Analytical Models for Managing and Predicting the Performance of Mature Waterflooded Reservoirs

Abstract

In this thesis, new mathematical models are developed for oil production and water injection wells operating in mature fields. In waterflooded reservoirs, production and injection wells should be monitored throughout field life to maximise recovery. This includes interpretation of pressure and flow rate data from wells. Effective management of mature fields results in balanced voidage replacement and the identification of damaged or underperforming wells for remediation. The motivation for this work originates from the author's experiences working on the mature Windalia waterflooded field on Barrow Island, Australia. While assets like this possess a wealth of production data, methods of direct analysis are often unavailable. Often mature fields are neglected in modern research, but managing these reservoirs is still crucially important for oil & gas operating companies. Application of state-of-the-art methods should not be ignored. For oil wells undergoing artificial lift, which are common in mature operations, mathematical models are derived for the case of cyclic or intermittent production. Using these models, it is possible to calculate Productivity Index and other reservoir properties from production data in cyclic wells. The transient flow regime is considered along with boundary dominated flow (steady or pseudo steady state cases). In the transient case, the superposition of linearised solutions is used to mimic the operation of well pump controllers. In the boundary dominated case, steady state harmonic theory is applied to solve the problem in a novel way. Type curves are presented to assist with interpretation of field data from Pump Off Controllers. Field case studies are presented; in some cases, the theory identified production wells that were later stimulated by a workover rig. For water injectors, the classical Pressure Fall Off transient analysis technique is reviewed and applied in its original format on Barrow Island. Yet there are cases when this simple approach is invalid when interpreting shut-in pressure data. Temperature effects when injecting cold-water into a reservoir are scarcely considered in the literature, with regards to effects inside the well itself. A case of this nature was observed in the field, and new theory developed to account for the heating period during a shut-in where cold water in the well equalises with the surrounding formation temperature. The theory combines the transient pressure from a composite reservoir with transient temperature changes during owing and shut-in periods. The effect on recorded downhole pressure is considerable in some cases. Finally, the method of Rate Transient Analysis (RTA) is extended to include delayed water injection at the outer boundary. In current practice, this effect is not considered when interpreting long term ow rate data from oil wells. In the proposed method, increased production is observed some time after water injection commences. Type curves are generated from analytical solutions derived in the Laplace domain. In addition to reservoir parameters obtained using RTA, the method allows characterisation of waterflood support in terms of effective injection rate and boundary condition (constant-pressure or constant-rate injection). The technique is validated using reservoir simulation and applied to field cases.