Management of a North Sea Reservoir Containing Near-Critical Fluids Using New Generation Sampling and Pressure Technology for Wireline Formation Testers

Abstract

Abstract This paper presents the first application of a new wireline formation tester, the Modular Formation Dynamics Tester (MDT), that successfully recovered bottomhole samples of a near-critical reservoir fluid. The tester device achieved this by controlling and minimising the pressure drop during sampling; in some cases just 0.5 bar was sustained. Full PVT laboratory study confirmed the purity and representativity of samples taken in this manner from two wells. Variations in the fluids' properties with depth as determined from samples taken during DST's on older wells were subsequently revised to take into account this newer, more accurate fluid description. The decision to sample a near-critical fluid using this wireline tester on the first well involved an element of uncertainty since such an operation had never been attempted before. However, in the case of the second well the decision was based on the knowledge and experience gained from the first, where overall performance of the tester and PVT quality of the samples were excellent. In fact, sampling in openhole with a wireline formation tester as explained above actually allowed dispensing with planned DST's in both wells, and hence proved to be a very cost-effective solution. The same wireline tester that was used for sampling comes equipped with a new single crystal quartz gauge, which displays quartz sensor accuracy and resolution but performs like a strain gauge in terms of dynamic response characteristics. Pressure profiles interpreted in all wells in the field, including data from the new quartz gauge, exhibit the typical characteristic of critical to near-critical reservoir fluids - no discernible gas/oil contact. The reservoir management implications of this result are discussed in the paper. Introduction Situated in the Southeastern part of the East Shetland Basin - U.K.C.S., the Alwyn Field produces oil and gas from Brent Group reservoirs. The structural style is of tilted and eroded fault blocks dipping to the West and aligned North-South. NE-SW cross elements further separate the hydrocarbon accumulations, and pressure communication may exist between adjacent panels. The hydrocarbon columns are restricted to the Tarbert and upper part of the Ness Formation of the Brent Group. One reservoir has been delineated by two wells located at two different structural depths. During DST's, four surface fluid samples were recovered, one in Well 1, and three in Well 2 as shown in Fig. 1. Analysis of the fluids indicated that, at initial conditions, the hydrocarbons were undersaturated, near critical point, and varied compositionally with depth. The fluid passes smoothly from a liquid at the base of the structure to a gas at the top with no phase contact. The absence of a gas-oil-contact was confirmed by the log and Repeat Formation Tester (RFT) data. P. 541^