MULTISTAGE HYDRAULIC FRACTURING WITH PRODUCTION INTERVALS—A PROMISING TECHNOLOGY FOR RESERVOIRS SUBJECT TO REVERSE FAULTING STRESS REGIMES

Abstract

The performance of hydraulic fracturing technology has not been so promising for some Australian tight-gas reservoirs. The existence of reverse faulting stress regimes (i.e. the vertical stress is the minimum one) in these reservoirs is found to be one reason among many others. Previous studies have established that the vertical hydraulic fracture initiated from a vertical well in a reverse faulting stress regime severely turns and twists to become horizontal while fracturing fluid is injected for further propagation of the fracture. This severely turned and twisted fracture impedes the fluid and proppant (engineered sand grains) injection and thus the fracturing job results in a short and constricted fracture. This is considered to be one of the major reasons for premature screen-outs that occur at extremely high-pressure on many occasions in the field, and the subsequent disappointingly low production rates. The aim of this paper is to present the results of an investigation with a model-scale gas reservoir to avoid this problem by carrying out the fracture treatments in a number of stages with production intervals. The basic mechanism that would allow the growth of a long, planar, productive fracture in such a manner is the production-induced stress change around the fracture tip. A simplified propped fracture configuration is modelled in a hypothetical small-scale reservoir with idealistic material properties. Production is simulated in time by varying different parameters and the production-induced stress changes are characterised by coupled fluid flow and deformation analysis. It is found from parametric results that the non-uniform reservoir pressure depletion induces a suitable stress state at the fracture tip for further planar propagation. The duration of production to induce the suitable stress state is found to be dependent on a number of parameters. The paper also highlights the implications and limitations of the concept for hydraulic fracturing in the mentioned reservoir conditions, and discussed further research directions.