Graded Proppant Injection For Well Stimulation In Unconventional Energy Production
Engineering Honours Degree, 2012
University of Adelaide
As conventional hydrocarbon reserves become less viable, global interest is increasing for unconventional reserves as an energy resource. These unconventional reserves, such as coal seam gas, do however present some production challenges with low reservoir permeability causing many fields to not be economically viable. In order to overcome economical hurdles, new production enhancing technologies need to be developed.
Stimulation by injection of proppant (which keeps the fractures open after switching to production) is widely used in unconventional energy reservoirs to increase well productivity. This is normally done using constant size proppant throughout the course of the stimulation process. The Australian School of Petroleum has recently proposed a new stimulation technique to increase the stimulation drainage area and incremental recovery, known as graded proppant injection. This involves initially injecting small proppant into the reservoir and then gradually increasing the size of the proppant during the injection process. This project has modelled fluid injection stage of this stimulation process accounting for the variation of stress within the reservoir during the injection period.
This project builds upon the initial research completed last year by the School of Mechanical Engineering for the injection of fluid and suspended particles into a natural fracture network. The project has involved the derivation of basic equations and the development of mathematical models describing the penetration of suspended particles into the fractured system of a coal seam. Models describing the change in permeability, cleat opening and productivity of a well have also been derived. The models developed have then been applied to a real coal seam field case assuming value of horizontal stress within the reservoir is not well known.
As the injection/production rate is increased in the developed model, dimensionless permeability and injectivity index also increases due to the increase in cleat opening. The horizontal stress has little effect on these dimensionless values; however it does have an effect on the injectivity index after proppant plugging and productivity index. This is a result of the increase in proppant concentration required in the cleat to minimise deformation during production resulting in lower cleat permeability. However this only the case when the horizontal stress is less than vertical stress as the stress normal to cleat is used to determine the permeability reduction factor. This factor is different for both the injectivity index after proppant plugging and productivity index. For the injectivity index it accounts for the reduction in permeability due to the presence of proppant within the cleat. The factor for productivity index also includes the reduction in permeability due to the deformation of the cleat around the proppant when injection ceases and fluid pressure is reduced.