Underbalanced Drilling (UBD) has become a popular technique in oil & gas industry, especially for well-developed hydrocarbon fields to minimize formation damage and maximize productivity. The key in UBD is to control bottom hole circulating pressure (BHCP). To maintain BHCP lower than the formation pressure, a light weight drilling fluid has to be used and probably with concurrent N2 injection. In the case of a tight reservoir, N2 injection rate is increased as high as 1500 SCFM, which will cause severe gas slugging and dramatic standpipe pressure (SPP) reduction when the gas is circulated though the choke. MWD signal will then be lost due to the pressure fluctuation. This paper will present a BHA optimization solution to maintain sufficient and stable SPP during UBD.
Two UBD cases will be discussed. Two main challenges were encountered in the first well: one was to reduce BHCP sufficiently low, the other was to maintain SPP high and stable. To reduce the BHCP, MWD pulser was changed from 180-280 GPM rating to 150-250 GPM to allow lower drilling fluid flow rate and increased N2 injection rate to 1500 SCFM. However, surface SPP dropped significantly due to gas slugging. To avoid this happen, the following optimizations were implemented in the second well: 1) added more HWDPs to reduce internal diameter of the drill string; 2) reduced bit TFA by using less nozzles; 3) installed flow rate restrictor in the drill string.
With reduced flow rate and increased N2 injection rate, BHCP was reduced to 300 psi below formation pressure. At the same time, with the optimized BHA, surface SPP was increased sufficiently to transfer MWD signal continuously. According to the simulation, among the 600 psi increase in surface pressure, 300 psi was from the restrictor, 150 psi was from the change of bit nozzles and 150 psi was from the added HWDPs. The main contribution was from the restrictor. Compared with the first well, which was drilled without restrictor, the data retrieved from the second well was smoother and more readable.
It was the first time that downhole flow restrictor was used in UBD well to maintain SPP. Combined with adding HWDPs and reducing bit TFA, SPP was increased sufficiently, whereby enabled continuous signal transmission and better data quality. Therefore, it is recommended to use this technique in the next UBD campaign.