Formation water samples are often collected via formation testing from appraisal wells to aid scale management planning. It is important that the samples, and analyses obtained from them, are of good quality. The risk of obtaining poor quality samples/analyses is, in part, determined by the methods, procedures and equipment selected by the operator. To reduce this risk, it is useful to simulate the anticipated sampling conditions ahead of formation testing. This aids selection of optimum methods, procedures and equipment to obtain formation water samples/analyses for scale management planning given the constraints of the drilling and testing programmes.
To demonstrate the approach, scale prediction software has been used to simulate the conditions experienced by samples obtained from low, moderate and high pressure and temperature reservoirs (all non-H2S-bearing). A typical North Sea formation water composition was assumed but the composition was varied to explore how this affected the results. A wide range of sampling scenarios have been modelled. For example, these have looked at the effects of sampling drawdown, sampling below the bubble point, in the transition zone and below the OWC, use of different types of sample chambers, and use of different post-sampling handling methods.
The simulation results show, for given reservoir conditions and formation water compositions, why, and how, the compositions can be affected by the choice of sampling methods, procedures and equipment. This has allowed recommendations to be made with respect to how to produce water from the test zone, what type of sampling tools should be used under different circumstances, and how the samples should be handled after collection for each category of reservoir. The same approach can be used after acquisition to help confirm the quality of the samples/analyses obtained (i.e. by simulating the actual sampling conditions encountered).