Reservoir Descriptions Via Pulse Testing: A Technology Evaluation

Abstract

Abstract Implementation of pulse testing technology as a tool to obtain reservoir descriptions has been limited to only certain types of reservoir heterogeneities. An integrated approach to evaluate pulse testing as a means of obtaining reservoir descriptions for multilayer heterogeneous reservoirs is presented in this paper. A method is developed to simulate pulse tests using a gridded model. Pulse test responses in multilayer heterogeneous reservoirs are investigated for unequal layer pressures, vertical layer heterogeneities, and wellbore storage effects both at the active and observation wells. The integrated approach is applied to a pulse test pilot completed in a San Andres reservoir. The usefulness of pulse testing is found to be highly reservoir specific. Pulse test data alone are not sufficient to derive reservoir descriptions for multilayer heterogeneous reservoirs and must be used in conjunction with geological, petrophysical, and single well pressure transient data. Pulse testing may not provide an adequate definition of vertical layer heterogeneities for tertiary miscible gas performance predictions and can be used only to refine the definition of high speed layers which control gas cycling in a tertiary miscible gas process. The pulse test derived reservoir description may be adequate for waterflooding predictions under certain situations. For primary depletion predictions, single well pressure transient data are sufficient when coupled with geological and petrophysical data. The integrated approach to evaluate pulse testing is recommended before actually conducting pulse tests in the field. Introduction The objectives of pulse testing are to derive reservoir descriptions and to determine interwell communication. This paper deals with the first objective. Reservoir descriptions are needed to predict the primary depletion, secondary, or tertiary response of a reservoir, to validate a simulator for a new process, or to optimize operations resulting in improved economics of the process.' The basic element of a pulse test consists of an active well (also referred to as a pulsing well) and an observation well (also referred to as a responding well or a passive well). A series of flow changes are introduced in the active well and the pressure response is recorded at the observation well using a sensitive pressure gauge. Johnson, et al., introduced pulse testing to the petroleum industry in 1966. They assumed an infinite, single layer homogeneous reservoir containing a slightly compressible, single phase fluid. The gravity, capillary, and wellbore storage effects were neglected. We will refer to such a system in the remainder of this paper as an "idealized" system. Based on the line source solution, Johnson, et al., showed how to compute reservoir transmissibility (kh/ 40) and storage () between a well pair by interrelating time lag, response amplitude, and cycle period for equal pulse and shut-in periods (Figure 1). They also devised a "tangent method" to analyze pulse test data which gives those properties independent of any linear reservoir pressure trends which may result from the pressure buildup of observation wells or instrument drift. Brigham presented a simplified graphical method for this procedure. Kamal and Brigham provided a method to compute the optimum pulse ratio to achieve a maximum pulse test response with unequal pulse and shut-in periods. P. 171^