Extending the Limit: Interval Pressure Transient Testing in Low-Permeability Reservoirs in the North Sea

The use of wireline-conveyed formation testers for sampling, downhole fluid analysis (DFA), interval pressure transient testing (IPTT), and producibility estimation is well established in the oil and gas industry. The increasing demand for hydrocarbons is driving the exploration and production of reservoirs previously deemed uneconomical. The low-permeability limit for such reservoirs is continually being reduced while the limit for reservoir temperature is increasing. IPTT techniques address these trends by extending the limits for high-quality data acquisition.The decision on whether to perform IPTT requires an estimate of permeability. This estimate usually comes from either pretesting or other well data such as nuclear magnetic resonance (NMR) logs. However, these measurements have a shallow depth of investigation and may underestimate the mobility of the reservoir fluids because of formation damage such as fines invasion, or high-viscosity fluid in the invaded zone. To displace invaded fluids in low-permeability formations can be challenging, usually requiring a high drawdown to pump the mud filtrate and fines from the formation. High-performance packers and pumps are essential in such environments that require a large number of high differential-pressure cycles.In this paper three case studies of IPTT in low-permeability reservoirs in the North Sea are presented. In the first, pretesting with a single probe and dual-packer indicated a tight formation (no fluid communication between the tool and formation was established). IPTT was attempted and fluid moved at a drawdown of around 80 bar. The subsequent influx of formation gas reduced the drawdown significantly and a large increase in mobility was observed.In the second tight gas case study gas influx was detected, but because of the large drawdown the gas compressibility prevented a large-volume cleanup and the test was aborted. This case shows the limit of what is achievable in this environment.The third case study illustrates a very low-permeability limestone that was straddled with a dual-packer-standard-probe testing configuration. The downhole pump was equipped with a high-pressure displacement unit (DU). This pump configuration handled the high initial drawdown required to clean up the damaged zone. Initial cleanup pumping was followed by 30 hours of producing time and then a final pressure buildup. The result was a high-quality estimate of deep formation properties and fluid samples with very low contamination levels. A lesson learned was that formations considered "tight" according to data available before formation testing may be productive and capable of flowing hydrocarbons when evaluated with IPTT. Compared with the cost of a drillstem test (DST) in low permeability conditions, the dual-packer tool provided a fast, cost-efficient, and flexible way of testing individual zones.