Enhanced Recovery of Coalbed Methane through Hydraulic Fracturing

Abstract

SPE Members Introduction Hydraulic fracturing has been used to accelerate removal of methane from coal seams ahead of underground mines. Originally, all of the gas that was produced was flared or vented. Over the past decade, however, there has been a growing past decade, however, there has been a growing awareness of the vast potential for recovery of methane gas from coal seams. This awareness is leading to the commercial exploration and development of coalbed methane reservoirs in many areas of the U.S. Coalbed methane production is viewed as a new and significant energy source. Associated methane gas from coal is now the primary source of natural gas for the state of Alabama and is rapidly becoming a major source of natural gas in the San Juan Basin of New Mexico and Colorado. The natural gas industry is still learnin how to complete wells and produce gas from coal-seam reservoirs. Compared to our knowledge of techniques for the completion and production of sandstone and limestone reservoirs, our knowledge in coal-seam reservoirs is minimal, at best. The success achieved in producing gas from shallow coal has encouraged some operators to explore for gas trapped in deep coal seams that will never be mined. One area of significant activity is the San Juan Basin. To optimize recovery from most of the wells drilled into deep coal seams, hydraulic fracturing treatments are required. Experience has shown that the design and execution of fracture treatments in coal seams is not straightforward. High injection pressures, complex fracture systems, screenouts and the production of proppant and coal production of proppant and coal fines after the treatment are typical problems facing the operator. The mechanical properties of coal are significantly different from conventional rocks. Conventional reservoir rocks typically have a value of Young's modulus in the range of 3 to 6 million psi. It is not uncommon to see laboratory measured psi. It is not uncommon to see laboratory measured values of Young's modulus for coal in the range of 100,000 to 1,000,000 psi. The low value of Young's modulus will result in the creation of very wide hydraulic fractures. Additionally, it has been found from mineback experiments that very complex fracture systems are usually created during a hydraulic fracturing treatment. Not only are multiple vertical fractures often created, but fractures propagating in multiple directions can be quite common. It is felt that this complex hydraulic fracture behavior is due to the presence of the cleat system, the complex structure of the coal matrix, and the stratigraphy of the coal seam with respect to the surrounding sediments. GAS PRODUCING MECHANISMS The mechanism of gas production from coal is substantially different than observed from conventiona reservoirs. There is littl or no free gas present in the coal. Most of the pore space in the cleat system is water saturated. Even though some free gas may exist in the cleat system, most of the gas is adsorbed on the surface of the coal. To produce this gas, the pressure in the cleat system must be reduced to cause the gas to desorb from the surface of the coal to the cleat system and diffuse through the coal matrix. Normally, significant volumes of water must be produced in order to lower the pressure in the produced in order to lower the pressure in the cleat system so that gas desorbtion can begin. Because gas desorption is the primary source of production, the gas flow rate from a coal seam may production, the gas flow rate from a coal seam may increase with time. P. 43

Lessons Learned From Modeling Hydraulic Fracture Treatments in Coals Using a Fully Functional Three Dimensional Fracture Model in a San Juan Basin Project

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] et al 1 investigated how well the fractures are contained in coal during a stimulation treatment. In this study, 12 vertical wells were drilled, completed, and subsequently evaluated via mining excavation.…”

Stimulation of coal seam permeability by micro-sized graded proppant placement using selective fluid properties

“…during the hydraulic fracturing. The primary role of hydraulic fracturing in coal seams is to create an improved connection of the natural fracture and cleat network to the wellbore according to [29]. However, based on highly instrumented experiments in coals, varying dimensions of the created hydraulic fracture are unlikely to accept the main body of proppant introduced into the latter stages of the treatment to maintain hydraulic fracture conductivity as discussed in [30].…”

Numerical simulation of hydraulic fracturing coalbed methane reservoir with independent fracture grid

“…Boyer et al [4] conducted Laboratory and field tests to establish criteria for containment of an induced hydraulic fracture. Holditch et al [5] utilized hydraulic fracturing treatments to optimize recovery from most of the wells that drilled into deep coal seams. Wright et al [6] summarized a few enhancements for coal seam fracturing technology, as well as the present limitations and the necessary advancements required for superior coal seam fracture performance in future.…”