Volumetric Growth and Hydraulic Conductivity of Naturally Fractured Reservoirs During Hydraulic Fracturing: A Case Study Using Australian Conditions

Abstract

Abstract The limitation of conventional hydraulic fracturing with two long wings of coplanar fractures is well recognized in the context of naturally fractured tight gas or Hot Dry Rock (HDR) geothermal reservoirs. This paper presents a 3D model for an alternative stimulation technology for such reservoirs. The model stochastically simulates actual reservoir representative natural fractures processing field data available from cores and logs. These simulated fractures are then analyzed for deformations with the combination of simple elastic structural mechanics and linear elastic fracture mechanics principles coupling with the injected fluid pressure and in-situ stresses. Finally, the hydraulic conductivity and the reservoir growth pattern are formulated as functions of fracture deformations. The applicability of the model has been verified using the data of actual fracture stimulation programs conducted in the Hijiori HDR site. It has been found that the model is capable of simulating actual natural fracture distribution in the reservoir. The model is finally applied to a series of numerical analysis with central Australian reservoir conditions to investigate the sensitivity of natural fracture parameters (e.g. size, density and orientation) and in-situ stresses to reservoir growth and conductivity. It is observed that the reservoir growth pattern is mainly influenced by fracture parameters and the relative magnitude and direction of in-situ stresses. Reservoirs with predominantly strike-slip and reverse faulting stress regimes and high deviatoric stresses are favorable for horizontally dominant reservoir growth - a pattern which is highly desirable for efficient HDR geothermal energy extraction. The information provided in the paper is directly applicable to HDR geothermal reservoir development with a high potential for new applications in tight gas reservoirs in which the abundance of natural fractures is so far posing significant complexity to conventional hydraulic fracturing, resulting in multiple fractures, high treatment pressure and premature screen-out. Introduction Despite many success stories of conventional hydraulic fracturing, where fractures are initiated and propagated by induced fluid pressure and retained by proppants, there are experiences of treatment failure in some regions, particularly those in Central Australia. Investigations suggest that these regions are usually subjected to high deviatoric stresses, i.e., the difference of maximum and minimum horizontal in-situ stresses is high, and pre-existing natural fractures1. Furthermore, conventional hydraulic fracturing has also been found to be inefficient in Hot Dry Rock (HDR) geothermal reservoirs - an emerging source of energy - because they contain naturally fractured high heat granite rocks. It is thus imperative that further studies will continue to improve the effectiveness of conventional hydraulic fracture technology addressing different issues which are believed to be responsible for treatment failures due to the above mentioned reservoir conditions. In parallel, an alternative technology should also be sought which would fundamentally incorporate such geological conditions and design effective hydraulic fracture stimulation taking the advantages of such geological conditions into account. This paper contributes to the evolution, understanding, modeling and application of such an alternative technology for hydraulic fracture design and evaluation. This alternative stimulation technology is known in various names, such as low-proppant, no-proppant, waterfrac treatments etc. Recently, it has been called ‘shear dilation’ treatment because of its underlying engineering principles.