Combining sonic while drilling and formation pressure while drilling for pore pressure analysis to reduce drilling risk: a case study in offshore Vietnam
The accurate prediction of pore pressure is important for safe drilling and can significantly reduce financial risk. The major focus during drilling in a high-temperature/high-pressure (HP/HT) environment is optimising the drilling operation to mitigate risk by utilising data such as sonic logging-while-drilling (LWD) and offset well information.
An offshore exploration well was being drilled in offshore Vietnam with drilling challenges of abnormally high formation pressure. In normally compacting sediments, water escapes through permeable sands or along fractures as overburden sediments build up, and fluid or pore pressure remains close to hydrostatic pressure. In such normally compacting sediments, under increasing effective pressure, porosity decreases and compressional velocity increases. However, if formation fluids cannot escape, for example due to the low permeability of overlying shales, then they bear part of the overburden load and hence become overpressured. These formations are called undercompacted, meaning they have a higher porosity than normally compacted shales. In overpressured shales, which contain pressured water, density is lower, porosity is higher, and compressional velocity is lower than normal.
Formation pressure can be the major factor affecting the success of drilling operations. If pressure is not properly evaluated, it can lead to drilling problems such as lost circulation, kick, stuck pipe, hole instability, and excessive costs. Therefore, knowledge of the pore pressure is of considerable value because it provides the means for improving drilling operations and designing better casing programmes to reduce those risks. Using the data gathered from well logs, it is possible to predict the probable pressure profile that will be encountered while drilling. LWD data enable monitoring pore pressure very effectively; this pore pressure prediction can be continuously updated using LWD sonic and formation pressure while drilling (FPWD) to make optimal decisions for the drilling operation. Once a suitable predrill pressure profile is established, it is monitored on the current wells with logging while drilling sonic and directly calibrated with formation pressures taken in sands.
In the offshore Vietnam well, pore pressure monitoring by LWD formation pressures and estimated pore pressure from LWD sonic data allowed drilling operations to be optimised. Employing this technique led to successful drilling without any incidences related to pore pressure. The computed pressures from LWD sonic matched the measured pressures obtained from FPWD. The real-time sonic matched the recorded-mode sonic closely. This technique could apply not only to the wells in this basin but also to the other wells in the locations with abnormal high formation pressure.
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