It is effective to drill horizontal wells in carbonate to increase production. However, the horizontal sections were limited and hard to be extended by means of conventional drilling due to high risk of drilling in fractured cavernous carbonate. Application of a precisely pressure controlled drilling system was developed to have successfully drilled and extended horizontal sections, reduced risk of drilling in the said kind of wells with narrow pressure margins in western China’s Tarim basin.

A comprehensive MPD technology with 9-operational situations, 4-controlling modes and 13-precise controlling techniques of emergent changeovers was built up through full-size laboratory test, field test and deployment. Under-, near-, or over-balanced MPD was selected and carried out to meet diversified requirements based on proper well trajectory design, build-up rate control, retention of margin for adjustment and control of the well track, dynamic prediction of bottomhole pressure alternation while drilling according to the stability of the formation and whether toxic gases existed.

Drilling in Tarim basin usually experienced complex geology, narrow density margin, low ROP, frequent downhole complex and accidents, and long drilling duration. After the precise MPD technology being applied in Tarim basin, the risk of kick-and-loss coexistence has been reduced and the well cost has been gained beneficial results. The application assisted not only the hydrocarbon discovery of reef flat geology of Ordovician system in China’s Tarim basin, but also the well deliverability enhancement, effective treatment of downhole complex, and improvement of ROP. Application of precise MPD in a Tarim horizontal well created a record for deepest horizontal well with MD at 8,008m, TVD at 6,327m, and horizontal section of 1,551m. Another Tarim horizontal well produced a new record of daily footage 150m and horizontal footage 1,561m, and 42 pay zones were discovered. The difficulty of horizontal section pressure control in fractured cavernous carbonate was effectively resolved and multi combinations of fractures and caves were drilled through by application of precise MPD technology. The horizontal sections, historically limited by 200m-300m, were largely extended in fractured cavernous carbonate horizontal wells, and the well capacities were advanced.

Drilling of fractured cavernous carbonate horizontal wells is an integration and innovation of the precise MPD technology, downhole risk control, and the optimization and control of horizontal well trajectory. The precise MPD technology could effectively avoid or alleviate downhole risk, cut non-productive time, extend horizontal section, promote rate of penetration, reduce drilling cycle, achieve safe drilling, and therefore reduce well cost. It would play a significant role in future challengeable drilling activities.

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