Research progress in natural gas hydrate reservoir stimulation

Natural gas hydrate (hereinafter referred to as hydrate) is a kind of future energy with great potential. At present, the apparent factor hindering commercial hydrate development is the unqualified gas production rate, so it is necessary to research stimulation means from the perspective of hydrate exploitation method, and reservoir stimulation technology is particularly important. After systematically reviewing hydrate reservoir stimulation theories and technologies, this paper divides the field of reservoir stimulation into three directions, i.e., hydraulic fracturing, near-well stimulation and chemical stimulation, summarizes the main research results, problems and challenges in these three directions, and carries out discussion and prospect. And the following research results are obtained. First, rapid progress has been made in the research of hydraulic fracturing and it results have verified the fracability of hydrate deposits. It is revealed that the fracturing mechanism of hydrate deposits is similar to the tensile failure of rocks. It is indicated that fractures can still make contribution to permeability improvement after they closed under confining pressure, which is beneficial to secondary opening. In addition, the effects of in-situ stress and fracturing fluid on fracturing behaviors of hydrate deposits are analyzed, the numerical model of fracturing process is established, the mechanisms of hydraulic fractures to improve the depressurization and heat injection effects of hydrate reservoirs are analyzed, and the improvement effects of hydraulic fractures, as high-permeability channels on gas production rate and other key production indexes are clarified. Second, the research of near-well stimulation reveals the washout mechanisms and laws of high-pressure water jet on hydrate deposits. It is verified by numerical simulation that water jet slotting, circumferential grouting column and stratified split grouting can effectively improve the production performance, but the research is in the stage of establishing gas production model to evaluate the stimulation effect and the feasibility and effectiveness needs verifying. Third, as for chemical stimulation, the potential of CO₂ caprock reconstruction and methanol huff and puff to improve the production performance of hydrate reservoirs is explored, and the feasibility of CO₂ caprock reconstruction and storage is analyzed. In conclusion, to deal with current problems and challenges, it is necessary to strengthen the construction of the reservoir stimulation theory and technology system with hydraulic fracturing as the leading role, near-well reservoir stimulation as the auxiliary and chemical stimulation as the supplement, so as to accelerate the industrialization process of hydrate resources.