Well Completion

After drilling operation the hole is provided with casing and cementing is done. A sketch of typical completed well is shown in the figure 1.

Figure 1 Typical completed well

By logging, formation evaluation, drill stem testing, coring it is determined wheather a well should be completed for commercial oil production. The Well completions are of the following categories:

1. Open hole completion2. Conventional perforated completion3. Sand exclusion type and 4. Permanent type completion

Open Well CompletionIn this case the oil string is set on the top of the pay zone (The reservoir rock in which oil and gas are found in exploitable quantities). This type of completion is possible if the zone is competent and the hole wall does not cave.

After the pay zone is reached, the rotary drilling is stoped and oil string is set. Using cable tool drilling the mud from the hole is bailed out. Formation damage is eliminated by adopting proper cementing and mud technology.

For oil to flow from the reservoir to the hole hydrofracturing of the reservoir is carried out. Earlier nitroglycerine shooting was carried out to stimulate the reservoir for The hydrofracuring or glycerine shooting is done in the open hole.

Conventional Perforated CompletionThis category of completions is in wells, in which the oil string is set through the pay section, cemented and subsequently perforated at the desired interval. Such completions are extremely common and are feasible in all formations except those in which sand exclusion is a problem. The main factor in this type of completion is the performing process.

Earlier (before 1930) casing could be performed in place by mechanical perforator search perforator used a single blade or wheel type knife which could be opened at the desired level to cut vertical slots in the cosigned.

In the early 30s bullet perforating equipment was develop. In the bullet perforating a multi-barrels firearm is used. The devise is lowered into the well position at the desire level and fired. The high velocity bullet pierces through the casing and cementing. Large hole of 1.5” also can be obtain and it can facture the formation upto 2 meter. From the fractured reserve wire oil enters the production string through the perforated casing.

Bullet piercing is good in medium and soft rock. It gives maximum fracturing of cement and rocks. Generally it is cheaper.

Jet Perforating In this method penetration of the target is obtained from the jet stream’s high velocity impact. The velocity of the jet is in the order of 10000m/sec. There are retrievable and expendable jet perforating guns. The retrievable gun is composed of a cylindrical steel carrier with the charges opposite ports facing rdially from the vertical axis of the carrier. Expendable guns are composed of materials which disintegrate into small particles when the gun is fired.

Jet penetration is used for deeper penetration in hard rocks and multiple casing string. This cause minimimum shattering of the cementing.

Jet piercing is superior in dense formations and for penetration of multiple casing strings.

In softer targets bullet perforation is preferred. Bullet produces greater fracturing around and beyond the perforation which results in greater permeability, particularly in soft and/or shaly formations. Bullet gun is used in holes with temperature less than 275 degree Fahrenheit. Jet equipment can work up to 400 degree F.

For perforation job accuarate depth measurements are necessary. Proer well logging is essential for proper placement of the shot. The fluid presence in the hole plays a critical role in perforation practice. It is advantageous to perforate with well bore pressure lower than formation pressure.

Comparing Open Hole Completion and Perforated CompletionThe open hole method is initially cheaper, since perforation costs are not there. It also avoid contamination by cements. Perforated completion provide better control over the pay section as intervals can be perforated as desired. Individual sections can be isolated and selectively stimulated much more easily and satisfactorily through perforations than in open hole.

Hydraulic fracturing of zones is more successful in perforated completions. Productivity ratios of perforated wells are in general 50% higher than open hole completions.

Sand Exclusions Problem

The completion of well in an unconsolidated sand is not very easy. It is necessary to exclude production of sand along with oil. Sand can cause severe problem to the machinery due to its corrosive effects. The sand is eliminated by using screening. Screening is done by using slotted or screen liners or by using packing of the hole by aggregates such as gravels. The slotted or screen liner is run on tubing and hung inside the oil string opposite the producing zone. Figure 2 shows gravel packing for sand exclusion.

Figure 2

Sand may also be excluded by using consolidated plastic material which actually cements the sand grains together, thereby preventing caving.This reduces permeability and is not useful in low permeability dirty sand.

Permanent Type Completion

In this type the tubing run and the well head is assembled only once in the life of the well.

Well Stimulation

Well stimulation is means of increasing well productivity. It is done by

1. Nitro shooting2. Acidizing3. Hydraulic fracturing

The benefits and limitations of these methods are mentioned in the table below