Interference Testing in Reservoirs With Conductive Faults or Fractures

Abstract This paper addresses interference testing in reservoirs with conductive faults. Both cases of interference tests where the active and observation wells are at the same and opposite sides of the fault plane are considered. Fluid flow along and across the fault plane is allowed. A region of skin around the fault is also included to account for alterations in reservoir properties that may occur in the vicinity of fault thrusts. It is shown that there are fundamental differences in pressure transient characteristics of an observation well located at the same side or opposite side of a fault, and that the classical line-source solution does not apply. When the active and observation wells are located across a fault, the interference behavior resembles that of a hydraulically fractured well with bilinear flow characteristics. However the observation well response has the features of an active well when the wells are situated at the same side. Namely, an initial line-source behavior followed by a negative unit-slope (characteristic of a constant pressure linear boundary), then the bilinear flow signature on pressure derivative. An expression is derived to calculate fault skin factor or distance to the fault for this case. Because interference tests are function of fault properties and spatial arrangement of wells with respect to the fault plane, type curves for various regions are developed to account for different flow behavior characteristics in the reservoir. The determination of reservoir regions is based on the fundamentals of physics of flow occurring in this complex heterogeneity case. Practical guidelines for the interpretation of interference tests are provided. Field data from an interference test in a faulted carbonate reservoir are analyzed by the techniques presented in this paper, and fault parameters are estimated. It would have been virtually impossible to interpret the field data without the model of this paper. Introduction Faults are the most common form of heterogeneity in hydrocarbon reservoirs. The nature of fluid conductivity of faults, whether sealing or partially sealing or fully conductive, has a profound effect on oil and gas recovery from faulted reservoirs. While sealing faults block fluid and pressure communication with other regions of reservoir leading to reservoir compartmentalization, conductive faults can act as pressure support sources by allowing fluid transfer along and across their planes. Therefore, characterization of faults with respect to their conductivity characteristics, fluid transmissibility potential, orientation and geometrical attributes is important in many aspects of petroleum exploitation, field development and production. Pressure transient testing is a powerful tool for providing reliable and useful information about many reservoir characteristics. The use of single-well pressure transient tests in delineating faults is abundant in the literature. The simplest of the models address estimation of distance to sealing faults. Cinco-Ley et al. considered an infinite conductivity fault or fracture and derived analytical solutions based on the concept of source functions. The first attempt to represent a fault as a partial barrier was introduced by Stewart who numerically modeled the fault zone as a vertical semi-permeable barrier of negligible capacity. This model correctly imposed the linear flow pattern at the fault plane. Yaxely obtained analytical solutions for transient pressures by considering the fault as a semi-impermeable linear discontinuity, a generalization of the Bixel et al.'s solution for reservoirs with linear discontinuities. Ambastha et al. analytically modeled a partially communicating fault as a thin skin region in the reservoir. The work of Abbaszadeh and Cinco-Ley provided a complete generalization where a fault (or a fracture) is viewed as a separate porous medium with two-dimensional flow occurring inside the fault plane. P. 327^