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Category Archives: Horizontal drilling

Why Drill Wells That Are Non-Vertical?

Directional and horizontal drilling

Directional and horizontal drilling have been used to reach targets beneath adjacent lands, reduce the footprint of gas field development, increase the length of the “pay zone” in a well, deliberately intersect fractures, construct relief wells and install utility service beneath lands where excavation is impossible or extremely expensive.

Below is a list of six reasons for drilling non-vertical wells. They are graphically illustrated by the six drawings in the right column of this page.

A) Hit targets that can not be reached by vertical drilling.

Sometimes a reservoir is located under a city or a park where drilling is impossible or forbidden. This reservoir might still be tapped if the drilling pad is located on the edge of the city or park and the well is drilled at an angle that will intersect the reservoir.

B) Drain a broad area from a single drilling pad.

This method has been used to reduce the surface footprint of a drilling operation. In 2010, the University of Texas at Arlington was featured in the news for drilling 22 wells on a single drill pad that will drain natural gas from 1100 acres beneath the campus. Over a 25 year life-time the wells are expected to produce a total of 110 billion cubic feet of gas. This method significantly reduced the footprint of natural gas development within the campus area.

C) Increase the length of the “pay zone” within the target rock unit.

If a rock unit is fifty feet thick, a vertical well drilled through it would have a pay zone that is fifty feet in length. However if the well is turned and drilled horizontally through the rock unit for five thousand feet then that single well will have a pay zone that is five thousand feet long – this will usually result in a significant productivity increase for the well. When combined with hydraulic fracturing, horizontal drilling can convert unproductive shales into fantastic reservoir rocks.

D) Improve the productivity of wells in a fractured reservoir.

This is done by drilling in a direction that intersects a maximum number of fractures. The drilling direction will normally be at right angles to the dominant fracture direction. Geothermal fields in granite bedrock usually get nearly all of their water exchange from fractures. Drilling at right angles to the dominant fracture direction will drive the well through a maximum number of fractures.

E) Seal or relieve pressure in an “out-of-control” well.

If a well is out-of-control a “relief well” can be drilled to intersect it. The intersecting well can be used to seal the original well or to relieve pressure in the out-of-control well.

F) Install underground utilities where excavation is not possible.

Horizontal drilling has been used to install gas and electric lines that must cross a river, cross a road, or travel under a city.

Pilot Hole Directional Drilling

Pilot hole directional control is achieved by using a non-rotating drill string with an asymmetrical leading edge. The asymmetry of the leading edge creates a steering bias while the non-rotating aspect of the drill string allows the steering bias to be held in a specific position while drilling. If a change in direction is required, the drill string is rolled so that the direction of bias is the same as the desired change in direction. The direction of bias is referred to as the tool face. Straight progress may be achieved by drilling with a series of offsetting tool face positions. The drill string may also be continually rotated where directional control is not required. Leading edge asymmetry can be accomplished by several methods. Typically, the leading edge will have an angular offset created by a bent sub or bent motor housing. This is illustrated schematically in following Figure.

Bottom Hole Assembly
It is common in soft soils to achieve drilling progress by hydraulic cutting with a jet nozzle. In this case, the direction of flow from the nozzle can be offset from the central axis of the drill string thereby creating a steering bias. This may be accomplished by blocking selected nozzles on a standard roller cone bit or by custom fabricating a jet deflection bit. If hard spots are encountered, the drill string may be rotated to drill without directional control until the hard spot has been penetrated.


The History Of Horizontal Directional Drilling

Martin Cherrington first conceived horizontal directional drilling in the 1960’s. He first realized the value of underground drilling when he and another contractor were given the same job: lay down telephone lines in Los Angeles. The only difference was that Cherrington was using an open trench method while the other contractor was using drilling to lay down cables. That contractor arrived two weeks after Cherrington yet managed to finish two weeks before him. This led Cherrington to believe there was merit in looking at underground drilling methods.

In 1964 Cherrington founded Titan Contractors, which specialized in utility road boring. It was an opportune time for the company’s formation because of a building boom in Sacramento and a recent “beautification” decree from the First Lady, Lady Bird Johnson. The decree was instated to clean up America by getting rid of utility lines which were an eyesore and hazardous during seismic and extreme weather events. As a solution, Sacramento proposed placing all utilities underground. Despite a favorable environment and HDD’s merits, however, other, more familiar, tunneling technologies like jack and bore and auger boring were usually preferred.

One of the main problems was the lack of control when drilling. It was often very difficult to make a straight bore, and the drill bit would resurface in unexpected places (like the middle of the road). Cherrington realized a solution when an engineer from P G& E invited him to consider a project for placing a gas line underneath the Pajaro River. The project would require drilling underneath the river, and the variability of the drill bit’s direction would make it challenging. To find a solution, Cherrington experimented with angled bores on a similar river, trying several different angles. He observed that the steeper the angle of the bore, the greater the achieved distance. This relationship between angle and distance helped prove that with “optimum entry angle, proper drilling techniques and the right downhole tool assembly” (Cherrington) HDD could be used to cross a river. Since then, familiarity with HDD has increased, and it has become a much more routine method for projects requiring a non-evasive boring solution.


Horizontal Directional Drilling Process

The tools and techniques used in the horizontal directional drilling (HDD) process are an outgrowth of the oil well drilling industry. The components of a horizontal drilling rig used for pipeline construction are similar to those of an oil well drilling rig with the major exception being that a horizontal drilling rig is equipped with an inclined ramp as opposed to a vertical mast. HDD pilot hole operations are not unlike those involved in drilling a directional oil well. Drill pipe and downhole tools are generally interchangeable and drilling fluid is used throughout the operation to transport drilled spoil, reduce friction, stabilize the hole, etc. Because of these similarities, the process is generally referred to as drilling as opposed to boring.

Installation of a pipeline by HDD is generally accomplished in three stages as illustrated in Figure 1. The first stage consists of directionally drilling a small diameter pilot hole along a designed directional path. The second stage involves enlarging this pilot hole to a diameter suitable for installation of the pipeline. The third stage consists of pulling the pipeline back into the enlarged hole.

HDD Process

HDD Process

Trenchless Excavation Method-Horizontal Directional Drilling

Horizontal Directional Drilling (HDD) is a trenchless excavation method originated in the late 1960’s by merging technologies prominent in the water wells and utilities industry. HDD involves drilling along a desired pathway while simultaneously inserting a pipe into the new borehole.Horizontal Directional Drilling

The HDD process is completed in three parts, the first being the drilling of a pilot hole along the proposed centreline and the second being the enlarging of the hole with a reamer. The last step would be have the new pipe installed behind the reamer during the last pass of the reaming process.

HDD Scope of application
1 Horizontal directional drilling apply to pipeline and cable, fiber optic cable and other steel casing through lakes, swamps, rivers, highway, railway, building and other unfavorable for excavation of construction.
2 Traversing segment of stratum is not pebble soil.
3 The point of traversing has the area for installing drill rig and pipeline, underground without other unknown obstacle.

Horizontal drilling Methodology

Horizontal drilling is the process of drilling a well from the surface to a subsurface location just above the target oil or gas reservoir called the “kickoff point”, then deviating the well bore from the vertical plane around a curve to intersect the reservoir at the “entry point” with a near-horizontal inclination, and remaining within the reservoir until the desired bottom hole location is reached.

The initial vertical portion of a horizontal well, unless very short, is typically drilled using the same rotary drilling technique that is used to drill most vertical wells, wherein the entire drill string is rotated at the surface. The drill string consists of many joints of steel alloy drill pipe, drill collars, and the drill bit itself.

From the kickoff point to the entry point the curved section of a horizontal well is drilled using a hydraulic motor mounted directly above the bit and powered by the drilling fluid.

The drill bit can be rotated by the hydraulic motor without rotating the drill pipe from the motor to the surface. Steering of the hole is accomplished through the employment of a slightly bent or “steerable” downhole motor. By orienting the bend in the motor and drilling forward without rotating the pipe, known as slide drilling, the hole can be steered around a curve from horizontal to vertical and/or to the left or right. The curved section typically has a radius of 300-500 feet. To return to drilling straight ahead, the pipe is rotated slowly while the downhole motor also continues to rotate the bit.


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