Effects of a No-Proppant Foam Stimulation Treatment on a Coal-Seam Degasification Borehole
Authors: Mahoney James V. , Stubbs P.B. , Schwerer F.C. , Dobscha F.X. 
Published: December 2007

A vertical borehole drilled into a friable* coal seam was stimulated by using 53,000 gal (200.6 m,) of foam. Proppant materials were omitted from the treatment fluids and colored, fluorescent pigments were included. The borehole produced 6.6 MMcf (0. 187 x 106 MI 1 of methane at an average rate of 49 Mcf/D (1390 m /d) during its 4.5-month lifetime. After the wellbore was mined through, both vertical and horizontal fracture surfaces were found decorated with fluorescent material. No significant penetration of the fluid into the overlying strata was found.

Introduction

Foam hydraulic stimulation treatments have been used to increase methane production from vertical degasification boreholes at U.S. Steel's Oak Grove mine, near Birmingham, AL. Fifteen boreholes, in an area approximately 2 miles (3.2 km) northeast of the active workings, have been stimulated using approximately 50,000 gal (190 m3) of sand-laden foam fluid. Despite very low daily gas output from two of these holes, the average production has been more than 70 Mcf/D (2000 m3/d) for the past 11 months. Cumulatively, more than 450 MMcf (1.07 × 10(6) m3) of methane have been removed from these degasification holes.

Before foam-stimulated degasification holes can be accepted as part of the standard sequence for development of gassy mines, two major problems must be solved. The most important of these is the potential detrimental influence of the stimulation treatment on the integrity of the strata overlying the coal. In this regard, previous experiences at Oak Grove have not been encouraging. Cracks were observed in the mine roof immediately above the coal-seam portions of the fracture around two boreholes that had been stimulated with small treatments [~20,000 gal (76 m3)]. Although no serious problems have been encountered to date, mine personnel are concerned that these cracks may present difficulties during pillar extraction. The second problem is the excessive operating expenses experienced for boreholes stimulated with sand-laden foam treatments. Sand flow-back into the wellbore has resulted in premature degradation of the dewatering equipment, abundant downtime, and repetitive cleanout procedures. Each of these occurrences could jeopardize the cost effectiveness of this treatment design.

Proppant and Injection-Rate Criteria

Sand, or an alternative proppant, traditionally has been blended into the stimulation fluid to prevent ground stresses of the formation from closing the induced fracture after the fluid pressure has been released. Production characteristics of the boreholes at Oak Grove, however, suggest that the closure stresses in the Blue Creek coal seam are significantly less than those encountered in the strata for which stimulation treatments traditionally have been designed. It is, therefore, conceptually possible that a treatment conducted without proppant might provide gas productions comparable with those recorded on the existing sand-propped boreholes. Obviously, such a treatment would eliminate the sand-related operating problems that have been encountered.

JPT

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