Oil, gas, groundwater and mineral brines all reach the surface through a borehole whose wall acts as a cylindrical filter. Regardless of the commodity being produced, every well follows a similar production cycle: output rises after completion, reaches a peak and then gradually declines as reservoir pressure drops and the near-wellbore zone becomes plugged with fines, scale, paraffin, or other formation damage. Operators attempt to slow that decline using hydraulic fracturing, acidizing, pressure pulsing, thermal treatment and other stimulation methods, each with technical, economic, or environmental limitations.

Borehole Mining (BHM), originally developed for extracting unconsolidated mineral deposits, offers a fundamentally different approach. Rather than relying on chemical treatments or induced fractures, BHM mechanically enlarges the producing interval using high-pressure (HP) waterjets while simultaneously removing the damaged material surrounding the wellbore. The result is a larger filtration surface and lower flow resistance.

This article presents a field demonstration of BHM used to stimulate coalbed methane (CBM) production in the San Juan Basin of New Mexico. Although conducted more than three decades ago, the project remains the only publicly documented field application of BHM for well stimulation.

According to Darcy’s law, production rate depends on reservoir permeability, pressure differential, filtration area, fluid viscosity and flow path length. Reservoir pressure and fluid properties are largely fixed by nature. The parameter engineers can most directly influence is the filtration area — the interface through which fluids enter the well.

Conventional under-reaming enlarges this surface and can improve productivity. Borehole Mining goes considerably further. Using HP waterjets and continuous slurry removal, the BHM tool hydraulically excavates the damaged zone surrounding the well while enlarging the producing interval into an underground stope that may reach 10–16 m (33–52 ft) or more in diameter. The process simultaneously removes plugged material, increases filtration area and reduces flow resistance.

Unlike hydraulic fracturing, BHM achieves these improvements mechanically, without chemical additives and with precise control over the excavation geometry.

In August 1993, BHM was field-tested in four coalbed methane wells in the San Juan Basin, New Mexico, at depths ranging from 2,930 to 3,304 ft (893–1,007 m). The wells, owned by different operators, were completed with 7-in. casing, requiring a single-string BHM tool (Figure 1).

The casing itself served as the outer hydraulic conduit delivering HP water to the bottom-head assembly. A packer isolated the working interval, directing water through a dual-wall bottom head consisting of a drill bit, hydromonitor and Venturi eductor. Slurry generated during excavation returned continuously to the surface through the inner pipe for separation and recirculation in a closed-loop system. The entire operation was carried out using a conventional drilling rig equipped with a top drive (Figure 2).

The most successful test targeted three coal seams within a broader stratigraphic interval. Excavation required approximately 18 hours and selectively enlarged the coal horizons while leaving the surrounding waste rock largely undisturbed.

Production records from Phillips Petroleum’s Well 30-5#205 showed an immediate increase following the BHM workover. During the initial monitoring period, gas production reached nearly four times the well’s previous peak production of approximately 450 thousand standard cubic feet per day (Mscfd), well above the 1993 economic cutoff of about 180 Mscfd.

The CBM reservoir in this portion of the San Juan Basin is naturally over-pressured. Once the damaged near-wellbore zone had been removed and the filtration area substantially enlarged, methane entering the excavation generated a natural gas-lift effect that further enhanced production.

A longer production history obtained in 2002 presents a more complete picture of the well’s behavior. Rather than sustaining the sharp initial production spike, gas output increased steadily for approximately eight to nine months, reaching production peaks of about 850 to 900 Mscfd in 1994-1995, approximately double the well’s historical maximum before stimulation.

The discrepancy between the early monitoring results and the longer-term production record is most likely explained by a partial collapse within the excavation shortly after completion.

An additional factor may also have contributed. Prior to the BHM workover, the well had undergone an experimental coal-blasting treatment, meaning the reservoir had already been partially conditioned before hydraulic excavation began. Similar drill-and-blast followed by BHM was previously applied to anthracite deposits in Kazakhstan during the 1980s, although those projects were never documented in the English.

Regardless of the exact mechanisms, the field results were significant. The stimulated well remained above its economic production limit for more than a decade. Productive life increased from an expected 3.5 years to approximately 12 years, while cumulative revenue grew from roughly $730,000 to about $3.3 million. When production finally ceased in 2002, a second BHM cleanout was technically feasible but was never attempted.

Originally developed as a mining technology, Borehole Mining also proved capable of serving as an effective waterjet-based mechanical well-
stimulation method.

As demand grows for lower-impact methods of resource development, technologies that can leverage existing drilling infrastructure become increasingly attractive.

Dr. Gregory Abramov is the president and CEO of Borehole Mining International, Inc., based in Denver, Colo.