Effect of Scaleup and Aggregation on the Analysis of Interference Tests

Abstract

Rajagopal Raghavan, SPE, Philllips Petroleum Co. (retired); Ralph R. Roesler, SPE, Southwestern Energy Co.; and O. Inanc Tureyen, SPE, Stanford U. SummaryThe objective of this paper is to demonstrate the influence of detailed, small-scale heterogeneities on interference tests. Specific issues encountered when interference tests are analyzed in reservoirs with complex geological properties are discussed. These issues relate to questions concerning the use of low-resolution models, the degree of aggregation, the methodology of scaleup, and the reliability of conventional methods of analysis. This paper demonstrates the importance of capturing fine-scale heterogeneities to replicate the true transient behavior of interference tests at both active and observation wells. The paper shows the effects of aggregation and scaleup as used routinely in the industry on evaluating transient responses. The consequences of using low-resolution models in systems with complex geology is also demonstrated. If low-resolution models are used, reservoir properties may be adjusted unrealistically to match the transient behavior observed in high-resolution models. Though scaleup preserves pore volume, estimates of storativity predicted by lowresolution models will have a significant effect on reservoir behavior and resource management. If porosity values are not regressed, significant changes in vertical permeability values are observed. This is an important observation with potentially dramatic effects on reservoir performance, especially in processes involving mobility differences. Regression on a single-layer model (homogeneous, or based on aggregation) was also shown to yield totally different geological outcomes. This also shows the need to use geological constraints during inversion, aggregation, and scaleup.

Complex geology and pressure tests

“…was incorporated. Our attempts are summarized in Raghavan et al (2001Raghavan et al ( , 2004, Thomas et al (2005) and Raghavan (2004).…”

“…2 and 3 were analyzed after ignoring the internal architecture of the reservoir by the methods outlined in Section 3. Other methods to analyze data are discussed in Raghavan et al (2004). All structural variables such as total thickness, well spacing, well radii, etc., however, are identical to the heterogeneous model.…”

“…We caution that the values of 〈T〉 and 〈S〉 are unique to each well pair. It is to be noted that it is usually not possible to evaluate all responses simultaneously to come up with values for 〈T〉 and 〈S〉 that would reflect the entire system; see Raghavan et al (2004). If we consider Well 3 and Well 4 (located at the same distance from the pulsing well), the difference in 〈T〉/T up is much smaller than the difference in 〈S〉/S up .…”

“…Synthetic drawdown data (forward simulations) were generated using a general-purpose, finite-difference, 3D model specifically tailored to estimate well responses at both flowing and observation wells in reservoirs with complex geology using local-grid-refinement techniques (Blanc et al, 1997(Blanc et al, , 1999. Details of the model used here and the methods to ensure the accuracy of the calculations are discussed in Raghavan et al (2004) and Thomas et al (2005). Each test is run for 3 h starting with initial conditions at which time the pressure is uniformly distributed with the system at equilibrium; Eq.…”

Appraisal of complex reservoirs with short-time tests

“…The permeabilities in the x and y directions of the Tarbert formation follow bimodal lognormal distributions, and the permeability distribution in the z direction, k z , is lognormal and somewhat unimodal. Permeability distributions in the porosity cube may be found in Raghavan et al (2004). Two horizontal slices of the model are shown in Fig.…”