All over North America, coalbed methane (CBM) is viewed as a tough way to make a buck. Margins are tight, and the very nature of the target makes it difficult to drill and produce profitably. Challenges include multiple thin zones, poor quality coals and seemingly endless volumes of formation water that must be bailed before the gas can flow to surface. Many operators have regarded coalbed prospects as risky ventures, only to be attempted as economically as possible. And they’re right. It’s an economic game, but like any game, there are two sides to every coin.

Alberta’s massive Manville coal is a deep, difficult target that spans the southern third of the province. Hard to get at, even harder to drill, the Manville has proved to be a formidable challenge, particularly where the coals are structurally complex. The first step in achieving a commercial well is to drill a wellbore where you want it. If you’re not in the coal, you can’t intersect any cleats and therefore the well won’t produce any gas.

Accordingly, the traditional approach has been to attempt to land wells in the coal seams and drill laterally as far as possible to achieve maximum coverage. The keyword is coverage. Heretofore, operators had experienced limited success in these structurally complex areas, achieving poor, 30% to 40%, to medium, 60% to 70%, coverage. Unfortunately, current economics dictates at least 90% coverage in the Manville to declare the well a commercial success. CBM in Alberta has largely been a low-tier play using low tier technology. Unfortunately, the results have been rather low tier too.

Too much hindsight
Many coalbed wells have been drilled that intersect the coal seams vertically. Production from these coals is largely dependant on the cleats and natural fractures, providing conductivity paths for the gas to flow to the well bore. Some are often augmented with the aid of hydraulic fracturing, but this can drive up costs and may narrow the profit margin. Accepting that greater coal exposure is needed, operators have invested in directionally drilled wells to try to intersect coal seams tangentially and drill along them. But without a way to tell where the bit was, this was like a shot in the dark.

Logging-while-drilling (LWD) systems offered a solution, and notwithstanding the added cost, some wells were attempted using conventional geosteering devices. Still, results were disappointing. With unerring hindsight, the logs could alert drillers that they had drilled out of the coal, but many valuable feet of coverage were lost trying to drill back into it. In some cases, particularly when crossing a fault, the coal seam was lost and the drillers were unable to find it again (Figure 1). These reactive geosteering technologies were unable to deliver the results that operators required.

Up Scope!
Like the famous submariner’s command implied, viewing targets through the periscope provided visual identification and ranging and was an invaluable tool for the silent service. It is appropriate that the name was borrowed to describe the world’s first real-time well placement and bed boundary mapping tool, the PeriScope 15 LWD system (Figure 2). Designed to confidently position well bores within thin and unpredictable zones, the system seemed to offer the optimum solution to the coalbed drilling problem. Although
it had never been used to drill coal before, indications were positive that
it would deliver on its promise and enhance the geosteering process.

Characterized by its transverse, axial and radial transmitters, the tool images the region surrounding the borehole to a radial distance of about 15 ft (4.6 m). Not only does it detect bed boundaries and fluid contacts characterized by their abrupt change in resistivity, it also measures the radial distance and direction to them. Data are presented as traditional log curves, but also, after inversion, in an axial borehole cross-section so drillers and operators can see where the borehole is headed with relation to the boundaries. A unique feature of the service is its ability to present polar plots whereby the operator can get an estimate of regional formation dip. This allows steering in any direction — up, down or sideways to avoid intersecting the boundary. Directional drillers have said, even when drilling at high rates of penetration, they typically get plenty of advance warning to execute their steering commands.

The Manville coal particularly
warranted this tool, mainly because there were some challenges known in advance by EnCana that conventional geosteering could not overcome. Not only were we challenged to land the well bore in the coal and keep it there, but we needed to be able to identify the highest quality zones within the coal seam and steer through them.

As with most coals, the Manville quality varies both laterally and vertically.

It is heterogeneous, with many undesirable areas of high ash content we wanted to avoid. From offset logs we’ve examined, we have been able to see the stratigraphy and identify
the “sweet spots” within the seam where we’d like to be. The forward looking technology makes this possible in addition to its ability to maintain over 90% coverage.

The cost of exiting the coal was substantial, even to the point of denying the well’s economic viability. Because of formation heterogeneity, the relationship of coverage to value is non-linear. For example, using a conventional LWD string, we might drill out of the seam, and take 330 ft (100 m) to drill back into it. In a linear world, that would cost us 10% of our production in a typical 3,300-ft (1,000-m) well, but because of the coal’s non uniformity, we might just miss the sweet spot in that exact section where we were out of the seam, and essentially kill any hope of commercial production. We believe that this has actually happened in other wells, and that’s the problem of drilling with a reactive tool. Our goal was 100% coverage and the tool is the only system that gives us that opportunity.

A different mind-set
Because the tool “scans” the region around the borehole, it must be operated in rotary mode. Accordingly, for best results it is run in conjunction with a rotary steerable system (RSS) which has the added benefits of improving penetration rate, steerability and borehole quality. To run the tools in combination requires a mind-set not commonly found in CBM country. Operators must change their focus from minimizing costs to maximizing profits. Fortunately, the added cost of the technology is more than compensated by improvements in drilling efficiency, borehole quality, and most of all, coverage in the productive interval. Every well on which the tool combination was run turned out to be commercially viable. More importantly, operators are requesting the system on subsequent coalbed wells, testifying to its applicability in this market.

Two cases point the way
EnCana planned to drill a horizontal well targeting the Mannville coals in central Alberta. The target zone was comprised of the Mikwan A and Mikwan B coals, which were parallel seams 23-ft (7-m) and 16.4-ft (5-m) thick, respectively, and separated by about 2 ft (0.6 m) of shale. Figure 1 shows the general relationship of the coal seams from a conventionally drilled offset well as it clearly illustrates two undesirable excursions from the target. Indications were that the coal seams were generally flat, “layer-caked” beds. Reality proved otherwise. EnCana drilled three 3,300-ft (1,000-m) wells in the lower Manville coal to test its commercial potential. Two wells were attempted with conventional geosteering tools. The third well represented the worldwide debut of the PeriScope 15 system in a coal well, and was a complete success with more than 90% coverage (Figure 3). Pre-job planning using experience gained from offset wells enabled the company to determine in advance if the resistivity contrast at the bed boundaries was sufficient to attempt well placement using the system. Besides candidate selection, planning tools were used to model the best wellbore trajectory, as well as project and quantify drilling risk. The plan was to land the well bore tangentially in the top of the Mikwan B using conventional geosteering with a retrievable resistivity tool, then switch to the LWD tool and PowerDrive RSS combination in a 4.75-in diameter bottomhole assembly (BHA), which would drill a 61¼8-in borehole. The job was monitored 24/7 using the Schlumberger Operation Support Center (OSC) located in EnCana’s office in Calgary.

The beds were of variable thickness, faulted and undulating, and it would be tough enough to stay in the coal let alone seek out the highest quality zones. There were several structural surprises, and a total of 14 steering commands were required. But the real-time display in the OSC enabled us to make decisions quickly, and with confidence. We felt we were in control of the situation at all times.

Figure 4 is a vertically exaggerated well sketch that illustrates the challenges we faced. Had we blindly followed the well plan we would have missed the coal entirely. However, the hi-tech BHA allowed us to find the top of the coal, enter it tangentially and stay within the undulating bed 99% of the time. Figure 5 is the actual real-time inversion correlated with the log curves (on top) that we were watching in the OSC. Sharp features, thought to be mega cleats, were identified and are labeled A, B and C.

It is very encouraging to see uptake of this technology in a market that traditionally has tight margins. Many operators in Western Canada have felt that their operations could not support the added cost of high technology solutions, but these initial cases suggest differently. Services like this can be economically justified in terms of their positive impact on profitability evidenced by prolific production volumes and slower decline rates. It’s essential to understand how the tool system is different and why it’s different. Then, given the right scenario, it is the only tool that gives you a chance at 100% coverage and as a bonus, allows you to find and stay in a chosen portion of the seam.

This paper, AADE-07-NTCE-13 Proactive Geosteering with Directional Deep Resistivity and Rotary Steerable Tool in Thin Coalbed Methane (CBM) Reservoirs, was presented at the 2007 AADE National Technical Conference and Exhibition.