1. n. [Drilling]

An adaptive drilling method used to precisely control the annular pressure throughout a wellbore. After determining the downhole pressure environment, drillers manage wellbore pressure constrained by the limits of formation properties. The annular pressure is kept slightly above the pore pressure to prevent the influx of formation fluids into the wellbore, but it is maintained well below the fracture initiation pressure. Rapid corrective actions can often be implemented in order to deal with observed pressure variations. The MPD process may utilize a variety of techniques including control of back pressure, adjusting mud density, modifying fluid rheology, adjusting the annular fluid level, controlling circulating friction and incorporating hole geometry in the well construction. 

The use of MPD to control the risks and costs of drilling wells that have narrow downhole pressure limits by actively managing the wellbore pressure profile has become a common practice. The dynamic control of annular pressures enables drilling wells that might not otherwise be practical.

See: underbalanced,  overbalance