Understanding Variable Well Performance in a Chalk Reservoir

Abstract

Summary Analyzing well performance is a complex process that increases in difficulty when multiple reservoir-drive mechanisms are in play in the same reservoir. This paper explores an overpressured, compacting chalk reservoir with high porosity and high oil saturation at initial conditions. The diverse drive mechanisms, experienced through the long production history of Valhall Field in Norway, are caused by different degrees of reservoir compaction across the field and the recent waterflood at the crest and northern areas of the field. The purpose of this study is to illuminate the various drive mechanisms experienced in this field. The underlying objective is to understand widely varying Arps b-factors in decline-curve analysis (DCA) that support production forecasting and project evaluation. The performances of inactive wells with long production histories were used as analogs to analyze active wells. Other analytical tools also were used to augment overall understanding of a type well's performance, including rate-transient analysis (RTA) and capacitance/resistance modeling (CRM). This study demonstrates that the proposed work flow for reservoir-performance forecasting can be adopted in highly complex reservoirs with different rock-mechanical properties, drive mechanisms, production scheduling, and field-development strategies. Specifically, the work flow entails establishing energy support for individual wells by use of Arps b-factor with DCA; collapsing shut-in periods, if any, and using the cumulative production curve for DCA to retain solution objectivity; performing RTA to gauge pressure/rate coherence and system's linearity; and using CRM to establish injector/producer connectivity.