Typical wells in the Gulf of Thailand are drilled using a slim-hole design that requires 6⅛-in. hole for their reservoir interval. The slim and highly deviated well geometry, potential of high acid gas contamination, and high-temperature environment requires a well-engineered non-aqueous drilling fluid (NAF) to meet the overall well objectives. These conditions always place severe limitations on the drilling fluids design and often lead to failures in openhole wireline logging operations, which is the most important well objective.The key fluid properties to ensure the success of wireline logging operations under this high temperature are hightemperature, high-pressure (HTHP) filtration control, filtercake quality, and thermal stability of the drilling fluid under extreme static condition. Selected emulsifier and HTHP filtration control agents work synergistically in providing excellent hole condition which allows the hole to be fully logged and evaluated. Clear benchmarks were set and monitored in all the field applications: (1) Successful wireline logging operation; (2) Variation of mud weight after static condition; (3) Variation of mud weight compared to formation tester data point.This paper details the design, development, and field applications of an ultra-high-temperature NAF used for drilling deep and hot wells in the Gulf of Thailand. In addition, the extensive laboratory work required to optimize the formulation for extreme high temperatures, the lessons learned, and the critical engineering guidelines for running a NAF in such harsh conditions will be described.

The definition of ultra-high temperature (uHT) conditions varies depending on regional average well temperatures. In the Gulf of Thailand (GOT), uHT is defined as a well with a bottom-hole static temperature (BHST) exceeding 450°F (230°C). Recently, a national oil company began requiring a drilling fluid stable up to 520°F (270°C) for exploration wells, especially when the actual BHST is uncertain. This paper describes the development and stability of a drilling fluid approved by the operator for these extreme well conditions. Typical exploration wells in the Gulf of Thailand require 6 1/8 -in. hole for their primary target interval. Beside the extreme BHST, the slim and deviated well geometry (up to 60°inclination), and the potential for highly acidic gas (CO 2 and H 2 S) contamination, presented extreme conditions for prolonged successful open-hole wireline logging operations (up to 4 days), which was the most critical objective for the exploration wells. Successful open-hole wireline logging is also imperative for perforating and further production from the typical thin layers of sandstone reservoir.New additives provide a thermally stable non-aqueous fluid (NAF) enabled logging and evaluation under the severe down-hole conditions. The novel emulsifier and fluid-loss-control agents were introduced after undertaking an extensive laboratory-testing program. Apart from the newly introduced products, fluid engineering and improved field practices also played an important role in the success of these exploration wells.The successful achievement with this novel NAF facilitated the planning of additional uHT exploration wells for the near future. In addition to the successful logging program, the novel NAF minimized non-productive time and mets the objectives set for the exploration wells comparing to the conventyiaonl NAF.

PTTEP in the Gulf of Thailand faced costly challenges while conventionally drilling several narrow operating window ultra high temperature (ultraHT) wells with formation temperature up to 220°C (428°F). In the ultraHT sections, the operator encountered serious ballooning issues that resulted in severe non-productive time (NPT) and difficulty reaching well total depth (TD). Additionally, formation pressure uncertainty in the steep pressure ramp region posed additional drilling challenges due to high risk of influxes.PTTEP decided to utilize managed pressure drilling (MPD) to overcome the complex ultraHT wells. The strategy involved designing a hydrostatically underbalanced mud weight to prevent ballooning and to enable optimal drilling flowrates. The automated MPD was used to continuously maintain bottomhole pressure (BHP) above pore pressure to avoid influxes. Furthermore, the MPD system was also used to safely identify formation pressures by performing static flow checks (SFC). Dynamic formation integrity tests (DFIT) and dynamic leak-off tests (DLOT) conducted while drilling accurately identified the losses limit and ballooning gradient. At well TD, special rollover procedures were implemented to displace trip mud weight in order to safely control the well prior to pulling out of hole, taking into account extreme thermal effects on bottomhole pressure (BHP) reduction.PTTEP was able to drill through the narrow operating window and avoid problems associated with typical ultraHT condition, including ballooning, loss and influx events. Moreover, MPD allowed the operator to drill efficiently while identifying the actual drilling window to establish wellbore pressure boundaries. Importantly, the operator was able to log the well to obtain the necessary geological data. As a result, the operator cut well costs by 50%, a total of USD 5 million, and saved 20 days of drilling time.The paper will share the success story of MPD application in drilling challenging ultraHT wells in the Gulf of Thailand. The paper will describe the drilling solutions to solve the well problems and the lessons learnt as part of knowledge sharing.

Drilling narrow window wells conventionally have been well known to cause major wellbore issues to the Operator in the Gulf of Thailand. Therefore, managed pressure drilling (MPD) has been deployed since several years ago to mitigate the drilling problems. To ensure safety and optimize drilling time, it is necessary to identify the actual pore pressure while drilling wells with narrow marginin order to eliminate kick, ballooning and loss events. The use of automated MPD system to precisely control bottomhole pressure (BHP) during connections combined withthe evaluation of bottom up gas trend while drilling enabled the pore pressure to be predicted accurately in almost real-time condition without the need to stop drilling in the Gulf of Thailand. Thus, the narrow window wells were drilled faster with MPD, which was beneficial for the Operator in the area where the fast factory drilling was necessary to make wells more economical. Furthermore, the use of this new method in effectively determining the actualpore pressure provided solutions to the Operator in mitigating wellbore issues at the same time improving drilling timeafter several years of MPD technology implementations. The estimated pore pressure results acquired with the new methodon several narrow window wells were even comparable to the actual wireline logging measurement results. The objective of this paper is to introduce an effective and efficient method in determining pore pressure (PP) while drilling challenging wells with tight reservoir characteristic by utilizing MPD technology in the Gulf of Thailand. In addition, this paper also describes the drilling strategy, detail procedure and lesson learnedin verifying pore pressure with the new method.