Using the Complete History Flow to Avoid the Misleading Boundary Effects in Well Testing

Abstract

The objective of this paper is to correct the inappropriate common practice of using the incomplete flow history before the main Build-up in well test interpretation and pressure transient analysis for eliminating the misleading understanding of boundary effects behavior. Generally, during a short-term well test, wells are usually in the transition flow regime. And bottom hole and well head pressure change noticeably with time especially in the lower permeability reservoirs. The length of the flow period should be long enough to attain a radius of investigation and stabilize the well production and fluid composition for evaluating well deliverability and flow performance as well as determine critical rate for water or gas coning etc. The unknown duration of flow period mainly is based on the inaccurate analogy in well test design. It is widely recognized that the determination of enough Build-up period is more important in the reservoirs with low permeability for investigating of boundary effects. While the significance of the proper length of pressure drawdown or flow period is usually overlooked in PBU interpretation. PBU interpretation with main flow period and final Build-up period in the well surveillance stage is a common practice. After the pressure gauges are retrieved, the on-site interpretation will be conducted in accordance with data collection with no regard to the flow history prior to PBU. During this process, the misleading boundary effects are observed which might lead to misinterpretation that the pressure behavior is influenced by boundary configuration, interference from offset wells, reservoir heterogeneity and fluid OWC in late time regime. In addition, the determination of reservoir permeability and skin factor as well as drainage area and boundary effects identification all require the proper input of complete flow history. Therefore, the complete flow history must be utilized appropriately to avoid the misleading boundary effects in well testing for more accurate interpretation of reservoir and its boundary characterization as well as hydrocarbon volume estimation. The significant difference will be clarified and demonstrated by comparison and sensitivity analysis between the different length of flow duration scenarios for better understanding the misleading boundary effects so as to avoid the misinterpretation and inappropriate decision-making.