17. Heavy Weight Drill Pipe (HWDP)

HWDP is an intermediate-weight drill string member with drill pipe dimensions for easier handling. Its heavy wall tube is attached to special extra-length tool joints. These provide ample space for recutting the connections and reduce the rate of wear on the OD.

A joint of HWDP has a greater wall thickness and longer tool joints than normal drill pipe. Midway between the tool joints is an integral wear pad which acts as a stabilizer. 

HWDP is less rigid than DCs and has much less wall contact. Chances of differential sticking are reduced. 

Its three-point wall contact feature solves two serious problems in directional drilling :

It permits high-RPM drilling with reduced torque. HWDP can be run through hole angle and direction changes with less connection and fatigue problems.
 

Today, the trend in BHA design is to minimize the number of DCs in the BHA and use HWDP to comprise a major portion of available weight on bit.

Note: Heavy Weight Drill Pipe should not be used as bit weight in vertical holes larger than those listed in the table below.

The BHA in a directional well may have 20 or more joints of HWDP between the drill collars and drill pipe.

Figure below shows the HWDP Dimensional Data Range :

 

 (All dimensions are given in inches)

Spiral Heavy Weight Drill Pipe

Processing of Heavy Weight Drill Pipes :

(click on the image to get high resolution preview of the image)

Heavy Weight Drill Pipe Material :

7. Directional Drilling Survey Calculations

When drilling a directional well, surveys are taken at regular pre-determined intervals in order to determine the present downhole location compared to surface location.

In directional survey we note down the inclination and azimuth at the survey point for a particular survey depth and using these data we calculate the North-South (N-S), East-West (E-W) coordinates and TVD using few mathematical calculations.

There are several methods that can be used to calculate the survey data, of these some are accurate while are other may produce some error for a given situation.

Some of the most common methods used for survey calculation in the industry are:

Tangential method (Least Accurate)

Balanced Tangential method

Average Angle method

Radius of Curvature method (Most Accurate)

Minimum Curvature method (Most Accurate)

The Tangential, Balanced Tangential and Average angle method are based on the trigonometry of a right angle triangle.

TANGENTIAL METHOD

As shown in the figure below, I₁I₂ is the actual wellbore course. To calculate the inclination at I₂, we draw a tangent to I₂. The tangential method states that the tangent drawn at the survey station I₂ is the assumed wellbore course and angle A is the required inclination which is similar to inclination at I₂.

It uses only the inclination and direction angles measured at the lower end of the survey course length.

Now applying trigonometry to the right angle triangle ABI₂, we have :

Directional Drilling : Tangential Method

angle A = angle I₂

AI₂ = assumed well course = ΔMD (change in measured depth for this interval)

AB = AI₂ Cos I₂ = ΔTVD (This will be equal to the TVD for this interval)

BI₂ = Departure

ΔNorth = ΔMD SinI₂ x Cos A₂ .

ΔEast = ΔMD SinI₂ x Sin A₂ .

It is clear from the above figure that the Tangential method gives a noticeable error in Measured Depth (MD) and Departure.

In Type I, III and IV holes, the error will be significant.

With the tangential method, the greater the build or drop rate, the greater the error. Also, the distance between surveys has an effect on the quantity of the error. If survey intervals were 10 feet or less, the error would be acceptable. The added expense of surveying every 10 feet prohibits using the tangential method for calculating the wellbore course especially when more accurate methods are available.  

"With my study and practice works performed on live well data, I observed that the calculations based on Tangential Method gives a considerably large value of departure and in some cases the well appears to be too shallow. In some deviated wells, the error in TVD was more than 40-50 feet."

BALANCED TANGENTIAL METHOD

The balanced tangential method uses the inclination and direction angles at the top and bottom of the course length to tangentially balance the two sets of measured angles. This method combines the trigonometric functions to provide the average inclination and direction angles which are used in standard computational procedures.

Directional Drilling: Balanced Tangential Method

From Balanced Tangential Method, following values are obtained:

ΔTVD = ΔMD/2 . (CosI₁ + CosI₂ )

ΔNorth = ΔMD/2 . [(SinI₁ x CosA₁) + (SinI₂ x CosA₁)]

ΔEast = ΔMD/2 . [(SinI₁ x SinA₁) + (SinI₂ x SinA₁)]

This technique provides a smoother curve which should more closely approximate the actual wellbore between surveys. The longer the distance between survey stations, the greater the possibility of error.

AVERAGE ANGLE METHOD

When using the average angle method, the inclination and azimuth at the lower and upper survey stations are mathematically averaged, and then the wellbore course is assumed to be tangential to the average inclination and azimuth.

Directional Drilling: Average Angle Method

From Average Angle Method, following values are obtained:

ΔTVD = ΔMD . Cos(I₁+I₂)/2

ΔNorth = ΔMD . Sin(I₁+I₂)/2 . Cos(A₁+A₂)/2

ΔEast = ΔMD/2 . Sin(I₁+I₂)/2 . Sin(A₁+A₂)/2

Since the average angle method is both fairly accurate and easy to calculate, it is the method that can be used in the field if a programmable calculator or computer is not available. The error will be small and well within the accuracy needed in the field provided the distance between surveys is not too great.

RADIUS OF CURVATURE METHOD

The radius of curvature method is currently considered to be one of the most accurate methods available. The method assumes the wellbore course is a smooth curve between the upper and lower survey stations. The curvature of the arc is determined by the survey inclinations and azimuths at the upper and lower survey stations as shown in Figure below. The length of the arc between I₁ and I₂ is the measured depth between surveys.

Directional Drilling: Radius of Curvature Method

The following values are obtained using radius of curvature method:

ΔTVD = [(180) (ΔMD) (SinI₂ – SinI₁ )] / π (I_{2 -} I₁)

ΔNorth = [(180)² (ΔMD) (CosI₁ – CosI₂) (SinA₂ – SinA₁) ] / π² (I_{2 -} I₁) (A₂ - A₁)

ΔEast = [(180)² (ΔMD) (CosI₁ – CosI₂) (CosA₁ – CosA₂) ] / π² (I_{2 -} I₁) (A₂ - A₁)

DEP = [(180) (ΔMD) (CosI₁ – CosI₂ )] / π (I_{2 -} I₁)

r = 180 / π (DLS)

ΔMD = (I_{2 -} I₁) / B_r .

Where,

π = 3.1415926

DLS = Dog Leg Severity

B_r =Build Rate

Note: In these equations, the inclination and azimuth are in degrees.

Since the calculation using this method becomes a tedious job, this method is not used in field practise unless a programmed software is available based on this method.

I have hands-on experience on few well planning softwares based on this method. In the coming blogs, we will discuss on "Directional Well Planing" and for calculation purpose, I will show you that how these softwares are utilized to design a directional well.

Here, its worth mentioning that when the value of inclination (I) and azimuth (A) are same at both survey stations, then the denominator for some of the above equation becomes zero and hence the equation is not defined. This is the ERROR we found using "Radius of Curvature Method".

SO HOW CAN WE OVERCOME THIS ERROR ??

One of the way to overcome this error is to use "Minimum Curvature Method" which has been discussed below.

The second method i would suggest is that, we can add any small number (say 1 x 10^-4  or 1 x 10^-5) to either survey points. The result thus produced will be insignificant .. !

MINIMUM CURVATURE METHOD

3. Applications of Directional Drilling

What is Directional Drilling?

Directional drilling is the science of deviating a wellbore along a planned path to a target located at a given lateral distance and direction from vertical. This includes drilling as vertically as possible from a given TVD.

The figure below shows a vertical well and a deviated well.

As a starter one can consider that any well which gets deviated from the vertical axis to achieve the desired target (hydrocarbon reserve in our case) may be termed as deviated well (or Directional well). 

Vertical Versus Deviated (Directional) Wells

What are the applications of Directional Drilling?

1. SIDE TRACKING

Sidetracking is one of the primary uses for directional drilling. Sidetracking is an operation which deflects the borehole by starting a new hole at any point above the bottom of the old hole as in Figure below. 

The primary reason for sidetracking is to bypass a fish which has been lost in the hole; however, there are several other reasons for sidetracking. A sidetrack can be performed so the bottom of the hole can intersect a producing formation at a more favorable position such as up dip above the oil-water contact. A well can be sidetracked to alleviate problems associated with water or gas coning. A sidetrack can be performed in an old well to move the location of the bottom of the hole from a depleted portion of the reservoir to a portion that is productive, such as, across a fault or permeability barrier. 

Most often, a sidetrack is accomplished by setting a cement plug in the hole and dressing off the plug to a depth at which the sidetrack will commence. The sidetrack can be either "blind" or "oriented". In a blind sidetrack, the direction of the sidetrack is not specified and is not considered a directional well. In either case, a deflecting tool is used to drill out the old hole and start a new hole.

Sidetracking a stuck BHA

2. STRAIGHT HOLE DRILLING

Straight hole drilling is a special case of directional drilling where an attempt is made to keep the hole vertical. Some reasons for wanting to keep the hole vertical are:

a.To keep from crossing lease lines;

b.To stay within the specifications of a drilling contract;

c.To stay within the well spacing requirements in a developed field (Figure below).

Straight hole Drilling

3. CONTROLLED DIRECTIONAL DRILLING

Controlled directional drilling is used when drilling multiple wells from an artificial structure such as offshore platforms, drilling pads, or man made islands (Figure below). The economics of building one offshore platform for each well would be prohibitive in most cases. However, since wells can be directionally drilled, forty or more wells can be drilled from a single platform. Without controlled directional drilling, most offshore drilling would not be economical.

Multiple Wells from an Artificial Structure

4. DRILLING MULTIPLE SANDS WITH A SINGLE WELLBORE

There are special cases when multiple sands are drilled with a single wellbore. Where steeply dipping sand zones are sealed by an unconformity, fault, or salt dome overhang, a number of vertical wells would be required to produce each sand, which are separated by a permeability barrier. However, all the sand zones can be penetrated with one directionally drilled well thereby greatly reducing the cost of production (Figure below).

Drilling Multiple Sands from a Single wellbore

5. INACCESSIBLE LOCATIONS

There are times when oil deposits lie under inaccessible locations such as towns, rivers, shorelines, mountains, or even production facilities (Figure BELOW). When a location cannot be constructed directly above the producing formation, the wellbore can be horizontally displaced by directional drilling. This allows production of an otherwise inaccessible hydrocarbon deposit.

Inaccessible location

6. FAULT DRILLING

Directional drilling is also applicable in fault drilling (Figure BELOW). It is sometimes difficult to drill a vertical well in a steeply dipping, inclined fault plane. Often, the bit will deflect when passing through the fault plane, and sometimes the bit will follow the fault plane. To avoid the problem, the well can be drilled on the upthrown or downthrown side of the fault and deflected into the producing formation. The bit will cross the fault at enough of an angle where the direction of the bit cannot change to follow the fault.

Fault Drilling

7. DRILLING SALT DOME REGION

Many oil fields are associated with the intrusion of salt domes. Directional drilling has been used to tap some of the oil which has been trapped by the intrusion of the salt. Instead of drilling through the salt overhangs, the wells can be directionally drilled adjacent to the salt dome and into the underlying traps as shown in Figure BELOW. However, since the development of salt saturated and oil based muds, the amount of directional drilling has decreased. It is difficult to drill long intervals of salt with fresh water muds. Directionally drilling around the salt, alleviates a lot of the problems associated with drilling salt.

Salt Dome Drilling

8. RELIEF WELL

A highly specialized application for directional drilling is the relief well. If a well blows out and is no longer accessible from the surface, then a relief well is drilled to intersect the uncontrolled well near the bottom (Figure). Water or mud are then pumped through the relief well and into the uncontrolled well. Since it is sometimes required that the relief well intersect the uncontrolled well, the directional drilling has to be extremely precise and requires special tools. Survey data is not accurate enough to intersect a wellbore at depth. Proximity logging is required when drilling relief wells.

 

Relief Well Drilling

9. DRILLING HORIZONTAL WELLS

Horizontal drilling is another special application of directional drilling and is used to increase the productivity of various formations (Figure below). One of the first applications for horizontal drilling was in vertically fractured reservoirs. In fractured reservoirs, a significant quantity of the production comes from fractures. Unless a vertical well encounters a fracture system, production rates will be low.

Horizontal drilling is used to produce thin oil zones with water or gas coning problems. The horizontal well is optimally placed in the oil leg of the reservoir. The oil can then be produced at high rates with much less pressure drawdown because of the amount of formation exposed to the wellbore.
Horizontal wells are used to increase productivity from low permeability reservoirs by increasing the amount of formation exposed to the wellbore. Additionally, numerous hydraulic fractures can be placed along a single wellbore to increase production and reduce the number of vertical wells required to drain the reservoir.

Horizontal wells can be used to maximize production from reservoirs which are not being efficiently drained by vertical wells.

Horizontal Drilling

10. DRILLING MULTILATERAL WELLS

Directional drilling can also be used to drill multilateral wells. Multilaterals are additional wells drilled from a parent wellbore as illustrated in Figure 1-11. Multilaterals can be as simple as an open hole sidetrack or it can be more complicated with a junction that is cased and has pressure isolation and reentry capabilities. Multilaterals are used where production can be incrementally increased with less capital costs. Multilaterals can be used offshore where the number of slots are limited. It is also used to place additional horizontal wells in a reservoir.

Multilateral Wells Drilled From a platform

11. EXTENDED REACH DRILLING
Another application of directional drilling is what is commonly termed extended reach drilling. As illustrated in Figure below, extended reach drilling is where wells have high inclinations and large horizontal displacements for the true vertical depth drilled. Extended reach drilling is used to develop reservoirs with fewer platforms or smaller sections of a reservoir where an additional platform cannot be economically justified. Extended reach drilling will become more popular as the cost of platforms in deeper water and severe environments becomes more expensive.

Extended Reach Drilling

Advances in technology have allowed operators to drill extended reach wells with very high HD/TVD ratios (the ratio of the horizontal displacement to true vertical depth). Wells have been drilled with HD/TVD ratios in excess of 6/1. In these wells the horizontal departure was more than six times the true vertical depth with the total measured depth exceeding 32,800 feet (10,000 m).