Real-Time Pore Pressure Gradients: An Example from the Pattani Basin

Klahan, Isara¹; Nopsuri, Watanyu²
¹Thailand Exploration, Chevron Thailand Exploration and Production, Ltd., Bangkok, Thailand.
²Drilling, Chevron Thailand Exploration and Production, Ltd., Bangkok, Thailand.

Prolific hydrocarbon fields in the Pattani Basin, Gulf of Thailand (GOT) are the result of syn-rift deposition of fluvial-delta plain sands and shales filling the basin in Miocene. The majority of reserves in this region have been found in small reservoirs, which are over-pressured in many areas. Drilling exploration and delineation wells is challenging in the overpressure areas where there is insufficient offset well information to define the pore pressure profile. Well design, casing design, mud program and cementing program are all dependent on the expected pressure profile, and a conservative pre-drill design is needed for safety. However, during drilling operations, the program can be modified to reflect actual pressure profile and, in some cases, reduce mud weight (MW), extend total depth (TD), and limit invasion from drilling over-balanced.

The A-17 was the first delineation well drilled in the outboard fault block of Pattani Field A and the deepest well drilled in the Pattani Basin in the last 39 years. It also was one of the hottest wells in the GOT and the first well in the Field A operating area to drill into the Lower Miocene, which was targeting analogous reservoir-quality pay sands in nearby platforms. In other fault blocks, formation pressure is high and our current three-string well design limits the well TD. Previously, all of the wells in the Field A were drilled to only the Middle Miocene, which is the main pay reservoir unit.

During the planning phase, A-17 drilling program was designed based on the pressure profile predicted from RFT data obtained in nearby wells. Two alternative pressure models were created, one for the "low trend", controlled by data points from wells in a similar structure (an outboard fault block), and one for the "high trend", controlled by the nearest well, one fault block away. The well was planned using the "high trend" with a four-string well design and a detailed well execution strategy developed to obtain pressure data at critical stages to make decisions on MW and TD. The actual pressure profile of the well ended up following the "low trend", which indicates that future drilling in this block can be done with a lower-pressure, less costly three-string well. The well was drilled efficiently with incident-free operations. Besides pressure data, important information was obtained from the A-17 well on pay window, reservoir quality and hydrocarbon resource potential for future development in the Field A.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012