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This case study highlights a major step change in fluid sampling technology while drilling. The decision to deploy a Logging While Drilling (LWD) fluid sampler was made because of the S-profile well, which would have required pipe-conveyed wireline fluid sampling. This would have added rig days to the project and increased the risk of stuck pipe. One run of the LWD fluid sampler, during a wiper trip, was sufficient to acquire the necessary 60 pressure tests, fluid gradients, down hole fluid identification and analysis and PVT samples. Six single phase oil samples (four liters total volume) were acquired. Lessons learnt offer additional best practices and risk mitigation techniques for future LWD sampling projects.The thin bed succession comprises centimeter to meter scale sandstones, inter-bedded with shales. The sandstones have a variable measured mobility between 1.2-and 296-mD/cP. This type of succession poses significant challenges to obtain a gradient for each sand layer, particularly since each layer may have different fluids. Moreover, inherent depth uncertainties in these measurements, affects the fluid gradient. The excellent repeatability of the pressures allowed adjusting formation tops by reconciliation to pressure profiles. This reconciliation process could only be achieved by taking many pressure tests (upto 60). A consequence of this process was the high-grading and selection of representative locations for sampling. There were zero lost seals during the pressure testing and sampling operation. Laboratory analysis results showed about 15% WBM filtrate contamination, which was comparable to wireline runs in the same formation.In conclusion, this case study shows that LWD pressure and sampling technology provides high quality results over complex thin bed reservoirs with different fluid types; Operators no longer need to consider a well's deviation or complex geometry to collect fluid samples while drilling; the demonstrated operational efficiency and good sample quality resulted in significant savings in rig costs (USD 2 million) and the LWD sampling application reached the same sample integrity and safety standards as compared to existing wireline methods.

Sampling While Drilling has been successful in recent times covering numerous applications including difficult boreholes, horizontal wells, influencing drill stem test plans, cost savings in marginal fields, high sample volume collection, aid in quantifying non-hydrocarbon content for facilities design and deployment in carbonate under total losses with pressurized mud cap drilling conditions. This case study discusses one more application, which is in weak unconsolidated sands of a field located in Tatau, Sarawak, Malaysia. The principal objective of the sampling-while-drilling run discussed in this paper was to collect representative water samples from an aquifer, the laboratory analysis of which would yield invaluable information about compatibility for future water injection. The secondary objective was to collect low contamination single-phase oil samples if the well penetrated hydrocarbon reservoir layers at virgin pressure. Determining fluid gradient was an inherent requirement as well. This paper describes the challenges, best practices and learnings following the deployment of this technology in a deviated infill well in a mature oil field with sanding history and multiple layers of poorly consolidated sandstones with uncertainties on the pore pressure, due to production from the field and adjacent fault blocks that might have a certain level of connectivity.

This paper discusses the application of selective slug sampling using the wireline formation tester – RCX* (Reservoir Characterization eXplorer), in a clastic heavy oil reservoir and its significance to fluid characterization. The low mobility ratio between the reservoir fluid and water-based mud filtrate results in a long cleanup time to obtain samples with an acceptable level of contamination. Tool sticking and wellbore stability issues add to the challenges in open-hole fluid characterization and testing. Increasing the flow rate to reduce cleanup time does not help mostly due to formation of an emulsion between the viscous oil and the mud filtrate. The emulsion is hard to disperse, leading to a contaminated sample. A higher flow rate results in sanding and loss of seal. In selective slug sampling, phase segregation is achieved by lowering and optimizing the flow rate, reducing the pressure drawdown that curtails emulsion formation and movement of grains thus increasing the sealing efficiency. The segregated oil slugs can be selectively collected in sample tanks multiple times downhole. Even a water cut of 60 to 70% is sufficient to collect high-quality samples, which is a significant improvement in sampling time compared to conventional fluid sampling, where 10% or less water cut is required. The technique was applied in an unconsolidated clastic heavy oil reservoir in Vietnam. Four 14- to 22-°API single-phase oil samples were collected in two hours each. Laboratory results indicate less than 10% contamination; water cut being 50 to 75% at the time of the sampling. It was the first time that successful heavy oil samples were collected from these layers. Fluid sampling is critical to identifying compositional variation of reservoir fluids along the depth, resulting in appropriate sampling strategy and eventually correct fluid characterization. Sampling in unconsolidated heavy oil reservoirs is improved using the selective slug sampling technique, making it possible to secure fast, yet high-quality samples, aiding fluid property characterization.

Since the advancement of Focused Sampling techniques, wireline formation fluid sampling has undergone a dramatic change. This has primarily been due to the promise of acquiring representative formation fluid samples with minimal mud filtrate contamination and large sample volumes, thereby adding value to the PVT laboratory studies as well as reducing the fluid sampling time, thus aiding significantly to the cost savings. This paper demonstrates the contribution of focused sampling technology for reservoir fluid mapping in numerous exploration and development wells in South East (SE) Asia, by optimized selection of different packer types based on varying reservoir properties. For the exploration wells, the primary objective was to determine the non-hydrocarbon (non-HC) content (CO2 and H2S in this case) of the single-phase reservoir fluid samples, which were expected to be close to the saturation pressures. Following the 3D near-wellbore simulations, an elongated and an extra-elongated focused packer were selected due to expected low permeabilities, reservoir thickness and wellbore conditions. The wells were drilled in managed pressure drilling (MPD) conditions, with overbalance ranging from 900 to 4,300 psi. The development campaign consisted of five producers with key objectives of determining fluid type and the non-HC (CO2 in this case) content along with assessing the reservoir/block connectivity. The concentration and uncertainty in CO2 distribution would have a major impact in developing the production strategy of the area. A standard focused packer was selected for the sampling jobs which were carried out on pipe due to high overbalance conditions (~2,400 psi). In the exploration wells, 30+ samples (gas, oil and water) were collected with the time-on-wall ranging between 1.5 and 7 hours. In the development campaign, 50+ samples (gas and oil) were collected with the time-on-wall ranging between 45 minutes and 2.5 hours. Given the depths and low permeabilities of the reservoirs with high overbalance, this resulted in significant time savings. The larger flow area of the elongated and extra-elongated focused packers ensured minimal contamination in the collected samples given the challenging sampling conditions, where restrictions to pressure drawdown existed. The PVT laboratory results showed ‘insignificant’ oil-based mud filtrate contamination in the samples. In addition, the large sample volumes provided flexibility for additional PVT studies and improved resource assessment. The focused sampling technology was successfully applied in both exploration and development campaigns in the SE Asia region. The pre-job simulations ensured optimal packer selection between the three focused packer types. The comparison between the actual sampling results and the 3D near-wellbore simulation would help optimize future sampling operations in the area. In addition, the two campaigns have reiterated a clear value of information in saving cost, reducing contamination in the samples and technology success in the given environments.