A Novel Approach to Estimate Reserve and Forecast Production for Unconventional Gas Reservoirs by Developing New Reserve Correlation and Decline Model
Authors: Khalil Hazem Mohamed , Abdel Fattah Khaled Ahmed , Abdel Aziz Abdel Aziz Mohamed , Dahab Abdel Sattar Abdel Hamid 
Published: October 2022

With the continuous increase in the worldwide demand for natural gas due to continuous natural decline of the conventional gas reservoirs, petroleum companies are turning their attention to unconventional gas resources. Reserve estimation in unconventional reservoirs is the most important during the development of unconventional reservoirs because the reserve affects project economics. The estimation of reserves in unconventional reservoirs is challenging due to the long production period required to estimate reserve accurately. The main goal of this work is the development of a new Decline Curve Analysis (DCA) models and a new Estimated Ultimate Reserve (EUR) correlation to forecast production data in unconventional gas reservoirs early and accurately.

Recent decline curve analysis methods suffer from the limitation in use as they need a long production period to give accurate results and their calculations don't depend on the data of a reservoir's properties, the well and completion parameters, which are different from one reservoir to another. Several simulation models that model production from Multi Fracture Horizontal Wells (MFHW) are used to generate data representing different unconventional gas reservoirs. This data is used to study the effect of the reservoir and hydraulic fracture properties on the performance of unconventional gas reservoirs, then the most important parameters are used to build the EUR correlation and new decline models.

The traditional decline models do not take into consideration the reservoir and completion parameters and need a long production period to achieve a good forecast. The new decline models were developed to eliminate these limitations through developing new decline models that include reservoir parameters such as reservoir permeability and initial reservoir pressure. The novel EUR correlation is different from other published studies in that, the focus was not on the estimating EUR using well production data, but the focus was on estimation of EUR using reservoir and completion properties. Using this newly developed correlation, reservoir engineers can estimate the reserve before drilling using well design, completion, and reservoir parameters. The newly developed DCA and EUR correlation has been tested and verified using different numerical simulations and field data. For this validation cases, the production data is used to compare the new approach and the other widely used decline models, which indicate the accuracy of this new approach.

The advantages of these new developed DCA models are good accuracy compared to other DCA models because the new developed models depend on reservoir properties and this accuracy is achieved using short production history. This advantage helps better economic evaluation and well optimization of these unconventional resources as one of the important applications of these new approaches.EUR correlation is one of the first correlations developed for UGR and will help engineers for better economic evaluation and well optimization of these unconventional resources.