The practice of directional drilling traces its roots to the 1920s, when basic wellbore surveying methods were introduced. These methods alerted drillers to the fact that supposedly vertical wells were actually deflecting in unwanted directions. To combat this deviation, drillers devised techniques to keep the well path as vertical as possible. The same techniques were later employed to deliberately deflect the well path to intersect hard-to-access reserves.
The first intentionally drilled directional wells provided remedial solutions to drilling problems: straightening crooked wellbores, sidetracking around stuck pipe and drilling relief wells to kill blowouts (see Figure 1). Directional drillers used rudimentary survey instruments to orient the wellbore. By the 1930s, a controlled directional well was drilled in Huntington Beach, California, USA, from an onshore location to target offshore oil sands.
Today, operators use sophisticated drilling assemblies to drill complex geologic structures identified from 3D seismic data. Previously unreachable reserves have become accessible and economical to produce.
Horizontal directional drilling includes three main specialized applications: extended-reach drilling (ERD), multilateral drilling and short-radius drilling. Operators have used ERD to access offshore reservoirs from land locations, sometimes eliminating the need for a platform. As of 2013, the world’s longest ERD well is the 12,345-m [40,502-ft] well drilled from Sakhalin Island, Russia, to the offshore Odoptu field. Multilateral drilling helps increase wellbore contact with hydrocarbon-producing zones by branching multiple extensions off a single borehole. The first multilateral well was drilled in 1953 at Bashkiria field, Bashkortostan Republic, Russia. The main borehole had nine lateral branches that increased penetration of the pay zone by 5.5 times and production by 17-fold, and cost only 1.5 times that of a conventional well. Short-radius drilling produces wells with a curve of 44-m [144-ft] radius or smaller.
Principles of Directional Drilling
Most directional wells begin as vertical wellbores. At a designated depth, known as the kickoff point (KOP), the directional driller deflects the well path by increasing well inclination to begin the build section. Surveys taken during the drilling process indicate the direction of the bit and the toolface, or orientation of the measurement sensors in the well. The directional driller constantly monitors these measurements and adjusts the trajectory of the wellbore as needed to intercept the next target along the well path.
Directional drilling applications
Directional drilling applications. Reservoirs that are not readily accessible from available surface locations can be exploited through directional drilling.
Initially, directional drilling involved a simple rotary bottom hole assembly (BHA) and the manipulation of parameters such as weight on bit (WOB), rotary speed and BHA geometry to achieve a desired trajectory. Changes in BHA stiffness, stabilizer placement and gauge, rotary speed, WOB, hole diameter, hole angle and formation characteristics all affect the directional capability and drilling efficiency of a BHA.
By varying stabilizer placement in the drillstring, directional drillers can alter side forces acting on the bit and the BHA, causing it to increase, maintain or decrease inclination, commonly referred to as building, holding or dropping angle, respectively.