Abstract
This paper reviews pressure behavior at an observation well intercepted by a vertical fracture. The active well was assumed either unfractured or intercepted by a fracture parallel to the fracture at the observation well. We show that a vertical fracture at the observation well has a significant influence on the pressure response at that well, and therefore wellbore conditions at the observation well must be considered. New type curves presented can be used to determine the compass orientation of the fracture plane at the observation well. Conditions are delineated under which the fracture at the observation well may influence an interference test. This information should be useful in designing and analyzing tests. The pressure response curve at the observation well has no characteristic features that will reveal the existence of a fracture. The existence of the fracture would have to be known a priori or from independent measurements such as single-well tests.
Introduction
In this work, we examine interference test data for the influence of a vertical fracture located at the observation well. All studies on the subject of interference testing have been directed toward understanding the effects of reservoir heterogeneity or wellbore conditions at the active (flowing) well. Several correspondents suggested our study because many field tests are conducted when the observation well is fractured. They also indicated that it is not uncommon for both wells (active and observation) to be fractured. To the best of our knowledge, this is the first study to examine the influence of a vertical fracture at the observation well on interference test data. Two conditions at the active well are examined: an active well that is unfractured (plane radial flow) and an active well that intercepts a vertical fracture parallel to the fracture at the observation well. The parameters of interest include effects of the distance between the two wells, compass orientation of the fracture plane with respect to the line joining the two wellbores, and the ratio of the fracture lengths at the active and observation wells if both wells are fractured. The results given here should enable the analystto interpret the pressure response at the fractured observation well.to interpret the pressure response when both the active and the observation wells are fracturedto design tests to account for the existence of a fracture at one or both wells, andto determine quantitatively the orientation and/or length of the fracture at an observation well.
We also show that one should not assume a priori that the effect of a fracture on the observation well response will be similar to that of a concentric skin region around the wellbore-i.e., idealizations to incorporate the existence of the fracture, such as the effective wellbore radius concept, may not be applicable.
Mathematical Model and Assumptions
In this study, we consider the flow of a slightly compressible fluid of constant viscosity in a uniform and homogeneous porous medium of infinite extent. Fluid is produced at a constant surface rate at the active well. Wellbore storage effects are assumed negligible because the main objective of our work is to demonstrate the influence of the fractures. However, note that wellbore storage effects may mask the early-time response at the observation well. Refs. 1 and 2 discuss the influence of wellbore storage on interference test data. We obtained the solutions to the problems considered here by the method of sources and sinks. The fracture at the observation well was assumed to be a plane source of infinite conductivity.
SPEJ
P. 933^
The Discrete Numerical Models and Transient Pressure Curves of Fractured-Vuggy Units in Carbonate Reservoir