After the ball is at the surface, lower the tubing string slowly while reverse circulating to clean out the casing to bottom

Monitor pump pressure closely. Large pieces of coal can plug the small opening at the bottom of the tool and cause a rapid increase in pump pressure.

*

You may circulate nitrogen instead of water for slotting. Nitro-gen can enhance cutting because it removes cuttings more quickly than water. It also ensures returns to the surface.

Though most coal seams can support the hydrostatic pressure of a column of water, some seams may not. If a seam cannot support a water column, you would not be able to observe the returns at the surface. In such cases, using nitrogen will lower the hydrostatic pressure on the seam and allow you to get returns to the surface. Though nitrogen is more expensive than water, the additional cost may be offset by reduced slotting time.

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Interseam Completion Restricted Access Completion

A special formation access technique called “restricted access completion” was developed at the Rock Creek project to propagate more effective fractures. Restricted access refers to the technique of perforating only one seam in a group of thin, closely-spaced seams. The primary objective of restricting access to a single seam is to stimulate multiple seams (via the single seam) without the expense

of perforating and treating each thin seam individually. The tech-nique also offers the following potential benefits:

❖ Helps prevent high pressures during stimulation

❖ Helps prevent propagation of multiple parallel fractures

❖ Reduces excessive fracture height growth

❖ Reduces migration of sand and coal fines

Tests conducted at the Rock Creek project demonstrated the effective-ness of the restricted access technique. Monitor well data, interference testing, dye tracing, and reservoir modelling all confirmed that frac-ture treatments initiated at the bottom of the multi-layer Black Creek interval are at least as effective as fracturing stimulations conducted each individual layer of the interval. Figure 4-6 illustrates the fracture communication created in the Black Creek Coal Group by the re-stricted access completion technique.

Special Formation Access Techniques

In the Black Warrior Basin, three specialized formation access techniques have been used to complete multiseam wells. These techniques are:

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Restricted Access Completion

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Limited Entry Completion

The research from the Rock Creek project showed that restricted access completions are effective in a closely-spaced group of coal seams if:

❖ There are no barriers to fracture height growth between the coal seams (i.e., the in-situ stresses of the formations be-tween the seams are not significantly higher than the stress in the coal.)

❖ Stress profiles yield vertical rather than horizontal fractures

❖ Barriers to fracture height growth exist above and below the bottom coal seam

Figure 4-6

Fracture Communication Created by the Restricted Access Completion Technique

Limited Entry Completion

The limited entry completion method allows you to simultaneously stimulate a group of coal seams in a well instead of stimulating them separately, which requires downhole equipment and sand plugs for isolation. You can use the limited entry method only in perforated cased hole completions.

Figure 4-7 shows a typical limited entry multiple-zone completion.

You can use the limited entry technique to hydraulically fracture several zones with different rock properties and in-situ stresses.

You control stimulation treatments that require different initiation and propagation pressures by the number and size of perforations

Figure 4-7

Limited Entry Multiple-Zone Completion

you place across each zone. Adjusting the number and size of perforations at each zone controls the friction pressure through the perforations, which gives you some control over the treating pres-sure at each zone.

The main benefit of a properly designed limited entry completion is the ability to fracture multiple seams with one treatment. To be successful, a limited entry completion must be designed to provide sufficient rate into each seam to adequately widen and extend the fracture.

The ability to widen and extend the fracture is a function of the mechanical properties (Young’s Modulus and Poisson’s Ratio) of the formations. To properly design a limited entry completion, you must analyze the mechanical properties of each coal seam and its adjacent formations and account for the differing properties of each zone in the design. A design with perforations based solely on the thickness of the coal seam will likely not be an optimum design.

An improperly designed limited entry completion could cause the problems listed below:

❖ Propagation of a fracture at each set of perfora-tions may result in a shorter fracture length and a greater fracture height than desired

❖ Inadequate injection rates into each set of perfo-rations which could cause poor proppant trans-port, excessive fluid leakoff, and potential bridg-ing in the fracture because of insufficient fracture width

The limited entry completion will likely be most successful when used to fracture multiple coal seams in the same coal group where there are confining barriers between the seams to be fractured. (A confining barrier is a zone that has a higher stress than the zone to be fractured and is thick enough to restrict the height growth of the induced fracture.) If the seams do not have confining barriers between them, a restricted access completion may work more effectively than a limited access completion. The restricted access completion is discussed in the next section.

The limited entry completion has not been as successful for fracturing multiple coal groups as it has for fracturing multiple seams within one

coal group. This variation in performance might be due in part to the degree of difference in the mechanical properties of the formations between the coal groups.

Interseam Completions

This completion technique involves perforating through casing into rock partings above, below, or between coal seams rather than directly into the seams.

Interseam completions have been attempted in the Black Warrior Basin to complete coal zones for which conventional fracture stimu-lation pressures were excessive. In the basin, unusually high treating pressures are sometimes encountered while attempting to fracture coal seams. In some cases, treating pressures may reach the safe limit for the casing before the operator has established any substantial injection rate.

At the Rock Creek project, two separate coal groups in Well P5 were completed using the interseam technique. Though high treating pressures have not presented problems at Rock Creek, the interseam technique was used to assess its effectiveness. Despite having no coal directly connected to the wellbore, Well P5 became one of the highest sustained gas producers in the field following its interseam comple-tion. In addition, Well P5 required no pump repairs for 2 1/2 years.

(On average, wells in the Black Warrior Basin require pump work 2-4 times a year). The results of this one test certainly cannot be considered conclusive; however, they do indicate that interseam completion may be effective for some wells.

Figure 4-8 shows the lithology and location of perforations for the Well P5 interseam completion.

The decision to complete wells with the interseam technique should be based on a thorough understanding of the in-situ stresses of the coal seams and surrounding strata.

Though the usefulness of the interseam completion technique requires further investigation, the technique may offer the advantages listed below:

❖ High treatment pressures may be avoided

❖ Several target seams may be connected with a single treatment

❖ A propped fracture initiated in interseam strata may inhibit plugging by migrating coal fines and proppant during produc-tion

Figure 4-8

Lithology of the Well P5 Interseam Completion

In document A Guide to Coal Bed Methane Operations (Page 157-164)