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Formation testing while drilling (FTWD)
Formation Testing While Drilling (FTWD) has a broad interest in all the different disciplines involved in drilling and evaluating the well. For the drilling engineer and the geologist, a number of different approaches to the problem of acquiring formation-pressure data while drilling have been tried.
A sophisticated subindustry has evolved aimed at pore-pressure prediction using proven methods such as
- “D exponent”
- Connection gas
- Cuttings analysis
Real-time formation-pressure data will, at a minimum, allow more-frequent calibration of pressure models. For the reservoir engineer, it opens the possibility of “barosteering”; where there is doubt in mature fields about whether a compartment has been drained, immediate measurements can be taken, and a decision can be reached about whether to geostop or geosteer for a more-promising compartment. It allows immediate testing to verify whether geological barriers are sealing, and it opens the possibility of pressure profiling to identify (from gradient information) types of fluids present and contact points. For the drilling engineer, the precise identification of mud weight needed offers potential for improvements in ROP (Rate of penetration). For all, particularly in high-angle wells, it offers the prospect of eliminating the need to acquire costly pressure measurements by pipe-conveyed wireline techniques. All will have concerns about the time required to take a test, especially if no circulation is permitted, because those conditions increase the likelihood of tool sticking.
Methods used to address the problem of data acquisition
Two different approaches have been taken to the problem of acquiring the data.
Traditional approach associated with drillstem tests (DSTs)
The first adopts the traditional testing approach associated with drillstem tests (DSTs). In this manifestation, dual inflatable packers are mounted on the outside of the tester. When a zone of interest is reached, a command is issued from the surface, and the packers are inflated to isolate the zone of interest. The drawdown pump is activated to remove a controlled volume of fluid from the annulus between the packers. Circulation above the tool can be maintained with a diverter sub, and pressure data continue to be pumped to the surface until sufficient data are acquired. One advantage of an approach of this sort is that it investigates a greater depth in the formation and is not susceptible to seating problems in laminated formations in which there may be a chance of landing a probe on a hard streak. Conversely, the exposure of relatively large areas of inflatable packers to the wellbore environment calls for careful design and handling to avoid damage.
Traditional wireline approach
An alternative approach to acquiring data follows the traditional wireline approach. In this approach, a small extendable probe with an elastomeric seal is applied to the formation on command from the surface. An internal piston is then actuated to draw down the pressure by as much as 8,000 psi below hydrostatic pressure. Formation fluids then flow into the probe and build up the pressure in the probe to the formation pore pressure. Pressure measurements are taken both with fast-acting strain gauges and high-accuracy quartz gauges. Tests can be acquired either with the pumps on or off. The drawdown and buildup profiles also provide information used in the determination of formation permeability. The reduced area associated with the probe should reduce the chances of drilling damage, and the smaller volumes involved in the test should provide reasonable data in a shorter time period (although from a shallower depth) than the DST-type design.
Future of FTWD
Early indications are that FTWD tools will be adopted quite rapidly by the industry provided that they can be shown to provide high-quality, reliable measurements without significantly increasing loss-in-hole risks.
- Proett, M., Walker, M., Welshans, D. et al. 2003. Formation Testing While Drilling, a New Era in Formation Testing. Presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, 5-8 October. SPE-84087-MS. http://dx.doi.org/10.2118/84087-MS.
Notworthy papers in OnePetro
Rashad M., Asbjorn G. et al. 2009. Using Formation-Testing-While-Drilling Pressures to Optimize a Middle East Carbonate Reservoir Drilling Program, SPE Middle East Oil and Gas Show and Conference, 15-18 March. 120715-MS. http://dx.doi.org/10.2118/120715-MS
Mark Proett, Mike Walker et al. 2003. Formation Testing While Drilling, a New Era in Formation Testing, SPE Annual Technical Conference and Exhibition, 5-8 October. 84087-MS. http://dx.doi.org/10.2118/84087-MS