The Challenge of Water Sampling with a Wireline Formation Tester in a Transition Zone

摘要

Water sampling in Saudi Arabia has been a priority due to the importance it can play in reserve estimates and completion decisions. To accurately estimate the oil and water saturations in a producing zone it is essential to know the water chemistry and accurately estimate the water conductivity. What can make this particularly challenging is when a well is drilled with water-based mud (WBM). In this case the filtrate is miscible with the formation water making it difficult to determine when to sample with a pumpout wireline formation tester (PWFT). Traditionally, a resistivity sensor is used but in cases where the salt or calcium chloride concentrations are high the resistivity can be relatively insensitive to the difference between WBM filtrate and formation water. Other methods, such as optical, have limitations as well. Furthermore, it is desirable to obtain the formation sample near the transition zone which can cause oil and water to be mixed while sampling. To meet these challenges new methods for sampling and fluid identification are employed. Using a new dual-inflatable Straddle Packer Section (SPS) it is possible to separate the oil and water phases while sampling. Gravity can separate the oil and water phases within the SPS sealed interval, and by using two spaced inlet ports it is possible to obtain distinct segregated water and oil samples. In the sampling process the fluid phases can be mixed which can complicate the fluid identification. By using a new high resolution density sensor this mixing process can be more clearly understood. Fluid segregation within the straddle packer interval and PWFT pump can be verified and methods of obtaining separate oil and water samples are shown using field case studies. The new density sensor is based on a vibrating tube where the fluid passing the tube changes the natural frequency of vibration. Because of its high accuracy it is possible to determine additional fluid properties such as salinity and the difference between the WBM filtrate and the formation water.