The Effects of Size, Shape, and Orientation of an Impermeable Region on Transient Pressure Testing

Abstract

ABSTRACT A practical consideration of the effects of the shape, size, and orientation of an impermeable reservoir region on transient pressure testing is presented. The constant rate production well is external to the impermeable region. Impermeable regions may be in the form of sealing fractures of finite length, that have little volume associated with them and are only a local restriction to fluid flow. These regions may also be shale lenses or reduced permeability regions that in addition to being a restriction to flow occupy a significant reservoir volume. This paper considers a single impermeable region with various sizes, shapes, and orientations with respect to the active source well. The transient pressure response of the constant rate well is generated by replacing the impermeable boundaries by a set of line sources, and then applying the method of superposition. This method can be extended to include pressure responses of interference wells. The method is validated by matching the pressure responses of simple cases like a linear no-flow boundary and an internal circular no-flow boundary. The key issue of this paper is the detectability of large scale impermeable reservoir regions by transient pressure analysis. The presence of an impermeable region causes the pressure response to deviate from the homogeneous line source response. This pressure deviation is the indication that the reservoir is heterogeneous. Four major parameters affect the pressure response of the active source well in the presence of an impermeable region: 1. the shortest distance between the well and the impermeable region, 2. the size of the region, 3. the shape of the region, and 4. the orientation of the region. This paper presents combinations of these four parameters that significantly affect the transient pressure response as well as some criteria for deciding what impermeable regions may be uniquely detected.