Abstract
Enhanced Oil Recovery (EOR) methods are inherently dynamic and challenging. Thermal recovery presents a unique set of challenges which Petroleum Development Oman (PDO) has managed to successfully mitigate, or at least minimize, potential adverse impact. At the "A" East Field the development is currently dominated by Cyclic Steam Stimulation (CSS). This is a well established thermal recovery method and typically is a precursor for Steam Drive (SD) in many thermal fields. Successful CSS operation provides effective reservoir pre-conditioning for subsequent SD operation. If CSS is compromised, SD will not yield the anticipated optimum volumes. This paper will therefore focus on steam breakthrough mitigation in thermal operations and various approaches to manage this inevitable risk.
Steam breakthrough (SBT) occurs when steam finds its way from an injector through the reservoir and into an adjacent producing well. Steam breakthrough is an expected phenomenon; however, if it occurs prematurely in a thick vertical reservoir, it can lead to poor sweep which, if unmitigated, will compromise ultimate recovery output by increasing Steam to Oil Ratios (SORs). Inadequate sweep may be exacerbated by operational conditions, well type selection (vertical vs. horizontal wells), or geological and petrophysical characteristics (well spacing, high permeabilitystreaks,etc.). Instrumentation and active surveillance methods are critical components in identifying indicators of compromised sweep. Surveillance methods, including temperature logging and pressure data, are essential for performance evaluation. Early signs of impending steam breakthrough can be identified by producer well hydraulic support from nearby injectors, followed by a rise in temperature. Various operational and mechanical methods were utilized to minimize sweep inefficiency and creative well management concepts were employed including Group CSS (GCSS), where a set of communicating wells are operated as if they are one CSS set.
Steam breakthrough is a common concern in most thermal operations. Identifying the best approach to mitigate or minimize the associated adverse impact requires an understanding of well dynamics and behavior. This allows the most suitable and cost effective measures to be implemented. Within "A" East Field, most wells experiencing SBT improved once mitigations were introduced. For example, pump performance and efficiency improved after implementing SBT management controls. A decision tree hierarchy was key in establishing effective and economic decisions based on actual well behavior. The optimal mitigation choice strikes a balance between cost and operational efficiency.
CSS wells pose unique operational challenges in deep thick reservoirs compared to a more typical thin, shallow reservoir. This paper describes the risk and also lays out mitigation steps to minimize steam breakthrough impact on production and overall project economics.
