Carbonate reservoirs hold a significant percentage of the world’s oil and gas reserves. However, geologic characteristics such as natural fractures often present substantial challenges during conventional overbalanced drilling programs.

Many approaches to overcome these challenges—especially when they require a hydraulic workover rig, a snubbing unit or killing the well to stop the flow of formation fluids to the surface—drive up costs and risks and minimize production, all of which compromise the profitability of the well.

Underbalanced drilling (UBD) can address these limitations by intentionally maintaining bottomhole circulating pressure (BHCP) lower than the pore pressure of the wellbore. The reduced pressure in the wellbore eliminates near-wellbore damage; increases well productivity and recovery; and mitigates drilling problems such as differential sticking, fluid losses and low ROPs. As the well is being drilled, formation fluid flows into the wellbore and then to the surface, where it is processed accordingly.

With its significant natural fractures and a high degree of fracture-enhanced porosity and permeability, the Pha Nok Khao (PNK) Formation in Thailand exhibits excellent gas production potential but is not a good candidate for conventional drilling. Upon embarking on a project to drill a deviated exploration gas well in the PNK Formation, an operator chose a UBD program to minimize formation damage, maximize productivity and determine the gas-flow rate while drilling.

The objective was to test the formation’s upper limestone-encased dolomite corridor to determine whether there were sufficient gas reserves to support production, establish reservoir characteristics and sustained maximum flow rate, and acquire real-time pressure data and formation fluid samples to optimize reservoir management.

The operator partnered with Weatherford to deploy a closed-loop UBD package with two downhole deployment valves (DDVs) to provide ultrareliable isolation during tripping and minimize formation damage. This eliminated the need to snub or kill the well or to flow the well while tripping, any of which would lead to potential HSE issues, increased mobilization and setup time, reduced tripping speeds, and additional crew requirements. Weatherford has successfully applied tandem DDVs elsewhere in the world, but this was the first time the approach had been used in Thailand.

Safe isolation, efficient tripping
Because maximum surface pressures were projected to exceed the UBD operating pressure rating of 5,000 psi, even while staying below the BOP stack rating, the UBD job was designated as high-pressure and thus required significant pre-job safety considerations. The team opted to apply a single-phase drilling fluid UBD operation to a highly fractured 6-in. 271-m-long (889-ft) hole section to prevent formation-damaging fluid losses.

The full UBD package included:
• A SafeShield rotating control device to contain annular fluids while drilling and to divert mud and cuttings through a dedicated choke manifold into the separation equipment;
• A choke manifold to control the flow of well fluids into the surface separation equipment; and
• A four-phase vertical separation system that handled all flow from the well.

Upper and lower DDVs, each rated to 5,000 psi working pressure, were installed in the 7-in. liner tieback section at 1,251 m (4,104 ft) and 1,272 m (4,173 ft) measured depth. Engineered with a flapper-type seal mechanism to contain reservoir fluids, the DDV is a casing-deployed downhole barrier that allows fullbore passage of the drillbit and prevents backflow of reservoir pressure to surface.

In this case, the tandem DDVs were installed to isolate the formation while tripping in closed-loop underbalanced conditions to enable efficient trip in/trip out of the hole and to minimize formation damage. An additional DDV was installed as a backup. The DDV functionality was tested and verified at the rig floor at 576 m (1,890 ft) and at 1,276 m (4,186 ft).

The UBD operation was successfully completed in just three days. The operator drilled the entire hole section in UBD mode using one bottomhole assembly. After drilling conventionally, the team circulated 10 lb/gal of water-based mud, which was later reduced to 9.6 lb/gal for UBD initiation.

After intersecting the gas production zone, the team collected formation and fluid samples to determine reservoir characteristics and production potential. Drilling continued until the target formation depth was reached with a gas production rate of 283,300 cu. m/d to 424,900 cu. m/d (10 MMscf/d to 15 MMscf/d) achieved during drilling. The target BHCP range was achieved with a single-phase drilling fluid, later lightened by the produced gas. After penetrating the wellhead, the operator cycled (opened and closed) the lower DDV nine times and the upper DDV three times.

No nonproductive time, HSE issues
To eliminate the risk of a washout below the casing shoe using conventional wireline logging, Weatherford deployed the Compact well shuttle (CWS) messenger system.

The CWS messenger system protects logging tools from harsh wellbore environments, which enables real-time data transmission in UBD conditions without interrupting well flow.

After running the lower and upper completions, the team used the same UBD package to conduct isochronal testing—a series of well flow/shut-in test intervals—to gain additional information about the reservoir’s productivity potential through detailed, comprehensive testing conducted over a 144-hr period.

The application of the closed-loop UBD system, configured with the tandem DDV, enabled the entire 271-m section to be drilled with zero nonproductive time and no HSE issues.

No fluid loss or formation damage was observed during the UBD operation as gas flowed from the reservoir to the wellbore and to the surface. Gas production met expectations, and client objectives were achieved.

The tandem DDVs provided consistent pressure control throughout the process, facilitating safe, efficient tripping without snubbing or killing the well during drilling, logging and running the lower completion.

Continued flow while drilling underbalanced enabled the operator to collect real-time reservoir characterization data of the fractured block’s zone contributing to gas production as well as the nonproducing zone.

The project further validated the UBD with a tandem DDV approach as the optimal solution for drilling fractured carbonate reservoirs in Thailand safely and efficiently while enhancing reservoir productivity.