A systematic Approach to Sampling During Well Testing

Abstract

The industry is focusing on cost reductions by saving on expensive rig time and on reducing the impact on the environment during well testing. Decisions regarding field development are increasingly taken based on the results of samples and data obtained by wireline formation tester/samplers (WFT/S) and not on data obtained by flow to surface during a drill stem test. The use of oil-based mud to increase the drilling rate is also becoming more common. These measures reduce the likelihood of obtaining quality fluid samples and increase the uncertainty in field development projects related to fluid data. A systematic approach to fluid sampling is presented which discusses the different aspects related to the quality of fluid data obtained depending on the sampling method, type of reservoir fluid system and formation properties. Recommendations for the decision making process are presented. Introduction Managing efficiently the production of natural gas and oil requires accurate data on the characteristics of the reservoir fluid and the phase and property changes as the fluid moves from the reservoir through the transport and production systems. The objective of reservoir fluid sampling is to collect a sample that is representative of the reservoir fluid. These samples are subsequently used for laboratory studies of the physical and chemical property change that occur during production. A non-representative sample will not reflect the true properties of the reservoir fluid and may result in costly errors in design and reservoir management regardless of the accuracy of the laboratory data. One should also keep in mind that the sample may only represent a single point in the reservoir where it was obtained and there is no assurance that the sample is representative of the fluid throughout the reservoir. Planning A successful sampling program in a well requires good planning, the right sampling equipment and the right sampling technique. Also the timing of the sampling operation is important. In most situations the best conditions for taking a representative sample of the reservoir fluid are during the exploration phase before the formation pressure has started to drop. Some specialised fluid studies may be identified later and the required samples taken successfully during the production phase. There will be differences in the challenge depending on whether the reservoir fluid is an oil, a near-critical fluid, a gas condensate or a dry gas. The well will normally be logged prior to the commencement of any reservoir fluid sampling. The logging will give information that is very useful in the planning of the sampling operation. It has become increasingly more common in offshore wells to take open hole samples by use of wireline formation testers in order to save on expensive rig time and to reduce the impact on the environment from standard drill stem testing. The selected sampling intervals will be based on logs. Intervals with good permeability and good hole quality increase the chances for a successful sampling run with a WFT. The height of the hydrocarbon column may indicate whether a compositional change with depth is likely to be encountered and whether it will be important to sample several intervals. One should try to take advantage of the bubble point gradient (typically 0.2–0.4 bar/m) in a situation were the fluid is close to saturation. The pressure gradient in the hydrocarbon column together with the reservoir conditions will identify the type of reservoir fluid, Figure 1. The degree of undersaturation may be evaluated by the use of correlations. Wire line fluid samples should and will in most situations be taken as a part of the well logging operation. These samples will usually not be truly representative due to the difficulties with well conditioning and an effective clean up. There may also be effects on the reservoir fluid from the decreased temperature in the vicinity of the well caused by the mud circulation. Each fluid system presents special challenges, for example a gas condensate can drop below the dew point pressure and high molecular waxes/resins may deposit from oils.

Reservoir Fluid Sampling Revisited—A Practical Perspective

“…Isokinetic or split-phase sampling can be used to sample twophase well-head fluid or to measure the volume of entrained liquid in a gas stream [11][12][13] . Once a stable GOR is established, the two-phase fluid mixed using a mixing head.…”

“…Previously it was assumed that the mixing head perfectly distributed the liquid droplets across the pipe cross section. This assumption was found to be not exactly correct, so to improve the GOR measurement and minimize the error due to this effect, the probe now samples at three different points across the pipe and averages the result 13 .…”

“…This is done by determining the composition of the contaminated oil sample, then a sample with a known added volume of OBM, and the full OBM composition. Based on this the OBM components can be mathematically removed from the contaminated oil composition 13 .…”