By Petroleum Technology Transfer Council

Coiled tubing, horizontal underbalanced drilling has advantages in British Columbia

Operational ease, safety and reduced damage are among reasons for using this method in underpressured, compartmentalized carbonate reservoirs.

Trevor Dufresne, Penn West Energy Trust, Canada; and Don McClatchie and Tim Leshchyshyn, BJ Services Company, Canada

The underpressured, compartmentalized nature of the Upper Devonian, Jean Marie carbonate reservoir in northeastern British Columbia, Canada, lends itself to development with underbalanced horizontal drilling. More than 900 horizontal wells were included in a recent study, covering all well operators in the area. Both jointed pipe and coiled tubing (CT) have been used for drilling, and each method has its advantages and disadvantages. Costs for each approach are about the same, so operational ease, safety and reduced reservoir damage are the main drivers for drilling using coiled tubing. Penn West Energy Trust has drilled a sufficient number of wells, using both techniques, to make a valid comparison.

Fig. 1. Typical underbalanced CTD rig in Canada and the US.

THE JEAN MARIE FORMATION

A large amount of underbalanced drilling has been done in the Jean Marie formation. The principal area of activity is in northeastern British Columbia, a winter-only access region.

Regionally, the Jean Marie includes of stacked patch reef or barrier reef build-ups, and is from 15 m (49 ft) to over 100 m (328 ft) thick. The best Jean Marie reservoir is present where the zone has undergone porosity and permeability enhancement through leaching and compactional fracturing. The formation is an underpressured gas reservoir, with porosity up to 9% and permeability between 0.5 and 10 md, as determined from core analysis.

The reservoir is characterized by higher permeability gas flow paths through vuggy porosity and fractures, as well as low-permeability gas storage in leached limestones and dolomites. Patch reefs range in size and density but are connected through lower-permeability debris fans. Being underpressured with low water saturation, the Jean Marie is extremely sensitive to fluid invasion. Historically, vertical wells were drilled with a water-based mud that resulted in highly overbalanced pressures and mud invasion, hence lower production rates.

Underbalanced horizontal drilling technology is effective in the Jean Marie, because it helps overcome problems with compartmentalization, a dual-permeability system, underpressured gas and low water saturation. Horizontal length allows the wellbore to contact more than one high-permeability patch reef. Drilling underbalanced preserves the natural high permeability flowpaths already present by keeping damaging drilling fluid out of the reservoir.

CT VS. JOINTED PIPE UNDERBALANCED DRILLING

More than 1,000 horizontal wells have been drilled and are producing from the Jean Marie at an average vertical depth of 1,650 m (5,413 ft), with single-, dual- and triple-leg horizontal sections from 300 m (987 ft) to 1,000 m (3,281 ft) in length. Horizontal Jean Marie wells are generally two to four times more productive than vertical wells, when the first 9,000 operating hours are analyzed in year-on-year comparisons.

For over a decade now, CT units and jointed pipe rotary rigs have competed head-to-head in the underbalanced drilling market. The strength of jointed pipe rigs lies in their mechanical capabilities. Greater “push” and “pull,” and the ability to rotate the drillstring, often result in greater horizontal reach. Jointed pipe also has a few more options for dealing with hole problems and fishing operations. CT rigs, on the other hand, enjoy the luxury of a continuous drillstring. No threaded connections to make or break result in faster trip times and reduced safety risks to drilling crews, as pipe handling is eliminated. More importantly, the absence of threaded connections means there is no need to stop circulating to make a connection. For underbalanced drilling, it is commonly accepted that continual circulation results in a more controllable and steady BHP.

In fact, the ability of underbalanced CT drilling to limit pressure fluctuations is one of its greatest advantages when drilling easily damaged formations like the Jean Marie. When drilling underbalanced with jointed pipe and using a foam system, the BHP drops when a connection is broken to pick up the next section of pipe. The foam, which is 60% to 85% gas, breaks down over time when not being sheared due to pumping. The foam then has the potential to partially separate into two phases, gas and free water, in the wellbore.

Once the connection is complete, the mud pumps are turned back on, and a slug of foam, gas and free water is pushed up the wellbore, resulting in a downhole pressure spike until the denser slug is pushed out of the hole. These pressure spikes, if they exceed reservoir pressure (see Fig. 2), can quickly damage the Jean Marie’s fragile permeability system. In comparison, when using mist, 80% to 85% gas, in conjunction with coiled tubing, no connections result in fewer pressure fluctuations and therefore less formation damage.

Fig. 2. Pressure profile: Coiled tubing vs. jointed pipe.

GENERAL BASIN COILED TUBING DRILLING EXPERIENCE

When several years of horizontal underbalanced drilling in the Jean Marie are analyzed, some consistent trends emerge. In jointed pipe operations drilling with foam, gas production rates while drilling generally decline as the hole length is increased. This can be attributed to the number of hours spent with the formation exposed to the drilling fluid, as well as higher friction pressure, due to the long horizontal length.

It is difficult to determine the most significant damage mechanism, as it could be fluid breakout, poor control of foam quality, a higher percentage of liquid in the drilling media or pressure spikes during pipe connections. Yet, wells drilled using the underbalanced CT approach with a mist drilling fluid system typically do not see this decline. A typical CT drilling operation using mist sees production rates steadily increase while drilling to TD.

SIDETRACKING WITH COILED TUBING

An additional advantage of the CT technique is the ability to perform open-hole sidetracks quickly, easily and often with higher build rates than conventional, jointed pipe drilling. The reservoir’s faulted nature, in some cases, results in discontinuities along the wellbore. In crossing a fault, the wellbore may abruptly pass from productive Jean Marie limestone to tight shale in just a few meters. When the real-time gamma sensor in the CT bottomhole assembly detects this transition, the bit can be pulled back to immediately begin an open-hole sidetrack at a geologically suitable point. Thus, the wellbore’s azimuth or vertical depth can be adjusted to drill below the shale and back into the productive Jean Marie porosity.

SUMMARY

In this application, the principle advantages of CT over conventional jointed pipe have a direct impact on field development economics. Advantages, such as rig safety, result in reduced operating costs and lower crew turnover. Increased net rate of penetration reduces the time on the well. Better control of BHP reduces formation damage, yielding improved production. Quick and easy sidetracks allow the wellbore trajectory to be redirected when faults are encountered, and thus, allow more contact with a productive reservoir. Disadvantages lie in having higher move costs, operational limitations with push/ pull strength and shorter horizontal reach. The ultimate test of any technology is whether it improves the economics of a field development. 

THE AUTHORS
	Trevor Dufresne is a senior geologist (Geol. I.T.) at Penn West Energy Trust. He has six years in the oil and gas industry, and has worked properties in northeastern British Columbia for three years. He has a geology degree from the University of Alberta. E-mail: trevor.dufresne@pennwest.com
	Don McClatchie is the global business development manager for Coiled Tubing Drilling at BJ Services Company. He has 11 years in the coiled tubing industry, in a variety of operations, research and management roles. He has authored a number of technical papers on coiled tubing applications. Mr. McClatchie has a mechanical engineering degree from the University of Alberta. E-mail: dmcclatchie@bjservices.ca.
	Timothy Leshchyshyn is the senior technical specialist for Special Projects in the Stimulation Department at BJ Services Company Canada. He is a professional engineer with 10 years of experience. He manages BJ Services’ PayZone historic treatment archive and maintains their library of engineering design software. An active author and speaker for professional societies, Tim has a chemical engineering degree from the University of Alberta. E-mail: timleshchyshyn@bjservices.ca.