Major challenges faced by today's industry in drilling fractured carbonate reservoirs are formation damage and potential drilling problems such as fluid loss and differential sticking. To drill such formations, underbalanced drilling (UBD) technology has been used as it can intentionally keep the bottomhole pressure (BHP) lower than the pore pressure (PP), which will eliminate the near-wellbore damage, and mitigate drilling problems like fluid losses. This technology was also used onshore in Thailand, mostly for development wells with low-pressure reservoirs. Well-A, located in the fractured carbonate Pha Nok Khao (PNK) formation of northeast Thailand is an exploration well with expected reservoir pressure around 6,000 psi (13 ppg in equivalent density), whose operator was determined to utilize UBD for the production section in order to minimize formation damage and maximize and test productivity.
The higher reservoir pressure leads to higher operational risks for UBD. While drilling, single-phase water based mud (WBM) was used to keep proper drawdown for this section. In order to have safe and efficient tripping operations, tandem downhole deployment valves (DDV) were utilized to provide ultra-reliable isolation of the wellbore for all the trips during drilling, logging and completion, which eliminated the need to snub or kill the well—any of which would lead to potential formation damage and safety issues. After completion, isochronal well testing was conducted using the same UBD package and later, in order to obtain more data to determine whether the operator could commercially develop the field. After that, a long-term well-test operation was conducted by continuously flaring the well at variable rates for 15 days using the same package.
As the well was in an underbalanced condition through all the operation phases, no formation damage, fluid losses and NPT happened during the whole operation. The use of same UBD package from drilling till well testing for the full operation cycle also minimized costs and rig time.
This paper details the UBD approaches used to minimize formation damage and evaluate reservoir productivity for the Well-A in the PNK formation of northeast Thailand.