An Effective Reservoir Pressure Measurement and Fluid Sampling Methodology in Tight Formations

Abstract

One of the main challenges in exploration / appraisal phase is to measure the reservoir pressures and sample down-hole single phase fluids to identify the fluid types in tight layers (permeability <1md). This challenge escalates when multiple reservoirs are exposed in same open-hole with varying differential pressures across them. A novel wireline formation tester (WFT) technique was applied in an open-hole condition to collect in-situ representative fluid samples faster and cost effectively. It also significantly reduces the risks associated while sampling across multiple reservoirs in comparison to the conventional techniques. Conventional approach of using dual packer module requires longer set-unset time as well as pumping out the trapped volume between the packers before receiving fluid from the formation. On the other hand, the probe module is more effective for sampling in high permeability layers. In tight reservoirs, the fluid sampling is very challenging due to low formation withdrawal rates, high drawdown and elongated station times. These challenges were overcome with a radically designed WFT that utilizes a 3D elliptical probe module with an optimum reservoir contact area. These probes enable circumferential flow from the formation with a faster cleanup process and hydrocarbon breakthrough. The new sampling technique showed an improvement, over the conventional methods in several aspects: (1) Tight zone pressures are obtained faster with lower supercharging effects. (2) In-Situ representative fluid samples are acquired in tight reservoirs above saturation pressure enabling proper fluid characterization PVT studies. (3) Stationary times during sampling are reduced due to negligible trapped volume. (4) Set and retract timing is shorter so that a new sampling method of retract-move-reset is developed to minimize the mud invasion between multiple settings. (5) Reduces the differential sticking risk in longer operations with multiple reservoirs exposed in the same open-hole. (6) Minimizes the sampling runs resulting in significant cost saving. Logging operation proved to be successful with the new sampling method by acquiring representative fluid samples in tight formations, which was not possible earlier. This paper describes a case history and recent achievements made in acquiring representative single phase fluid samples in tight reservoirs, overcoming the challenges and risks associated with conventional sampling techniques.