Fluid identification is an important objective to resolve key uncertainties of a complex reservoir prior to perforation in the developed fields of Eastern Kalimantan. This paper explains how using a formation tester equipped with two downhole fluid analyzer modules helped understand reservoir fluid characteristics, identify production zones and optimize perforation zone selection.
Relying only on open hole log data and performing correlations among nearby wells may be inconclusive since the channel sands under study have limited lateral extent and hard to correlate. Several layers are potential pay zones and may contain oil or gas. However, water zones and secondary gas cap formation in a few layers are also common. Nonetheless, unexpected fluid production, such as water or excessive gas is an undesirable outcome.
A formation tester equipped with an extra large diameter probe and two downhole fluid analyzer modules was used to identify reservoir fluids in newly drilled wells. Two fluid analyzers were placed above and below the downhole pump module. The fluid analyzers monitored downhole oil based mud filtrate contamination, free gas presence, water or oil flow at selected depths. The surveys identified the downhole fluids and clarified oil, gas and water bearing zones. Some zones were identified to have gas and possible oil presence. Few stations, which were clearly identified as oil were perforated and produced oil/dry oil with natural flow. The survey helped optimize perforation zone selection, avoided unwanted fluid production and helped the operator to find and produce oil in a complex setup.
In developed and ageing fields, it is essential to understand the reservoir and fluid characteristics for optimum reservoir management. A common method is to integrate all existing information on reservoir rock, fluid and production data. These range from seismic, geological and petrophysical data, core analysis, well tests and production data. However in complex reservoirs, despite the number of wells drilled in a development scenario, correlating / integrating such data is not always enough to avoid unexpected results. Missing productive intervals in a new well, zones with unexpectedly low / high pressures, undesirable fluid production and presence of additional reserves or bypassed hydrocarbons are common occurrences in complex reservoirs. At a given location, layer or compartment, reservoir fluids may change with time; water encroachment, secondary gas cap formation/gas cap expansion, reservoir re-pressurization are some of the reasons of changing fluid characteristics. For certain fluids, pressure decline causes thermodynamic changes (such as solids precipitation or significant liquid dropout) which can significantly alter well productivity, ultimate recovery and project economics.
In aging reservoirs, fluid movements are of constant focus and routine cased hole logs are common to track such changes. Location of news wells for bypassed/remaining oil is equally important. In certain environments, conventional open hole logs may not fully resolve the fluid content of stacked reservoirs. In Kalimantan, Indonesia, it is common to have low resistivity pay zones which can contain significant amount of hydrocarbons. Also, the well known density-neutron separation may not always result in water free hydrocarbon production. Coupled with reservoir and fluid complexities above, often zones with unwanted fluids are perforated. Selectively testing each producing layer to identify fluids using conventional surface test equipment is a viable approach but can be costly.
In this paper, direct pressure and fluid identification measurements using a wireline formation tester (WFT) tool will be outlined. The Modular Formation Dynamics Tester (MDT)* tool and its downhole fluid analysis methodology using Live Fluid Analyzer (LFA)* and Composition Fluid Analyzer (CFA)* were used in the ageing Sangatta field. The main objective was to clearly identify fluids downhole and give conclusive results on detecting water, gas and oil zones and oil bearing formations with a secondary gas cap. Several zones were identified with WFT were later perforated and tested. Well test results following WFT surveys from four wells are presented.