Wireline formation testers (WFT) are used for pressure measurements, fluid sampling, downhole fluid analysis and interval pressure transient testing. In three of these four applications how well we design and implement the flowrate requirements will determine if we will achieve the program objectives. In designing and executing even simple WFT jobs the inter-dependence of flow rate with drawdown, sand failure, phase change and contamination clean-up all must be considered. Until recently there has been relatively little "degrees of freedom" with respect to optimizing flow rate. There was typically one pump with one or two displacement units and one or two probe sizes to choose from. Now, however, when designing a typical WFT tool string one is faced with the choice of at least five different probe types, four different displacement units, and four different hydraulic pumps. Additionally, modern WFT strings now allow the use of two pumps simultaneously giving even more flow rate options.
In this paper we discuss the interplay between the four different hydraulic pumps and the four different displacement units and how to optimize tool string design to achieve the desired flow rate and drawdown characteristics. We show field examples from a near saturated low perm reservoir in Angola and from the Niger Delta during the acquisition of a sample using focused probe technology that requires careful balancing of two pumps operating simultaneously.
One of the implications of the pump design in the WFT is the ability of the pump to segregate two phase fluid as it passes through the pump. We show how to maximize the utility of this characteristic and provide field examples where the ability to segregate allows for large reductions in sampling times in the case of two phase fluids.
Finally, with all the work on the optimization of the hardware for maximum flow rate flexibility we introduce a newly developed software algorithm that provides a real time flowrate measurement.
The defining characteristic of the third generation of advanced wireline formation testing (WFT) tools is their modularity and their ability to pump reservoir fluids downhole. Whereas previous generation formation testing tools relied on natural flow into a sample chamber at some pressure below formation pressure, the current generation of tools is able to move fluids from the lower pressure formation into the higher pressure borehole. Today most formation tester tools accomplish this by means of a displacement unit (DU) which is driven by a hydraulic pump. Figure 1 shows an example. The DU is moved by hydraulic fluid being pumped alternately into one side and then the other. On any given stroke the DU is pulling fluid from the reservoir on one side and expelling it out the other side.