Strengthening mylonitized soft-coal reservoirs by microbial mineralization

Abstract

Mylonitized-soft-coal is common throughout central China and results from the crushing of intact coal into fine particles under extreme tectonic stress. The fine particles result in very low permeability and hydraulic fracturing to recover natural gas is difficult due to the very low mechanical strength. Enhancing structural integrity and mechanical strength of these coals are the keys to efficiently extracting coalbed methane (CBM). The following explores using microbially-induced calcium carbonate cementation to strengthen and stiffen the coal – and therefore enable successful hydraulic fracturing. We explore the impact of this cementation on the mechanical properties and on microscopic mechanisms of failure for granular assemblages of four sizes. Results indicate significant strength gain after only short periods of biochemical reaction (hours to days) with four cycles of microbial injection (~2 days) yielding a maximum compressive strength (UCS) of ~12 MPa and a brittleness index of 0.17 exceeding that of hard coal. Notably, a higher calcium carbonate content does not automatically guarantee a higher strength- indicating that the distribution of the mineralization and the quality of the particle-particle bonding exerts key control. Also, for identical injection volumes, the resulting calcium carbonate content differs significantly with particle size – larger particle size samples can accommodate larger masses of calcium carbonate. Imaging by SEM indicates that precipitation first occurs on the particle surface, partially enveloping it, before creating particle-particle bonds – thus maintaining grain-pore and pore-pore fluid transport connectivity. As the void is occupied, the cementation process slows, halts, as bacteria and nutrient are expended, and further supply is limited. Therefore, for a fixed duration of supply, the interparticle space of the smaller particle-size will be the first to be bonded and the carbonate content of the smaller particle-size samples will be lower than that for larger particles.