One of the advantages of MWD mentioned previously is that it helps an operator steer a drill in different directions. The ability to steer a drill in directions other than straight down has been one of the most significant advancements in the history of oil drilling.
Because many oil reservoirs are spread out horizontally, vertical wells may not extract enough oil efficiently from them. A horizontal well is drilled deep down vertically at first, but then changes direction (at what is called the kick-off point) before it encounters the reservoir (at the entry point) and extends horizontally through it. But the advantages of horizontal drilling go beyond increasing well productivity. It also allows wells to be dug safely under environmentally sensitive and protected land.
Although the first horizontal well was drilled in 1929, they were expensive, and the development of hydraulic fracturing soon improved the productivity of vertical wells. Advances such as MWD and steerable motor assemblies, however, made horizontal drilling a more viable option by the 1980s.