Stimulation Techniques of Coalbed Methane Reservoirs

Abstract

<jats:p>Coalbed methane (CBM) plays an important role in securing world energy supply and transiting electricity generation from fossil fuel to renewables. CBM reservoirs are generally very tight and require effective stimulation to achieve economic extraction. In recent years, an increasing number of coal seam stimulation techniques were developed, but selecting the most suitable stimulation technique for a particular CBM reservoir condition is becoming increasingly challenging. Therefore, it is deemed very important to compare the effectiveness of different stimulation techniques in a meaningful way to guide future research directions in this area. In this paper, the stimulation techniques were firstly classified into different categories according to the stimulation mechanisms. Then, the associated principles, the history of advances, and challenges of different stimulation techniques were comprehensively reviewed. Two indexes were proposed to compare the stimulation effectiveness at the laboratory and field scales, respectively. Finally, the comparison and evaluation of each stimulation technique in respect to the stimulation effectiveness, influence range, duration, and environment were conducted in detail; the cryogenic liquid nitrogen stimulation technique receives the highest total score among the discussed laboratory-scale stimulation techniques. Hydraulic fracturing and gas injection stimulation techniques gain the highest total score among key field-scale stimulation techniques. Considering the time required for each stimulation method to take effect, high-voltage electric fracturing may have a greater potential in the future. This work is expected to help better select the optimal stimulation technique for reservoir specific conditions.</jats:p>

Reserves Estimation for Coalbed Methane Reservoirs: A Review

“…The techniques used in CBM stimulation can be categorized into three classes [37]: (i) mechanical stimulation, which includes hydraulic fracturing technique and formation blasting; (ii) thermal stimulation, such as using the cryogenic liquid nitrogen and heating, which intends to increase the rate of methane desorption, deteriorate the integrity of coal body, and weaken the mechanical strength of the coal as a result of temperature effect, which in turns lead to an improvement of permeability; (iii) chemical stimulation, such as acidification and microbial stimulation techniques, which focus mainly on dissolving the minerals in the coal to alter the porosity and enhance permeability.…”

“…Presently, hydraulic fracturing has become a standard stimulation method for tight oil and gas reservoirs. There is a challenge associated with such a stimulation technique, that is, the permeability of the coal reaches a peak value just after the fracturing process, but then the gas recovery declines as a result of the closure of the initiated fracture [37]. Thus, different improved hydraulic fracturing techniques have been proposed to overcome the problem, such as a graded particle injection and pulse hydraulic fracturing [39][40][41][42].…”

“…The permeability in the first two zones can be significantly improved. There are some obstacles associated with blasting technique, such as [37] (i) the inability to use deep-hole blasting in a high gas content coal seam; (ii) the possibility of causing roof and floor damage to the coal seam, which can make water to flow into the coal seam; and (iii) difficulty to construct an adequate borehole due to collapse, mainly in a soft coal seam.…”

“…The microbial stimulation technique involves using the microbial to destroy the small molecular substances in a coal seam, which in turn enhances the permeability and gas production. However, few field applications of microbial stimulation have been reported [37]. There are some challenges associated with microbial stimulation, such as (i) an appropriate microorganism needing to be selected, (ii) the difficulty of degrading high-rank coal by microbial stimulation, (iii) the long time it takes to notice the effect of the stimulation.…”

Study on Borehole Arrangement Methods for Gas Extraction by Hydraulic Slotting in Long-Distance Through-Coal Seam Tunnel

“…Cheng et al [7][8][9], Lu et al [10], Feng and Kang [11] Wang et al [12], etc. have studied preextraction coal seam permeability enhancement after coal mine underground hydraulic slotting, and practices have shown that this technology can quickly and extensively enhance the coal seam permeability and the preextraction effect, laying a foundation for quickly eliminating the danger of gas outburst [13,14].…”