Reverse Engineering; Evaluating In-Place Volume From Reservoir Performance and Rock Properties in a Reservoir with Limited Well Spread

Abstract

The evaluation of hydrocarbon in-place volumes in any reservoir depends principally on structural uncertainty, fluid contact as well as rock and fluid properties distribution. This paper focuses on integrated approaches used to estimate hydrocarbon in-place volumes in a reservoir with very limited well spread. This study highlights the importance of carrying out further studies on our brown fields to re-evaluate the earlier estimated hydro-carbon Inplace volumes and its ultimate recovery by integrating historical information (performance data etc) and emerging development plan. About five wells have been drilled in the field and their penetrations are restricted to the eastern flank of the reservoirs. Well test data exist in some of the reservoirs and continuous BHP data were also taken in most of the producing reservoirs. The Mangu field reservoir ‘A’ has over 10 years of production (with limited well spread and few producers). It has produced more than the initially estimated ultimate recovery, over 85% of its initially estimated in-place volumes. Hence the integrated technical team decided to to re-evaluate the in-place using performance-based information and re-evaluation of the expected ultimate recovery (Buckley Leverett Approach). The determination of hydrocarbon in-place volumes in Mangu field was also premised on the re-estimation and prediction of rock and fluid properties variation away from area of well penetrations. In order to determine the average reservoir properties and their associated uncertainties as well as their use for in-place volumes estimation, multidisciplinary approaches have been applied and they include (a) evaluation of well by well logs & rock properties (b) generation of reservoir property maps (c) analysis of pressure and performance data and (d) Static/Dynamic modelling and (e) Determination of Recovery Factors using Buckley Leverett approach and use of Decline Curve Analysis. The result of this study highlights the significance and impact of limited well data spread and its associated uncertainties on reservoir in-place volumes estimation and ultimate recovery determination. An integrated approach has helped to significantly reduce the uncertainties associated with in-place volumes estimation in the Mangu field thereby leading to a commercially viable and timely Field Development Plan.