HORIZONTAL DRILLING Horizontal drilling makes re-entry attractive in US Gulf

Applications show quick payback from re-entries involving behind pipe zones, attic oil, and bypassed wellbore damage

Mike Cooney
Keith Fisher
Baker Hughes INTEQ

Since the mid-1980s, horizontal drilling has helped operators solve production problems and improve well productivity in a variety of applications. Once a novelty, horizontal drilling has become a mainstay of development programs in oilfields around the world.

More recently, horizontal drilling has been used as a re-entry method to improve production of existing offshore wells. This article presents applications and case histories of special horizontal drilling techniques being used in mature reservoirs in the Gulf of Mexico.

Horizontal re-entry technology permits production of reserves that otherwise could not be recovered economically. Successful re-entry projects require a multi-disciplined approach that includes reservoir analysis, casing exit, directional drilling, fluids, formation evaluation and completion technology.

When properly implemented, horizontal re-entry programs can reach otherwise unattainable reserves and extend the economic life of offshore fields in the Gulf of Mexico and other areas.

US Gulf of Mexico

The first Gulf of Mexico horizontal well was drilled in 1990. Since then, more than 100 horizontal wells have been completed in the Gulf, mostly offshore Louisiana, to tap both gas and oil reservoirs. Baker Hughes INTEQ has been involved in 75% -80% of the Gulf of Mexico horizontal wells to date. Interest in horizontal drilling in this offshore province accelerated in 1994, with approximately 35 horizontals completed.

A number of factors contribute to the increase in horizontal drilling in the Gulf of Mexico:

Productivity: A horizontal well provides productivity that is 2-5 times higher than typically achieved by a directional well in the same area. The long horizontal section has more contact with the reservoir and has a Kh (permeability height term) that is 5-20 times greater than a standard directional well. Horizontal wells also are ideal candidates for low cost open-hole completions that don't require cemented casing or perforations.
Recovery: Because of their increased exposure to the reservoir, horizontal wells can maximize ultimate recovery. One reason for this is that for a given production rate, a horizontal well will produce with a lower drawdown pressure at the sand face than a conventional well. This lower drawdown pressure results in reduced coning problems and enables the well to maintain economic production rates even after the reservoir pressure has been depleted significantly.

Re-entry application

Horizontal drilling also can be used offshore to re-enter and recomplete existing wells and extend the economic life of the platform and production infrastructure. Horizontal wells can access reserves that are beyond the reach of conventional wells. In short, this technology enables operators to make money by producing hydrocarbons that otherwise would be lost forever.

Re-entry wells - because they utilize an existing production platform and a previously constructed wellbore from the surface to the kickoff point - are inherently less expensive than new wells.

Because of the smaller hole sizes, a re-drill can require a smaller, less-expensive rig, such as a workover unit. With thorough planning and skilled rigsite personnel, one Gulf of Mexico operator recently drilled and completed a series of horizontal re-entries for an average of 37% of the cost of the original development wells from the platform.

Re-drills also benefit from better geological control. The original well data, as well as new 3D seismic interpretation and reservoir modeling, can be used to identify new target objectives and limit the risk of drilling a dry re-entry hole.

For example, horizontal re-entry techniques can be used to produce zones that were not targeted in the original development program. These may be shallower zones behind existing casing strings. In older domed structures, existing wells can be sidetracked and drilled up structure to recover attic oil flight along the flanks of the dome.

Horizontal drilling also can be used for remedial purposes. A horizontal sidetrack can bypass near-wellbore damage (such as mud invasion, precipitates, or migrating fines) more effectively than a simple remedial treatment or expensive hydraulic fracturing job. Horizontal sidetracks can redirect the well around coned zones of water or gas to increase oil production. A horizontal sidetrack also may be less risky and more economical than a major workover for salvaging a junked well and bringing it back on production.

In all these cases, incremental production can significantly improve the return on investment for existing structures (e.g., platforms, pipelines, production facilities) without major new capital investment.

Re-entry drilling

Once the reservoir is evaluated, targets are chosen, and the completion is designed, the horizontal re-entry process begins at the rig with the casing exit procedure. Depending on the application, the sidetrack can be initiated through the existing casing string, either by milling a section or by cutting a window using a casing whip stock.

In a section mill operation, the entire casing is milled up for a length of 60-100 ft. A cement plug is then placed across this section to be used as a positive kickoff plug to exit the casing.

When cutting a window, the whipstock is set at the desired kickoff point using a permanent packer. A milling assembly then cuts a section out of one side of the casing in the desired direction. This method has been used most frequently in Gulf of Mexico wells.

After casing exit, a steerable drilling assembly with a measurement while drilling (MWD) system is used to build the curve and drill the horizontal section. Tricone or PDC bits are selected to match the formation and the motor.

The drill string may be inverted, with HWDP run above conventional drill pipe. Proper bottonhole assembly and drill string design requires accurate modeling of drillstring torque, drag, and buckling loads.

The MWD system can include formation evaluation sensors, such as gamma ray and resistivity devices. These readings are important in horizontal drilling because they are used to geosteer the well. Prejob log response modeling enables the directional drilling team to detect bed boundaries or fluid contacts before the bit crosses them. This method has enabled drillers to land the well in zones as thin as six ft and keep it within the tight target for the entire horizontal interval.

In short radius re-entries, the bottomhole assembly includes an articulated motor and a flexible MWD system, for drilling radii of 40-100 ft. High yield strength tubing is used most frequently as the drill string in short radius wells.

Drill-in fluids

Some industry estimates suggest that fluid and cuttings infiltration in Gulf of Mexico wells typically results in skin damage in the 10-20 range, even after perforating. This problem could be compounded in horizontal wells where drilling mud is in contact with the reservoir for longer periods of time.

Additionally, the preferred open hole completion does not provide for perforations which could penetrate through a mud-invaded zone. Non-damaging drill-in fluids, specially formulated with bridging agents to minimize invasion, are therefore extremely important in horizontal applications.

These fluids protect the payzone from damage while the horizontal section is being drilled and their impermeable filter cake typically can be removed by produced fluid without use of a breaker.

Completion options

Most horizontal wells in the Gulf of Mexico are completed with some form of open hole completion with sand control capabilities. The decision to eliminate the cemented production casing is driven by both technical and economic issues.

For example, the objective may be to produce relatively small reserves that remain in place, so the long-term remedial options offered by a cased and cemented wellbore or a gravel packed open hole may not be advantageous. Under proper reservoir conditions, the advantages of the barefoot approach would be:

Eliminates cementing, perforating, and gravel packing long intervals of high angle wells.
Minimum rig time required to install prepacked screen and/or slotted liner.
Minimal flow restrictions.
Relatively low cost.

When completing medium-radius wells with an open hole method, some means of zonal isolation usually is required, because the typical Gulf Coast medium radius well will cross several zones between the casing exit and pay zone.

A cemented intermediate casing string usually is set in the objective sand or the shale immediately above it. The drawbacks to this approach are the cost of this step and the reduced hole size of the final hole section.

External casing packers (ECP) can be used as an alternative isolation method. In these cases, the entire re-entry well from casing exit to total depth can be drilled as a single hole section. Casing, ECP, and prepack screen are run simultaneously. The ECP is inflated at the top of the sand, and the casing string above it may be cemented in place through a staged cementing collar.

In a short radius well, the completion may not require zonal isolation. For example, in a well drilled with a 40 ft radius, the casing can be exited either in the upper portion of the pay interval or in the adjacent overlaying shale. In these cases, the required isolation can be provided by the overlaying shale and/or the original casing.

Case histories

Case 1: One operator has successfully re-entered and horizontally drilled from eight wells off a single Gulf of Mexico platform, which was suffering from high water production. The operator used horizontal re-entry techniques to drill along the formation strike and recover the attic oil trapped along the upper flanks of the structure between the water and gas cap at about 9,000 ft true vertical depth. Horizontal intervals varied from 400 ft to 1000 ft, and were completed with a prepacked screen.
On average, the horizontal sidetrack wells were about six times more productive than low-angle conventional sidetracks drilled and completed from the same platform. Production from these wells recovered redrilling costs within two months of recompletion.
Case 2: In another re-entry application, a different Gull of Mexico operator wanted to exit 7- 5/8 -in. casing at 9,000 true vertical depth and drill horizontally through two oil bearing sand lobes separated by a thin shale interval. The entire re-entry section was drilled in 6-3/4-in. hole, and the completion assembly - consisting of liner, prepack screen and external casing packers - was run in one trip.
Two sets of tandem external casing packers were used in this completion. The upper set served to isolate the overlaying water zone. The lower set was placed across the intermediate shale interval to provide for shutting off any future water production from the lower sand by setting a through-tubing inflatable bridge plug between the packers.
Before re-entry, this well was producing 80 b/d of oil with a water-oil ratio of 3:1. In the first two months after the horizontal recompletion, the well averaged 1,000 b/d with a 0.2 WOR.
Case 3: Another operator drilled a complex wellpath to align the horizontal well with the formation strike after exiting a window through seven-in. casing. The build-and-turn curve was drilled in six-in. hole, then isolated with 5-1/2-in. casing. The 1,000 ft horizontal section was drilled in 4-3/4-in. hole with a drill-in fluid at 85 of inclination to angle across a laminated sand.
Completed with a pre-packed screen, the well produced an average of 60 MMcfd of gas for the first six months of production and paid out in 35 days. Two wells drilled downdip in the 1970s had initial production rates of 15-20 MMcfd.

Future of re-entry

By combining low cost with high productivity, horizontal re-entry technology promises to extend the economic life of many offshore fields.

However, projects must be carefully evaluated, planned and executed to match the right technology to the specific application. Experts from a full range of technical disciplines will contribute to the success of this process. Although it is no a panacea, the re-entry approach has the potential to increase production and improve profits for oil company operators.