Petroleo

Drilling and Completing Shale Gas Wells

Horizontal Drilling and Well Construction
Natural gas will not readily flow to any vertical well drilled through it because of the low permeability ofshales. This can be overcome to some extent by drilling horizontal wells, where the drillbit is steered from its downward trajectory to follow a horizontal trajectory for one to two kilometres, therebyexposing the wellbore to as much reservoir as possible. By drilling horizontally, the wellbore may intersect a greater number of naturally existing fractures in the reservoir – the direction of thedrill path is chosen based on the known fracture trends in each area. The trade-off between drilling horizontal versus vertical is increased access to the reservoir, but at a far higher cost. During wellconstruction proper casing and cementing of oil and gas wells is very important to protect water supplies and ensure public safety. Initially the surface hole is drilled to below the depth of anypotable groundwater or subsurface aquifers. Pieces of steel pipe, called casing, are connected together, and then run into the wellbore. The surface casing is then cemented in place. The cement fills thespace between the outside of the casing and the hole. The cement, once hardened, is tested for such properties as hardness, alignment and pressure integrity. Drilling only resumes once the cement ishardened and the surface casing has passed a pressure test. Next the main hole is drilled by drilling out the bottom of the surface casing down to the reservoir. Another casing string is run into thewellbore inside the surface casing to the bottom of the well and cemented in place. A third string, intermediate casing, is run in the same manner. At a determined depth, the well is then drilled at anincreasing angle until it meets the reservoir interval in a horizontal plane. Drilling then continues to predetermined length. Once drilling is complete, production casing is placed in the...