Application of Tracers in Oil-Based Drilling Mud for Obtaining High Quality ReservoirFluid Samples

摘要

Oil-based drilling fluids are extensively used in drillingactivities world-wide. During the drilling process, due toover-balance pressure in the mud column, the filtrate ofoil-based mud invades the formation. This hydrocarbon-basedfiltrate mixes with the formation hydrocarbon, which cancause major difficulties in obtaining representative reservoirfluid sample. Despite the recent improvements in sampling,obtaining a contamination-free formation fluid is a majorchallenge, particularly in open hole wells.Depending on the type and conditions of the reservoir, theoil-based mud filtrate is totally or partially miscible with theformation fluid. Oil-based mud filtrate completely dissolvesin reservoir oil, therefore the captured sample contains the truereservoir oil with added filtrate. Gas condensate (lean gascondensate in particular) is often not fully miscible with mudfiltrate. In this case, the mass exchange between gascondensate and mud filtrate makes the sampleunrepresentative of the reservoir fluid.In this study the impact of sample contamination withoil-based mud filtrate on different types of reservoir fluids,including gas condensate and volatile oil samples, isinvestigated. Two simple methods are suggested to retrievethe uncontaminated composition from a contaminated samplewhere mud filtrate is totally dissolved in the formation fluid(I.e. reservoir oil samples). A tracer-based technique is alsodeveloped to determine the composition of uncontaminatedreservoir fluid sample from a sample contaminated withoil-based mud filtrate, particularly for those cases where thetwo fluids are partially miscible. The tracers are added to thedrilling fluid with the additional cost to the drilling mudpreparation being negligible. The capability of the developed techniques has been examined against deliberatelycontaminated samples under controlled conditions andcontaminated field samples. The results indicate the reliabilityof the proposed methods.