The Challenge of Water Sampling with a Wireline Formation Testerin a Transition Zone

摘要

Water sampling in Saudi Arabia has been a priority dueto the importance it can play in reserve estimates andcompletion decisions. To accurately estimate the oil andwater saturations in a producing zone it is essential toknow the water chemistry and accurately estimate thewater conductivity. What can make this particularlychallenging is when a well is drilled with water-basedmud (WBM). In this case the filtrate is miscible withthe formation water making it difficult to determinewhen to sample with a pumpout wireline formationtester (PWFT). Traditionally, a resistivity sensor is usedbut in cases where the salt or calcium chlorideconcentrations are high the resistivity can be relativelyinsensitive to the difference between WBM filtrate andformation water. Other methods, such as optical, havelimitations as well. Furthermore, it is desirable to obtainthe formation sample near the transition zone which cancause oil and water to be mixed while sampling.To meet these challenges new methods for samplingand fluid identification are employed. Using a newdual-inflatable Straddle Packer Section (SPS) it ispossible to separate the oil and water phases whilesampling. Gravity can separate the oil and water phaseswithin the SPS sealed interval, and by using two spacedinlet ports it is possible to obtain distinct segregatedwater and oil samples. In the sampling process the fluidphases can be mixed which can complicate the fluididentification. By using a new high resolution densitysensor this mixing process can be more clearlyunderstood. Fluid segregation within the straddlepacker interval and PWFT pump can be verified andmethods of obtaining separate oil and water samples areshown using field case studies. The new density sensoris based on a vibrating tube where the fluid passing thetube changes the natural frequency of vibration.Because of its high accuracy it is possible to determineadditional fluid properties such as salinity and thedifference between the WBM filtrate and the formationwater.