Investigation of the artificial fracture conductivity of volcanic rocks from Permian igneous in the Sichuan Basin, China, with different stimulation method using an experiment approach

Abstract

Stimulation of oil and gas volcanic reservoirs is quite topical, while particular stimulation mechanisms of various fracturing technologies applied to volcanic rocks need to be clarified. This paper explores volcanic rock sample fracture surfaces' morphology, the acid etching depth, and/or proppant embedment depth under propped, acid etching, and propped-etching. The conductivity is an important evaluation index for the effect of stimulation treatment such as fracturing and acid fracturing, therefore, fracture conductivity under different conditions, such as proppant size and concentration, acid concentration and activity, the separate or joint application of acids and proppant, was evaluated with the samples from Permian igneous in the Sichuan Basin, China. The results indicate that the reaction between acid and rocks is hindered by the complexity and heterogeneity of mineral distribution, forming discrete dissolution holes in local areas instead of flow channels. The maximum embedding depth was 67 μm without acid etching, while those obtained after gelling and cross-linking acid etching were increased by 2.3–3.2 times, reaching 178 and 227 μm, respectively. The etching depth of the samples with the acid can reach 1000 μm. By the effect on conductivity, the increasing order was as follows: acid etching fracture, propped-etching fracture, and propped fracture, while there was a slight difference in conductivities of propped and propped-etching fractures. Therefore, hydraulic fracturing with proppant provides the volcanic reservoir's most effective stimulation. Acid-carrying-proppant fracturing would also be a lucrative stimulation technology if natural fractures are developed and contain acid-dissolved minerals. Acid fracturing failed to form the main fracture with high conductivity, and therefore is not recommended. This study is considered instrumental in clarifying the mechanism of different stimulation methods and substantiating the most efficient development of volcanic reservoirs. The research results can provide a reference for the optimization of stimulation method and the design of treatment parameters of volcanic reservoir.