theory has been used to evaluate RFT tests in a fractured reservoir and good agreement between the estimate of block size using this technique and that obtained from a statistical analysis of core was found. A finite-element numerical model has also been used to simulate pretest buildups in cubic and parallelepiped matrix blocks. The late-time deviation from linear spherical buildup commences when the nearest constant pressure boundary is encountered. Dimensionless buildup type curves for such matrix blocks are given. The question of supercharging of the block pressure as a result of mud filtrate invasion has also been considered. The proximity of constant pressure fractures limits the degree of over-pressure which can develop at the sand-face. Supercharging only becomes a serious problem at very low matrix permeabilities and the RFT gives successful pressure measurements in fractured reservoirs with a tight matrix.